Philips PM 2520 Service and user manual

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led. Ver. v. Historie v/d Radio

Met dank aan Hein Ros

PHILIPS

Amplifier Voltmeter PM 2520

66 403 45.1-10

20/564/01

Manual

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Survey of figures

1 Block diagram 6
2 Rear view 10
3 Front view with controls 13
4 Various ways of connection 14
5 Attenuator circuit 17
6 Quadratic network 17
7 Current-voltage characteristic 18
8 Parabolic curve
9 Removing the knob 22
10 Bottom view of interior 25
11 Right-hand side view 27
12 Left-hand side view 29
13 Pulse shape 30
14 Mains-transformer 35
15 Bottom view of exterior 41
16 Top view 41
17 Rear view without rear plate 42
18 Switch segments 43
19 Unit U1. (left), switch plate 44
20 Unit U1, (right), switch plate 44
21 Unit U2, calibration voltage generator 4 4
22 Unit U3. pre-amplifier 45
23 Unit U4, supply unit 40
24 Unit U5, output stage 47
25 Unit U6. quadratic network 48
26 Measuring cable 49
27 Circuit diagram 53
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Contents

GENI ERAL PART 5
I. Introduction 5
H. Description of the block diagram 6
III. Technical data 8
IV. Accessories 9
DIRE CTIONS FOR USE 10
V. Installation 10
VI. Operation 12
SERV ICE DATA 15
VII. Description of the mode of operation 15
VIII. How to gain access to the parts 22
IX. Maintenance 23
Х. Adjusting devices and their functions 23
XI. Checks and adjustments 24
XII. Replacing parts 32
XIII. A few data for fault analysis 35
XIV. List of parts 36
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GENERAL PART

Introduction

By means of the amplifier voltmeter PM 2520 a.c. voltages of 100 µV...300 \mu V can be measured in a frequency range from 10 c/s to 1 Mc/s.

The apparatus has the following special properties:

  • a. Irrespective of the wave form the indication is always proportional to the r.m.s. value of the input voltage.
  • b. The circuit is perfectly symmetrical with respect to earth. This renders it possible to measure the voltage between two points which are at a high potential with respect to earth.

The apparatus is equipped with an accurately indicating meter having three scale divisions, viz.:

  • 1. 0...100
  • 2. 0...300
  • 3. a dB-scale for direct indication of the damping factor.

For the purpose of checking the input voltage, an oscilloscope can be connected to two sockets at the front of the apparatus. These sockets can also be used as amplifier output.

The apparatus moreover has a socket for connection of a recorder, so that it is possible to record the voltage under test by means of an automatic compensator PR 2210A or PR 2400A.

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II Description of the block diagram (Fig. 1)

In principle the apparatus consists of two amplifiers with a high input impedance and which are perfectly symmetrical with respect to earth.

The voltage under test is amplified by means of two amplifier stages (B1', 11', 2' and 12') preceded by an impedance-converting preliminary stage (B1, 11) and followed by an output amplifier (B3', 13', 4', 14'). According to the measuring range selected the voltage under test is attenuated at the input or between the amplifier stages by the measuring-range switch. The three amplifier stages of this switch are coupled mechanically and are carried by the same shaft.

Via the Graetz rectilier the voltage is supplied to a quadratic network and indicated by the moving-coil instrument.

A calibrating-voltage generator is provided for calibrating the measuring instrument. This generator delivers constant sinusoidal voltages of 10 mV and 3 V with a frequency of 1000 c/s.

Fig. 1. Block diagram

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The supply voltages for the amplifier and the calibrating voltage generator are electronically stabilized by the valves B5...B7. The amplified input voltage is available at the sockets "OUTPUT". The sockets "RECORDER" at the rear of the instrument can be used for connecting a recorder. During the connection of a recorder the moving-coil instrument is cut out.

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III Technical data

Properties given as figures with tolerances are guaranteed by us. Numerical values without tolerances serve as a guide for the user and apply to the properties of an average instrument.

Unless stated otherwise, the numerical values refer to nominal mains voltage.

Measuring range 100 µV300 V subdivided in 12 ranges
with max. values: 1, 3, 10, 30, 100, 300 mV
and 1. 3. 10. 30. 100 and 300 V.
dB-range -20 dB to +20 dB
0 dB - 0.775 V across 600 Ω = 1 mW.
Input impedance Input resis stance Input capa Input capacitance
in measuring range symm. asymm. sy m m. asymm.
1 mV 1 V
3 V300 V
8 Μ Ω
20 Μ Ω
4 ΜΩ
10 ΜΩ
15 pF
7,5 pF
30 pF
15 pF
Accuracy

After calibration for a sinewave voltage of 10 mV

(f.s.d.) and 1 Kc/s ± 1%
Attenuation ± 1%
Meter ± 1.5 %
Frequency response
20 c/s - 200 Kc/s +- 1 %
10 c/s - 1 Mc/s 30

for non sinusoidal voltages following accumulative errors should be taken into account

for crest factor 1: +- 34%
for crest factor 15: 3%
Pre-deflection < 50 µV (in the 1 mV-range, input termi-
nals terminated with 100 kΩ and screened).
Calibrating voltages 10 mV and 3 V.
Frequency 100 c/s.
Proportion between peak and ŭ
r.m.svalue U rdmas = max. 5.
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Common mode rejection I.e. the attenuation of signals which are in phase with respect to earth and applied between the input terminals and 60 dB for 1 V. 50 c/s. earth Output for recorder Measuring range on sockets "RECORDER" 0 - 100 mV Internal resistance 1 k\Omega Potential of sockets "RECORDER" with respect approx. 100 V to earth Max. amplification on sockets "OUTPUT" 40 imesInternal resistance 100 \ \Omega Supply By means of a voltage selector the apparatus can be adjusted to mains voltages of 110. 125, 145, 200, 220 and 245 V. frequency 40...100 c/s (frequencies < 50 c/s only at nominal mains voltage). Measuring error at mains-volt--- 1 % age variations of + or -10% Drift of calibrating voltage at mains-voltage variations of 1 or -10% - 0.5 % Power consumption approx. 70 W Dimensions and weight height 24 cm, depth 22 cm, width 36 cm weight 9.7 kg.

Mains cable Manual Measuring cable

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DIRECTIONS FOR USE

V Installation

A. ADJUSTING TO THE LOCAL MAINS VOLTAGE

By means of a voltage selector the apparatus can be adjusted to mains voltages of 110, 125, 145, 200, 220 and 245 V. The voltage value adjusted can be read through the round opening in the rear wall.

Adjustment to other mains voltages is effected as follows:

  • a. Remove the four screws C and lift the covering plate (Fig. 2).
  • b. Pull out the selector, turn it till the correct voltage value is up and press it back.
  • c. Refit the covering plate and check whether the correct voltage value is indicated.

Fig. 2. Rear view

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B. CONNECTIONS
Earthing

Earth the instrument according to the local safety regulations. This can be done via:

  • · the earth screw at the rear of the instrument
  • the earth socket at the front of the instrument the mains cable if the instrument is equipped with a 3-core mains cable fitted with a plug with rim-earthing contacts.

Avoid double earthing; it may increase the risk of hum.

Mains cable

Proceed as follows:

  • a. Check whether the mains-voltage selector has been properly adjusted.
  • b. Earth the apparatus.
  • c. Set the mains switch "0-----" to "0".
  • d. Connect the mains input socket to the mains using the mains cable supplied.
  • e. Check whether the pointer of the meter indicates "0"; if necessary, readjust by means of the plastic screw at the front of the instrument.
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VI Operation

A. SWITCHING-ON

  • a. Switch the instrument on by means of knob "0 \sim ".
  • b. Check whether the lamp beside the mains switch glows. After about 15 minutes the instrument has obtained the required stability.
B. CHECKING AND CALIBRATING
1. Zero adjustment and pre-deflection

The pre-deflection is adjusted as follows:

  • a. Short-circuit the input terminals to earth.
  • b. Set the measuring-range switch to position "10 mV".
  • c. Adjust the indication of the meter to zero by means of potentiometer "0".
  • d. Set the measuring-range switch to position "1 mV".
  • e. The meter must now indicate less than 50 \mu V.
2. Calibration

For calibrating purposes the front plate of the apparatus has two sockets at which stabilized calibrating voltages of 10 mV and 3 V (1000 c/s) are available.

The 10-mV voltage serves to check and calibrate the amplifier. while the 3-V voltage can be used for checking the pre-attenuator.

Calibration is effected as follows:

Set the measuring-range switch to 10 mV.

Connect socket "10 mV" to one of the sockets "1 mV-300 V": the free socket must be connected to the earth socket.

Check whether the instrument indicates exactly 100 on the scale 0-100. If necessary, readjust potentiometer R37 (screw-driver adjustment, right-hand panel).

Checking the pre-amplifier

  • -- Calibrate in the above-mentioned way.
  • Set the measuring-range switch to 3 V.
  • Connect socket "3 V" to one of the sockets "1 mV-300 V": the free socket must now be connected to the earth socket.
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- Check whether the instrument indicates a value between 296 and 304 V on the 0-300 scale. If the meter indicates a different value, the adjustment of chapter XI E must be carried out.

3. Adjusting the common mode rejection (balance)

The instrument has a provision for symmetry adjustment, by means of which the effects of a.c. fields near the measuring arrangement can be suppressed to a maximum degree.

This adjustment is carried out in the following way by means of potentiometer "BALANCE" on the front plate:

  • Connect the sockets "1 mV-300 V".
  • Connect a test cable of 1 metre to these sockets; one end of this cable must lie free on the table or workbench.
  • Set the measuring-range switch to position 3 mV.
  • Adjust the indication to minimum by means of potentiometer "BALANCE".

Fig. 3. Front view with controls

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C. MEASURING METHODS
1. Symmetrical with respect to earth

The voltage under test is connected to the upper terminals "1 mV - 300 V" (Fig. 4a).

2. Asymmetrical with respect to earth

The voltage under test is connected to one of the upper terminals "I mV – 300 V" and the terminal "*". The free terminal is connected to socket "*" (Fig. 4b).

3. Amplifier output

The amplifier output "OUTPUT" can be used as input both for the symmetrical and the asymmetrical methods.

4. Output for recorders

CAUTION!

A recorder must be connected free from earth, as both sockets carry a voltage of about 100 V with respect to earth.

At the rear of the instrument there are two sockets for connection of a recorder (Fig. 2).

If a plug is inserted into one of the sockets, the instrument is switched off automatically.

The output voltage is 0-100 mV (100 mV corresponds to full-scale deflection on the meter).

When connecting a recorder, the internal resistance of the measuring instrument must be taken into account.

Fig. 4. Various ways of connection

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SERVICE DATA

VII Description of the mode of operation (Fig. 27)

A. AMPLIFIER

The a.c. voltage under test is applied to the input attenuator via sockets "1 mV – 300 V".

In the measuring ranges 1 mV · 1 V the input signal is not attenuated. In the case of a symmetrical input the input resistance is 8 MQ.

In the higher voltage ranges 3 V - 300 V the input signal is attenuated by the voltage divider R1-R3 (R101-R103), as a result of which the input resistance in these ranges is 20 MQ with symmetrical input.

Via capacitor C32 (C132) and resistor R4 (R104) the signal is applied to the control grid of valve B1 (B11). These valves act as cathode followers and convert the high input impedance to a low impedance. Resistor R5 (R105) causes a negative feedback which increases the input impedance. Symmetry is obtained by means of potentiometer R93.

The signal now having a lower impedance is applied to the second attenuator R8-R12 (R108-R112) (Fig. 5).

Via capacitor C4 (C104) and resistor R16 (R116) the signal now reaches the control grid of valve B1' (B11') of the pre-amplifier and is subsequently amplified by two stages: in this case as well the output is a cathode-follower stage.

The RC network consisting of R94-C40 and R25 (R194-C140 and R125) constitutes a negative feedback to the cathode potential of valve B1' (B11'), on account of which the frequency-response curve is corrected.

On the low side of this curve the filter represents a higher impedance than on the high side.

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The common mode rejection is adjusted by means of resistors R99 and R58. The grid bias is symmetrically adjusted by means of R99, while the decoupling is adjusted by R58.

The third attenuator determines which voltage range. 3 mV or 10 mV, will be used by switching over the resistors R26 and R27 (R126 and R127).

The amplified signal is applied to the control grid of value B3' (B13') of the output amplifier via capacitors C9 (C109) and resistors R35 (R135).

The two following stages then amplify the signal which is then given a low impedance by cathode-follower B4' (B14') and applied to the Graetz rectifier GR1...GR4.

Electrical zero adjustment is effected by adjusting the control-grid voltages of the valves B3' and B13' symmetrically by means of potentiometer R31 ("0" on the text plate).

The sensitivity of the indication and of the amplification is determined by the value of potentiometer R37 in the cathode lead of values B3' and B13'.

The frequency-response curve is adjusted at high frequencies by means of trimmer C11 (C111).

B. QUADRATIC NETWORK

The r.m.s.-value rectification is effected by a network consisting of a special circuit of diodes and resistors (Fig. 6). The special properties of this network ensure that the current flowing through the indicating meter is proportional to the square of the voltage under test.

This quadratic function corresponds to a parabolic curve. In this way it is ensured that for indicating the r.m.s. value the scale of the meter can be calibrated almost linearly.

To explain the r.m.s.-value rectification the mode of operation of the diode circuit will first be described in detail.

Fig. 7 shows the current/voltage characteristics for average-value rectification (A), peak-value rectification (B) and r.m.s.-value rectification (C). The uninterrupted line always indicates the practical value and the dotted line the theoretical value.

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Fig. 6. Quadratic network

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Fig. 7. Current-voltage characteristic

These figures reveal that the curve is parabolic for the r.m.s.value rectification and represents a value between the average and peak values. As with a parabola the proportion \frac{u}{i} differs in all points. r.m.s.-value rectification cannot be achieved with a linear circuit. For practical purposes the test circuit must be arranged so that the current flowing through the instrument corresponds as accurately as possible to the parabolic curve, so that it is proportional to the square of the voltage.

The circuit of the network including the diodes GR5...GR8 bends the linear characteristic (working line) in specific ratios, as a result of which the parabolic shape of the curve is approached (Fig. 8).

On account of this step-wise adaptation it is possible to arrive at such an approximation of the parabolic curve that the deviations lie within fixed tolerance limits.

The resistors R50-R51. R49-R52 and R58-R53 are potential dividers delivering the intermittently occurring voltage to the relevant diode, thus determining the point of deflection.

After the input voltage has been amplified and rectified by the Graetz circuit, the diode in the relevant circuit becomes conductive when the deflection voltage is obtained, so that the resistance value of this circuit decreases.

The same process occurs in one or more diode circuits, dependent

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on the voltage: every time the curve will be deflected at the moment when the relevant diode becomes conductive, so that the curve will follow a parabolic path (Fig. 8).

Fig. 8. Parabolic curve

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Diode GR9 is provided for limiting the blocking current; but for this diode the blocking current would be a factor 4 higher. The heating of the diodes GR5...GR8 is compensated by the diodes GR10 and GR11.

Zener diode GR17 protects the indicating meter against too high a voltage, while diode GR16 limits the meter current to about 100 µA.

C. SUPPLY UNIT

All supply units are electrically stabilised by valves B5. B6 and B7 in the classic way. The 110-V voltage is supplied by B3.

The supply transformer is protected by a thermal fuse which interrupts the mains voltage if the transformer becomes too hot.

D. CALIBRATING-VOLTAGE GENERATOR

For calibrating the instrument calibration unit U2 delivers voltages of 10 mV and 3 V; the frequency of the voltage is 1000 c/s.

The R.C. oscillator consists of pentode B8' in conjunction with the capacitors C34, C35 and C36 and the resistors R81 and R82, constituting the tuning circuit. Triode B8 is connected as a diode and serves to control the amplitude of the oscillator voltage.

The 3-V calibrating voltage is adjusted by means of resistor R84 or R87 and the 10-mV calibrating voltage by means of potentiometer R92.

E. OUTPUT FOR AN AUTOMATIC RECORDER

The output sockets "RECORDER" are provided for connection of a recorder. If a plug is inserted into one of these sockets the indicating meter is automatically cut out.

Care must be taken, however, that the sockets "RECORDER" are connected to a d.c. voltage with respect to earth. This means that the connection of the recorder must be made free from carth.

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F. AMPLIFIER OUTPUT

At sockets "OUTPUT" the input signal (B13') amplified by channel 2 is available. This output signal is given a specific value by the cathode follower.

The output is independent of the test amplifier and the measuring procedure cannot be interfered with by external influences.

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VIII How to gain access to the parts

  • 1. Removing the switch knob (Fig. 9)
    • a. Remove cap "A" (for instance with the nails of thumb and forefinger).
    • b. Slacken screw "B" and tap it lightly.
    • c. Pull the knob from the spindle.

Fig. 9. Removing the knob

2. Removing the panels (Fig. 2)

The cabinet consists of a number of separate panels that can be removed one by one.

Rear wall

  • a. Loosen the screws "A" and the earth terminal.
  • b. Remove the rear wall.
Bottom plate and side panels

  • a. Loosen the screws "B" belonging to the plate.
  • b. Slide the plate forward and lift it out of the frame.
Cover

  • a. Remove the four screws from the grip.
  • b. Loosen the screws belonging to the cover.
  • c. Slide the cover forward and lift it out of the frame.
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IX Maintenance

The amplifier voltmeter PM 2520 requires little maintenance, as the instrument does not contain parts that are really liable to wear. If necessary, the switch shaft can be lubricated with some thinly fluid oil (sewing-machine oil).

The switch contacts may only be treated with a special switch oil in the case of break down. (The ordering number of this oil is stated in the list of mechanical parts).

The panels are made of aluminium coated with a layer of P.V.C. and can be cleaned with water and soap after having been detached.

ltem to be adjusted Adjusting Required Recommended
devices auxiliary apparatus PHILIPS apparatus
Stabilised d.c. voltage R 70 Valve voltmeter GM 6020
Electrical zero-adjustment R31 none
Sensitivity R37 L.F. generator
Valve-voltmeter
GM 2317
GM 6012 (calibrated)
Frequency-response curve СП, СП Ł.F. generator
H.F. generator
ZV 2312
GM 2883
Pre-attenuator R3, R103 L.F. generator GM 2317
C1, C101 Valve-voltmeter GM 6012 (calibrated)
Calibrated-voltage generator R84. R87 L.F. generator GM 2317
R92 Valve-voltmeter GM 6012 (calibrated)
Common mode rejection R58, R93 L.F. generator GM 2317
R98, R99 Valve-voltmeter GM 6012 (calibrated)

X Adjusting devices and their functions

The above sequence is completely arbitrary. The sequence stated in Chapter XI is recommended for complete adjustment.

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XI Checks and adjustments

The tolerances stated below are factory tolerances which apply to the readjustment of the instrument. They may differ from the data stated in the "General Part" (point III).

In Chapter X all adjusting devices, selector resistors and selector capacitors are stated and their functions are described; this chapter also contains a survey of the auxiliary equipment required.

A. CHECKING THE SUPPLY UNIT
1. Mains current

  • Check whether the mains-voltage selector indicates 220 V and connect the apparatus to this voltage (mains frequency 50 c/s).
  • Set the mains switch to position "--". The pilot lamp must now light up.
  • Measure the current consumption. This may be maximum 350 mA.
2. D.C. voltage

Adjust the mains voltage to 220 V. + or - 1 % by means of a variable transformer or a stabilizer.

Measure the voltage with respect to earth on point 7 of unit U4. It must be 215 V. + or - 2%. If necessary, select another value for R70.

B. ZERO ADJUSTMENT
1. Mechanical

  • -- Set the mains switch to position "0".
  • Check whether the pointer indicates zero: if necessary, readjust the pointer by means of the black plastic screw below the indicating meter.
2. Electrical

Set the mains switch to position "~".

The apparatus remains switched on during all following test measurements.

The heating-up time is at least 15 minutes.

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  • Short-circuit the input terminals to earth.
  • Set the measuring-range switch to position "10 mV".
  • Check whether the pointer indicates zero; if necessary, readjust by means of potentiometer "0" on the front plate. (Tolerance: maximum 0.5 scale division.)
C. SENSITIVITY

  • Connect the left-hand socket "1 mV 300 V" to terminal "*".
  • Set the measuring-range switch to position "10 mV".
  • Connect a voltage of 10 mV, frequency 1000 c/s between the right-hand socket "1 mV 300 V" and earth.
  • Adjust the indication of the meter to 100 by means of potentiometer R 37 (Fig. 10).
  • Connect the right-hand socket "1 mV 300 V" to terminal " \neq ".
  • Connect a voltage of 10 mV, frequency 1000 c/s between the left-hand socket "1 mV – 300 V" and earth.

The meter indication must now also be 100. If necessary, check the zero adjustment.

Fig. 10. Bottom view of interior

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D. FREQUENCY-RESPONSE CURVE

  • Connect the left-hand socket "1 mV 300 V" to terminal "*".
  • Set the measuring-range switch to position "10 mV".
  • Connect a voltage of 10 mV, frequency 1 Mc/s between the right-hand socket "1 mV 300 V" and earth.
  • Adjust the indication of the meter to 100 by means of trimmers C11 and C111 (Fig. 11). See to it that both trimmers are given approximately the same number of turns.
  • Check the other channel by reversing the connection of the left and right-hand input socket. If necessary, correct the deflection by means of the above-mentioned trimmers.
  • Check the frequency response curve as follows:
freq uency of input signal indication on the meter
10 c/s 97103
20 c/s 99101
100 c/s 99101
1 kc/s 100 reference point
10 kc/s 99101
100 kc/s *) 99101
200 kc/s *) 99101
500 kc/s *) 9810 2
1 Mc/s *) 97103

*) These four ranges must also be checked in the positions, "1 mV", "3 mV", "30 mV" and "3 V" of the measuring-range switch.

E. PRE-ATTENUATOR

  • Connect the right-hand input socket "1 mV 300 V" to socket "*".
  • Connect a voltage of exactly 3 V, frequency 70 c/s between the left-hand input socket "1 mV – 300 V" and earth.
  • Adjust the deflection of the meter to 300 by means of potentiometer R3 (Fig. 10).
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  • Connect the left-hand input socket "1 mV 300 V" to socket "+".
  • Subsequently connect the input voltage between the right-hand input socket "1 mV – 300 V" and earth.
  • Adjust the deflection of the meter to 300 by means of potentiometer R103.
  • Increase the frequency of the input voltage to 100 kc/s.
  • Adjust the deflection for both channels to 300 by means of trimmer C1 or C101.

Fig. 11. Right-hand side view

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F. CALIBRATING-VOLTAGE GENERATOR

  • -- Set the measuring-range switch to position "3 V".
  • Apply an accurate calibrating voltage of 3 V, 1000 c/s and the internal 3-V calibrating voltage of the instrument to the input terminals via an external change-over switch. The internal or external calibrating voltage on the instrument

can now be read at option, so that it is possible to compare both voltages.

Adjust the internal calibrating voltage by means of the selector resistor R84 or R87 so that it is equal to the external voltage within 0.3% (Fig. 12).

Check the internal 10-mV calibrating voltage in the same way using an external calibrating voltage of 10 mV.

The adjustment is effected by means of potentiometer R92 (Fig. 12).

- Check the oscillation frequency and the wave form by means of a calibrated oscilloscope (frequency limits 800...1200 c/s).

G. AMPLIFIER OUTPUT

Set the measuring-range switch to position "1 mV".

  • Connect a voltage of 1 mV, frequency 1000 c/s, between the right-hand input socket "1 mV 300 V" and earth.
  • Measure the voltage between the output sockets "OUTPUT" "+" and "-". This voltage must be 50 mV. ± 10%.
H. MAINS-VOLTAGE DEPENDENCE
1. Sensitivity

Set the measuring-range switch to position "10 mV".

Adjust the indication of the meter to 100 by means of an external constant voltage.

Vary the mains voltage by + and -10%.

At intervals of 1 minute the deflection of the pointer must not deviate more than half a scale division.

2. Drift of the calibrating voltage

  • -- Connect a voltmeter to the calibration sockets.
  • Vary the mains voltage by + or -10%.
  • -- The calibrating voltage may vary maximum 0.5 %.
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J. COMMON MODE REJECTION

  • 1. D.C. voltage adjustment of the working-points of valves B1 and B11
    • Adjust potentiometer R93 (Fig. 12) so that the cathode resistors of B1 and B11 (connecting points U3/13 and U3/18) are at exactly the same potential.
  • 2. Set potentiometer R58 "BALANCE" to its central position.
  • 3. Adjusting the differences between the amplifiers
    • Connect the left-hand socket "1 mV 300 V" to the right-hand socket.
    • Connect a sinusoidal voltage of 1 V. 1000 c/s, between one of the input terminals and earth.
    • Set the measuring-range switch to position "3 mV".
    • Adjust the deflection of the pointer to minimum by means of potentiometer R98 (Fig. 10).

Fig. 12. Left-hand side view

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4. Phase-adjustment at low frequencies

  • For connections, see point 3.
    • Adjust the frequency of the input voltage to 50 c/s.
  • Set the measuring-range switch to position "3 mV".
  • Adjust the deflection of the pointer to minimum by means of potentiometer R99 (Fig. 12).
5. Checking the symmetry of potentiometer R58 "BALANCE"

The maximum deflection in both extreme positions of the potentiometer must have approximately the same value.

If this should not be the case, the preceding adjusting points must be checked.

  • 6. Fine-adjustment of the minimum deflection (see point 4) is effected by means of potentiometer R58.
  • 7. The pointer must indicate less than 1 mV.
K. R.M.S.-VALUE INDICATION

The network for indication of the r.m.s. value can be checked best by comparing this indication with the indication of a thermocouple instrument. If such an instrument is not available, it is also possible to carry out a comparative measurement with another calibrated PM 2520. Should differences be found, possible tolerance overlaps must be taken into account, however.

Apply pulses to the left-hand input socket "1 mV – 300 V" and to terminal "#" by means of a square-wave generator. The pulses must be so strong that full-scale deflection is obtained, while the iterative frequency must be 1000 c/s.

Fig. 13. Pulse shape

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The deviation from the indication of a calibrated thermo-couple instrument must not exceed 4 %.

A defective diode in the network can be readily detected by measuring the blocking and forward resistances. A simple way of doing this is to use an ohmmeter; the diode need not be removed from the circuit.

L. RECORDER OUTPUT

Connect a voltage of such a value that the instrument shows full-scale deflection.

Connect a millivoltmeter with a high input impedance (Ri ≥ 100 kΩ) free from earth to the sockets "RECORDER" at the rear of the instrument. The indication of this instrument must lie between 95 and 105 mV d.c.

- Check whether the indicating meter is cut out when a plug is inserted into one of the sockets "RECORDER".

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XII Replacing parts

The parts used in the manufacture of the instrument have not been specially selected or given a particular value.

When replacing electrical components the data stated in the parts list must be strictly adhered to, as otherwise the accuracy of the instrument will be affected.

For the accessibility of the parts, see Chapter II.

A. THERMAL FUSE

When this fuse blows, the cause of this blowing must be traced before replacing the fuse.

Fitting a new thermal fuse.

Fit the thermal fuse to the spring and pull it over the hook on the transformer. The fuse becomes accessible after removing the cover.

B. VALVES AND DIODES
Valves

In the case of replacement the valves must be aged for 100 hours. The simplest way of doing this is to leave the instrument with the new valves switched on for 100 hours.

The valves can also be aged outside the instrument, however. For this purpose the valves are connected as diodes; all grids are then connected to the anode of the relevant system. The anode voltage is selected so that at normal heater voltage a quiescent current of 1/6 × the maximum permissible cathode current flows. This quiescent current has the following values for the various types of valve:

B1/11 PCF80 == 2.3 mA Both pentode and
triode
B2/12: B3/13; B4/14 ECF80 = 2.3 mA ditto
B8 E80CF = 3 mA ditto
B6 EF86 = 1 mA
B5 EL86 = 1.6 mA
B7 85A2 = 1.6 mA
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Valve Chapter XI, paragraph
B1/11, B2/12, B3/13, B4/14
B5, B6, B7
= A, B, H
= A, B
B8 = E

Diodes

For the replacement of diodes no special measures are required. In this case as well it is recommended to check the relevant part of the circuit.

C. PILOT LAMP

  • Remove the lens from the text-plate.
  • The lamp fitted by means of a bayonet catch can now be gripped and removed with the aid of a piece of rubber hose having an internal diameter of 5 mm.

D. INDICATING METER

  • Remove the cover, the bottom plate and side panels. Remove the switch knob.
  • Remove the mains switch (16-mm spanner).
  • -- Unsolder the connecting wires of sockets "100 mV", "3 V" and of the two sockets "OUTPUT". If a well-fitting screwdriver is available, the threaded ring of the sockets on the text-plate can be unscrewed.
  • After removal of the screws "A" (Fig. 12) lift the front panel, together with the loose edge of the cabinet. from the frame.
  • Unsolder the connecting wires of the indicating meter.
  • Remove the 4 fixing screws and remove the instrument from the chassis.

E. TEXT-PLATE

In principle the same method should be adopted for replacing the text-plate as the one described under D.

  • Remove the 6 fixing screws at the side of the frame, the black plug sockets, the emblem and the lens.
  • Take the cast-aluminium meter frame out of the old text-plate and fit it into the new one.
Page 33
F. MEASURING-RANGE SWITCH UI

Remove the cover, the bottom plate and the side panels.

  • Unsolder all connecting wires from the mounting plates of the switch.
  • Remove the knob from the switch.
  • -- Remove the fixing screws A (Fig. 12) from the front frame, so that it can be pulled slightly forward, together with the textplate.
  • Remove the two hexagonal screws (8-mm spanner) from the switch frame. The entire switch unit U1 can now be removed from the instrument.
G. SWITCH SEGMENTS

  • Remove the measuring-range switch from the instrument according to Chapter F.
  • Remove the stop of the switch and one of the fixing strips of the segments.
  • Pull the shaft of the switch out of the segments.
  • Replace the relevant segment.
Important

When carrying out repairs on switches always see to it that the wire forms are not displaced or deformed.

This also applies to the arrangement of the resistors fitted to the switch.

Such changes will cause capacitive variations and have a detrimental effect on the frequency-response curve of the instrument.

It is recommended in any case to check the instrument according to Chapter XI and, if necessary, to adjust it again, after the measuring-range switch has been taken apart.

Page 34

XIII A few data for fault analysis

The following voltage values apply to an average apparatus and are given for your information only.

1. Supply transformer

The supply transformer and its connection diagram are shown in Fig. 14.

The voltage values in the relevant table have been measured at no load.

The numbers stated between brackets represent the wire colours according to the international resistor colour code.

2. Operating voltages

The principal operating voltages of the instrument are stated in the circuit diagram. These voltages have been measured with respect to earth or between the indicated connecting points by means of a valve-voltmeter.

Fig. 14. Mains-transformer

Page 35

XIV List of parts

A. MECHANICAL
– Minimum basic sic stock
Fig. Item Number Code number Description S for 1 3 5 10 apparatus
15 1 I ZD 202 84 Instruction plate ** - ł
3 2 1 M7 076 17 Grip 44 - _ 1
3 3 2 E2 742 67 Bracket ** - _ 2
15 .1 1 P5/560/16/NB Lens ** - I
16 5 1 M7 751 78 Switch socket * i 2 3 5
15 6 1 973/53 Knob 30 mm dia. ı k i 2 2 3
15 7 I 973/D52 Cap for knob ** I 1 [
15 8 I 973/P51 Arrow head for knob ** - _ 1 2
15 9 3 M7 694 87 Terminal * 1 2 3 5
15 10 1 08 521 10 Mains switch * - t 1 2
16 П I M7 415 67 Lamp holder ٠ - _ _ 1
15 12 4 P5 655 14 Rubber stud * * - 2 4
17 13 I M7 737 11 Voltage selector * - 1
17 14 1 978/M2 19 Mains connection * - 1
17 15 П 976/PW9 > 12 Valve socket, noval * ב .4 ( ) 10
17 16 ŧ 976/PW7 - 10 Valve socket, miniature * I 2
17 1 ZD 413 40 Print UT (left) *
18 ť ZD 412 94 Print U2 *
19 l ZD 412 99 Print U3 *
20 1 ZD 413-04 Print U4 *
21 Ι ZD 413-26 Print U5 ٠
22 i ZD 412 92 Print U6 *
23 I. ZÐ 413-42 Print U1 (right) সা
17 24 109 A3 320 36 Soldering tag ** 10 10 15 25
25 10 cc 971/7t Switch oil ** - _ _ 10 cc
26 1 4822/075/00261 Measuring instrument শা _ Ι 2
12 27 2 909/V9 , 4 Base for C6/C106 * 1 1
Page 36
-8411 imai DUSI * SIOCK
Pos. Number Code number Description S for 1 3 5 10 apparatus
1 1 Housing
2 l 4822 216 00411 Plug pin * - - i 2
3 ב M7 343 37 Plug pin * L 2 3
4 Ι 4822 216 00407 Mounting block 新神 - - i 2
5 ι 4822 216 00409 Rubber sleeve * - Ι 2
6 1.2 m 4822 128 00439 Coaxial cable 1,2 2,4 2,4 4.8 m
7 I 4822 216 00408 Rubber sleeve * - ] 2
8 2 978/1 × 4AP Plug * - 1 2
' ) l 978/8 Crocodile clip * - - 1 2
Measuring cable (fig. 26)
Purpose of the column S
Components not marked

These should be present at the Service Department in the country concerned or at the customer's who is using the apparatus. They include:

  • a. nearly all electrical components:
  • b. mechanical parts which are vulnerable, or which are subject to wear.

- Components marked with one asterisk

These components generally have a long or unlimited service-life, but their presence is essential for the correct working of the apparatus. Stocking up of a few of these components depends on the following factors:

  • a. the number of apparatus present in the country concerned:
  • b. the necessity of having the apparatus working continuously or not;
  • c. the time of delivery of the components with respect to the import restrictions in the country concerned and the duration of the transport.

- Components marked with two asterisks

These components have a long or unlimited service-life and they are not essential for the correct working of the apparatus. Generally there is not a local stock.

.

Page 37
B. ELECTRICAL PARTS
Explanation of the electrical parts list

Only non-standardised parts are stated in the electrical parts list. In the circuit diagram the standard components are indicated by means of identification marks, each representing a certain number. A list of marks with corresponding numbers is provided on the diagram. The order number of a certain standard part may be determined as follows:

is ordered under number 901/120K. is ordered under number 904/4E7. 4.7

The correct values of the adjustment resistors and capacitors have been fixed during adjustment at the factory.

All resistors are vaporised carbon resistors, unless otherwise stated.

Item Unit Code nu mber ŀ' alue Volt Description
C3 : l i 3 C 435 E )F/H320 320 μF 64 Electrolytic
C5 1 U 3 909/X16 I 16 μF 40 Flectrolytic
C6/C106 I UB 909/A40 ю 400 μF 4 Electrolytic
C8 I U 5 AC 8324 4/100 100 μF 150 Electrolytic
C10/C110 ł U 5 909/T64 0 640 μF 4 Electrolytic
C12 ١ U 6 C 426 A E/G12.5 12.5 քե 25 Electrolytic
C13/C14 ļ U 4 C 435 D F/F800 800 μl ; 25 Electrolytic
C15/C16 I U 4 AC 8209 9/16 - 16 10 5 - 16 μF 4(X) Electrolytic
C17/C18 l L! 4 AC 8209 9/16 - 16 Ie n4 16 μF 400 Electrolytic
C21/C22 I L' 4 AC 8220 5/25 + 25 23 - 25 μF 250 Electrolytic
C23/C24 l 13 AC 8220 5/25 - 25 25 55 25 μI 250 Electrolytic
C41 1 LU -4 AC 860. 3/25 25 μF 100 Electrolytic
Resistor S
liem – Unit Codenum ner Value Watt Tol. Description
R3/R103 ιı F 088 DG i:10AO3 5 kΩ lin. 0.1 20 ", Carbon pot entiometer
R8/R108 υī 48 063 95 4K.55 1550 Ω 0.4 0.5% Wire-wound l resistor
R10/R110 U 1 48 760 95, 155E 155 Ω 0.4 0.5% Wire-wound l resistor
R11/R111 U 1 48 760 95 /49E 49 Ω 0.4 0.5 "; Wire-wound f resistor
REZ REE U I 48 760 95 22F7 22.7 Ω 0.4 0.5 % Wire-wound I resistor
Capacitors
Page 38
Item Unit Code nummer Value Wan Tol. Description
R19/R119 U 3 B8 305 23D/190E 190 Ω 0.1 1 " Carbon resistor
R26/R126 48 063 95/1K 55 1550 Ω 0,4 0.5 ° u Wire-wound resistor
R28/R128 48 760 95/227E 227 Ω 0.4 0.5 ° " Wire-wound resistor
R31 E 199 AA/C21B1K l l 10 Wire-wound potentiometer
R37 U 5 4822 071 005 84 5 kΩ log. 0,1 20 0 .0 Carbon potentiometer
R39/R139 U 5 4822 070 003 21 43 I 5 "., Carbon resistor
R44 U 5 E 003 AG/D6K2 6200 Ω ł 5 °4 Carbon resistor
R45/R145 U 5 £ 003 AG/D15K 7.5 2 ۵, Carbon resistor (par.)
R46 U 6 B8 305 23D/55K 55 0.1 1 ° µ Carbon resistor
R 47 U 6 901/82K 41 0,1 1 ". Carbon resistor (par.)
R48 U 6 901/36K + /2K4 38400 Ω 0,1 ° ا Carbon resistor (ser.)
R49 U 6 901/10K × /11K 21 0.1 ، ا Carvon resistor (ser.)
R50 U 6 901/10K ±2K2 12200 Ω 0.1 1" Carbon resistor (ser.)
R51 U 6 901/1K2
B8/305/17D/1K65 2850 Ω 0.1 I "" Carbon resistor (ser.)
R52 U 6 B8 305 23D/21K2 10600 Ω 0.1 ] " Carbon resistor (par.)
R53 U 6 901/100K /82K 45 Ω 0.1 1 % Carbon resistor (par.)
R58 U 6 916/GE20K 20 kΩ lin. 0.125 20 ° n Carbon potentiometer
R70 U 4 901/33K…1M 33kΩ1 ΜΩ 0.25 5 "" Adjustment resistor
R83 U 2 48 123 93/15K 15 1.2 1 "" Wire-wound resistor
R84 U 2 901/5K…1M 5kΩ1 МΩ 0.25 5 °" Adjustment resistor
R 85 U 2 48 123 01/85K 85 1.2 1 "。 Wire-wound resistor
R86 U 2 B8 304 94D/34K 34 1.2 ۳. Wire-wound resistor
R87 U 2 901/100K1M 0,11 0.25 5 °a Adjustment resistor
R88/R89 U 2 48 123 01/110K 110 1.2 a" ا Wire-wound resistor
R91 U 2 48 760 95/190E 190 Ω 0.4 0 ا Wire-wound resistor
R92 U 2 901/1KIM TkΩT 0.25 5 "., Adjustment resistor
R93 U 3 E 088 DG/10AO3 5 kΩ lin, Carbon potentiometer
R98 4822 128 001 59 100 Ω lin. Carbon potentiometer
R99 U 3 E 097 AD/1M ł MΩ lin. Carbon potentiometer

Valves and semiconductors

Item Unit Number Туре Description
BI/BI1 U 3 2 PCF80 Valve (noval)
B2/B12 U 3 2 ECF80 Valve (noval)
B3/B13 U 5 2 ECF80 Valve (noval)
B4/B 14 U 5 2 ECI-80 Valve (noval)
85 U 4 1 EL86 Valve (noval)
B6 U 4 1 EF86 Valve (noval)
B7 U 4 ł 85A2 Valve (miniature)
B8 U 2 Ι E80CF Valve (noval)
Page 39
hem Unit Number Турс Description
GRI U 5 1 OA5 Diode, gold-wire
GR2 U 5 1 OA5 Diode, gold-wire
GR3 U 5 ł OA5 Diode, gold-wire
GR4 U 5 Ι OA5 Diode, gold-wire
GR5 U 6 l OA5 Diode, gold-wire
GR6 U 6 1 OA5 Diode, gold-wire
GR7 U 6 1 OA5 Diode, gold-wire
GR8 U 6 1 OA5 Diode, gold-wire
GR9 1 OA5 Diode, gold-wire
GR10 U 6 3 OA5 Diode, gold-wire
GR11 U 6 1 OA5 Diode, gold-wire
GR12 U 4 1 OA214 Silicon diode
GR13 U 4 L OA214 Silicon diode
GR14 U 4 Ι OA214 Silicon diode
GR15 U 4 I OA214 Silicon diode
GR16 U 6 I O A 85 Germanium diode
GR17 U 6 1 OAZ213 Zener diode
GR18 U 4 1 OA210 Silicon diode
GR19 U 4 Ι OA210 Silicon diode
Miscellaneo DUS
Description Pos. Code number
Mains transfe ormer .1.1 ZD 601 24
Thermal fuse VL1 974/1125
Pilot lamo 1 A 1 6878 (6 V 100 mÅ)
Page 40

Fig. 15. Bottom view of exterior

Fig. 16. Top view

PEM 1226 E 904

Page 41

Fig. 17. Rear view without rear plate

Page 42

Fig. 18. Switch segments

43

Page 43

Fig. 21. Unit U2, calibration voltage generator

E 908

Page 44

Fig. 22. Unit U3, pre-amplifier

Page 45

Fig. 23. Unit U4, supply unit

Page 46

Fig. 24. Unit U5, output stage

Page 47

Fig. 25. Unit U6. quadratic network

Page 48

Fig. 26. Measuring cable

49

Page 49

Page 50

Page 51

Fig. 27. Circuit diagram PM 2520

Page 52

Fig. 27. Circuit diagram PM 2520

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