AVO CT-160 Service manual

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CHAPTER 1

MAINTENANCE INFORMATION

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C O N T E N T S

TEST EQUIPMENT REQUIRED SECTION 1 Page 3 FAULT FINDING & SERVICING NOTES SECTION 2 Page 3 VOLTAGE CHECKS WITH NO VALVE UNDER TEST SECTION 3 Page 7

SECTION 1 – TEST EQUIPMENT REQUIRED

(a) AVO Electronic Testmeter (or equivalent dc mean valve voltmeter). (b) Valve CV491 (Standardised for Mutual Conductance at 16mA anode current). (c) Model 7 or Model 8 AvoMeters (3). (d) Power Valve capable of passing 100mA anode current

CV428.

(e) Resistor 680kΩ ± 5%

SECTION 2 – FAULT FINDING AND SERVICING NOTES (See WARNING on page 2) NOTE: All measurements and tolerances stated do not include those of the testing

instrument, and where necessary, these should be ascertained particularly before commencement of the calibration procedure. Where possible the recommended instruments should be employed.

1. 500 c/s ac Supply Operation & its Relation to Servicing Whilst the instrument is suitable for use use on 50 to 500 c/s ac supplies, service

and calibration should normally be carried out using a 220/230V 50 c/s supply. The following features play a vital part in the correct operation of the instrument

on a 500 c/s supply. (a) The two electrostatic screens (S1 and S2) on the ht transformer prevent

spurious mA/V readings and care must be taken when replacing a transformer to ensure that these screens are connected as shown in the Circuit Diagram (see Fig. 4).

(b) The separate cable forms lying side by side across the instrument ensure

that the grid circuit and its associated wiring is kept well apart from the ht wiring to prevent the transference of energy from one circuit to the other at high mains frequencies. If, at any time, it is necessary to displace wiring within the instrument, great care must be taken to ensure that it is replaced in its original position.

insulation ranges when the instrument is used at high mains frequencies.

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2. To Check Accuracy of Instrument Before commencing servicing the instrument should be checked as follows:–

(a) Ensure that the mains ‘On/Off’ switch is in the ‘Off’ position. (b) Connect the instrument to an ac voltage supply of 220/250V 50 c/s of known

magnitude. (c) Set the instrument voltage adjustment to its appropriate position. (d) Switch on, noting that the panel indicator is illuminated. (e) Set the Mains Voltage Selector fine control such that the meter pointer lies

as near as possible to the centre of the ‘~’ zone. (f) Connect the AvoMeter in series with the A1 link. (g) Using the CV491 (standardised in accordance with Para.3) with 200V dc

anode volts, check that

i. For 16mA anode current the negative grid volts indication is within ±

5% of the standardised value.

ii. The slope (mA/V) is within ± 5% of the standardised value.

A reading of 8mA on the external instrument will be equivalent to a dc

current of 16mA through the valve (this is the value normally indicated by

the anode current controls when the meter is at its null position). (h) Remove valve and external meter. (j) Set the Neg.Grid Volts control at ‘40’. (k) Connect a resistance of 680kΩ ±5% between grid and cathode sockets on

the top cap connector panel. (m) Set the Circuit Selector switch to position ‘Gas’.

The panel meter should indicate full scale deflection ± 20%. (n) Disconnect the resistance and switch off.

3. To Obtain Standard Figures for a Valve Using dc Supplies Using the recommended AvoMeters, the valve should be connected as shown in

Fig. 1. If unable to use the recommended meters ensure that those used are of

sub-standard accuracy, the current meter having a maximum voltage drop of 100mV and preferably scaled 0–25mA and the voltmeter a sensitivity of 1000Ω/V. If rectified ac is used for the ht supply, it is essential that steps are taken to ensure that the supply circuit is adequately smoothed (the Solartron Varipack is a suitable source). The bias supply should be obtained from a suitable battery (note polarity of connection). The heater supply for the valve may be ac or dc, but must within ± 5% of the rated voltage.

(a) Set the grid bias voltmeter to read 9V. (b) Adjust the ht supply to 200V, then by means of successive adjustments of

the bias and ht voltage controls, set the anode current at 16mA (the anode voltmeter must read 200V). Note the new grid bias reading.

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The difference between the two anode current readings (i.e. 1mA) over the difference between the two grid voltage readings:—

Ia2 — Ia1

Vg1 — Vg

The result will generally be between 4 and 5mA/V (see Final Test Procedure). For greater accuracy it is suggested that readings of grid voltage be plotted against values of anode current between 10 and 20mA and the slope taken from the curve at 16mA.

The valve should now be labelled as follows:—

Va = 200V dc Ia = 16mA dc

Vg = . . . . . . . . . .

Slope = . . . . . . . .mA/V

Date . . . . . . . . . .

CV 491

The valve should be re–standardised daily when in use.

4. Construction The instrument comprises two units in a hinged transit case, the lid of which is not

detachable. Electrical connection between the two units is effected by means of two 5–way side by side cables.

5. Removal of the Instrument From its Case (See WARNING on page 2) To facilitate servicing or calibration of the instrument, it is necessary to remove

both sections from the casing, this being accomplished by the removal of four hexagonal headed bolts, which form the feet of the control unit, from the underside of the case. The control panel will then be released. The valve panel can be withdrawn from its section of the case by the removal of eight fixing screws around its periphery.

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6. Simple Faults

SYMPTOMS POSSIBLE FAULT ACTION

(a) No dial light indication

No dial light indication or meter deflection on SET ~ setting of Circuit Selector.

(b) No indication of meter

current

No indication of meter current and protective relay operates when testing tetrodes or penthodes.

7. Relay Operates and Fails to Clear Should the relay operate due to a suspected faulty valve and fail to clear after

switching of an on again with no valve in panel, set Roller Selector switch to read 000 000 000 and remove top cap connecting lead. Switch instrument off and on again.

No mains input. Dial light bulb burnt out.

Fuse blown.

No anode volts at valve pin.

No anode volts at valve pin but screen volts present.

Check mains connector. Replace LP1.

Check Mains Voltage Selector setting and replace F1 and/or F2.

Check that links A1 & A2 are tight and making firm contact.

Check that links A1 & A2 are tight and making firm contact

If fault clears the most likely cause of the trouble is a short on the valve panel, certain pin(s) being shorted out to earth by stray wire or solder, or a breakdown in insulation.

If the fault still persists, however, check ht line for breakdown to earth between Roller Selector Switch on Valve Panel and ht transformer on control unit.

8. Adjustment of Protective Relay The relay should seldom require attention, but if for any reason parts are

replaced, the adjustment is simple, it only being necessary to position two 4BA screws (see Fig. 3). It should be noted that the bobbins if replaced, should be positioned such that the flux which they produce is additive.

Operational limits are as follows:– (a) Anode overload — Relay should operate on 100V short circuit. (b) Screen overload — Relay should operate on 60V short circuit. (c) The relay should not arc excessively on a 200V short circuit on

anode or screen. (d) The relay should not operate when checking a 120mA rectifier. Before making any adjustments check that the lamp LP1 is operative.

When the instrument is used solely on a 110V supply, it may be preferable to replace LP1 with a 100V, 15W pigmy lamp.

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9. Servicing the Valve Holder Panel The Valve Holder Panel is connected electrically to the control panel by means of

two 5–way side by side cables. One of these cables embodies two thicker sections (16/.012) for H+ and H– leads. Connections to tag boards on either unit are shown in Fig. 2.

The wiring of the valve holders on the panel is in the form of nine separate loops, all pins comprising a loop and linking in roller 1 of the Roller Selector Switch. This form of loop connection is used likewise for pins 2–9, all nine circuits approximating in length and following a similar route around the panel. These loops are further loaded with beads of ferroxcube which sufficiently damp the loop to prevent the valve under test breaking into parasitic oscillation. A diagrammatic layout is shown in Fig. 2. Ferroxcube is also used on leads feeding the selector switch, as a precaution against lf oscillation.

Where it is necessary to replace valve holders, these with the exception of the B8B are fitted to the panel with nuts and bolts, and are thus easily removable. Care should be taken to replace all wire in its original position.

10. Removal and Replacement of Knobs and Setting of Knob Skirts To remove any knob, remove 6BA screw and spring washer. To remove knob

spindle an skirt, release locking pin. The switch nut is now accessible. To adjust skirt, slacken lock nut, rotate skirt to desired position an re–tighten lock nut. Reverse procedure to replace.

SECTION 3 — VOLTAGE CHECKS WITH NO VALVE UNDER TEST Connect instrument to known 220/230V 50 c/s supply, ensuring that the mains ‘On/Off’

switch is in the ‘Off’ position, and adjust coarse and fine settings of the mains voltage selector panel to match the supply voltage as accurately as possible. Set the Circuit Selector to ‘Test’ and the Electrode Selector to ‘A1’ and proceed to check the relevant electrode voltages as follows:–

1. Heater Voltages

(a) Connect the AvoMeter, switched to its ac voltage ranges, between H+ and H–

sockets on top cap connector panel.

(b) Switch on and rotate the Heater Voltage switch through the full range of values,

the external meter being set to the appropriate voltage range as required.

shown in the following table.

Due allowance must be made for the limits of accuracy of the measuring instrument for each particular reading:–

Nominal

Volts

5 10 20 40

117

Actual ac

Volts

2.34

5.5

10.4

21.0

42.0

125.0

Limits

2.2 / 2.5

5.3 / 5.7

10.2 / 10.8

20.5 / 22 41 / 44 120 / 130

(d) Switch off and remove the meter.

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2. Anode Voltages (a) Connect the AvoMeter, ensuring that the mains ‘On/Off’ switch is in the ‘Off’

position, and set to its ac voltage range, between A en C sockets on the top cap selector panel.

(b) Switch on and rotate the ‘Anode Voltage’ switch through successive positions, the

meter being set to the appropriate range as required.

switch –2 +6 per cent.

Due allowance must be made for the limits of accuracy of the measuring instrument for each particular reading, e.g., with the ‘Anode Voltage’ switch set to 100, the actual voltage reading should be 110V –2 +6%.

(d) Switch off and remove the meter.

3. Screen Voltages (a) Connect the AvoMeter, ensuring that the mains ‘On/Off’ switch is in the ‘Off’

position, and set to its ac voltage range between S en C on the top cap selector

panel. (b) Short the anode of V1(a) to cathode (pins 2 and 5. See Fig. 3). (c) Switch on and rotate the ‘Screen Voltage’ switch through successive positions, the

external meter being set to the appropriate range as required. (d) The meter readings obtained should be 1.1 × the voltage indicated by the ‘Screen

Voltage’ switch –2 +6%.

Due allowance must be made for the limits of accuracy of the measuring instrument for each particular reading.

(e) Switch off and remove the meter.

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CHAPTER 2

CALIBRATION

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C O N T E N T S

TEST EQUIPMENT REQUIRED SECTION 1 Page 9 CHECKING THE NEG. GRID VOLTS CONTROL SECTION 2 Page 9 CHECKING THE SET mA/V CONTROL SECTION 3 Page 9 CHECKING THE SET ~ INDICATION SECTION 4 Page 10 Ia CALIBRATION CHECK SECTION 5 Page 10 SETTING THE mA/V DIAL SECTION 6 Page 10 THE INDICATING METER SECTION 7 Page 11

SECTION 1 – TEST EQUIPMENT REQUIRED

(a) Avo Electronic Testmeter (or equivalent dc mean Valve Voltmeter)

NOTE: The Electronic Testmeter should be standardised at the appropriate voltages

before making any adjustment mentioned in the following paragraphs.

SECTION 2 – CHECKING THE NEG. GRID VOLTS CONTROL (See WARNING on page 2)

(a) Set mains ‘On/Off’ switch to the ‘Off’ position. (b) Open LK (Fig. 3) and set the panel controls as follows:–

CIRCUIT SELECTOR to TEST, ELECTRODE SELECTOR to A1 and NEG. GRID VOLTS to 40.

(Fig. 3) until a voltage reading of 20.8V is obtained. Remove the Testmeter.

(d) Connect the Testmeter between G1 and C sockets on the top cap connection

panel, or if the panel has been disconnected for servicing, to the G1 and C positions on the tag board at the back of the Unit.

(e) Check that at the 13 and 4 marks on the dial, readings of 6.75V and 2.1V

± 5% are obtained. If either or both readings are out of tolerance, the dial should be adjusted

mechanically to split the error. If it is necessary to make an adjustment, slacken the three counter–sunk screws on the top of the dial which will then be free to move within the latitude of kidney shaped slots. After adjustment, re–tighten screws and check readings. The areas marked 0, 5, 15 and 40 should correspond within the indicated area to 0V, 2.6V, 7.8V and 20.8V ± 5% respectively.

(f) Switch off and remove the Testmeter.

SECTION 3 – CHECKING THE SET mA/V CONTROL

(a) With LK still open, connect the Testmeter set to a suitable dc range, across

R5.

(b) Switch on and check that when the dial is advanced to its 10, 5 and 2mA/V

positions, readings of 52.5mV, 105mV and 250mV ± 3% are obtained.

been upset, the procedure ‘Setting the mA/V Dial’ given in Section 6 should be adopted.

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(d) Switch off and remove the Testmeter.

SECTION 4 – CHECKING THE SET ~ INDICATION

(a) Standardise the Testmeter at 47V dc. Close the link LK and set the panel

controls as follows:–

CIRCUIT SELECTOR to ‘SET ~’, and ELECTRODE SELECTOR to A1. (b) Connect the Testmeter across RV2 and switch on. (c) A reading of 47V should now be obtained, whilst the meter on the the

instrument panel should indicate within the ‘~’ zone.

If voltage reading is correct, but panel meter indication is outside ‘~’ zone,

check Resistors R3 and R4. (d) Switch off and remove the Testmeter.

SECTION 5 – Ia CALIBRATION CHECK

(a) Open the A1 link on the valve base panel and insert a Model 7 AvoMeter, set

to a suitable dc range, into the circuit. (b) Set up the instrument and place under test any power valve capable of

passing 100mA anode current, e.g., CV428. (c) Set the Anode Current controls to 100mA (90mA and 10mA), switch on and

with the instrument set to its test position, allow the valve to warm up. (d) Set the panel meter pointer to zero by means of the Neg.Grid Volts control. (e) The external meter should indicate between 47.5mA and 52.5mA

(0.5 × indicated value on Anode Current control ± 5%) the panel instrument

indicating zero. If required repeat this test at any other settings of Anode

Current controls. (f) Switch off and remove the meter.

SECTION 6 – SETTING THE mA/V DIAL

(a) With the link open and the ‘SET mA/V’ dial at rest, set RV1 at its maximum

anti-clockwise position (viewed from the front panel) and adjust friction tight

the locking nuts of the U-shaped stirrup. (b) Connect the Testmeter, set to a suitable range across R5. (c) Switch on and advance the ‘SET mA/V’ dial to a reading of 5. (d) Rotate the RV1 spindle further, by means of the stirrup, in a clockwise

direction until the Testmeter gives a reading of 105mV. (e) If this reading is achieved without further clockwise advancement of the

stirrup, or if its procurement necessitates an anti-clockwise movement of the

stirrup, then investigate the accuracy of R1, R2, R5 and RV1. (f) The locking nuts on the stirrup should now be tightened and the reading of

105mV on the voltmeter checked. (g) Again check that the dc millivolts developed across R5 at the 2mA/V and

10mA/V settings of the dial are 260mV and 52.5mV ± 3%. (h) Check that the dial can now be rotated to its 1mA/V position and that the

motion is eventually arrested by the stop screw on the dial and not by the

stop at the end of the potentiometer track. (j) Switch off and remove the meter.

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SECTION 7 – THE INDICATING METER This is a self–contained unit which may be withdrawn from the control panel by the

removal of two 2BA screws (see Fig. 3). When used in the instrument as an anode current null indicator, the meter has a full

scale deflection for approximately 10mA (not critical).

When removed from the instrument, the meter has a full scale deflection of 30µA and

internal resistance of 3,250Ω. When shunted by R9 only (see Circuit Diagram) the meter has a full scale deflection of 39.8µA

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CHAPTER 3

FINAL TEST PROCEDURE ================

C O N T E N T S

TEST EQUIPMENT REQUIRED SECTION 1 Page 12 FINAL TEST DETAIL SECTION 2 Page 12

NOTE: When all repairs have been carried out and the instrument is re–assembled,

carry out the following final test detail.

SECTION 1 – TEST EQUIPMENT REQUIRED

(a) AVO Electronic Testmeter (or equivalent mean dc Valve Voltmeter). (b) Model 7 or Model 8 AvoMeter. (c) Valves CV428, CV491 and U52. (d) Resistor 1 megohm ± 1% (e) Resistor 680kohm ± 5%

SECTION 2 – FINAL TEST DETAIL (See WARNING on page 2)

(a) Apply a 500V megger test between the mains input and frame. (b) Apply a known ac 50 c/s voltage in the range 200–250V to the instrument

and with Circuit Selector at ‘Set ~’, set mains voltage selector until needle

on panel registers as near as possible to the centre of the ‘~ mark’. Check

that selector settings show ± 5V of the actual mains voltage.

It is most important to ensure that the meter reading in the ‘Set ~’ position

is maintained at the centre of the ‘~ mark’ on the scale for all subsequent

tests. (c) Set the Circuit and Electrode Selector switches as given in the table overleaf

and connect a 1 megohm ± 1% resistor across the electrodes on top cap

board as detailed under the heading ‘Condition’. Check that a leakage of

1 megohm ± 10% is indicated on the panel meter in each case:–

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Circuit Selector

Switch

A/R A1 Resistor connected across

S/R A1 Resistor connected across S

C.H/R A1 Resistor connected across

C/H C/H Resistor connected across C

NOTE: In positions C.H/R and C/H check that heater volts appear across H– and H+.

(d) Check operation of the overload cut–out with:

i. An anode/cathode short at an anode voltage of 100.

(Connect a short across A1 and C top cap board).

Electrode Selector

Switch

Condition

A1 and any of the following S, H–, C, A2 or G.

and any of the following H–, C or G.

C, H– or H+ and any of the following A1, A2, S or G.

and either H+ or H–.

ii. A screen/cathode short at a screen voltage of 60.

(Connect a short across S and C top cap board).

(e) Check the following unloaded anode volts with the Testmeter

(on appropriate ac range) connected across A1 and C on top cap board).

Nominal Volts: 60 100 150 250 400

Actual ac Volts: 66 110 165 275 440

Limits: –2 +6% (f) Check the following unloaded heater volts with the Testmeter

(on appropriate ac range) connected across H+ and H– on top cap board.

Nominal Volts Actual ac Volts Limits

5 10 20 40

117

(g) Insert a CV428 in the appropriate socket, set the instrument to the correct

Electrode Selector switch to A1 and obtain a balance the check that:

2.34

5.5

10.4

21.0

42.0

125.0

2.2 / 2.5

5.3 / 5.7

10.2 / 10.8

20.5 / 22 41 / 44 120 / 130

i. By varying the grid voltage, an anode current of 100mA is obtainable.

ii. The valve does not oscillate.

NOTE: The meter reading should not vary appreciably when the hand is

placed near or on the insulated anode lead.

iii. By varying screen voltage the anode current varies accordingly.

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(h) Insert a CV491 (12AU7) strapped as single triode and standardised for

mutual conductance at 16mA (anode current) with 200V applied to the anode, connect the H.R. Testmeter (on 10mA ac range) across the A1 link. Then check that:

i. For 16mA anode current on the CT160 the external Testmeter reading

is 8mA.

ii. Negative grid volts is ± 5% of the standard figure.

iii. Mutual conductance is ± 5% of the standard figure.

NOTE: The mutual conductance figure for the CV491 will normally be in the

order of 4mA per volt at an anode current of 16mA. The tolerance specified for measurement of mutual conductance on the instrument applies only to this point on the mutual conductance control.

(j) Remove the CV491 and with the bias control set at 40 connect a 680kΩ

±5% resistor between G and C connections on top cap board. Check that, with the Circuit Selector switch set to ‘Gas’ the CT160 meter

shows f.s.d. within 20%.

(k) Insert a U52 in the appropriate socket, set the Circuit Selector switch to test

and Electrode Selector switch to D1. Insert the Testmeter (on appropriate dc current range) in series with the load from the top cap board (D1) to the rectifier anode.

Check that for all load switch positions:

i. The CT160 meter reading is approximately in centre of ‘good’ scale.

ii. That the Testmeter readings are within ± 10% of nominal value.

(m) Repeat test as in (j) with anode load connected to D2 on top cap board and

Electrode Selector to D2.

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