Hamtronics R144 User Guide

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HAMTRONICS R144 VHF FM RECEIVER, REV. 4/94:

INSTALLATION AND MAINTENANCE

FUNCTIONAL DESCRIPTION.

The R144 is a premium commercial grade single-channel vhf fm receiver. It features a helical resonator front end with low-no ise dual gate fe t's, an 8-pole crysta l filter plus a ceram ic filter for superior i-f selectivity, and hysteresis squelch circuit to lock onto fading signals. The R144 kit is available for the 143-150 MHz band, and wired units are ava ilable fo r this ba nd and also the 150-174 MHz commercial band.

CRYSTALS.

The channel crystal plugs into sockets on the board. We can order crystals for any frequency desired. If you order your own, be sure to supply the following specs.

The receiver uses 32 pF parallel resonant crystals in HC-25/u holders. Crystals operate in the fundamental mode. The crystal frequency for the 143-174 MHz band models is (F-

10.7)/9. Frequency tolerance is .001%. We recommend that any new crystals be o rdered directly f rom us to be sure that they will perform properly over the -30 to +60 °C range for which the unit was designed. This is especially true for commercial receivers with the temperature-compensated crystal oscilla tor (TCXO) option, since the crystal must be matched exactly to the compen sation circuit in the receiver. If you use an OV-1 crystal oven, specify a crystal with a 60°C breakpoint.

INSTALLATION.

Power Connections.

The receiver operates on +13.6 Vdc at about 150 mA peak with full audio. Current drain with no audio is only about 35-40 mA. A crystal oven adds about 450 mA peak current drain when cold and only about 25 mA when warm. A well regulated power supply should be used.

Be sure that the power source does not carry high voltage or reverse polarity transients on the line, since semiconductors in the receiver can be damaged. The positive power supply lead should be connected to the receiver at terminal E3, and the negative power lead should be connected to the ground plane of the board

through the mounting hardware or the shield of the coaxial cable. Be sure to observe polarity!

Speaker.

An 8-ohm loudspeaker should be connected to E2 with g r ou n d retu rn to the ground plane through the mounting h ardware. Use of lower impedance speaker or shorting of speaker terminal can result in ic damage. The receiver can also drive higher impedances, like 1K to 10K input impedances of COR boards, etc. There is no need to load down the output to 8 ohms.

Note th at the audi o outpu t ic is designed to be heatsunk to the pc board through the many ground pins on the ic. When running moderately low audio levels as most applications require, it is no problem to use an ic socket; so we have provided one for your convenience. If you will be running high audio levels, check to see if the ic is getting hot. If so, you should remove the ic socket, and solder the LM-380 ic directly to the board for better heatsinking.

Antenna Connections.

The antenna connection should be made to the receiver with a phono plug. If you want to extend the antenna connection to a panel connector, we recommend using a short length of RG-174/u coax and a good phono plug with cable clamp (see catalog). We do not recommend trying to use direct coax soldered to board or another type of connector. The method designed into the board results in lowest loss practical. When soldering the cable, keep the stripped ends as short as possible.

Mounting.

Some form of support should be provided under the pc board, generally mounting the board with spacers to a chassis. 3/8-inch holes should be provided in a front panel for the bushings of the SQUELCH and VOL- UME controls. After s liding bushings through panel, washers and nuts can be installed on the outside of the panel. Be sur e to provide support for the board; do not rely on th e controls to support the board.

For repeater applications, the receiver should be mounted in an rf tight box, such as our model A16.

The receiver board relies on the mounting h ardware to provide the d c and speaker ground connections to the ground plane on the board.

OPTIONS. Squelch Circuit.

The squelch circuit has about 3 to 6 dB of hysteresis built in, so that once the squelch opens, the signal must drop 3 to 6 dB below the opening threshold before squelching again. This allows for so me fadin g on mo bile stations and prevents squelch pumping on heavy modulation. It also prevents cycling due to slight desense in repeater installations. Of course, this requires setting the threshold a little higher than if there was no hysteresis so that it will close with no signal. If you prefer the older type squelch, you can simply remove Q5 from the circuit; however, this is not recommended for repeater installations. If you want more or less hysteresis, you can decrease or increase the value of R25.

Repeater Use.

E4 provides a "carrier operated switch" output which may be connected to a COR module to turn a transmitter on and off. The output level is about 7V unsquelched and 0V squelched. There is a resistor in series with the output to limit current. Refer to COR module instructions for details.

If your repeater controller uses discriminator audio, rather than the speaker output, filtered discriminator audio is available at E5. The level is about 2V p-p. If you need audio which is squelched, take it from the input (right hand terminal on the VOLUME control.

Audio Muting.

If the receiver is used as a part of a transceiver, audio muting can be accomplished without switching the power or speaker lines. If the transmitter is keyed by applying B+ to the exciter, simply connect the keyed B+ through a 100K resistor to the junction of R25 and R27 on the receiver board. The dc level will be sufficient to trigger the squelch circuit in U2, regardless of the rf signal level coming into the receiver.

Of course, some means of discon-

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necting the receiver from the antenna must be provided, and we recommend our TRR Coax Relay Module if the power level is under 25 Watts. Otherwise, a larger coax relay will be required.

Discriminator Meter.

If you wish to u se a discriminator meter and you are handy in designing with op-amps, you can run a samp le of the dc voltage at DISCRIMINATOR output termina l E5 to one in put of a n op-amp and tie the other input to a voltage divider pot set to provide a reference voltage of about +3.5Vdc.

S-Meter.

There is no s-meter function, as such, available in i-f amplifier ic's made for professional receivers; however, a signal strength indication is available a t test point TP2. This voltage is a function of the noise level detected in the squelch circuit. It is about +3Vdc with no signal an d 0Vdc with full quieting . Y ou ca n ta p o ff this point with a high-impedance circuit, such as an op-amp, to drive a meter or a computerized repeater controller.

Subaudible Tone Decoder.

To use our TD-3 Subaudible Tone Decoder or a similar module, connect its audio input to DISCRIMINATOR terminal E5. If you want to use it to mute the aud io (in s tea d o f in h ib itin g a repeater transmitter as is normally done), conne ct the mute ou tput of the TD-3 to the righ t-hand lug of th e volume control.

ALIGNMENT.

Equipment needed for alignment is an fet voltmeter, an rf signal generator, a regulated 13.6Vdc power supply with a 0-200 mA meter internally or externally connected in the supply line.

The slug tuned coils in the receiver should be adjusted with the proper aluminum .062" square tuning tool to avoid cracking the powdered iron slugs. Variable capacitors should be adjusted with a plastic tool with a small metal bit on the end. Tools are available for adjusting the rf coils (model A28) and variable capacitor (model A2).

The squelch pot should be set fully counterclockwise. The volume pot should be set just a little clockwise.

a. Install channel crystal in socket.

b. Connect speaker and +13.6 Vdc. You should hear white noise.

c. Connect dc voltmeter to TP3 (top lead of R1 8 ). Ad ju st f irs t L4 , th e n L3 and L4 alternately for maximum response. (Approx. +1.2 to 2.2 Vdc typical.)

d. Connect stable signal generator to TP4 (right-hand lead of rf choke L9), using coax clip lead. Connect coax shield to pcb ground. Set generator to exactly 10.7000 MHz. Use a frequency counter or synthesized signal generator. Set level just high enough for full q uie tin g . (A t 1 uV , y o u should notice some quieting, but y ou need something near full quieting for the test.

e. Connect dc voltmeter to TP1 (top lead of R16). Adjust discriminator transformer L8 for +3.5Vdc. Note that the voltage changes very rapidly with tuning. Full swing of about 1 to 8V occurs within a few kHz, and a little drift (+/-1kHz) may be noticed d ue to the high sensitivity of the test signal. It is not necessary to maintain exactly 3.5V.

Note: There are two methods of adjusting the mixer and front end. One is to use an fet voltmeter with test point TP2, which is the top lead of CR3. The voltage at th is point is proportional to the amount of noise detected in the squelch circuit; so it gives an indication of the quieting of the receiver. A signal peak, therefore,

is indicated by minimum noise voltage, not maximum.

The other meth od is to us e a r egular professional SINAD meter.

In either case, a weak to modera te signal is required to observe any change in noise. If the signal is too strong, ther e will be no cha nge in the reading as tuning progresses; so keep the signal generator turned down as receiver sensitivity increases during tuning.

If you use TP2 with a voltmeter, the signal can be modulated or unmodulated. If you use a SINAD meter, the standard method is a 1000 Hz tone with 3 kHz deviation.

f. Check that signal generator is still on 10.7000 MHz. With weak signal applied to Q2 gate-1 as before, adjust L2 for minimum noise or distortion. This step is cr itica l to g et lo west distortion in the crystal filter.

g. Remove signal generator so the receiver hears just noise. Readjust L8 slightly so that the voltage at TP1 is +3.5V with just n oise coming thr ough the i-f.

h. Connect signal generator to J1. Adjust to exact channel frequency,

and turn output level up fairly high. Adjust frequency trimmer capacitor C16 to net the c rystal to channel f requency, indicated by +3.5Vdc at test point TP1. If you can't find the signal at all, tune your signal generator up and down the band slightly. (Also check that osc illator is peaked as per step c.) If your crystal has the wrong load correlation or is slightly out of tolerance, you may be able to compensate by changing the value of C15 so C16 can net the crystal on frequency.

The piston capacitor tuning range is restricted to provide best frequency stability; so sometimes it may be necessary to change the fixed capacitor.

The proper adjustment results in +3.5Vdc, the same as preset for the exact 10.700 MHz i-f frequency earlier.

Maximum capacitance (lowest frequency) occurs with the piston screwed in all the way, and minimum capacitance (highest frequency) is with the piston all the way up. Be careful not to completely remove the piston. If the piston screw becomes a little tight (squea ky), you can app ly a small amount of silicone oil to the threads.

i. Connect fet dc voltmeter to TP2 (top lead of CR3). Set signal generator for relatively weak signal, one which shows some change in the dc voltage indication at TP2. Alternately peak L5, L6, and L1 until no further improvement can be made.

j. Helical reson ator T2 supplied in the kit has been factory aligned to 145 (223) MHz. Re adjus tment o f the th ree screws may be necessary even if you are operating close to the preset frequency. Alternately adjust the three screws for best sensitivity.

Be careful not to adjust any screw more than just a few turns or you may unscrew the metal tuning slug from the end of its lead screw inside the resonator. If that should happen, you may be able to carefully disassemble the affected section and repair it.

k. Repeak L6 and L1 after the helical resonator is tuned to work out any interactions.

When properly tuned, sensitivity should be about 0.15 to 2.0 µV for12 dB SINAD.

TROUBLESHOOTING.

The usual troubleshooting techniques of checking dc voltages and signal tracing work well in troubleshooting the receiver. A dc voltage chart and a

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list of typical audio levels are given to act as a guide to troubleshooting. Although voltages may vary widely from set to set and under various operating and measurement conditions, the indications may be helpful when used in a logical tr oubleshooting procedure.

The most common troubles in all kits are interchanged components, cold solder joints, and solder splashes. Another common trouble is blown transistors and ic's due to reverse polarity o r po wer lin e tra nsie nts. Remember if you encounter problems during initia l testing that it is easy to install parts in th e wro n g p la ce . Do n ' t take anything for granted. Double check everything in the event of trouble.

If the receiver is completely dead, try a 10.700 MHz signal applied to TP4 (right-hand lead of rf choke L9) with a coax cable clip lead. You should be ab le to hea r the q uieting effect of a 10 uV carrier at 10.700 MHz. You can also connect the 10.700 MHz clip lead through a blocking capacitor to various section s of the crystal f ilter to see if there is a large loss of signal across one of the filter sections. Also, check the 10.245 MHz oscillator with a scope or b y listening with an hf receiver or service monitor.

TABLE 1. TYPICAL DC VOLTAGE S . Xstr E(S)B(G1)C(D) G2 .

Q1 0084 Q2 0.5084 Q3 2.7 2.4 8 Q4 2.2 0 8 Q5 Squelched 0 0 0.7 Q5 Unsq 0 0.7 0

U1 1 6 8 14

7 0 7 13.6

U2 1 2 3 4 5 6 7 8

7.9 7.2 7.2 8 1.1 1.1 1.1 8

U2 9 10 11 12 13 14

4.4 3.5 1 To 8 2.5 2.5 .3 To .7

U2 . 15 16 17 18

0(Sq), 7.2(Unsq) 0 0 2. 3

A signal generator on the channel frequency can be injected at various points in the front en d. If th e m ixer is more sensitive than the rf amplifier, the rf stage is suspect. Check the dc voltages looking for a damaged fet.

If audio is present at the VOLUME control but not at the speaker, the audio ic may have been damaged by reverse polarity or a transient on the B+ line. If no audio is present on the volume control, the squelch circuit may not be ope ratin g p rop erly . Ch eck the dc voltages, and look for noise in the 10 kHz region, which should be applied to noise detector CR2/CR3 with no input signal. (Between pins

12 and 13 of U2 is an op-amp active filter tuned to 10 kHz.)

Typical Dc Voltages.

The dc levels in Table 1 were measured with an 11 megohm fet vm on a sample unit with 13.6 vd c b+ ap plied. All voltages may vary considerably without necessarily indicating trouble. The chart sh ould be used with a logical troubleshoo ting plan. All voltages are positive with resp ect to gro und e xcept as indicated. Voltages are measured with no signal applied but oscillator running properly and with squelch open unless otherwise specified.

Typical Audio Levels.

Following are rou gh mea sur eme nts of audio circuits, using an oscilloscope. Measurements were taken under two conditions. The first is with no input signal, just white noise so conditions can be reproduced easily. The second is with an input signal having a 1000 Hz tone modulated ±3kHz.

TABLE 2. TYPICAL AUDIO LEVELS (V p-p)

Measured At On Noise On Tone U2 pin 10 (discrim.): 4 2 E5 (discriminator out): 3 1.5 U2 pin 13 (noise ampl): 4 0.4 Top of volume control: 0.8 0.2 Across 8Ω spkr term: 12 10

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PARTS LIST FOR R144

Note: Capacitor values and helical resonator vary with frequency segments. Above 150 MHz, values may vary.

Ref # Value (marking) C1 18 pf C2 62 pf C3 .001 uf chip capacitor C4 not assigned C5 .001 uf (1nK or 1nM) C6 4 pf C7-C8 .001 uf chip capacitor C9 .01 uf disc (103) C10 .001 uf (1nK or 1nM) C11 2 pf C12 2 pf C13 2 pf C14 3 pf C15 39 pf C16 Piston trimmer, 2-11.2 pf C17-C18 150 pf (151) C19 .001 uf (102 or 1nK) C20 68 pf C21 1 pf C22 7 pf (TXCO option only -

see text) C23 68 pf C24 .01 uf disc (103) C25-C26 .001 uf (1nK or 1nM) C27 22 pf C28 0.5 pf C29 22 pf C30 .01 uf disc (103) C31 .001 uf (102, 1nM, or 1nK) C32 0.5 pf C33 47 uf electrolytic C34 470 uf electrolytic C35 10 uf electrolytic

C36 0.15 uf (red) C37 0.1 uf monolithic (104) C38 0.15 uf (red) C39 .01 uf disc (103) C40-C41 .001 uf (102, 1nM, or 1nK) C42 .01 uf disc (103) C43 0.47 uf electrolytic C44 62 pf C45 220 pf (221) C46-C48 0.1 uf monolithic (104) C49 120 pf C50 0.47 uf electrolytic C51 20 pf C52 20 pf C53 .047 uf chip capacitor CR1 not used CR2-CR3 1N4148 (may not be

marked) E1 Not assigned E2-E5 Socket pins FL1-FL4 Matched xtal filter set FL5 Ceramic filter (blue) J1 RCA jack JMP-1 Jumper - see dwg L1 2-1/2 turns (red) L2 7A-691F IF transformer L3-L4 6-1/2 turns (blue) L5-L6 2-1/2 turns (red) L7 0.22 uH rf choke (red-sil-

red-red) L8 455kHz IF transformer

p/n 831-5 or YMC-15002

or T1003 L9 0.22 uH rf choke (red-sil-

red-red) OV-1 Crystal oven (option) Q1-Q2 N.E.C. 3SK122 or Philips

BF-988 dual-gate mos fet

(static handling pre-

cautions required)

Q3-Q5 2N3904 or 2N4124 R1-R2 100K R3-R4 not assigned R5-R6 100K R7 180 ohms R8-R11 not assigned R12-R13 15K R14 680 ohms R15 not used R16 330K R17 1.2K R18 470 R19 27K R20 100K pot R21 27K R22 100K pot R23 6.8K R24 510K R25 100K R26 3.3K R27 100K R28 330K R29 680 ohms R30 3Ω (orn-blk-gold-gold)

-or-

3.3Ω (orn-orn-gold-gold) R31 47K R32 15K RT1 Thermistor (TXCO option

only - see text) T1 Not assigned T2 Helical Res #1008

[#1080]

U1 LM-380N Speaker

Amplifier U2 MC-3359P IF Amplifier U3 78L08 +8Vdc Regulator Y1 Channel xtal (see text) Y2 10.245 MHz, 62 pf IF xtal Z1-Z6 Ferrite beads

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