Authored by VE3EFJ
12.2 TR7 MODS AND TECH
I have encountered few mods for this radio. This could be caused by the difficult nature of performing them or by the very fact that a stock TR7 is pretty good as it is. What makes this radio difficult to modify is the plug in board modules. This is good. This means that a TR7 is unlikely to be mucked with too severely. Your worst problem is likely to be alignment if your 'new' TR7 is a little sour.
 | Servicing A TR7 is a robust transceiver that is almost impossible to kill. It holds its alignment extremely well and generally is overbuilt and 'over designed'. Once brought up to specifications, it should stay that way almost indefinitely. However, should your TR7 require service, you are in a bit of a dilemma. To service a TR7 beyond the superficial, one needs a good oscilloscope, volt meter, service manual and the extender boards. The latter two are no longer available from Drake. A service manual may be purchased from: Antique Manuals, K7FG 1-800-807-6146 The telephone number is ... interesting. This organization sells manuals for a considerable number of examples of old(er) gear and a lot of BoatAnchors. The current price of the TR7 manual is about $34 US. Its not a bad deal. A TR7 is not difficult to set up, but one must be aware of what to tune and what not to touch. DO NOT align the first crystal filter unless you are prepared to go at it with a sweep generator. Quite a number of slugs, trimmers and trim pots are involved in an alignment and not all of the adjustments are immediately accessible. Especially in the case of a TR7, if it works, don't fix it. |
| Set Up The outlined procedures in the service manual are very well thought out and are presented in a linear progression. Follow them. The synthesizer set up is a bit tricky. Most important is to ensure that the 40, 13.695 and 8.05 MHz oscillators are exactly on frequency. If each one of these is within 100 Hz, then the readout, PBT and CLAR will 'naturally' fall very close to spec. Proper test equipment is essential to set up a TR7. |
| All Band Transmit Included with this article are additional TIF and TXT files outlining Drake synthesizer changes for full receive coverage and full frequency coverage transmit. |
| Digital Display For a while the DR7 digital display was an option. A TR7 is significantly less without the DR7 display. If you are looking at a TR7 to purchase, make sure that it does indeed at least have this option installed. Of all the 'options' available for the TR7, it is unlikely you'll ever find a loose DR7 unless someone is cutting up a TR7 for parts. |
| Early and Late Models The very early model TR7 was sold without the DR7 board. It is unlikely you will encounter one of these - few were made. ASK if it has digital display before purchase! The early model TR7 had a 3 transistor predriver on the PA heat sink. Additionally, the adjustment for TX/RX frequency required you to remove the DR7 and use extender boards. Very inconvenient. The later model TR7 uses a 2 transistor predriver. You need to pull the top cover and look at the circuit board closest to the front panel. If you see a U shaped aluminium heat sink, it is the later model. Additionally, this model TR7 had an access hole on the motherboard for the TX/RX frequency adjustment. AF/RF Gain Control This is the same control as used on the SPR4,
which was available, although I don't know the current status. The one
difference is that the TR7 control has a double switch for both AC and DC.
Depending on what is gone on the TR7 control - anything but the switch,
basically, you can graft the old control switch onto the replacement
control. This requires careful disassembly of the controls, but it can and
has been done. |
| TR7 Mixing Scheme The TR7 and TR7A is a dual conversion transceiver using a first IF of 48 MHz and a second IF of 5.645 MHz. The same path is used in reverse on transmit. For the BFO, there is no 5.645 MHz crystal as such, for it is synthesized from 2 crystal oscillators at 8.05 and 13.695. The first mixer is a DBM followed by a grounded gate post amplifier in to a 48 MHz 4 pole crystal filter. On transmit, the 48 MHz transmit signal is routed through the 48 MHz filters, through the post amplifier and into the DBM. The post amplifier has its inputs and outputs reversed through steering diodes. Output on transmit is taken directly from the DBM into the 3 stage high gain PA section (predriver, driver and PA functional blocks). ALC is achieved on transmit by use of a diode attenuator in a previous low level stage. In receive, there is a dedicated board for the IF filters followed by a 3 stage IF MOSFET amplifier employing forward AGC. The crystal filters are treated all the same - there is no gain compensation for bandwidth. The primary reason for the mixing scheme is so full coverage from .5 to 30 MHz can be achieved with a 5 to 5.5 MHz VFO. The synthesizer in the TR7 is a tracking synthesizer. The PTO at 5 to 5.5 MHz is used in the PLL with the divider chain to control a VCO operating at 48 to 78 MHz. If the PTO drifts, then the synthesizer will drift in step with it. |
| RF Tightness The radio cannot be aligned when extender boards are in use. Some adjustments must be done with the cover plate off. For the other adjustments, there are holes in the cover plate for access. These can only be accurately adjusted with the cover plate in place. Make sure the cover plate is screwed down snugly with all those screws - not just a few. Some boards have grounding fingers. While re-installing these boards, make sure the fingers and tabs make chassis contact. If the above is not adhered to, mediocre alignment and operation will result. There will be RF leakage into the IF section of the receiver. This will have a dramatic effect on S meter, AGC and spurious responses. |
| AUX 7 Programming See the separate section covering this option in detail. |
| The FA7 Fan Some manufacturers do not provide for forced air cooling of their PA stages. Ten Tec is a good example. Their PA stages can run so hot that it really hurts to grab the heat sink. I've never seen one 'melt', but having them get that hot gives me the willies. Heat and electronics do not happily co exist (ref TR4 above). While the transistors may take it and good design compensates for it, thermal run away is a concern. Its an ugly event to watch and once started, the event is catastrophic and usually expensive. The FA7 was an option on the TR7 for heavy duty cycle use. Experience has shown that without a fan, even on SSB, the PA gets inordinately warm. Regardless of mode, some form of forced air cooling should be employed. The requirement is to provide air circulation, not necessarily air cooling. The fan should be set up to blow in, not out. This is contrary to the FA7 direction, but seems to afford much better cooling. I mount the fan so it blows in, under the theory fans move more air on the blow side than the draw side. It does seem to be noisier blowing in, though. I really do not think it matters all that much, so long as you can get the temperature down and the hot air out. If you mount it to draw, you should feel warm air coming out and the top of the cabinet 'cool'. The FA7 fan runs from 110 VAC and is meant to be run 'through' the PS/7. If you have a PS/7, a 110 VAC 'muffin' fan will bolt right on. If you use a generic power supply use a 12 volt version and power the fan off the TX Vcc from the PA stage. 24 volt DC fans will push a fair bit of air quietly and these are readily available surplus. |
| Digital Operation All Drakes with the exception of the TR5 use a free running VFO. This may not be stable enough for RTTY as the long term drift is a few hundred cycles. If you must use a Drake for digital operation, your best bet is a TR5 or a TR7 with an RV75 remote VFO (not the RV7). I have no T/R switching times for any of the Drake equipment, but it is reasonable to assume that none of it switches fast enough for AMTOR. * note readers comment on TR7 switching times |
| Receiver Sensitivity Check Properly aligned, the S Meter should rest just off zero, for the AGC detector must be in the 'on' state slightly, otherwise the AGC will pop. The calibrator should provide an S9 signal on 10 meters with no antenna attached if the alignment is close. Without an antenna, a properly operating TR7 should appear almost to be dead. If the RF gain is rotated fully CCW, the S meter should rest at the S9 +80 db mark - no higher or lower. Since there is no preselector to peak, the calibrator test assumes the S Meter is set up in accordance with the alignment instructions. The other alternative 'sign of life' tests you can do is to scratch the center pin of the SO/239 with a metallic anything. The S Meter should respond and you should hear the scritch noises most plainly in the speaker. You can also connect almost any antenna to the SO/239 and you should hear an increase in background noise, however slight - even on 10. |
| 8.05 MHz Osc Won't Net This oscillator is varicap controlled and is used in conjunction with the 13.995 fixed oscillator to develop the BFO. In doing it this way, there is little chance that there will be BFO leakage, or what leakage there is, can be controlled. There is a trimmer adjustment to net the 8.05 MHz crystal, but what the manual fails to tell you is that this adjustment is also affected by the trim pots for the injection frequencies for the BFO. If you try to set this trimmer up and it just won't trim, try an arbitrary setting of the trimmer screw and see if, say, on LSB you can get it to the proper frequency with the trim pot for that mode. |
| Receiver AGC Set Up Notes Aside from alignment, set up in this area has considerable affect on the receivers sensitivity and AGC 'personality'. Also important is the 10 volt regulator adjustment, for it too will have an effect on oscillator alignment, AGC and sensitivity. Tests indicate that at 9 volts, the receiver and AGC setup is quite 'mushy'. For all practical purposes, the 10 volt regulator adjust is the one adjustment that will determine how 'crisp' the radio is. Adjust the 10 volt regulator from measurements taken on the motherboard. |
| Transmitter Output Check A TR7 should produce 150 watts output on 80 meters if set up properly. Current draw will be 22 amps at 13.6 volts. Use no smaller power cable than #12 for short runs and #10 for 15 feet or more. You should be able to disconnect the transmitter load and key the transmitter to full output. Properly set up ALC will limit the output 'power' to 20 watts or so. If you pull the blue wire from the ALC board (the one between the shielded cable and the red wire on the LHS), the PA stage will run wide open and I've measured over 225 watts output on 80 meters. Not recommended as a normal practice, but this is a good test of final transistor health. Set to its nominal 150 watt output, a TR7 is definitely loafing along. |
| Won't Transmit The TR7 has a separate pin on the power connector for +13 volts to the PA. Out of the 4 pin power connector pins, 2 are ground and one pin each is for the radio proper and the PA. Ensure that the PA stage does have 13 volts. The transceiver will make all the right noises (relay closure, etc), but won't generate any RF. This is a common oversite. Its comparable to not having plate voltage for the PA stage in the TR4. |
| Accessory Filters The TR7 filters are not interchangeable with the R4C filters. The R4C accessory filters are 5695 kHz and the TR7 are 5645 kHz. The factory supplied SSB filter is a 'fidelity' filter. Your transmitted audio with a properly set up radio and a microphone should sound like FM broadcast. The skirt roll off is just a little 'soft'. You need to go to a 1.8 kHz filter to get much RX improvement. The stock SSB filter is quite good in receive. The TR7 always transmits through the SSB crystal filter supplied with the radio. You can put the other 3 filters where ever you want, but don't mess with this filter in this position. |
| AM Filter An AM filter is almost impossible to find. You can fake an AM 'filter' by putting a 390 ohm resistor through the input and output pins of any blank crystal filter position. It actually isn't bad. What is determining the selectivity is the 48 MHz first IF filter. |
| Transmit Power Pay particular attention to the SWR balance trimmer, C1901. ALC action is affected adversely by an improper null. This null trimmer also affects the watt meter calibration, so if you change the trimmer setting, R2001 and R2002 will need adjustment also. Essentially, the FOR output is used for ALC and the REV output is used for shutdown. This is independant of the watt meter setting. When you set up the ALC null, use a high impedance analogue meter, a non metallic alignment tool and a good 50 ohm load. There are 2 control settings that affect the ALC. The obvious one is the 'ALC' control on the ALC board in the bottom of the transceiver. The other setting control is the gain pot on the predriver. This control sets the gain by setting the feedback on one of the driver stages (old driver board) or the current in the preamp stage (newer driver board). Properly set, you should have just enough ALC on 10 and as expected, a controllable abundance on 80 meters. Improper set up of the ALC usually means no ALC or will make the mic gain setting overly sensitive and the ALC clamp early on the lower bands. There is additional ALC/drive compensation from the band switch for the 10 and 15 meter bands. Extra resistors are switched in on these band settings to provide more drive/higher ALC threshold to provide gain compensation. These resistors have only a very minor effect on drive compensation. If you are having upper band drive problems, these resistors should not be the first suspects. For proper transmitter ALC action it is essential for the PA driver and final stages to be in good condition. 150 watts output should be easily attained on 40 and 80 meters. |
| External Speakers Unlike the '4 line, the TR7 employs an LM380 audio power stage. This IC is load tolerant and 8 ohm speakers may be used without problem. |
| Microphone Later series TR7 provided for both high and low impedance microphnes through the use of different pins on the connector. High impedance mics may be connected to pin 4. Input Z is about 750K, but this port is much less sensitive than pin 1. High Z mics are expected to be high output (> 100 mv). The above is a factory change on the later series TR7. Early models had a jumper on the circuit board for microphone impedance. |
| PA Driver Stage At least 2 different sets of boards were used in the driver stage next to the power amplifier. Early TR7s used 3 transistors; the late model board used 2 transistors. In this board, the last transistor is an MRF47 |
