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.
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
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.
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
Is unavailable from Drake.
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.
As for replacement switches, about the best you can do is rummage through someones surplus parts bin. These types of switches were used in old AC/DC televisions and AM/FM radios.
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.
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.
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.
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
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.
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.
Adjust the 10 volt regulator from measurements taken on the motherboard.
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.
This is a common oversite. Its comparable to not having plate voltage for the PA stage in the TR4.
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.
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.
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.
The board with the MRF476 predriver most likely was changed because it was much cheaper to make. This in itself is not a fault, but the way it was executed presents some problems that will be dealt with in a separate area. On this board the driver is an MPS-H20. I've used the MRF237 as a replacement because the transistor is biased for about 20 ma - about .3 watt. In my mind this is a little heavy for a TO92 transistor. The MRF237 may also be used as replacements for the SRF2331. These transistors are somewhat unique - the case is the emitter and the collector and emitter pins are interposed. If you orient the transistor so the base (center lead is furthest away from you) is in the centre and the transistor is held by the leads underneath, the emitter is the left hand lead, NOT on the right where you would expect a TO5 to be. The case in question is a TO39.
If it is necessary to change any of the transistors in this area, you must use heat sink compound on the mounting bases. Most folks use far too much of this stuff. The purpose of this compound is to ensure a good thermal contact between the transistor and the heat sink by filling in the (natural) pits in the metal faces. Thats all its used for. Too much is just as bad as none - its a metal filler only. Do not over goop this stuff!
The cure is to lift the emitter off ground with a resistor. Make a tight bundle of 3 - 1.8 ohm 1/8 watt resistors in parallel. Cut the emitter lead of the MRF476 about where the lead changes width. Remove the stub from the circuit board and put this resistor network between the emitter and where the the stub went into the circuit board. Removing the stub can be interesting for its soldered on both sides of the board.
Yes, raising the emitter will decrease the gain. The degenerative feedback also makes the MRF476 easier to drive, so the net result is a wash. This one change for this specific board type is highly recommended, especially if you're thaving problems with MRF476 longevity.
MRF475/2SC2092 will replace the SRF2338 driver transistors. The collector is the mounting tab, so don't forget the insulating wafer.
MRF476/2SC2166 will replace the TO220 predriver. The driver board changed over the years. The collector is the mounting tab, but its board placement is isolated from the circuit board. Do not use tab isolation hardware. The collector choke makes collector contact through the bolt.
The cost of all of the above is about $90 from RF Parts. One final transistor alone is over $63 from Drake.
The TR7 will shut down 50% at a 4:1 SWR. This provides more than adequate protection. However, the transmitter draws considerable current from a 13 volt supply. The supply should be rated at 30 AMP ICAS minimum. Marginal supplies and DC power cords will not provide enough current under load and likely will drop in and out under full carrier condition jeopardizing the PA. It is important that a stiff high current supply be employed with the TR7.
I recommend this change for those SSB operators that would like to operate VOX on their TR7's but haven't for lack of VOX gain.
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