• Stefan, DL7MAJ, whose YouTube videos inspired me to actually start a job I had been putting off for too long and for his 'cheat sheet' of bandswitch BCD values.
• Tom Evans for his 'Parent Board pinout' document and Jim Shorney's diagram of 'Parent Board Pin Numbering', both in the 'Files' section of the group, were / are essential reading.
• Mike, G3ZCC, whose hi-res scans of the circuits were just right for printing and scribbling on.

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Hi to all in the group. Below some findings from my repair journey with a TR-7 that you may find interesting.

A couple of years ago, I acquired a Drake 'job lot' on eBay. Until this week, the TR-7 transceiver (an early one in the low 1000 s/n range) had just gathered dust. I looked at the radio when I got it and knew it was going to be trouble. Externally there were missing case screws; some screws broken off in their bottom chassis nuts; and sheet metal screws of various sizes used in threaded holes. Internally, I could tell just by looking at the power supply board that the person(s) who had worked on it previously should never have been allowed to use a soldering iron. Maybe is was a gas torch they actually used:-(

This week, I decided the time had come to fix the thing. Starting symptoms were panel lights only, no signs of life in IF and AF stages and the frequency display tumbling uncontrollably. Here's what I found:

On the Power Supply board. Series pass transistor for the 10-Volt rail and the 5-Volt regulator were floating in free space; Ferrite core on the inverter transformer had come apart, giving no / low 24- and -5-Volt supplies; Tracks lifted / open-circuit where the +10 V SET and USB PBT pots had been replaced. Could really use a new power supply board, but patched it up for now.

On the IF Selectivity board. Several large solder splashes bridging PCB tracks, presumably put there when additional IF filters were installed, and which appeared to be the source of a short on the +10-Volt R rail.

On the Transmit Exciter board. Switching transistor for the 10-Volt R rail open-circuit. 2N4402 replaced with ZTX550 from junk box.

On the chassis. The scrapes around the band-switch on the rear panel told me the band switch had been removed previously. The switch wafers along the shaft were FUBAR, causing the synthesiser to be mis-programmed; the VCO to be on 'HI' range when the radio was set to the 'low' bands and loss of RF signal paths through the LPF and HPF. So bad was the mess, I almost made a service tool that could be used to flip the indexing on individual wafers 0 / 180-degrees. Eventually I made do with using the Drake band switch shaft, inserting in alternately (and only part-way) through the rear- and front-panel holes until all wafers were indexed correctly. It took a long time, not helped by not being able to see where the wipers on many of the wafers were pointing. The tension on the bandswitch as it turns is very weak. I can't understand how the single piece of (spring steel?) wire can exert much force on the switch shaft to improve matters. Perhaps there's something missing / broken here? Any knowledge from the group here?

On the Pass Band Tuning board, the CMOS switch that processes the PBT voltages was faulty. MC14016 replaced with CD4066, in IC socket, just in case;-) The PBT voltage being fed to the VCXO was nonsense with any position of the mode switch; whether any of the PBT pots on the power supply board were adjusted; and whether PBT was engaged, or not. With the fault present, the 'toppy' / restricted frequency response of the noise out of the receiver sounded like the PBT control was set hard to one end.

On the HPF daughter board, bridged tracks around the PIN diodes and a brown stain on the board where something had got hot. The UM9401 PIN diodes had been replaced with an assortment of silicon rectifier diodes. I have some sympathy here. Access to the rear of the board in-situ to make a repair job is not easy. I removed all three diodes, cleaned the rear of the board with solder wick and re-built the three diodes in a Christmas tree on the front of the board. I did not have any of the original PIN diodes. Looking at this article I substituted 1N4007. They work well on RX, but remains to be seen whether they will be good enough on TX. Any group experiences here?

On the Digital Display board, dry solder joints on the rear Molex connector.

Evidence of contact cleaner spray sticky residue near all Molex connectors in the card cage. Can't believe this was the right stuff for the job. Cleaned up and selective use of DeoxIT D100L from a needle dropper.

With all of that done, the receiver is now fully working and sounds good. The next part of the journey will be the transmitter - I feel I am overdue some luck here!

My thanks to:

* Stefan, DL7MAJ, whose YouTube videos inspired me to actually start a job I had been putting off for too long and for his 'cheat sheet' of bandswitch BCD values.
* Tom Evans for his 'Parent Board pinout' document and Jim Shorney's diagram of 'Parent Board Pin Numbering', both in the 'Files' section of the group, were / are essential reading.
* Mike, G3ZCC, whose hi-res scans of the circuits were just right for printing and scribbling on.

Regards,

Mark, G4FPH.

ISTR that I spotted an error in Tom's very nice Parent Board document but I never got back to it. I'll have to take another look.

Thanks for catching the errors in the parent board pinout doc. No matter
how many times a person rechecks something, a different pair of eyes can
spot unseen errors.

The Heathkit Shop used to make a nice-looking, replacement power supply board for the TR-7, but not sure whether they still do.

Mine does not have any ball and leaf spring.

I do have another TR-7 - a very nice, late model, made just prior to when Drake started producing the TR-7A. Plus / minus some minor connector oxidation issues, that one has always worked well. Problem is, when stuff just works, there's little incentive to learn much about what goes on inside. Fixing up this 'basket case' TR-7 has forced me to understand the design, which will be good for maintaining the 'fleet' in future. Having needed to get to the LPF and HPF areas of the radio, I now understand why Drake decided to split the rear panel in the later models!

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@Mike: Thanks for you post. I hope you are in good health. I will send you an e-mail regarding the power supply board. TI and others have some amazing switching regulator IC's available these days. If I had time for a design challenge, it would be to have a go at replacing all of the linear regulators with them. The +24 and -5 Volt rails must run hardly any current. Has anyone measured the current draw on the +10 and +5 Volt rails. In any case, must be less than 5-Amp and 1-Amp respectively.

@Group: I now have the TR-7 transmitter working OK. The Drake 7077 mic that came with the radio sounded somewhat average, so I plugged in the Shure 444 that I use on my other TR-7. It didn't work - what!? The reason was that this low s/n TR-7 did not have the 470k resistor installed between pins 1 & 4 on the rear of the microphone socket. It now has, and the 444 sounds good.

The transmitter produces 100 W RF out up to and including 20 metres. On 15 metres, it's down to around 85 W. On 10 metres, it's down to about 25 W, so some work to do. Despite having a low s/n of just over 1000, this TR-7 has the MkII pre-driver board in the PA, whic I guess is a good thing.

I have some 'talkback' / low-level undemodulated SSB getting back into the speaker when I transmit. Think I read somewhere that the early models were prone to this and that separating out the Hi- and Lo- Amps lines in the DC power lead (like Atlas did!) helps. I will try this. Any other tips for getting rid?

I did have a nice find when I went digging to do the transmit carrier balance - an AUX 7 card installed, with the three Drake diode modules fitted for full 0 - 1.5 MHz receive coverage, plus a previous owner had made some, Christmas-tree style, modules for 30, 17 and 12 metres. So, I did have a bit of luck after all the pain!

I'm wondering what to do with the chassis screws that are broken off in their pem (I think that's what they are called) nuts. I can't imagine the nuts are designed to be pulled out and replaced. Anyone have experience to share here, please?

I was listening to the 20 m band on the TR-7 last night, enjoying signals from the US booming in, when the synthesiser unlocked and the digital display spun off to 17.xxx.x.

Jim,

I think you may be right! The issue is not directly a heat-related one as, when I first turned the radio on from cold this morning, the loop was unlocked. Quite the opposite of the 'unlocked when hot' symptom I was seeing the other day.

After warming up a little, reception came good. I then hooked up the scope to various pins on the motherboard belonging to the VCO board, willing it to go wrong again! After a while, it obliged. In the fault condition, all the power rails stayed good, as did the 500 kHz reference signal from the PBT reference board. The 500 kHz pulses coming into the VCO from the translator board were almost non-existent in amplitude, however. If I wound the scope sensitivity right up, I could see the frequency of the pulse train from the translator board changing in a believable manner with the position band switch, which was good.

At this stage, I started physically pressing on the parent board in the space between the VCO and translator boards and, specifically, on either end of the Molex on the translator board carrying pins 10-18. Doing this caused the loop to lock and unlock for a while until it settled in a locked condition and would not fault again.

Prior to ripping the DR-7 board out again, and as the rear connector of the DR-7 lands on the translator board, I started applying pressure to the DR-7 rear connector. Lo and behold that would bring the fault on again.

So, I suspect 'Molexia' on the translator and DR-7 boards, or maybe a bad joint on the pins somewhere. I will inspect, clean and re-solder as necessary...

Regards,

Mark, G4FPH.