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.
