I bought a non-working TR-7 from a well know website, it received quite well, but the digital display didn't work amongst other things.
After my initial fault find and usual cleaning of connectors and interconnections, the fault remained. I have a large stock of TR-7 parts including the DR-7 display board. I swapped it and sure enough, the display worked - so it's the DR-7 that is faulty.
So faulty board back into the unit. Next check that the switch on the back of the unit which allows the counter to be used as a frequency counter (a nice touch that most people forget about!) - check that it works - the switching worked correctly- on the display when switched to external the display should blank - this did for the most part - but for the 10's digit which showed a 0
The TR-7 has a service manual which is very good - easy to read with good circuit descriptions (in my opinion!) - I'm lucky I have an original one which is wonderful but some good scans are available if you look carefully - it does recommend that you have a scope to fault find on this board with 100Mhz I can vouch that if you don't it might be a little tricky as the waveforms really do need that sort of bandwidth.
I have an order which I check the DR-7s , effectively, check voltages, and then work backward from the display - multiplexers, and drivers first, then timebase/gate, dividers and finally if needed input controllers and other inputs.
So after checking the voltages (all ok) I skipped the display and multiplexers as the display was working (although not displaying the right frequency of course!) and started on the timebase /gate.
I start by checking the 500Khz input (present) and then work through the dividers, first one -U9014 divides by 5 (100Kz or 10us) - this was fine, then further divided by U9017, 9019, and 9022 to divide by 10,000.
Check the output of U9017 - all OK - Check input into U2019 (pin 10) - all ok.
Check the output of U9019 at the point indicated by the manual (collector Q9003) - nothing like the right waveform - ahhhhh.
Work back - check the output directly at U9019 pin 14 (q4) - OK , pin 13 (q13) - nothing
Now without the pin 13 output via the two diodes (D9003, D9004) the transistor won't switch properly and so the following divider stages (U9020) and U9018B won't work correctly.
So this definitely cause problems with the reset and loading pulses which would explain why the digital display wasn't working perhaps, it wasn't loading and resetting correctly! Lucky I guess that it was this early on in the check.
I carried on and checked the gate / timebase area, as well as the input and internal/external switching, were OK - which they were.
So I ordered the correct chip - 14158 CMOS chip - MC14158BCP which is a dual BCD counter - I even managed to find a Motorola version of the chip which was the same brand the original ones, I order two just in case at the grand cost of £5.98 in total!
Chips arrived, chip removed, and carefully replaced with a new one ic (Circled below) before cleanup the diodes are just to the right of pin 14.
Interestingly enough the date code on the other chips are 7904 making them 1979 vintage with the new chip being 1998 vintage putting them 24 years apart !
And so carefully put the DR-7 back into the TR-7 and .........
Success!
I checked it worked on all positions of the band switch - it did and that the up / down / store buttons functioned correctly - they did.
I then check the rest of the divider chain - and as expected all waveforms etc. were as per the manual
So now that's fixed onto the next stage for repair - perhaps the store button (you can see top left how it's broken) together with checking the transmitter and full re-alignment, together with the change of the blue film - this one has faded rather badly.
However I spent a lovely hour or so listening around on 80M, 60M, and 40M, the receiver is sensitive and all other functions PBT, etc. work well even the USB /LSB balance doesn't seem too bad!
Overall about an hour to find the fault and about 45 mins to fit the new chip, put the board back in, and finalise testing.
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