After compiling all the precious informations I got from the previous threads, I did my homeworks on the weekend,
I decided to go deeper in the issue, and start to check components (usual suspects).
Step 1 : Check BC328
This is the worst area in comparison with diagram. Every voltage reading is wrong, around BC328. I unsoldered the transistor and my component tester said it is a 13 Ohms resistor between B and E...
RIP BC328....
Step 2 : Check C275
Since this is the only capacitor in // on the amp citruitry, I was wondering if it was working properly and not leaking. Well results were
C= 222 uF (220uF blueprinted)
OK but....
4.5 % Vloss!
I'm afraid this is a bit too high, is it
??
Step 2 : Check C277 (last 3 pics)
This is the output cap. 800 uF blueprinted.
It was pointed me out as being a possible cause for the death of final stage transistors.
Getting access to the capacitor was a pain in the A
It is sunk between the Ge final stage heatsink and the DC input filtration block, with the power antenna remote on the top ...
Finally I managed to get the heatsink out of the PCB. Not for the faint hearted
The results: Good cap, bad cap... ?
C=990 uF (800 uF blueprinted)
Vloss = 1%
Even if this component is now definitely out of spec, my guess is this is the NOT root cause for the no amplification issue (correct me if I'm wrong
). I've seen fancy values on schematics based on AD156/157 pair, ranging from 470 to 1000 uF and it seems that the number itslef is not that important, assuming it stays in the same order of magnitude, say from 300 to 1000uF.
Vloss seems quite acceptable (lower reading was 0.9%, highest 1.2%), but once again, I miss some feedback to compare.
Any other checking I SHOULD do ?
Thank you in advance for your contribution
Best regards
Sebastien