1965 Fisher 200-T one channel not working

[gui_tarzan]

Well, if the problem is on that board, I'd compare transistor voltages from working side to nonworking side. I also see some small value electrolytic coupling caps there. Maybe one of those has dried out. There isn't much on that board.

I went ahead and ordered a full compliment of new electrolytic caps yesterday for the audio boards and power supply. I knew I would eventually figure out where the problem is with all your collective help so it just makes sense to replace them. I'll check the voltages tomorrow also, Fridays are very slow work days so I should be able to test during the day. Another advantage of working from home.



-- Enzo, you have NO idea how powerful your words are. When you told me several years ago "it's just an amp", that also fits in many other areas of life. Those words have popped into my head on so many different occasions it's not funny. It's highly appreciated.

[gui_tarzan]

Well, if the problem is on that board, I'd compare transistor voltages from working side to nonworking side. I also see some small value electrolytic coupling caps there. Maybe one of those has dried out. There isn't much on that board.

Q101 is within a tenth of a volt. Q103 is two volts high. Q102 is a volt high but Q104 is way low, only 2.1 volts instead of 10.6 on the drawing.

[Helmholtz]

Transistors have 3 legs and all 3 voltages matter.
Also with BJTs it's always a good idea to directly measure base to emitter voltage. Should be between 0.5V and 0.7V for proper operation.

Check/replace C106 (500µ).

[gui_tarzan]

Transistors have 3 legs and all 3 voltages matter.
Also with BJTs it's always a good idea to directly measure base to emitter voltage. Should be between 0.6V and 0.7V for proper operation.

Check/replace C106 (500µ).

Of course they do, I was just checking against the schem. For a "service manual" it doesn't list some information that I thought should be there, exactly what you mentioned.


Edit to add chart:

[Helmholtz]

Q102 collector voltage and Q104 base voltage must be the same.

[gui_tarzan]

Yep, you're right. I must have had a couple of dirty connections. I just re-checked all of them and here are the updated numbers.

[gui_tarzan]

Parts are here! Once I get the actual cause narrowed down it's getting a recap on the audio and power sections. The #1 filter cap is going to need something to help it fit.

[Helmholtz]

I just re-checked all of them and here are the updated numbers.

O.k., transistors are working. Voltages look good now.

[gui_tarzan]

That's good. I'm going to re-cap that board (and the power supply first) and see if one of the preamp caps is the culprit.

[gui_tarzan]

Well, I'm going to admit I made a rookie mistake on this. I learned a lot about troubleshooting this type of amp from you guys and I am grateful to you for that. But I'm going to have to eat crow on this because I completely missed something that I never should have missed. In the end it matters a lot to me because I wasted a bunch of your guys' time and I feel bad about it. The problem was...

...the wire connection on the positive side of the 1000mfd cap (C18) that sits between the pre-driver/driver audio board and the output selector. It must have been touching enough by moving it around when I did the audio tests on the separate components and then moved again when I got to the preamp board. It LOOKED like a good connection! When I moved the wire to desolder it it came right out of the connection on the cap. Unbelievable. I feel so stupid.

(sigh)

[g1]

So it's all working now?
That is good news, however you got there.

[gui_tarzan]

Yes it is. Both sides are working normally now.

[Leo_Gnardo]

Yes it is. Both sides are working normally now.

YAY!

[gui_tarzan]

Funny story...

I replaced a few of the electro caps last night. I turned it on and it now has a static noise in all input selections. WTH??? Ok so I started tapping things and I got to the preamp board and the right side went silent again! Come on! So I tapped the input connection once more and got sound with no static. Huh? Ok, the board itself has a bad connection as well so I wasn't totally nuts. I wiggled the wire and it cut out and back in and so on so I pulled it and am going to re-cap, check all the resistors and solder connections and we'll see what happens then.

Weirdness abounds with this unit. Of course it's only two years younger than I am and I have my share of bad connections and weirdness.

[gui_tarzan]

Another question - I haven't seen a capacitor on an input with the negative side pointing to the incoming signal. I circled them in the attachment.