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Very Not Good Voltages in a Fisher 80c

wmaio57

wmaio57

Active Member
Hey guys!

About two years ago I bought a non-working Fisher 80c and recapped everything, except for the two cans. When I fired it up, I got almost no volume out of it and decided to mess with it another day. Well, tonight I got it off my shelf and replaced SR-1, fired it up and again, almost no volume. I checked the manual and decided to test some of the tube pin voltages. That's when I discovered that I a major problem and I am unsure how to proceed. Here's what I'm seeing:

1P5BZIt.jpg


I'm really not sure where to go from here. I'm not sure if SR-2 is my issue, but I'd like to replace it as well. If anyone knows a suitable replacement, please let me know. I'm thinking that my transformer might also be open. If so, is there a suitable replacement? Any help would be greatly appreciated!

-Bill
 
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Here's a schematic, if anyone needs to see one. Unfortunately, no voltages are listed. The previous post with pin voltages is on a separate page of the manual.

tuxxOFH.jpg
 
Check the output voltage of SR-2, 1st. Should be slightly less than the working voltage of C-23A. Then RECHECK all of your pin 4 to chassis ground. Series string. Should be 6 volts apart with V4 at 6VDC. V1 should be about 24VDC. if same as before, then replace SR2. Pull one leg of each power resistor check against spec replace if out by 5%. Got a feeling one or more went High Z. R35, 38,39,41, & 43. I'd also replace C21 in it's entirety, C-23 and C27.
 
SR2 is the heater string. I'd be looking at output off SR1, which is the high voltage supply. Both can be a problem but since all of the posted voltages show low B+, I'd probably start in that chain. Bad first caps or bad rectifier would be my guess. C27 specifically has to be good or all of the voltages are going to be low
 
Hey guys,

So I pulled and checked all of the power resistors, R35, 37, 39, 41, and 43. All of them check okay. The C27 10mf cap has been replaced and checks good. The two can caps were disconnected and test within reason (for now). I will replace them later. SR1 has been replaced with 2x 1N4007 diodes and last night I replaced the bridge SR-2 with 4x 1N5408 diodes. Voltages on the pins came up a little but not enough. I'm thinking that my transformer may be the issue. I've never tested one before and with this one, I'm not even sure I know what the windings should read. Anyone happen to know and could led me any guidance?

Thanks!

-Bill
 
gadget73 said:
ohm check from ground to either end, I'm guessing under 200 ohms each side.
Click to expand...

I'm not 100% sure what I need to do. Are you saying that I should disconnect the transformer windings and measure each side separately to ground? For example, it has two red, two orange, and two green wires. Should I disconnect each pair one at at time and measure each wire to ground? If it's bad, what am I looking for? Sorry, I've never tested a transformer before.
 
should be able to check it in circuit. One meter lead to chassis ground, the other to each end of the transformer. Thats just for the HV winding. The center tap is grounded, so each end of the winding will read to ground. Since the tubes light and only the B+ supply seems way low, the only thing I'd be immediately suspecting is a problem in that specific winding.

If it does read funny, lift the two wires going to SR1 and ohm check from chassis to each of those. Digital meter in ohm mode shouldn't put out enough voltage to switch on the diodes but just in case.
 
other thought, check the DC voltage from C27 to ground. Guessing it ought to be somewhere around 250-300v. Highest tube voltage listed anywhere is 195v, so it has to be higher than that.

If thats reasonable, verify the resistors between C27 and C21C and C21D are good. If those have drifted way off value the voltage will end up low.

also C27 has to be good and properly connected or all of the voltages are going to be way low.
 
Thanks for everyone who helped me out with this diagnosis. As it turned out, the transformer was the problem. The windings that supply the B+ were open. Now I need to figure out a suitable replacement.

hsznFIj.jpg
 
that sucks

also shame its potted. Could probably find a common square frame transformer that would fit in the can so it could be hidden. Could likely get a transformer for a Dyna PAS to work in here. Doesn't have a 6v winding for the lights but thats not a huge issue to deal with.
 
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