Dead Fender Stage 185-replacing thermisto

[Phostenix]

I finally found a decent pic of the front panel. Both of the volume controls are before the preamp out. There is no master volume that I can see.

[kepeb]

no, no your right. no master.
i didnt consider the second channel for a second.

[Phostenix]

still absolutely silent on all of that.

You clipped your audio onto all of those resistors & no output? Is there any hiss from the speakers?

[kepeb]

no. none at all
rechecked all the speaker wires again.

[kepeb]

the speakers are definitely functioning too i checked thart.

[kepeb]

i know one of the most common to go on tube amps are the smoothing capacitors/filter bit.

would that voltage check i did earlier eliminate the large electrolytic's as a possible source of problems?

[Phostenix]

There are 2 things that don't make sense. The audio should be there on pin 1 of U9. If it's not getting to there, it's not getting into the power amp. But, if you inject a signal at R86, it should go into the power amp. Try injecting the audio onto either side of R136 just to make sure, then we should get some voltage readings from the power amp.

It could be that the protection circuit in the power amp is clamping the output, but that audio should be getting past U9 even if that were true.

[Phostenix]

Check pins 4 & 8 of U9 and make sure they have -16V DC & +16V DC, respectively.

R139 & R156 should have +50V DC on one side of them. Check to see that.

R143, R155 & R166 should have -50V DC on one side of them.

I don't know how to predict what the other DC voltages in the power amp would be to know if they are right or wrong.

[kepeb]

i got -2.7 on pin 4 and + 2.3 on pin 8

+voltages check out

really cant find r166... still looking but all the others check out

[Phostenix]

We haven't checked another obvious thing. L1 & R174 are a coil & resistor right at the power amp output to the speakers. Check those with your ohm meter with the power off. They're in parallel, so checking either should look like a short. Then check from either side of those components to the speaker wires. One speaker lead should show open & the other show be 0 ohms or thereabouts.

[Phostenix]

really cant find r166... still looking but all the others check out

Meaning the -50V is there on R143 & R155?



Those voltage on U9 are way too low. Check pins 4 & 8 of U7 and U8. They should also be +-16V DC

[kepeb]

0.01 on l1 and 1.700 on r174 on the 2k meter setting

yes its there on those two resistors

theres 2.2 and -2.7 on u7

on u8 theres -2.7 and 2.2+

[kepeb]

0.01 on l1 and 1.700 on r174 on the 2k meter setting

yes its there on those two resistors...R143 R145...!

theres 2.2 and -2.7 on u7

on u8 theres -2.7 and 2.2+

..

[Phostenix]

OK, that gives us something to track down. It doesn't explain why the power amp is silent, but it gives us something....

I have to go get some propane for the grill. Let me know if you are going to keep working on this tonight. I'll need to think about what the next step is.

[kepeb]

Cool.
thanks for all your guidance up to now. i really wouldnt know where to start.
i hope i can get it sorted. not sure how long i'll be up but i'll likely be back on it tomorrow if you think its possible to find the problem, i'd love to have it working again.
cheers man.
kev.