Dead Fender Stage 185-replacing thermisto

[Phostenix]

It looks like the Mute circuit is ahead of the Pre out, so it shouldn't affect inserting into the Power Amp in, but let's do it anyway just to be sure.

[kepeb]

ok.
i did a couple of things.
i changed the resistor which was wrong anyway to another one.
a 0.22ohm (correct value) 7w original was 20w but much better match at least

then i checked there was still continuity with speakers cable etc

then i ran a signal in through the power amp in.
the speakers are still completely dead. not a sound. not even a little hiss, hum or buzzing. no click when you turn it on... nothing.

[Phostenix]

Before we declare U9 dead, let's check a couple of things. Unplug your cable from the power amp input. With the power off, check continuity between R73 and C48. One side of R73 and one side of C48 should be 0 ohms (or very close to it). Without seeing the board, I can't tell you which sides, so just check both sides of R73 to one side of C48, then both side of R73 to the other side of C48.

[Phostenix]

Oh, and you did confirm that the audio signal comes out of a cable plugged into the Pre out?

[kepeb]

yep, confirm signal from pre out jack.

i checked continuity where you said tho.
no reading there. on either or.

[kepeb]

couple of questions.
would the section i am examining now be whats causing the speakers to make no sound at all? seems strange for any amp to have no sound at all. even when idle...?

also.
will the fact that my reverb tank is not connected affect any of this testing or the output?
kev.

[Phostenix]

The reverb tank should make no difference. It's being mixed into the signal with U8, so it's before the Pre out.

Yes, it would make sense for there to be at least a little hiss from the speakers with no input.

The lack of continuity between R73 & C48 is a problem. What range of the ohm meter are you set to when checking that?

I want you to insert your audio source on pin 1 of U9. Use a small solid conductor wire so that you can touch it to just pin 1 of the IC or clip the audio source onto R86. I can't tell what the value of R86 is on that schenatic. Try it on both sides of the resistor. Be careful with the level of that signal. If the power amp is OK, it's like having the amp turned all the way up - there's no volume control - you're going straight into the power amp.

Then, try inserting the audio onto R149 & R153 - either side. That will tell us whether the power amp is working at all.

How much later are you going to stay up. I'm guessing it's 10pm there now?

[kepeb]

hey
yea its 10.20 here, i'm a night time person tho.
i was using the lowest range on the ohmeter.

i will do that now, hopefully not waking everyone up, is that after the master control dial on the front then?

[Phostenix]

Originally Posted by kepeb

is that after the master control dial on the front then?

I don't see a master volume on this schematic, just CH.1 & CH.2 volumes. ???

[kepeb]

no,
still absolutely silent on all of that.
must be something else on the output then?
sorry, your right... no master either

[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]

Originally Posted by kepeb

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]

Originally Posted by kepeb

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]

Originally Posted by 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.

[Phostenix]

The obvious next thing is to check the other op amps & see if they are all that low. I suppose you could check your meter just to be sure, too. Check a fresh 9V battery & make sure it's close. Then, if they are all that low on pins 4 & 8, we need to find CR54 & CR55. They're 5W zener diodes, so they should be bigger than "regular" diodes like you'd find in an effects pedal. The PCB layout drawing isn't clear enough for me to find them. If there isn't +16 & -16 on them, we've got to figure out why.

[Phostenix]

In the 3rd picture you posted, R175 & R176 are the big 270 ohm resistors just to the right of the big power filter caps. Check the DC voltage on both sides of those. R175 should have +50 on one end & +16 on the other. R176 should have -50 on one end & -16 on the other.

[Phostenix]

The next thing that I'm curious about is that thermostsat switch (?) that is attached to one of the output transistors. I can't find it on the schematic, so I'm not sure what it triggers if it reaches its target temperature. I'd like to know that it's not doing something.

[kepeb]

CR54 & CR55 read 2.3 and -2.7 vdc, so that seems very low.

how do you know what voltage values to expect on these components?

r175 has 50v then 2.7v
r176 has 50 then 2.6v

does this mean these two arent doing thier jobs?
kev.

oh and the thermostat switch, one end goes to the power switch and the other end goes to cp7

[Phostenix]

Those low voltages mean that something is pulling the 16v rails down, but it's not pulling the 50v rails down. Since the power amp is also down, I'm trying to think of something that is common. It's also strange that both rails are down, so whatever is happening is affecting both sides.

Did you check pins 4 & 8 on the other op amps?

[kepeb]

hey yea.
they're all (all the opamps) around the 2.0 - 2.5 range.

[kepeb]

so, if something is 'pulling the voltage down'
what kind of thing is that usually caused by?
does that mean something after those two power resistors is more likely open or shorted?

[Phostenix]

It means something is partially shorted. We're running out of ways to divide & conquer this since it's all on 1 PCB. I was hoping someone else might have an idea of something else to check before I say that you need to pull the board & start checking components. The 2 100uF filter caps are the first thing to check - C80 & C81 it looks like. You could just check them for low resistance, but ideally you need a cap tester. If those are good, the next thing is to start checking the op amps. I'll channel Enzo for a moment and say that you shouldn't just start throwing parts at it, but I think we're at the point where you need to pull the board & start removing parts and testing them. If someone else has another idea, I'm all ears. I would start looking at the +16 & -16 volt rail traces to see if there are any jumpers that could be removed temporarily to separate some of the components that are attached to those voltages. If there are no jumpers, then you just have to start removing op amps one at a time to see if the voltage comes back up. I don't know if there is anything other than op amps or inverter ICs attached to the 16v rails. I don't think there's anything in the power amp attached to those rails, so you may still have another problem there once you get the +-16V back. I won't be able to spend time with it while I'm here at work now, but I'm hoping to print the schematic on 11X17 paper today so that I can get a better look at it all.

[kepeb]

well, i dont see anyone else jumping in.
thats great then. i'll make some space so i can leave the board out safely.

so you need to get the op amps out in order to check them?

also how can i make a cap tester? or is that something i have to buy?

[52 Bill]

I have not read through the entire thread to see what you've checked so far but here are a couple of things to look for.

Originally Posted by Phostenix

It means something is partially shorted. We're running out of ways to divide & conquer this since it's all on 1 PCB.

Heat could be one indicator of a shorted IC. Are any of them warm or hot?

Voltages on output pins could be another. Read voltages on the audio chip output pins. There should be little or no voltages there.