Dead Fender Stage 185-replacing thermisto

[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.

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.

[kepeb]

cool cheers.
so thats pin 6 of all the opamps? i'll check now
is there a safe way or good way to check for heat from them, how hot is too hot?

[kepeb]

all the opamps have 0.0 and 0.02v on pin 6

[52 Bill]

cool cheers.
so thats pin 6 of all the opamps? i'll check now
is there a safe way or good way to check for heat from them, how hot is too hot?

Dual opamps have outputs on pins 1 and 7.

For the heat test, I touch the top with my little finger. Be careful, sometimes bad ones will get really hot. Most will feel cool to the touch, some will feel warmer. It's just one way to help find a bad one on a board full of chips. The warmer ones get voltages checked first, as that will be the real test.

[kepeb]

ok cheers
i'll check.

i also just checked the zener diodes cr55 cr54 that had 2.0 instead of 16v outs with the diode setting and got readings both ways.

and on the ohm setting
in forward bias theres about 600 ohm and in reverse theres 800.
is that nornal for zener diodes?
kev.

[kepeb]

theres about 1volt on u9 pin 7 all the others are about 0.01-2v
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