Dead Fender Stage 185-replacing thermisto

[Phostenix]

yea down 1-4 then up 5-8 on the right with the notch at the top. right?

Yep. So, find the 8 pin TL072 op amp U9 and check for audio on pin 7 and then pin 1. Be careful with pin 7 because pin 8 next to it is the positive voltage (+16V). You don't want to short that to anything.

[kepeb]

ok found it by the pre out.
no audio on either of those pins

[Phostenix]

Interesting. How about on pin 3 of U9?

[kepeb]

Interesting. How about on pin 3 of U9?

no nothing there either

[Phostenix]

Also, leave a cable plugged into the input (to keep the mute circuit off) and plug your music source into the power amp input & see if that gets you audio to the speaker. It looks like the Gain Offset knob should give you some control over the volume and also a 3 position Level switch marked Low, Medium, and High. Try changing those controls & see if it makes any difference to the sound coming out of the speaker.

[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]

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