86 Carvin X100 trouble.No Volume,cooked resistor

[gary]

You can barely hear the sound from the speaker with both channels on 10.There was a cooked 10 ohm resistor after the 22uf 500 volt caps.The reverb wasnt working and it had a hum.Now the hum is gone,along with the signal.I replaced the resistor with a higher watt rating but nothing happened.There is hardly any where in the preamp stage getting voltage after the tubes.

HELP!

[Enzo]

I don't see a 10 ohm resistor. Is there a part number? Like R121 or something?

Did you read a color code or is the value printed on this part?

Resistors don't burn out on their own, resistors burn out because something draws too much current through them. SO we want to look for a shorted cap or tune or whatever downstream from that resistor.

[gary]

That resistor would be R132.

[Amp Kat]

Two things to check on that amp are the power supply PI circuit resistor that acts as a choke and the screen resistors. All of the power supply feed wires are on the left side of the board and all of the feed wires on this amp are directly soldered to it making it tough to get the board out. My guess is a screen resistor.

[drewl]

Well that R 132 is the feed for the -16v line.
Measure voltage on both sides of that resistor. Without the -16v supply the preamp may not function.
Have you tried plugging into the fx return line to test the power amp?
How about the signal at the fx send or preamp out jack?

[Gtr_tech]

There's an op amp just before the PI in those so if the LV supply is not right it'll still have no output with a signal into the return jack.

[gary]

R132 is at -15.4 volts.

The red pannel LEDs arent coming on either.

Connect pin 3 and 4 of op amps 1,2 and 3,the speaker pops loud and the LEDs come on.Found that by accident,I dont have a surgeons hand...

[Amp Kat]

Try checking the op-amp for +15 & - 15 volts usually on pins 4 & 8 from ground and see if you have voltage there.

[gary]

V- pins are around -15.V+ pins are -4.

[Enzo]

So the V+ is not getting to the chips.

R132 is the series resistor for the -15v, but R133 (22 ohm) is the series resistor for the +15. Is there more or less +15v at BOTH ends of that resistor? If not is it open? Or is it too hot to touch? if open, there will be no voltage on the outer end of it. If it is hot, then the working end is grounded - like a shorted cap or shorted op amp. You can usually measure shorted cap on the line using an ohm meter. A shorted op amp usually gets hot - touch each one with your finger after the amp has been on a little while.

And of course the two supplies could both be OK, and ther is a break in the circuit between the supply and the rail traces through the preamp. An actial crack or broken solder on a jumper wire.

[gary]

I replaced R133 because it read 0 ohms.Replaced the following cap,C75,too.When you turn the amp on now you get a hum ( the first problem I had before ) from the speaker but R133 starts to SMOKE!!!!

[Enzo]

I doubt the resistor itself would have measured zero ohms if you measured it off the board. You might have gotten zero ohms in circuit, or from one end or the other to chasis.

What resistance do you measure to chasis from the C75 end of R133? Is C75 installed the right way - negative to ground?

Look right above C75 and V+ on the drawing - C53 is also across that V+ rail. Is it shorted?

And right above R133, see the five small caps? One each sits across each IC more or less, and one could be shorted.

Any of the op amps could be internally shorted. You may or may not be able to tell that with a meter.

Logic IC A6 runs off the V+ rail, does it look distressed at all? When R133 smoked did A6 get hot?

V+ and V- run off board to the EQ circuit. Possible trouble over there.

[gary]

A6 didnt get hot,IT WAS A2 !Burned the hell out of me too!

Replaced a2.Now R133 is no longer smoking.Now pin V- is -15 and v+ is up to 13.5.

Still no LED and no sound.

Its getting a little better,though.No smoke is good.

[gary]

Something new.R133 is smoking again,flipped the bypass and something popped sounded like a cap or transistor going.I unplugged the EQ wiring harness and it stopped smoking.Found a bad cap ( CB124 ) smoking too.There was a slider on the EQ replaced before it come to me,it was halfway installed.

Could this cap be causing the short?I'll have to go for a drive tomorrow and get one.I dont have one that size on hand.

[Enzo]

Of course it could.