frontman 212r cranky sound

[hutsch]

Hi guys
I need a help in a fender frontman 212r repair
Heres the deal
Client brought me this amp, and wat it have is a distortion sound on the clean channel
When you take of the gain in the distortion channel , the sound remains with this distorted sound
So I started to look and the R1 was opend, and the led of the drive channel was opend to
changed both, but no goal
so i changed my attentions on the the Q20 and Q21
Replaced both and no goal, so i looked on R79 and notice that when I literaly took it off the circuit functions better, the cranky sound disapears, BUT the amp starts to function a little bit and then it mutes, than functions a little bit and mutes again.
So anyone can Help?
I checked also Q24, and I'm getting -11 on the norm channel and -16 on the more drive channel

[loudthud]

... and the led of the drive channel was opend to
changed both, but no goal ...

Which is the LED for the drive channel? DS52-B?

I checked also Q24, and I'm getting -11 on the norm channel and -16 on the more drive channel

This points to where the problem is. Check the test points in the switching circuit for the voltages indicated on the schematic. If the voltages are correct, replace Q24.

Q20, Q21 and R79 probably are not part of the problem.

[hutsch]

yes its d5-b yellow led
q 24, i already changed
on the more drive channel everything is ok, all dc are normal, the problem is on the norm and drive channel

[loudthud]

Check the voltages at TP20, TP22 and TP23 in both modes. Do the voltages agree with values on the schematic?

[hutsch]

ok
checked tp 20- norm channel 0,7 V , drive channel 2,5 V
checked tp 22- norm +15,7 V , drive channe -15,1 V
checked tp 23 - norm -16 V , drive channel -32V
Just tp 22 is normal, the others are wrong

[loudthud]

This amp seems like it has multiple problems, but they may have a common cause. If you can find what is causing one problem and fix it, that may fix all the other problems.

Problems:
R1 was burned out. Could be caused by touching a guitar to a bad ground.
Mute circuit seems to be acting up.
Drive mode circuit is not functioning correctly.
"Cranky sound".

Do you have the correct voltages at TP5 and TP6? (+/- 16V)

Have you inspected the foil side of the circuit board for any cracked or burned tracks. I'm thinking whatever burned out R1 may have burned a ground track. Check for continunity between the chassis and ground at several points like the input and footswitch jacks.

Is R49 ok? It's part of the mute circuit. Does connecting a cable from the Preamp Out jack to the Power Amp In jack cure the muting problem?

I can't see where -32V could be getting to TP23. The only possibility would be a short on the circuit board or some kind of ground problem. Any cracks or burned spots on the PCB?

If either U7 pin 1 or U7 pin 7 are at +16V, the yellow drive LED will not be allowed to be on. When both points are -16V, the LED should be on.

The connection to D9 looks like a mistake on the schematic. I think it should connect to the Mute line instead of R18/C10.

[hutsch]

TP5 and TP6 are ok. Someone before me tried to repair the amp, but the guy took of 2 pots, and he broke some tracks, but i have already fixed it,
R49 is ok, connectin the cable dont solve the problem,
when i connect another amp in the pre out it works fine on the other amp, i checked again the board and didnt see any cracks or burns,
U7 is normal for me, led is on when i changed for drive channel, and off on the clean channel,

[hutsch]

well, what I didant undertand, is why when I take out R79 de sound of the amp become clean, anybody can help?

[Enzo]

R79 is the feedback resistor for the power amp. When you removed it the amp has no way to correct itself so it becomes unstable and cycles on and off. It needs to be there.

TP23 should go from 0 to -16v in operation. Both readings are now 16 volts off. Perhaps the readings were taken with the meter connected to something other than ground.

I agree with loudthud about D9.

[hutsch]

tp23 is going, -16v norm, and -0,0016v on the drive

[hutsch]

hi guys, did it, finally, problem was r90 blown, so change it and it worked fine, BUT sometimes it starts to get little noises with the guitar on mute, anybody already experienced that?