Fender Frontman 212R Distortion

[themusic]

I know this topic has been pretty well discussed , and after reading all the other threads , I'm still stuck.
Amp has a distorted sound clean or drive channel. Check speakers ok , checked pre-out works great.
Power-in, distortion . Pulled board and look for cracks, cold solder ect. looked ok.
With schematic, started with test points. TP 1-25 ok TP 26 -9.0vdc -9.0vdc -2.40vdc. that seems off.

TP 27 -0 vdc -0vdc -17vdc seems good. Now the trouble TP28 16.3 vac and finallyTP29 9.2vac.

That's were I'm at, thanks

[The Dude]

Here's a schematic link for any takers.

http://support.fender.com/schematics..._schematic.pdf

Edit: Can you verify that this is the correct schematic and that test points are what you have? It will save us from going in circles.

[The Dude]

So we have a power amp problem or power supply to the power amp. I would first check your rails +-42V (TP3+4). Check them both AC and DC. Make sure both are up and clean. The fact that you have 9.2VAC on the output makes me think possibly one of your rails is not filtered properly. Do you hear a hum at the output?

Of course this all assumes we are talking about the same test points.

[Jazz P Bass]

I'll bite.

First off, if the Preamp Out test good & the Power Amp In is bad, please consider confining the tests to the power amp.

Test the power rails for a steady + & - 42Vdc.
Also test the Vac ripple on those two rails.

Then test the + & - 16Vdc supplies for stability.
Also, test for unwanted Vac.

The first place I will test is the power supply (if there is not a gross short presented).
If the power supply is not up to snuff, the power amp cannot function properly.

If the PS checks good, then the fun begins.
Finding the fault in the power amp section.

Edit: simulpost

[themusic]

Same schematic. TP3 +41.7vdc TP4 -41.7 vdc what should my ac readings be ? no hum

[The Dude]

Same schematic. TP3 +41.7vdc TP4 -41.7 vdc what should my ac readings be ? no hum

You should have very little AC at those test points, but since your DC is up to snuff, we're probably good there. Also, check your +-16V supplies as Jazz suggested. Do you have a scope?

[themusic]

+- 16 is good...no scope...thanks

[The Dude]

Can you give us the DC voltages on U6 pins 1-8. Be careful not to have a "probe accident" and make things worse.

[Steve A.]

These amps are selling used for as low as $150 on the GC site right now. If this is for a customer he might be better off replacing it than repairing it. If its your own amp you might want to give it a shot. It would help to know its history, like what it was doing before the problem developed.

BTW the Frontman amps are the last solid state guitar amps that Fender still makes that are 100% analog. The designs are similar to the various Champion, Princeton and Deluxe ss amps from the 90's before they went digital in the early 00's. (I prefer to work on the lower powered analog amps that have a power amp module rather than discrete components.)

Steve Ahola

[Jazz P Bass]

and finallyTP29 9.2vac.

Whoa!.
I missed that.
With no signal in & no load on the output, measure the Volts dc on TP29.
If you get any reading above 1/2 Vdc then your output stage is compromised.

[J M Fahey]

I suspect those 9.2 VAC were measured while playing guitar or injecting some kind of audio.

Please set all controls to 0, plug nothing at the input, and re measure V AC and DC at amp output , TP29 .

If there was audio present there with no input, the OP would have mentioned "loud hum/buzz/noise/squeal" which he did not.

An above 20kHz oscillation would not be heard by itself, but doubt a common multimeter would catch it either.

So please confirm/deny.

Thanks.

[52 Bill]

Or is that 9vac high freq oscillation that is also causing the overall distortion?

Are R113/C58 connected and working? Make sure that all of the small value caps that are connected to the op amps and to the driver transistors are not broken loose.

[themusic]

R113/C58 ok. with no input TP 29 7.5vac-8.1vac .374vdc.
when I pug into input sporadic light hissing ,popping, when unpluged nothing.
U6 1- .006vdc 2- .006 3- .006 4- -17.16 5- .000 6- .000 7- .351 8- -16.62 thanks

[J M Fahey]

1) So you have minus 16.62V on U6 pin 8?

You should gave roughly plus 15V there.

2) when you measure important (7.5 to 8.1 VAC ) on speaker out and you plug a speaker there, do you hear anything?

Hiss (not the common weak one which is only audible close by but quite strong, noticeable a few feet away) , "ghost" sounds, buzzy hum?

3) do the power transistors heat up even playing nothing?

[themusic]

My bad U^ pin 8 -16.62vdc.
No sound with speaker, only with input then the sporadic popping hissing every minute or so, not constant.
Nothing seems to be getting hot.
Here's the weird thing , I turn on this yesterday morning and good as gold. ran it all day still good.
Turn on this am still good "still has sporadic hissing and popping. After about 5 min went to distortion then another back to clean,
been that way for about 2hrs. I've chopsticked , shook, bounce, to try to get it to fail but no luck??? I'm lost...thanks