Fender Frontman 212R Distortion

[Steve A.]

Tapping on each component with a *plastic* chopstick can indicate a defective part or a bad solder joint.

I have not tried this myself but if it is a thermal issue heating the pcb a little bit with a hair dryer or heat gun might help with the troubleshooting. (I paid $60 for a Craftsman heat gun 15+ years ago but Harbor Freight is selling them for $14.99 and I think that you can use a 20% off coupon to bring that down to $12. It sure beats using your wife's hair dryer and screwing it up! )

http://www.harborfreight.com/1500-wa...112-96289.html

Steve

[vincenzodesanctis]

in my case there is VAC on TP3 and TP5, what does it means?

When I power on the amp there is a bump. And when I play guitar the sound is reduced and distorted, like a transistor fuzz.

I tested the power rails:

TP1 is 32VAC
TP2 is 32VAC
TP3 is 41VDC but there is also 93VAC!!
TP4 is -41VDC but no VAC
TP5 is 16VDC and there is 36VAC!!
TP6 is -16VDC but no VAC

I tried to change the electrolitic caps C71-C72 (4700) but same problem. I tested the diodes value and I think they are good (D53-D54-D55-D56-D57-D58). Even C75 and C76 are good.
I don't know what is the problem and what kind of test I can do.

[Enzo]

Hi, Vincenzo, welcome to the forum.

May I suggest you start a new thread for your amp, so it doesn't get confused with the earlier posts about a different amp. In other words, we don't want two patients in the same room with the doctor.

A turn on thump is normal.

But as to your situation. I think your meter is being fooled. Is your meter a basic inexpensive one? Try this test: get out a 9v battery, verify it has 9v (or 8v or 7v or whatever) on your DC scale. It is a battery, so we KNOW there is no AC coming out. Now set your meter to AC volts and touch the battery terminals with the probes. It will usually spike a reading and quickly settle to zero AC volts. Does yours? Now reverse the probes, does the same zero result? Or does the meter show zero volts AC one way, but a large AC reading the other way? If the meter does not measure zero volts AC both ways on the battery, then the meter cannot be used to check for AC on top of a DC voltage. it will measure AC when ther is no DC present, and it will measure DC.

If you have 40v and 16v rails that close to spec, I will wager my lunch money your power supplies are OK, and the low power and distortion are elsewhere.


Isolate the problem. Plug the guitar into the power amp in jack, next to the preamp out jack. Does that come through clear and strong, or is it fuzzy and weak? Likewise play into the regular input jack up front, and run a cord from the preamp out jack over to some other amp. How does the preamp sound coming out the other amp? Clean and strong, or fuzzy and weak?

[vincenzodesanctis]

Thank you Enzo, I opened a new thread on http://music-electronics-forum.com/t41201/ (Fender 212r repair: distorted sound output and bump on power on)

[Saggiatore]

Hi guys!

I have from a friend of mine, another Frontman 212 with a similar issue (distortion in clean channel and dirty output). First I discovered that a couple of power (Q20-Q21) was changed in past by another technician. I solved this placing the original transistors, and some resistors as R104/R102/etc (value Fu47) were uot of value.
Ok, by changing all this the amp cleaned the sound, but scoping it loaded with a dummy load resistor, I measured the output and before the clipping there were not the 80W rms (using as input sinewave 1khz). But... this was all that I could do and my friend was using the amp for some time (about a month).
He called me and return me the Frontman again with more distortion and low power. I scoped it again an realize that the negative semicycle is clipped when the signal grows, otput clip inmediately. Positive cycle can grow to the maximum value then it clip normally.
Scoping the whole power section I realize that signal is clipped before power transistors, but I donīt realize who is the defective part.

Apart from this consultation, I would like to take this opportunity to greet a colleague from Argentina, a renowned builder of amplifiers, Mr Juan Manuel Fahey. A big hug!

Thanks a lot if someone could help me!
Regards

[J M Fahey]

Ouch!!

I saw "35 answers" so thought it was already solved, it turns out those were old ones.
The problem of sticking a question at the end of an old worn thread, you should have posted a new one

Ok, start by checking these voltages.

Amp on, preferrably through a lamp bulb limiter, no signal, no speaker on the output.


As always: red and blue show where to place red and black probes; red only means black goes to ground.

[Saggiatore]

Gracias Juan.
Dear Friend.
I thought about to open a new thread, but I decided finally to continue with it because it's another reason of distortion in these amps.

Here the radings in the conditions that you suggested:
1: 0 Vcc
2: 0.349 Vcc
3: 0.283 Vcc
4: 0.002 Vcc
5: 0.002 Vcc
6: 0.560 Vcc
7: 0.001 Vcc
7: 0.001 Vcc
9: 0.560 Vcc
10: 38.59 Vcc
11: 0.326 Vcc
12: -38.51 Vcc

Thanks a lof fro your help. A big hug

[spunko]

Hello, I had this same problem with a frontman 212r.

Solution: In my case Q12 was faulty, replaced it and no more distorted output.

The weird thing is that the faulty Q12 reads good out of the circuit. Is it posible for a transistor to pass diode test and still be faulty?

[Enzo]

Yes. Common diode test is to check base to collector and base to emitter. But don't forget to test emitter to collector. An emitter/collector short will still check OK from the base to each other. It is also quite possible for a transistor to test OK for junction voltage and still be extremely leaky when current is flowing.