Fender Frontman 65R - distorsion on clean canal

[Mr.Migoo]

Hi there,

I'm trying to fix different issues on this complicated schematic amp...
I had a 2,3 sec buzz at power on. The 10V D17 zener diod was dead. I replace it and it fixed the problem.
I had also strange and unpleasant "hiiisssss" and hummm when playing.
I replaced C79 capacitor, dead to.
It's not sounding like before but still not expected sound. I almost checked all components but I missed something.
When I'm playing in clean canal I have distorsion and very loud level even with the volume at minimum.
Still have unpleasant over high tone sound if high frequencies knob is up on guitar and/or on amp.
I didn't have this over level when I first checked the amp.

DC voltages seem ok.
I haven't checked yet AC sinusoidal signal path, waiting for a dummy load not to loose audition....

Any ideas where to check?

Thanks

[DrGonz78]

You don’t need to run the amp on a load to be able to trace the audio. It’s just a basic solid state amp and running it without a speaker or dummy load won’t hurt it.

[Mr.Migoo]

Tracing audio is ok until TP18. 37 mV AC is arriving until U7-A in+.
TP19 value on the schematic ??? 37 mAC? Is it a current value or the the V of mV is missing. I mesured 173 mVAC in this point.
In TP20, I have 360mV in stead of 270mV. The next step is the hit: I have 9.6 VAC in TP21!!! Something is going mad here...

DC voltages check is OK.
Any advice to go on with this?

The 3" humm at power on was fixed replacing fuse resistors R80-81-82-83 and Q9 and Q10.
D17 diode was dead too. C79 also, and I replaced some other resistors R53, R52, R75, R77, R78.
I also replaced clean vol R16 knob.

[The Dude]

1) None of those designation numbers mean anything to us without a schematic, so I've attached one. It's a good idea, when you start a new thread to attach a schematic or schematic link.
2) You fixed some things via post #3. What is yet to be fixed, or what is still wrong with the amp?
3) When you changed R16, did you use an original part? I ask because some of the fender amps use the outer mounting lugs of the pot to carry a ground path. If your replacement pot did not have these lugs, you may need to run a jumper wire where the lugs were soldered into the board.
4) Does the clean volume control have any effect at all, or is the amp wide open all the time?

[Mr.Migoo]

1) ok, it's good sense...
2) The Humm starting noise is fixed. But not the over level. Still have distorsion on clean channel.
3) I just checked with a standard knob , waiting to order an original. I will add the jumper for the ground path and check again with a guitar signal.
4) the clean volume control was working but not correctly in it's half course. The knew knob has effect on volume control (just checked with voltmeter value since I replaced it).

Is it normal to have a (x 5 ) gain through U7A?
I don't really understand the U7 function... (detail of schematic attached)

[Helmholtz]

Is it normal to have a (x 5 ) gain through U7A?

From the signal levels at TP18/TP19 I'd expect a gain close to 1.

[g1]

There is NFB applied near U7A (via R73 to R65) so I wonder if you might be getting more gain there because you have no load connected. You could try with the load and see if it makes TP19 agree with TP18.
It is important that you do not connect the P2 'speaker -' terminal to any ground (scope ground for example).

[x-pro]

I don't really understand the U7 function... (detail of schematic attached)

U7-B is a circuit to maintain zero potential at the amplifier output and simultaneously control speaker current at low frequencies, to improve the system efficiency.
U7-A is a limiter amplifier, controlled by the feedback voltage from the amplifier output.
a kind of compressor.

[Helmholtz]

.. and simultaneously control speaker current at low frequencies, to improve the system efficiency.

I think the main purpose of the load current NFB is to increase the output impedance and thus emulate some tube amp performance.
Maybe that's what you mean?

Higher output impedance means less speaker damping which in turn emphasizes low bass and treble response.

[x-pro]

Helmholtz, agree

[g1]

I think the main purpose of the load current NFB is to increase the output impedance and thus emulate some tube amp performance.

Yes, usually referred to as 'current feedback', a good description here: https://sound-au.com/articles/guitar-amps.htm#s3

[Mr.Migoo]

Thank all of you for the "current feedback" lesson. Very interesting link in the article.
I attached pics of my oscilloscope. Signal mesured (with a 8ohm load) at different test points. Signal is ok until U7A-3 input. The Output U7A-1 looks a bit strange, no? Still gain x4 and the signal is not a sinus anymore.
Someting wrong in the NFB or the U7 itself in trouble?
TP20 is quite interesting too...
And the final signal in TP21. Something is not going well
Just after taking those nice pics, the two transistors Q18 et Q19 smoked..........
I guess they get to hot with bad heat dissipation during my tests???
I need to order new ones to go on.
If the pics inspire you something...
I have not much background in electronics. Trying to learn by doing things. But this amp is not for beginners I guess.
But I'm quite stuppborn!

[x-pro]

Just after taking those nice pics, the two transistors Q18 et Q19 smoked..........

Have you ever wondered why transistors need heatsinks?
Why did they overheat?
Judging by the pictures, the output power on the load is very low.

[x-pro]

amplifier circuit with errors
That's how it is: https://music-electronics-forum.com/...r-frontman-65r

[Mr.Migoo]

the two transistors are on a small aluminium heatsink bar which is normally in contact with the metallic frame of the amp trough thermal paste.
I unmounted the mainboard, so the heatsink bar was not in contact with the amp but on my wood desk. Plus that the thermal paste has melted between transistors and the metal bar.
That's why I supposed they over heat even if there was not so much power on the load with 5 mV input signal. But it was really hot
and maybe there is another reason for this overheat.