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Solid State Amplifiers => Amplifier Discussion => Topic started by: Superpunky on March 01, 2021, 10:39:48 AM

Title: Fender Frontman 212R with distorted sound
Post by: Superpunky on March 01, 2021, 10:39:48 AM
Hi forum,

I have a Fender Frontman 212R (not FM212R) with an issue of distrorted/crackling sound when the guitar is played. The distortion/crackling happens only when the guitar is played, when the source is muted there is no sound on the speakers. Both speakers are working correctly.
The issue persists connecting the guitar on the Power Amp also, so there's something wrong in the power amplifier section (lower right section in this schematic). https://idoc.pub/documents/fender-frontman-212r-schematic-ylyxvo69mznm

I tested all the capacitors on board and they seem to be ok. Solderings seem to be ok as well.

Did this happen to any of you? What tests should I do to find out the issue?

Title: Re: Fender Frontman 212R with distorted sound
Post by: g1 on March 04, 2021, 08:02:22 PM
FM212R is the same as Frontman, just cosmetic differences.  Even the schematic you linked is labelled as FM212R (bottom right corner).
Have you tried tapping around on the board with a chopstick?
With modern lead free solder, you often can not spot bad solder joints by eye.
Title: Re: Fender Frontman 212R with distorted sound
Post by: Superpunky on March 07, 2021, 07:43:30 AM
A little update on what I have done:

- I tried the chopstick method as suggested, unfortunately I had no change in the output
- I measued the DC power supply: they are OK
- I reflowed every single soldering: no changes
- I swapped the OA U6: no changes
- I unmounted and tested every single electrolitic capacitor from C40 to C60, they are all ok

How should I proceed? should I unmount all the transistors and measure them? Is there a way to measure if Q18 Q19 Q20 Q21 are working correctly?

I attach the audio of the amplifier with a guitar attached directly to the PWR IN, maybe it could help

Title: Re: Fender Frontman 212R with distorted sound
Post by: Jazz P Bass on March 07, 2021, 12:37:27 PM
Please check the power supply voltages for stability, sameness & lack of ripple.
+ & - 42Vdc
+ & - 16Vdc
Title: Re: Fender Frontman 212R with distorted sound
Post by: Superpunky on March 13, 2021, 09:32:17 AM
TP3 is +42,7V DC  0,56V AC
TP4 is -42,8V DC   0,6V AC
TP5 is +16,5V DC
TP6 is -16,3V DC

I can't measure the ripple on TP5 and TP6 because the measure is floating, anyways I unmounted and tested C75 and C76 and they are good.
Title: Re: Fender Frontman 212R with distorted sound
Post by: Jazz P Bass on March 13, 2021, 02:55:19 PM
Next step is to measure the output transistor voltages.
The first stage opamp (pin #1 & #7), the driver bases & the output transistor bases. (the left leg of the component)
No signal. Volts dc.
Title: Re: Fender Frontman 212R with distorted sound
Post by: Superpunky on March 13, 2021, 04:31:26 PM
U6 OA voltages:
Pin 1: 5,1 mV DC
Pin 7: -235 mV DC

Drivers base voltages:
Q18: 1,225 V DC
Q19: -1,17 V DC

Output transistors base voltages:
Q20: -0,62 V DC
Q21: 0,56 V DC

Is it possible that the distorted output is the effect of this little asymmetry in polarization?
Title: Re: Fender Frontman 212R with distorted sound
Post by: Jazz P Bass on March 14, 2021, 12:42:42 AM
The dc voltages don't look bad at all.
You don't have access to a scope do you?
Title: Re: Fender Frontman 212R with distorted sound
Post by: Superpunky on March 14, 2021, 05:46:52 AM
SOLVED: I borrowed another working 212R to make some cross tests and it turns out that the distortion depends on bad speakers (both of them). I had them tested in first place, but I didn't realize they are bad (lack of experience). I feel so stupid  :-\
Thanks for your precious support  <3)

PS: I ordered a low cost function generator and an oscilloscope because I would like to keep up with this hobby
Title: Re: Fender Frontman 212R with distorted sound
Post by: Jazz P Bass on March 14, 2021, 01:17:25 PM
When I listened to the mpg file my first thought was "that's a bad speaker".
But then I saw that you verified them as good.
Glad that you got it sorted out.
Title: Re: Fender Frontman 212R with distorted sound
Post by: g1 on March 14, 2021, 04:30:50 PM
When I listened to the mpg file my first thought was "that's a bad speaker".
But then I saw that you verified them as good.
Exact same thing here.
Quite relieved to see it was actually the speakers and our instincts are still good.   :)