This amp recently began to sound like dirty distortion no matter the channel, at moments almost to a muffled/drowned out sound. I first swept through all of the pots hoping to find a dirty one causing the issue, but none changed the conditions of the distorted tone. I checked and cleaned both inputs for connection issues, no change. Plugged the Pre-Out to another amp and got the clean tone the amp had since it was new. Then plugged the speakers into another amp to see if the speakers were blown, but again got a clean and clear tone. I pulled down the circuit board to see if any capacitors were swelled or leaking, or if I saw something burned or broken visually, but all looked good from the top end (haven't yet pulled it completely out to see the bottom of the board). Does anyone know of a common issue/repair worth trying, or advise in next step diagnosing of this issue?
I have experience in circuit board soldering, DMM, schematic of this board, and have been an ASE certified electrical diagnostic specialist for many years. I may be new to circuit board component testing but I can follow directions with the best of them. Any help I can get with this would be great.
I have experience in circuit board soldering, DMM, schematic of this board, and have been an ASE certified electrical diagnostic specialist for many years. I may be new to circuit board component testing but I can follow directions with the best of them. Any help I can get with this would be great.
Schematic:
http://www.amparchives.com/Amp Archives/Fender/Schematics/Fender Frontman 212R Schematic.pdf
From your description it sounds like the preamp is OK but you have a power amp issue.
Initial checks should be:
1) Check the + and - 42V supply rails
2) Check the speaker output for DC offset. With no signal the speaker output should have no more than say 100mV of DC on it. Any more indicates a problem.
Cheers,
Ian
http://www.amparchives.com/Amp Archives/Fender/Schematics/Fender Frontman 212R Schematic.pdf
From your description it sounds like the preamp is OK but you have a power amp issue.
Initial checks should be:
1) Check the + and - 42V supply rails
2) Check the speaker output for DC offset. With no signal the speaker output should have no more than say 100mV of DC on it. Any more indicates a problem.
Cheers,
Ian
Okay, I checked the 42V and found it at an average of 41.6 on each of the contact points shown on the schematic. I used the + lead at the contact points and the - on the casing ground of one of the volume pots.
I checked for DC at the speakers, with speakers plugged in it shown 92mV and without speakers it was at 192mV. I was a bit curious with that I put my + lead with the + side of the speaker and same for the -, but my reading on the dmm shown -92mV. Is the polarity wrong or just my understanding?
I appreciate the help, would you be able to give me a next step or correct me if I've missed these two?
I checked for DC at the speakers, with speakers plugged in it shown 92mV and without speakers it was at 192mV. I was a bit curious with that I put my + lead with the + side of the speaker and same for the -, but my reading on the dmm shown -92mV. Is the polarity wrong or just my understanding?
I appreciate the help, would you be able to give me a next step or correct me if I've missed these two?
The DC offset is fine. Well within the average of 120mV of either polarity. Plug a known good amplifier into the pre out socket and listen. If clean the power amp has a fault, if dirty/distorted there is a pre amp issue. Once you have verified where to look for your fault, all will be easier.
Did you end up fixing the amp?
I have one now that had -7V on output. I changed Q13, and it got it to 450mV with nothing on the input, but dropped to 47mV when plugged in and 4mV with signal. The faulty Q13 was measuring as a jfet on my tester!
But now there is distortion. I am thinking when the other parts show up it may resolve.
I have not had a chance to separate the stages yet to see if it’s in the amp or pre.
I have one now that had -7V on output. I changed Q13, and it got it to 450mV with nothing on the input, but dropped to 47mV when plugged in and 4mV with signal. The faulty Q13 was measuring as a jfet on my tester!
But now there is distortion. I am thinking when the other parts show up it may resolve.
I have not had a chance to separate the stages yet to see if it’s in the amp or pre.
Same problem here. The transistors measured "open" in-circuit because of oxidation. I changed test probes for stainless steel pinpoint-type, then all transistors measured ok.
Turned out the auxiliary +/- 15V supply was noisy/unstable because of cracked solder joints of the zener diode circuit D57/D58 and R144/145. This is caused by very high temperature of these components, which operate close to their limits really, and accelerated agring of the solder joints as a result.
Turned out the auxiliary +/- 15V supply was noisy/unstable because of cracked solder joints of the zener diode circuit D57/D58 and R144/145. This is caused by very high temperature of these components, which operate close to their limits really, and accelerated agring of the solder joints as a result.
Hello everyone,
I am a new to this group and working on another frontman 212 that's driving me nuts.
R144/R145 are 330 ohms in my amp. I think Fender must have changed the value with later manufacturing and the old schematic still shows 200. They still get very hot and manage to unsolder themselves. So I had to take them both out, clean the oxidation off the leads and resolder them. Voltages are good now.
In the past, I worked on one of these amps where one of these resistors going open caused DC to go out to the speakers, resulting in a shorted voice coil. Speaker had to be replaced, obviously.
The amp I am working on now is distorted on the clean channel. I have a clean sine wave on the input to U6-B pin 5. The output on pin 7, has this ratty looking spike coming out of the sine wave. I have replaced U6 - same problem.
First picture is pin 5 input Second picture is pin 7 output.
..
![20240407_152310[1].jpg](https://www.diyaudio.com/community/data/attachments/1204/1204876-f97a6b3e6f0c6ce4b1cc44ead32f9feb.jpg "20240407_152310[1].jpg")
![20240407_152438[1].jpg](https://www.diyaudio.com/community/data/attachments/1204/1204877-b888d4668f90b3e40a1f1e18b6b70552.jpg "20240407_152438[1].jpg")
The spike will enlarge as I turn up the volume.
So I know the problem is in the amplifier circuits, because the preamp is putting out a clean signal. I believe this has something to do with the negative feedback coming off the output of the amp, but I can't figure it out.
Any thoughts or ideas would be greatly appreciated.
Sincerely,
Bob Savino
RES Electronics LLC
Bergenfield, NJ
I am a new to this group and working on another frontman 212 that's driving me nuts.
R144/R145 are 330 ohms in my amp. I think Fender must have changed the value with later manufacturing and the old schematic still shows 200. They still get very hot and manage to unsolder themselves. So I had to take them both out, clean the oxidation off the leads and resolder them. Voltages are good now.
In the past, I worked on one of these amps where one of these resistors going open caused DC to go out to the speakers, resulting in a shorted voice coil. Speaker had to be replaced, obviously.
The amp I am working on now is distorted on the clean channel. I have a clean sine wave on the input to U6-B pin 5. The output on pin 7, has this ratty looking spike coming out of the sine wave. I have replaced U6 - same problem.
First picture is pin 5 input Second picture is pin 7 output.
..
The spike will enlarge as I turn up the volume.
So I know the problem is in the amplifier circuits, because the preamp is putting out a clean signal. I believe this has something to do with the negative feedback coming off the output of the amp, but I can't figure it out.
Any thoughts or ideas would be greatly appreciated.
Sincerely,
Bob Savino
RES Electronics LLC
Bergenfield, NJ