Problem Fender Frontman 212R

[binito]

Hi ,
i´ve a problem with power amp in a fender frotman 212R. The clean sound dirty . I think the problem was a U6(TL072) and Q9-Q10.
I changed a u6 - q9-q10-q11-q12 and q13 and the problem is same.
+42 -42 +16 -16 Volt is Ok.
bias problem?

Schematic:
https://elektrotanya.com/fender_fron.../download.html

INPUT SIGNAL


PIN 5 - U6 (TL072)


TP18 - PIN7 - U6 (TL072)


Q9 COLLECTOR _ D23



Q12 COLLECTOR

[binito]

Any ideas ? Thanks in advance

[Simpson]

D16 and D17?

[Jazz P Bass]

What input value have you used.

I would start with what the schematic says. 5 Mvs.
Then check the values stated on the schematic.

You do not show what voltage level the scope shots are at.

How about posting what the output looks like.

fender_frontman_212r_sch.pdf

[binito]

D16 and D17?

i tested with meter and seems ok.

[binito]

What input value have you used.

I would start with what the schematic says. 5 Mvs.
Then check the values stated on the schematic.

You do not show what voltage level the scope shots are at.

How about posting what the output looks like.

[ATTACH=CONFIG]47530[/ATTACH]

I dont know if i put the signak correctly but the measures are:

TP3 : 41,2v
TP4: -41,2v
TP5: 16,6vdc
TP6: - 16,48Vdc

TP7: 5mVAC
TP15: 91mVAC
TP18: 145mVAC ***
TP19: 33mVAC
TP29: 4,14VAC

TP22: 15,4Vdc
TP23: -16,5Vdc
TP27: 0Vdc

[g1]

So you have correct 91mV at TP15 (PA-IN) but 145mV where there should be 33mV at TP18 ?
R66 is good and connected to ground? R67 & R68 are good?

[binito]

So you have correct 91mV at TP15 (PA-IN) but 145mV where there should be 33mV at TP18 ?
R66 is good and connected to ground? R67 & R68 are good?

Yes , the resistors are ok.
Before, when i touch with sonde of oscilloscope in q9 colector or emite q12 or +42 line the sound was clean.
But now the problem is always.
Thanks in advance.

[Jazz P Bass]

Seeing that U6 has been replaced, I would go back and ohm out all of the connections to make sure they go were they are supposed to.

With that much gain on Pin 7, I would assume there is an open in the feedback path.

[Mikestorm]

Hi to all
i've gotan issuewith a frontman 212r.
The amp sounds fine for 30 minutes, then the clean channel starts to distort.
The problem appear to be q12 and q13. When the amp sound fine, the 2 transistors gets hot, till the issue come back and the 2 transistors gets cool.
I've pulled out the transistors and checked fine.
all voltages seems right
Anyone got same issue?
Any ideas?

[The Dude]

I've had this problem on several of those amps. Yes, the transistors will check good, but fail under operating conditions. Try replacing them regardless of how they test. FWIW: If you check voltages on the transistors when the unit starts to distort, you'll see that the transistors have failed.

[Mikestorm]

Thanks The Dude

I've changed a couple of transistors, and changed q12 and q13(they gets really hot till the amp stop working) with mje340/350 pair.
Now the amp seems to work fine, the two transistors replaced now gets barelly warm.

i think those caused the issue in my frontman.

Thanks again
Mike