Fender Frontman 25R Very low Volume problem

[CoopDaKill]

I’m honestly not sure what version this amp is. Manufactured in 2010. Every schematic I have doesn’t show diode D28. I think it’s bad. Can’t read the numbers on the diode. Im really puzzled. What caught my eye, was the white band on the negative end. Usually black band. Is this a Zener diode? I did install a new input jack. Made no change. Pic below

[Enzo]

It is a 15v zener, why do you think it is bad?

D28 is on the FM25R "import" version, series II revision B

My Frontman 25R was manufactured in July 2000.

Manufactured in 2010.

It can't be both.

[Jazz P Bass]

I would call Fender and see if they have the correct schematic.

To me, D27 & D28 appear to be the back to back diodes on the input pins.

See if you have continuity across the top & the bottom of both diodes.

Then see if they go to the input pins of the LM3876 ic. (+Pin 10 & -Pin 9)

LM3876.pdf

Or they could be the clamp diodes on the power pins.

[Enzo]

Does mine not open for you? It shows the diode as a power up mute circuit added to the earlier revision without it. The layout likewise shows it in agreement with his photo.

[CoopDaKill]

Enzo , sorry about my typo. This amp was manufactured in 2010. I shouldn’t work when I’m tired. Lol. I do see D28 on the FM25R schematic you posted. And thanks for sharing that with me. The reason I think D28 is bad is, I pulled it out of circuit, and I get continuity in only one direction. Shouldn’t a Zener Dode conduct in both directions?

I really appreciate all your guys input. Someday I hope to have the Electronics intelligence that you all have. You all cease to amaze me.

[diydidi]

No. Zener should only read one direction as most good diodes will.

[Enzo]

With your meter, a zener will test like any diode. It only conducts the other direction when the zener voltage is exceeded. So your meter does not put out 15v, hence the zener never "zenes".

[g1]

Have you tried with headphones? Same problem?
What is your DC voltage at pin 8 of the LM3876 ?

[CoopDaKill]

On pin 8 of LM3876, I get -3.2 VDC. With headphones, I get the same low volume.

Jazz P Bass - - I'm not 100% clear on your instructions to check top and bottom of D27 & D28, but I did attempt it. D27, D28 have continuity. And I also have continuity from D27 to D28. Each of these diodes also have continuity to pins 9 & 10 of LM3876

[Jazz P Bass]

Then it appears that the amp matches no, as yet, known schematic.
D27 & D28 are the input diodes.
Move on, as that is not the problem.

The amp is not in 'mute' if you have -3Vdc on pin 8.

I think the problem is before the LM3876.
The signal voltage coming into the IC is too low to drive it to full power.

[CoopDaKill]

Thanks Jazz P Bass. At least that helps me narrow down the problem. The only other thing I've found is, R46 is way out of tolerance. It's markings indicate it should be a 1.5 kohm 5%. It measures less than 1 ohm, in circuit. It looks to be part of the input path. But, then again, we don't have a fully matching schematic.

I just called Fender and asked for the correct schematic. They sent me the same one Enzo posted, the FM25r import. Made in Indonesia. This is weird. Cause my amp has the LM3876 IC .

[g1]

Then it appears that the amp matches no, as yet, known schematic.

They sent me the same one Enzo posted, the FM25r import. Made in Indonesia. This is weird. Cause my amp has the LM3876 IC .

The 'FM25r import' pdf has 2 versions of the schematic. The LM3876 version is on page 4 of the PDF.

[CoopDaKill]

Thanks g1 . I see that now. So, page 4 is my amp.

[Enzo]

It pays to look over the entire file.

[g1]

R46 is part of the footswitch circuit.
I'm not sure why it would measure 1 ohm unless the footswitch jack unless they have not shown the jack's shunt switch connection.
I think if you lift one end you will find it measures ok out of circuit.
In any case, like Jazz said, it looks like you are losing signal before the output IC, and you said it is weak even using the aux in.
That narrows it down quite a bit, IC4 and Q4 areas.
What is the gate voltage at Q4 ?