Fender Frontman 25R Very low Volume problem

[CoopDaKill]

Hello. I have a Fender Frontman 25R with very low volume. I can hear the reverb working when I bang on the amp. I pulled LM3876T for out of circuit testing. When I touch my DMM (set to diode function), to pins 1 & 4, it’s shorted. Am I correct? Thanks
CooP
I’m located in east TN.
Heres a link to the LM3876 data sheet pdf

https://media.digikey.com/pdf/Data%2...DFs/LM3876.pdf

Looks like someone has butchered the input jack too. Know where I can find a replacement?

[Jazz P Bass]

Pin #1 is the +Vdc connection.
Pin #4 is the -Vdc connection.
If they where truly 'shorted' you would be blowing fuses.

Crashing the reverb tank and being able to hear it, tells me the output ic works.

Look at Darren Riley's site for a new jack.
Fender amp jacks

[CoopDaKill]

Thanks Jazz P Bass for your reply and help. I like the Darren Riley site. I reinstalled the output IC, and connected a CD player into the AUX in RCA jacks on the amp. Very low volume there as well. From my understanding, the AUX in is seperate from the volume control. AUX volume is controlled from the audio source. Is that correct? I don't think the input jack is faulty, even though it looks chewed up. I pulled a few electrolytic caps, and found C12 way out of tolerance. Should be 0.47 @ 50v, it tested over 4uf out of circuit. I replaced it with a 0.47uf film cap. No change.

[CoopDaKill]

I think I've found a bad JFET. U4. (TL072CP) With meter set to diode function, positive meter probe on pin 1 & neg meter probe on pin 2, I get a shorted (constant beep). And if I compare this to U3, which is also a TL072CP, I don't get the shorted beep, but a meter reading. This test was done with components in circuit. I'm assuming this means U4 is faulty? Please forgive my ignorance, and let me know if I'm correct in my assumption? TL072CP datasheet link below. Also worth noting is, I have the correct negative voltages on U1, U2, U3, & U4. But none have positive voltage.

http://www.futurlec.com/Linear/TL072CP.shtml

I also get a short between pins 1 & 2 of U5, which is a BA4560. ???

[The Dude]

My guess is that you are reading across a parallel path in regard to the "shorted" IC's. Hard to say for sure without a schematic. If you don't have your positive supply on pin 8 of any of the op amps, that is indeed a problem. I'd be looking for the source of that supply and figure out why you don't have it. Check first one of the op amps pin 8 to ground and see if you have a short on your +15V supply. You could have a shorted op amp(s) loading the supply. If no short exists, something could be open. Or, it could be both- maybe an op amp shorted and burnt something in the supply. Also, are any of the IC's getting hot?

[g1]

Yes, find the problem with the missing positive supply to the IC's.
Pin 1 to 2 connection can be a common buffer circuit, so it may be normal depending on schematic.

[Jazz P Bass]

There are two versions of this amplifier.

Here they are: Frontman 25R.zip

[CoopDaKill]

I've made a silly mistake. I confused the pins on the TL072CP's, (U1,2,3,4). I have positive and negative 15vdc on these. Please forgive my ignorance. Now, I'm looking at Q1 & Q2 (J111 transistors - datasheet below). I get no DC voltage between the Gate and Drain on these transistors. Also, when I set my meter to diode function, they both test shorted between Gate and Drain. Is this normal? BTW, This is the Version 1 of this amp. I've uploaded the schematic. Thanks again guys for all your help.
Coop

http://www.futurlec.com/Transistors/J111.shtml

[g1]

This is the Version 1 of this amp. I've uploaded the schematic. Thanks again guys for all your help.

Is the LM3876 a valid sub? I assume someone replaced the TDA, otherwise there is a third version of this model.

With the fet's the gate should not be shorted to the drain.

[CoopDaKill]

I pulled Q1 & Q2 (J111) out of circuit. They test shorted between Source and Drain. That’s normal? Thanks again

[Jazz P Bass]

For a J111 jfet, yes, drain to source is normally conducting .
The datasheet (Static Drain-Source On Resistance) indicates an RDS-On of 30 ohms.

http://www.onsemi.com/pub/Collateral/J111-D.PDF

[g1]

So I guess no one has the 'Rev.B' schematic, that uses the LM3876 ?

[Enzo]

The only LM3876 one I have is the FM25 DSP.

If they made a non-DSP with that IC, I don;t see it here.

[CoopDaKill]

My Frontman 25R was manufactured in July 2000. It has the LM3876. I’ve only found two schematics for the Frontman 25R. No DSP. Just a spring reverb. I think the earlier version didn’t use Q1 & Q2. But, I’ve been known to make a mistake or two. Lol.

[Enzo]

I didn't really think you had DSP, but I posted the drawing for the LM3876 power amp.