CoopDaKill 12/17/2017 9:19 AM
Fender Frontman 25R Very low Volume problem
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
I’m located in east TN.
Heres a link to the LM3876 data sheet pdf

Looks like someone has butchered the input jack too. Know where I can find a replacement?
Jazz P Bass 12/17/2017 12:34 PM
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 12/17/2017 3:41 PM
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 12/18/2017 3:53 PM
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.

I also get a short between pins 1 & 2 of U5, which is a BA4560. ???
The Dude 12/18/2017 6:02 PM
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 12/18/2017 8:28 PM
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 12/19/2017 12:16 AM
There are two versions of this amplifier.

Here they are: [ATTACH]46190[/ATTACH]
CoopDaKill 12/19/2017 10:43 AM
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.
g1 12/19/2017 1:05 PM
Quote Originally Posted by CoopDaKill View Post
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 12/20/2017 6:14 AM
I pulled Q1 & Q2 (J111) out of circuit. They test shorted between Source and Drain. That’s normal? Thanks again
Jazz P Bass 12/20/2017 10:22 AM
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.
g1 12/20/2017 1:06 PM
So I guess no one has the 'Rev.B' schematic, that uses the LM3876 ?
Enzo 12/20/2017 1:55 PM
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 12/20/2017 3:55 PM
My Frontman 25R was manufactured in July 2000. It has the LM3876. Iíve only found two schematics for the Frontman 25R. No DSP. Just a spring reverb. I think the earlier version didnít use Q1 & Q2. But, Iíve been known to make a mistake or two. Lol.
Enzo 12/20/2017 4:24 PM
I didn't really think you had DSP, but I posted the drawing for the LM3876 power amp.
CoopDaKill 12/23/2017 2:14 PM
Iím honestly not sure what version this amp is. Manufactured in 2010. Every schematic I have doesnít show diode D28. I think itís bad. Canít 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 12/23/2017 2:32 PM
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 12/23/2017 3:22 PM
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)


Or they could be the clamp diodes on the power pins.
Enzo 12/23/2017 4:45 PM
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 12/24/2017 7:54 AM
Enzo , sorry about my typo. This amp was manufactured in 2010. I shouldnít work when Iím 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. Shouldnít 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 12/24/2017 8:32 AM
No. Zener should only read one direction as most good diodes will.
Enzo 12/24/2017 11:00 AM
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 12/24/2017 12:20 PM
Have you tried with headphones? Same problem?
What is your DC voltage at pin 8 of the LM3876 ?
CoopDaKill 12/26/2017 9:05 AM
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 12/26/2017 10:58 AM
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 12/26/2017 11:51 AM
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 12/26/2017 1:25 PM
Quote Originally Posted by Jazz P Bass View Post
Then it appears that the amp matches no, as yet, known schematic.
Quote Originally Posted by CoopDaKill View Post
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 12/26/2017 2:02 PM
Thanks g1 . I see that now. So, page 4 is my amp.
Enzo 12/26/2017 3:30 PM
It pays to look over the entire file.
g1 12/26/2017 3:55 PM
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 ?
CoopDaKill 12/27/2017 11:04 AM
Gate voltage on Q4 is +0.572 vdc
g1 12/27/2017 6:01 PM
At this point, if it were me, I'd clip Q4 off as close to the board as possible and see if the sound comes back. Hopefully the legs are long enough that it can be re-used if it does not solve the problem.
It could be something else in the Q4 circuit, but my experience has been that it's more often the fet than the circuit.
Then if the sound comes back you can measure the spot on the board where the gate was. If the +.5VDC is still there, there is some other problem in the Q4 circuitry.
CoopDaKill 12/28/2017 9:53 AM
g1 ,,, I clipped off Q4, and the amp works great. So, the amp doesnít need Q4? I measured the voltage at the Gate spot, and itís +10.1 dc. Can I live without doing anything else now? Thanks
Enzo 12/28/2017 10:28 AM
Q4 is a power up/down mute. it helps prevent thumps and noises at those times. If you amp is not making objectionable noises at power up or down, then leave it out.
Mark Hammer 12/28/2017 11:00 AM
I have a Champion 110, which is essentially the identical amp in all respects, but has an external speaker jack on the front panel, instead of the dual phono inputs found on the 25R.

What seems to have not been asked so far is whether the low volume problem is sopecific to the Normal channel or occurs on the Drive channel as well. If common to both then whatever the problem is will likely be found in their common input path or common output path.

I will note that the Volume control on the Normal channel is annoying, and attempts, quite poorly, to adjust the gain of the stage and degree of post-gain attenuation simultaneously. If the problem is specific to the Normal channel, I'd consider examining the volume pot.
g1 12/28/2017 11:27 AM
Quote Originally Posted by CoopDaKill View Post
I measured the voltage at the Gate spot, and it’s +10.1 dc. Can I live without doing anything else now? Thanks
The -10V there sounds like the correct voltage. The bad fet was making it collapse. So that -10 indicates that the rest of that mute circuit is ok.
Like Enzo said, it is not necessary, just if the turn on or off thump bothers you.
J M Fahey 12/28/2017 11:36 AM
Looks like the problem was a bad muting FET muting all the time, hence common to both channels.

That active volume control has been used for ages, it was actually invented by Philips for use in their cassette decks (think early 70īs), I still remember the Electronics World magazine where they introduced it and showed all design equations.
If well made itīs a quite acceptable Log pot approximation , and has been a staple of Peavey amplifiers, specially in the PA ones., also some Ampeg ones.

The main point is to make gain "on 5" 10/15% of "gain on 10" and then it mimics a 10/15% Log pot.
It also allows, as shown, an active Bright control with 0 effect on 10 ... as any decent Bright control does.
CoopDaKill 12/28/2017 4:49 PM
No loud thumps or pops during power up and power down. The amp sounds good. Both channels. Thanks again guys for your expert help and guidance. Iíll be donating a few dollars to this fantastic forum.
Bass 300 8/4/2018 9:30 PM
Hello all. New member here. I actually joined specifically to comment on this thread.

I had the same problem with the same amp. Made in in 2010 just like the OP. Low volume across both channels. I found this thread in search of the solution.

I pulled the j111 jfet at the Q4 position as suggested and it resolved the problem. No loud noises on power up or down and the amp plays normally now.

Thought I post here for anyone else with this amp that runs into this problem. Thank you to all that contributed to this fix!
glebert 8/4/2018 11:11 PM
Welcome to the forum!