Here's an easy circuit, with hopefully a simple diagnosis, but I'm still new at this.

[Kenrod]

Fender Frontman 15G guitar amp. Initially suffered from blown output transistor.
Replaced that and I think a diode (it's been a while). The output was very low though after that repair.

After studying the schematic a while, i figured i could make the troubleshooting a little easier, as it seems the signal either goes through U1 on "clean" or U2 on the "drive" channel, before hitting U3 and then the power stage.

So, I realize that it's super loud on the "drive" channel, even on lower gain settings, than the clean channel can attain wide open.
From that, I surmise that I can essentially ignore U2, U3 and the power amp portion of the amp, focusing primarily on U1. I checked input signal to verify 5mV, checked and found +/- 15V on U1, but only about half the output at pins 1 & 7 of U1. I do remember checking all resistors around the U1B section of the opamp, but not necessarily around the A portion of U1.

Even so, is it fair to say i should replace U1, or are more tests needed?

Thanks

[dwmorrin]

So you checked you had a 5mV input... and where you should have 20mV (TP8) you're only getting 2.5mV?
Give us the dc voltages, with no signal applied, at all the U1 pins.
If its easy for you to do, a quick swap of U1 is not a bad idea. But I don't think we can verify that it is bad from the info provided so far.

[Kenrod]

11mV at pin 7 (TP8), 30-ish mV at pin 1. -15V at pin 4, +15V at pin 8.
This from memory of yesterday.
Ill go check all pins with no signal and report back

[52 Bill]

11mV at pin 7 (TP8), 30-ish mV at pin 1. -15V at pin 4, +15V at pin 8.
This from memory of yesterday.
Ill go check all pins with no signal and report back

I'm so confused.

So the problem is low volume only on the clean channel? If that's the case, then IC U1A is the only stage to be a problem, as U1B drive both clean and distortion channels. Check the channel switch as well as the clean volume control. I've seen the control break a trace at the pc board.

As for your voltages, which ones are dc and which ones are ac?

[Jazz P Bass]

Yeah, those voltages in the circle are volts ac.
You must apply a signal into the input jack to measure them.
A guitar will do if you bang on it hard enough.

A pretty good indicator of a failed opamp is high volts dc on the output pin(s), though low dc voltages do not prove it good.

[Kenrod]

I'm so confused.

So the problem is low volume only on the clean channel? If that's the case, then IC U1A is the only stage to be a problem, as U1B drive both clean and distortion channels. Check the channel switch as well as the clean volume control. I've seen the control break a trace at the pc board.

As for your voltages, which ones are dc and which ones are ac?

Yes, low volume only on clean channel.

If Im looking at the schematic correctly, when you switch on the "drive" channel you take out U1 completely and signal goes through U2, leaving U3 common to both before hitting the power section.

Voltages referenced in mV were AC, +/- 15V was referencing DC supply for ICs

[Kenrod]

Yeah, those voltages in the circle are volts ac.
You must apply a signal into the input jack to measure them.
A guitar will do if you bang on it hard enough.

A pretty good indicator of a failed opamp is high volts dc on the output pin(s), though low dc voltages do not prove it good.

I was using my iPhone with a signal generator app (sine wave at 1kHz), adjusting the volume for 5mV.

[g1]

U1B is in circuit for both channels, like 52Bill said. AC signal at TP7 is 5mV, at TP8 it is 20mV. It is boosted to 20mV by U1B.

[Kenrod]

Ok, I see that now. Circuit splits afterU1B when you choose to send the signal onward through the other half of U1 or send it through U2. Sound right?

That makes my latest finding make more sense to me at this point; I just checked all the resistors around the U1A portion of the circuit and found R9 only reading 38k ohms instead of 180k. I also noticed that the measurement had to "climb" from 34k to 38k. Every other resistor checked much faster when auto ranging.

[Kenrod]

Here are the DC voltages on the pins of U1. No signal applied, no speaker connected, just the chassis on the bench.
Pin1: 32.7 mV
Pin 2: 37 mV
Pin 3: 37 mV
Pin 4 -15.7 V
Pin 5: 10mV
Pin 6: 3 mV
Pin 7: 6 mV
Pin 8: +15 V

[Jazz P Bass]

R9 is a Red Herring.

There is a parallel path between it and R8, R6, R5 & R3.
That's 180K & 49K in parallel which equals 38.5K.

[Kenrod]

I wondered if that might be the case. I can't yet "see" all things in parallel just yet.

Bad U1? I did check the clean volume control and it functions properly and I checked continuity from legs on the pot (above the board) to a common point further down the connected trace.

[J M Fahey]

U1 should have no more than 4 or 5mV on +IN pins , caused by input transistor bias current (they are bipolars) through external resistance to ground and OUT voltage about same, multiplied by DC gain.
Said bias current is stated in the datasheet.

DC voltage at -IN must be a couple mV away from that, the difference is called "offset voltage" and is also a parameter written in the datasheet.

Armed with that, we check whether suspect Op Amp behaves or not.

a) U1b is reasonable:
+IN pin5 10mV (justified by the large 510k resistor to ground)
OUT 6mV (within 4mV of +IN)
-IN 3mV (close both to OUT and +IN)

b) U1A is not:
+IN 37 mV ... with only 180k to ground but, more important, only 22k to U1B OUT, which sits at +6mV ... I would expect no more than a couple mV away from +6mV ... yet we have 30mV difference.

It makes me suspect U1a , so all of U1.

Carefully replace it (pamper the PCB, use a good solder sucker, etc) and it might be the problem.

That said, I still suspect the channel switching, if replacing Ic1 restores proper DC values but not clean channel sound, just for kicks set switch to CLEAN and bypass S1B , not right at the switch because we might have some cracked track in the path, but add a short wire from the right end of R14 (as seen on the schematic, in the actual PCB it might point anywhere) to U3b pin 6 or even better (since it's too small) to left end of R15 or C22 which is the same.

In fact you can do this without even pulling the PCB from the amp, just scratching resistor legs on component side and tack soldering the wire there.

[dwmorrin]

Kenrod, just curious what your test equipment thus far has been.
Are you able to scope the signal, or are your ac readings just from a multimeter? And if so, what kind of meter?
It is possible that the "low output" reading of TP8 may just be meter inaccuracy.
My standard injection signal is more like 30mVrms. You might want to double check with a stronger signal that will be easier for the meter to read, and if you're getting close to a x4 gain, then U1 is probably OK.
If you don't have access to a scope, then I'd suggest cobbling together an audio probe (in its roughest form, just a cap and alligator clips - probe with the open lead of the cap!) and listening to the signal.

Another common and basic thing you may have skipped, since you're "new at this," is to clean out the pots and switches with contact cleaner like Deoxit D5. And then exercise the controls really good. (on/off many times for switches and full CW, full CCW over and over for the rotary controls)
A gunky channel select switch or clean volume control could also cause your reported symptoms.

[Kenrod]

I was using the test parameters from the schematic (5mV @ 1kHz) generated from my iPhone, verified by my old Leader scope and DMM (some previously $100 unit caught on clearance at the Rat Shack) before and after inserting it into the amp.
I also recently put together an audio probe. What a handy thing that has proven to be. I fixed my son's wah pedal after learning signal was getting to, but not through the transistors (supply voltage issue, no transistor turn on)

I'll clean all the controls and switches today and see what I get.