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[buffalocomputer]

I have a Frontman 25r amp and the drive select does not work from the switch or the footpedal. The crunch channel is always on. I tested the pedal and the switch for continuity, and both work fine. Any suggestions on troubleshooting this problem?

An added note- The LED indicator DOES respond then to switch is depressed, but the amp still stays on the crunch channel.

Schematic is attached

Thanks!

[Enzo]

If pushing the button makes the LED go off and on, then the control circuit seems to work.

Look at the schematic you posted.   Center left is the footswitch and panel control circuit.  The output of that circuit is TP18 coming out of U5b.  Says right on there that that point changes between +13v and -13v.  Does yours?    Since the LED works I will assume it does, but check anyway.

From there, follow the circuit to thr right through 1meg R54, then through a couple diodes to the gates of two transistors, Q1, Q2.  Those transistors are what does the channel switching.

SO we have the switching voltages at TP18, do you also have it on the left end of R54?  And the right end?  DO you have it on both ends of the two diodes?

[buffalocomputer]

The +-13v drops down to about 6.5v after R54 and is present at the diodes. since the crunch channel is always on should I assume Q1 to be bad?

[J M Fahey]

The +-13v drops down to about 6.5v after R54

What does it mean?
You mention one voltage (with no polarity indication) when you should post two: with LED ON and with LED OFF.
Thanks.

PS: also momentarily short Q1's Drain to Source; does it mute the crunch channel?

[buffalocomputer]

   The voltage changes polarity correctly when switched from -13 to +13. The voltage immediately after the 1mrg resistor changes from -6.5v to +6.5. Shorting Q1 seems to mute the crunch channel. With the switch in or out, the clean volume has some effect on the overall volume output if adjusted.

[Roly]

Okay, it looks like the control voltage is swinging as it should between a +ve value and a -ve value, so how does the actual FET signal switching work?

In this circuit both FET's are either on or off, and the switching occurs because one, Q1 (ref: C50), is in shunt across the clean signal path, and the other, Q2 (ref: D5), is in series with the drive/crunch signal path and breaks it when off - and seeing you are stuck in "crunch" that is what isn't happening; it's on all the time.  The clean FET Q1 may be switching okay because the crunch path would be expected to dominate.

The action happens when the control signal goes negative, so you need to confirm that the negative control signal is getting to the gate of Q2 via D5 (that D5 isn't open circuit), and if so that the FET source, connected to the following op-amp U2-B pin 6 is at ground potential.  If so then the FET should be switched off, and if it isn't it's duff and should be replaced.

What with?  Well ideally a J111, but as a switching function it's hardly critical and just about any N-channel Junction FET like an MPF102, 2N3819, or whatever you can get hold of, should do the job, just watch the lead arrangement.

J111 connections


MPF102

The same as J111

2N3819

Gate and Source transposed to a J111, must be crossed over without shorting.

[buffalocomputer]

Great explanation- I ordered a cheap pack of j111's from eBay. Thanks all for the help. 

[joecool85]

Let us know if that fixes it!

[buffalocomputer]

Turns out it was Q1 at fault - Nice to have a clean channel again

[Roly]

Are you sure you mean Q1, the one across the clean channel, and not Q2, the one in series with the crunch channel?

[Enzo]

Q1 shunts the clean channel.  Shorted Q1, no clean channel.


Often the case, these JFETs are more or less connected together at their gates.  One failed JFET can sometimes cause the other one not to function.


Remember, JFETs remain ON until turned OFF with a voltage to their gate.

[Roly]

Quite so, but I've been pondering the circuit trying to rationalise a couple of things that don't gell here.

In post #4 OP says "The voltage immediately after the 1mrg resistor changes from -6.5v to +6.5."

Now if that is the case how could the crunch channel be on?  The original complaint was that it was stuck in crunch, post #1 "The crunch channel is always on.", i.e. Q2 wasn't turning off.  If this was because a failure of Q1 was preventing the control line going, say, more than a diode drop or two -ve, how do we have -6.5V after the 1 meg resistor?

If Q1 was shorted the amp should have been switching between crunch and nothing.

[Enzo]

Very reasonable questions.  One thing I have learned over the years in forums like this is to sometimes hold the discussion at arms length and not take all reported data as gospel.    Like a thread elsewhere wherein the OP reported that he had no B+ voltage but the preamp was working.   So we got him to the solution, regardless of the integrity of the reports during the process.

[buffalocomputer]

   I can stand by my voltage readings +-.5v or so (I have a crappy meter). I replaced Q2 1st but got no results. Replacing Q1 did the trick. One other symptom that I did not mention is with the crunch channel all the way down, (gain and volume) a signal could still be heard if the clean vol. was over 5 or so. I suspect the transistor must have been 'leaky' and not 'burnt' enough to shunt the channel.

[J M Fahey]

I'd like to add another possibility:
As Enzo said, FETS are "always" ON , even unconnected, and need a voltage (in this case negative) to be turned OFF.
So the "normal" state, without external voltage applied,  is both FETs ON, Normal Channel muted, Crunch channel enabled.
Which is what the user found.
He also found that there was no change, even when switching, even with having alternation between "+/-6.5V" on "the other side of R54".
BUT: those +/-6.5V may not reach the FET gates, which would cause the exact symptoms observed.
The track leading to gates (through diodes) may be cracked.
As of the 6.5V measured, 13V, through a 1M resistor, into a 1M input impedance meter , will read 6.5V
It does not need any other external load to bring it down; meter input is enough.