Fender Frontman - really strange problem

[nwguy]

Had a switching problem with my Frontman 25R....drive channel was always on. Replaced Q1....everything worked great....for a while, then same problem came back. Just removed the new Q1, thinking I'll just switch back and forth with the volume knobs. Turned the amp on and got a very loud clicking like the second hand on an old school clock. Immediately switched it off not wanted to damage the speaker. Turned all of the knobs off / counter clockwise and tried it again and same thing.

Anyone ever run into this?

[Mick Bailey]

I haven't had this particular fault with these amps, but I have had switching and other issues caused by bad joints in the power supply. I'd start with making sure the DC voltages are stable and correct as per the schematic inspect the board and chopstick it, but disconnect the speaker first.

[nwguy]

Thanks Mick,
I will check that out.

[nwguy]

Spent a little more time with this today. The oscillation seems to be coming from the first preamp chip - U1. When I pull that out of the circuit, everything seems fine. Perhaps that is why Q 1 blew. Going to try replacing U1 and see what happens.

[nickb]

It would be unusual if U1 were the cause, especially since it happened after right after removing Q1. Replace Q1, and then assuming the clicking noise is gone, start measuring voltages on the gates and troubleshoot the switching problem logically. If it is still intermittent, you may be able to find it by following Mick's advice.

[Enzo]

Keep in mind that oscillation is a feedback loop through one or more gain elements. Pulling U1 stops it, so all we know is that U1 is involved. As to it being defective, I tend to think not, but it is easy to find out. Pulling U1 breaks whatever feedback loop has occurred.

[nwguy]

Thanks all....this is helpful. So, I will re-install / replace Q1 and see what happens. A few other notes...... With U1 out of the circuit / unpowered, the signal passes through the rest of the amp - input jack to speaker without problem. I am getting the oscillation on both pin 1 and pin 7 of U1. Does this mean that I should start looking around the input of U1-A ? Also , when I go the amp, the 15K resistor R8 was broken in half and has since been replaced. Should there have been some clues there?

Thanks again everyone

[Enzo]

Wait. remove U1, turn the whole amp off, and you can still hear the guitar through the speaker???

Try this, look at the footswitching circuit, center left. U5. And the output is TP18 - the signal that controls the JFETs Q1, Q2. Is that TP changing like the schematic says? Are the voltage reaching the gates of Q1,2?

[nwguy]

No, I removed power to jus U1 and was able to operate the amp through the drive channel. Also, the switching circuit checks out all the way to the gates of Q1 & Q2.

[g1]

when I go the amp, the 15K resistor R8 was broken in half and has since been replaced. Should there have been some clues there?

Do you mean R6 (15K) or R8 (22K) ?

[nwguy]

Sorry guys.... a little clarification. I disconnected R14 from the junction of R14 and Q1 and then was able to pass a signal through the drive channel. The connected side of R14 was still showing the oscillation. I then disconnected R8 and got oscillation out of pin 1 on U1. My guess would be that its one of the components around Pin 1, 2 or 3 of U1.

[nickb]

Sorry guys.... a little clarification. I disconnected R14 from the junction of R14 and Q1 and then was able to pass a signal through the drive channel. The connected side of R14 was still showing the oscillation. I then disconnected R8 and got oscillation out of pin 1 on U1. My guess would be that its one of the components around Pin 1, 2 or 3 of U1.

Are you certain that all the reference designators of your PCB correspond to the schematic? It's not unusual for then to change with revisions. The schematic is Rev A. What Rev is your PCB?

[nwguy]

Looked all over the circuit board for some sort of designation but all it said was Fender

[Enzo]

Did it have like a copyright symbol and maybe a year?

[nwguy]

Will have to check again