Amp makes high picth sound but not feedback

[Jam]

Its a Fender stage 160 I replaced D47 and everything worked great for a about a week. Now when I have the dirty channel on I get a real high picth sound its not feedback but it even does it when you turn the guitar off and the volume on the channel is off it only stops when you when the distortion channel is off . Some time if the amp is and dist is on and If I tilt the amp back and only back it will start to make the noise some times it comes and goes I dont know where to start

[tubeswell]

Is the high-pitched sound an overwhelmingly loud, screeching, banshee-like howl? If so, its probably a microphonic pre-amp tube, in which case try swapping it.

If it is a keening, high-pitched, kind of piping sound that is not loud, but just super annoying, if might be unwanted oscillation relating to a number of causes, including possibly bad solder joints (re-flow the solder joints with leaded solder), or a faulty component like a bad resistor (track it down and replace it).

[Jam]

Amp makes high picth sound but not feedback

it's the keening, high-pitched . Whats the best way to track it down ? where to start to test? look for?

[Robert M. Martinelli]

Hi,
as tw said, it looks like you have some unwanted oscillation going on.....
First of all check the supply voltages, especially the +/- 16 V for the preamp section, if the supply voltages are not on spec the amp could behave erratically ( check the zeners+droppers R155-R156-D62-D63 ).
If the voltages are OK, the first place I would look for trouble is the NFB "area" - unwanted oscillation usually occurs when you get positive feedback instead of a negative one. Check Q21 and U6, if U6 is socket-mounted try to temporarily remove it to see if the squealing disappears.

If the above tests fail, look at the schematic and try to check the test points at the specified conditions, they're usually good starting points for troubleshooting.

HTH

Best regards

Bob

[yunger]

clean and retension tube sockets. i had this problem before and it worked for me. good practice anyway. if that doesn't work, i would try new tubes next. you'll kick yourself if you find out it was one of those after you've torn into your amp with a soldering iron.

[Robert M. Martinelli]

clean and retension tube sockets. i had this problem before and it worked for me. good practice anyway. if that doesn't work, i would try new tubes next. you'll kick yourself if you find out it was one of those after you've torn into your amp with a soldering iron.

.....but wasn't the Fender Stage 160 a SS amp?

The Fender 160 schematic I have is all SS with a "multiple" TIP142-147 output stage, unless I'm having SERIOUS sight/schematic reading problems

Regards

Bob

[Jam]

Amp makes high picth sound but not feedback

It is a SS Ok I checked D62,D63 and R156 and R155 read good. But I get so lost on checking the diodes and resistors is their a way check the parts? I also checked a lot of the test points and get nothing do they work at different times? like Op amps are they pins 4and 8 to check them?

[Jam]

Amp makes high picth sound but not feedback

One more question When I was taking the knobs off the nuts were not tight do the pots have to have to be tight ?

[Robert M. Martinelli]

Pins 4 and 8 on dual op-amps ( 4558 and the like ) are the negative and positive supply pins respectively. You should have some 30-32 VDC across them, or +/- some 15....16 VDC to GND.

I find it weird you get no readings on the test points, make yourself sure you've set your DMM correctly, the test-points, values ( AC/DC ) and measuring conditions are clearly indicated on the schematic diagram.

To check diodes and resistors it's usually better to disconnect one lead from the PCB to keep other components to alter the reading...though it's possible to get correct readings with the components on if nothing is in parallel with them on the PCB, so, try to take a reading with both leads on first, if it tests good, OK, otherwise disconnect one lead and repeat the reading....hope I managed to make it clear enough...

Start by checking the power supply voltages ( TP2 and 3, +45 and -45 VDC to GND for the power amp section, TP6 and 7, +16 and -16 VDC to GND for the preamp section ). Be aware that the Zeners+droppers arrangement used by FMIC on many amps is prone to failure, R155-156 can overheat and melt the solder joints under the PCB. If the power supply voltage for the preamp
section is not OK the op-amps could operate erratically and this would explain the occurring self-oscillation as the NFB loop ( which should keep the amp stable ) has an op-amp in it ( IC6 B/A ).

HTH

Best regards

Bob

[yunger]

.....but wasn't the Fender Stage 160 a SS amp?

hehe.....foot in mouth

[52 Bill]

Also on that amp, look for any broken off caps on the board. Look for small typically green mylar caps that are used to cut off high frequencies. Sometimes they will break a lead off and the amp will oscillate.

Also check the position of the wires that connect all of the boards and jacks together. There was a lead dress issue with some of these SS amps. Keep all output related wires bunched away from the pre-amp sections of the pc board.

[Jam]

Amp makes high picth sound but not feedback

I have a another issue today I was going to test some stuff and it keeps blowing fuses. I tried three fuses and I checked to see if any was touching I need help

[Jam]

Amp makes high picth sound but not feedback

Ok update I had to replace Q18 and Q15 now it powers up. But I was wondering do the pots nut have to be how tight? do they ground to the amp or what ever they call it

[Jam]

Stage 160 need help

I had it the amp apart to look for anything to do with the high picth noise. So the next day I went to test some more and I turned it on and it blew the fuse so I checked everything and still blew fuses . So I checked and replaced Q18 and Q19 So I turned it on and it stayed on so I hooked my guitar up and started playing it with the clean on and so I wanted to test it dirty and started playing and the it blew the fuse so I checked the six + -transistors and now Q15,Q17 Q19 read bad the three - rails what did I do wrong?

[Robert M. Martinelli]

+1 on what Bill said....
small value caps are used against self-oscillation, they can be usually found between BJTs' Bases and Collectors, they shunt the high frequencies with their low Xc at those frequencies, and, if one of them goes open, nothing will keep the amp from oscillating...the problem is that oscillation can occur at frequencies beyond your hearing range, so the only way to spot it is with a scope. If you don't have one, check all these caps. Bear in mind that TIP142/147 are Darlingtons, with a very high hfe, so they're very likely to self-destroy should oscillation occur.
HTH
Best regards
Bob