SWR Studio 220 Squealing Sound/Intermittant

[Hawk]

Hello Folks,

This amp sounds great, except in the last 30 minutes it's made two high-pitched squealing sounds which sound like sirens. This siren-like sound last for 1-2 seconds then stops. Thoughts? Cap issue (s)?

Thanks.

[Hawk]

There is a 12AX7 tube in the pre-amp, so wondering if microphonic to a degree, but even now, amp  still running, no squeal...I'm sure it will show up again at some point...

[J M Fahey]

Maybe microphonics, but not necessarily, poor grounding (bad/dirty contact)  somewhere may also cause oscillation.

Some preamps squeal when amp is turned off, voltage goes down gradually and at at certain value Op Amps become "crazy" , probably because of starvation, and amp "squeaks" loke a mouse a few seconds after turn off, that's why many add a turn off mute.

Of you suspect microphony, soft tapping here and there with a chopstick may find it.

[Enzo]

Or bad cord, input jack problem.  Usually those result in hum, but with any gain, a loss of ground can cause a high pitched shriek.

Ball up your fist and whack the top of it while it is playing, does that make it happen or cause other noises?

[Hawk]

Thanks gents. Yes, I tried whacking the top when playing and nothing happened. I tried wiggling the guitar chords, wiggled input jack but it seems to  be solid (I will do a continuity check). While doing all of that I played the bass and no problem.

But what I did discover was this: every time after I re-started amp the siren sound would start after anywhere from 10-14 seconds then stop and everything would  be fine. The siren sound would start and build quite rapidly then abruptly stop--this sound lasts approx.4 seconds every time. Then, the amp sounds fine and I play away no problem.

I repeated this  on/off process five times and got the same results: 10-14 seconds all good, nasty siren sound for four seconds, then all is fine.

Note: the above occurs without the bass plugged into amp.

Cap discharging/charging?

[Hawk]

Below is a quote from a website which got me thinking (it was in response to a vox amp making a siren sound when turned on, the same sound my makes)...so I checked the zeners in the power supply and they checked out okay. Measured the ESR of the caps in the 15V supply and they are good...15V at Opamps....tried chopsticking components, pushing down on circuit board to see if maybe some poor solder joints...hmmm

"It's one of the 15 volt regulators in the power supply. They go into a cycle of thermal shutdown, turn back on and charge up the filter capacitor after the regulator....Hence the weird noise. Some do it randomly others do it in regular cycles.
There are two regulators, the LM7815, positive 15 volts and the LM7915, negative 15 volts. I always change both with 1.5 amp rated regulators even though it's usually only one of them causing the problem (always the one that's not hot to the touch, which shows that it's not working). These is the only parts causing that noise, not a capacitor, op amp or anything else. They supply power to all the audio op amps, other analog circuits, and depending on model, they also supply bias power for the tube."

[Hawk]

This seems to be happening before the EQ but I'm still unable to find the source. Something thermally triggered? When the siren sound starts the opamp voltages stay the same. It sounds like a charging cap is getting into the audio signal  . Any suggestions? Does this sound like a cap issue?

[Enzo]

SO you get 4 seconds of symptom...

Try to isolate the problem.  In those brief moments, do ANY of the controls affect the noise in ANY way?  SO if you turn all controls to zero, do you still hear it?

Scope the 15v rails, the voltage may be stable, but that doesn't mean some odd crap isn;t happening there.

Can you link or post the schematics?

[Hawk]

I noticed that turning the third EQ knob all the way to left  immediately kills the sound. I will post schematic and check all other knobs to see if they have any effect on killing the sound. Away for weekend so will post schematic on Monday... I'll also scope rails then...thanks!

[Hawk]

Here are some schematics. I numbered them as 1,2,3 because I kept getting error messages.

[Hawk]

Single black band? A resistor?  Also, a pic of circuit board.

[Hawk]

Notes. Plugged guitar into PowerAmp, no siren sound.

From Left to Right  along  bottom of Circuit Board we have 7 OpAmps, From Left to Right, along bottom, A,B,D, F,G, H, I
When Siren Sound starts there is Distortion present on third OpAmp (D) from left but not first two (A and B), but when I put the Scope on pin 7 of OpAmp D the siren sound stops.

The following tested when Siren Sound starts: All pins distorted on F and G. On He, 5 and 6 are not distorted but pin 7 is. On I (last one on right) 5,6,7 do not distort, but all other pins do.

On Quad Comparator, no pins distort except for OUT2 and OUT 3. What exactly is it's function in this circuit and where is it shown on the schematic?

Distortion (with siren sound) all across faders. I've tried to trace backwards from the distortion using the traces (hard to see but possible) and have had some luck but I then I feel I'm missing something.

The OPAmps aren't numbered so I find it challenging to figure out which is which. Any insight to which is which?

Is this siren sound a cap charging late, then later on losing its charge and re-charging. Do caps make a siren sound? Is this a case of a cap going bad.

-15Volts/+14.9 at all op amps. Scope the rails  during siren sound but they appear stable.

[DrGonz78]

Single black band? A resistor?

A zero ohm resistor or essentially a piece of jumper wire.

https://en.wikipedia.org/wiki/Zero-ohm_link

[Hawk]

Thanks Dr Gonz.

[Enzo]

The zero ohm resistor is easier for the pick and place machines to handle while the board is built.  No difference from a wire electrically.