loud scratching noise on One Channel

[hebaton]

1968 Super reverb AB763. New tubes, just biased, New electrolytics.
Makes a loud noise on vibrato channel triggered on attack, gets worse on high volume settings. Not happening on normal channel, so cannot be related to Output or Phase inverter. Can happen with or without Reverb engaged.

I cleaned contacts on all preamp tube sockets. So I figure a bad contact somewhere ( verified all wires for loose one, couldn't find any ) tube swapping and the fact this is affecting only Vibrato channel suggests this has to be related to either V2 or V4 ( I think ) am I looking for a faulty coupling cap ?

[maxkracht]

What kinda scratchy noise? Did it do this before replacing tubes and caps? If it were tone stack caps you should get scratching when turning knobs and there aren't many other caps to go bad that aren't shared or reverb related. Don't entirely rule out reverb stuff until you try it after pulling the driver tube AND turning the reverb off. If you have a scope it might be easy to track down what stage the noise comes from.

[cbass]

Posting voltages might help. Have you played it with the trem and reverb tubes removed? I believe its v3 and v5. Those old boards can also become conductive over time.

[hebaton]

Just tried it with tube 3 and 5 removed. Attaching recording.

Don't really know if the issue was present before cap job.

[hebaton]

What kinda scratchy noise? Did it do this before replacing tubes and caps? If it were tone stack caps you should get scratching when turning knobs and there aren't many other caps to go bad that aren't shared or reverb related. Don't entirely rule out reverb stuff until you try it after pulling the driver tube AND turning the reverb off. If you have a scope it might be easy to track down what stage the noise comes from.

Did you listen to the audio I posted ? I was hoping that could help pointing to the issue...

[maxkracht]

Not getting any specific ideas from that sound. I would assume bad connection or a parasitic. Resistors running parallel to the board often pull loose, same with ground connections. Looking at the amp from multiple angles helps. If you add more solder than necessary when changing things on an eyelet board, sometimes solder will pool underneath and short things out. Diligent chopsticking can usually locate those issues. Check voltages, make sure none of the resistors are too out of tolerance, you might notice something while double checking everything.

If it's a conductive eyelet board issue it should be measurable, just poke your meter lead into the board and see if there's voltage where there shouldn't be. Mainly around eyelets with high voltage. Conductive backing board can be slightly harder to spot.

[hebaton]

Not getting any specific ideas from that sound. I would assume bad connection or a parasitic. Resistors running parallel to the board often pull loose, same with ground connections. Looking at the amp from multiple angles helps. If you add more solder than necessary when changing things on an eyelet board, sometimes solder will pool underneath and short things out. Diligent chopsticking can usually locate those issues. Check voltages, make sure none of the resistors are too out of tolerance, you might notice something while double checking everything.

If it's a conductive eyelet board issue it should be measurable, just poke your meter lead into the board and see if there's voltage where there shouldn't be. Mainly around eyelets with high voltage. Conductive backing board can be slightly harder to spot.

Thank you ! I really tried to find bad connections and finally came across two broken pins stuck in the V5 socket. Not related to the issue at hand but still !

(ONe thing I am wondering about is preamp voltage. V1 plate is at +/-270v but V2 is about 220v
I don't think that's right. What could be responsible for that difference ? )
Just double checked, that was a bad reading . Voltage is good at 268v

[Stevem]

Only the tubes and OT circuit around V2 and V4 section b wil have anything to do with the issue your having.

What do you read for the cathode voltage on V2 pins 3and 8 and V4 pin 8?

[hebaton]

Only the tubes and OT circuit around V2 and V4 section b wil have anything to do with the issue your having.

What do you read for the cathode voltage on V2 pins 3and 8 and V4 pin 8?

Hi and thank you again. Is it suspicious that lightly tapping a coupling cap produces a loud pop ?

Readings at V2 4 and 8 2.v at V4 3v

[maxkracht]

Is it suspicious that lightly tapping a coupling cap produces a loud pop ?

Depends on the pop and depends on the cap. Ceramic caps in old fenders are often microphonic. Wires and components connected to grids can also make noise when you tap them. Neither is necessarily a problem. A pop could indicate a bad solder joint or broken wire.

[TUBEDUDE]

Touch-up the cap connections. Your tap may have been hard enough to affect another joint, so if that cap
didn't do it, reheat connections closest to it. Of course the cap may have an internal intermittent contact.

[hebaton]

The POP noise is VERY loud.
I suspected the .1 and .022 cap at V2 and replaced both. no change.
WHen playing on that channel, Sometime all sounds great, but suddenly a very loud noise comes on, sort of scratching with White noise at the same time. Some times the sound will cut off completely.

All electrolitics just done. could this be related ?

[sluckey]

All electrolitics just done. could this be related ?

Not likely since the NOR channel and VIB channel use the same filter caps.

[Stevem]

Since you have no scope at this point your really down to going with process of elimination.

Unsolder the plate coupling cap from pin 1 on both V1 and V2.
Then take a wire and go from pin 1 on V2 to the end of the unsoldered cap from V1 .

After this does the problem still take place when you turn up the controls on the normal channel?

If so then you found that the issue is the circuit around V1A.

If not then hook things back up and do the same test out with V1b and V2b.

[maxkracht]

The POP noise is VERY loud.
I suspected the .1 and .022 cap at V2 and replaced both. no change.
WHen playing on that channel, Sometime all sounds great, but suddenly a very loud noise comes on, sort of scratching with White noise at the same time. Some times the sound will cut off completely.

All electrolitics just done. could this be related ?

Could be related to new cathode bypass caps. No bypass wouldn't cut off all sound, but no cathode ground would. Easy to measure if this is happening, you would lose cathode voltage when the volume cuts out. Remove solder from the ground point by those noisy caps, clean with alcohol, and then resolder. Make sure the wire going to chassis ground is intact and clean, or run a new ground connection.