Vibrochamp weirdness

[Randall]

Blackface vibro champ comes in for treble pot problem. As it is turned up, the signal gets weaker and then goes away. I measure ~ 75 ohms from the cap side of the pot to ground. I lift the wire off that pot lug and the wire/cap junction read open to ground, but the pot lug changes as I press on it. I move the wire twist to the bass pot and the reading also changes, Eventually, I splay and reposition these wires a bit and the short is gone and the treble now works. I never did see what was shorting, so I wonder if maybe it is a metal splinter that somehow worked it's way through the cloth covered wire.

Now I have another strange issue. When powered up the amp makes four pulsing sounds in a row at about the speed you can comfortably say. "one, two, three, four". Sometimes it is more noticable than others, and twiddling the treble knob from 1 to 10 makes it stop. There is usually a pause of maybe 10 seconds between these four pulses, but sometimes it does it without a pause. It almost seems like the trem is involved, but it does it with no input plugged in and with the trem jack shorted off.

Any ideas?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[nosaj]

Blackface vibro champ comes in for treble pot problem. As it is turned up, the signal gets weaker and then goes away. I measure ~ 75 ohms from the cap side of the pot to ground. I lift the wire off that pot lug and the wire/cap junction read open to ground, but the pot lug changes as I press on it. I move the wire twist to the bass pot and the reading also changes, Eventually, I splay and reposition these wires a bit and the short is gone and the treble now works. I never did see what was shorting, so I wonder if maybe it is a metal splinter that somehow worked it's way through the cloth covered wire.

Now I have another strange issue. When powered up the amp makes four pulsing sounds in a row at about the speed you can comfortably say. "one, two, three, four". Sometimes it is more noticable than others, and twiddling the treble knob from 1 to 10 makes it stop. There is usually a pause of maybe 10 seconds between these four pulses, but sometimes it does it without a pause. It almost seems like the trem is involved, but it does it with no input plugged in and with the trem jack shorted off.

Any ideas?

https://www.thetubestore.com/lib/the...-Schematic.pdf

Sounds like a cap to me because of the pulsing. Check for dc on that pot while it's pulsing.
nosaj

[Randall]

Turns out the pulsing follows the Speed control. If I let it set for a while and power up with the speed control on 5 say, the pulsing goes at that faster rate. And still, it stops when giving the treble control a full range sweep. I'm not seeing the connection.

[shortcircuit]

Turns out the pulsing follows the Speed control. If I let it set for a while and power up with the speed control on 5 say, the pulsing goes at that faster rate. And still, it stops when giving the treble control a full range sweep. I'm not seeing the connection.

and once you give the treble control the full sweep it goes away and all is good until you let it set again?? have you done any tube swaps or looked for an arch in the dark??

[Randall]

"and once you give the treble control the full sweep it goes away and all is good until you let it set again?? have you done any tube swaps or looked for an arch in the dark??"

Yes, it doesn't do it every time at power up, but when it does, the treble full sweep stops it. I haven't seen any signs of tube failure. Once it is cleared up, it will idle for quite a while without noises.

[nosaj]

"and once you give the treble control the full sweep it goes away and all is good until you let it set again?? have you done any tube swaps or looked for an arch in the dark??"

Yes, it doesn't do it every time at power up, but when it does, the treble full sweep stops it. I haven't seen any signs of tube failure. Once it is cleared up, it will idle for quite a while without noises.

Did you ever check for DC on the treble pot when the pulsing would happen?

nosaj

[Chuck H]

There's also a possibility of some oxidation that may have caused both issues. You probably inadvertently corrected the treble pot problem just by manipulation, but I'm sure the whole amp could do with some cleaning on of the pot's, jacks and tube sockets.

[Randall]

Jason, haven't checked for voltage on the pots yet, but I will and report back.

Chuck, the amp has had a thorough cleaning already, and a cap job.

I am now pretty sure the trem is partially on at times, causing the pulsating. I say partially because it's not as deep and full as when the intensity knob is dialed up. This is odd.

[Chuck H]

I am now pretty sure the trem is partially on at times, causing the pulsating. I say partially because it's not as deep and full as when the intensity knob is dialed up. This is odd.

Well the oscillator is wired to operate unless you plug in the footswitch to actually turn it off. You should be able to (pretty much) stop the oscillator with the controls, but sometimes not. If you're sure the pots are clean and of correct value then there may be a problem with the grounding. Maybe old caps are leaking and a skewed bias condition is preventing the controls from stopping it entirely?

[nosaj]

Jason, haven't checked for voltage on the pots yet, but I will and report back.

Chuck, the amp has had a thorough cleaning already, and a cap job.

I am now pretty sure the trem is partially on at times, causing the pulsating. I say partially because it's not as deep and full as when the intensity knob is dialed up. This is odd.

Haven't noticed this yet. IS the footswitch connected ?

nosaj

[g1]

Could also be the intensity pot isn't going right to zero resistance, and maybe intermittently. You don't hear a dirty/defective trem pot like other pots. You can try jumpering the wiper to ground. (also like Chuck's idea about bad ground)
The treble pot affecting it may just be an incidental effect, so I wouldn't focus too much on that for the time being.

[Randall]

It is suddenly not doing it today. As stated earlier, grounding the trem jack did not stop it, so I will look at the intensity pot tomrrow.

[Randall]

Well, it stopped pulsating, and I don't know why. I soldered the control lugs and oscillator wiring and components, checked all grounds, measured 3.4R to ground on the wiper of the intensity pot, and found only a slight wavering voltage on the treble and bass pots in the 25 - 35mV range. I assume this is the tremolo. But, the amp has worked for 3 days now without acting up, so home she goes. Thanks again everyone!