Ampeg V4 wierd channel 1 & 2 interacting

[boroman]

Yes, 4A/4B are grounds.

What you tell about horizontal shorts is right (checked now)


1A+1B measures 1 Mohm on all three rocker sw. settings (-/N/+)
1A+2B measures infinite only in MINUS and NEUTRAL. It shows 1Mohm when it's in PLUS
2A+1B measures infinite only in MINUS and NEUTRAL. It shows 1Mohm when it's in PLUS

in the PLUS position 3A and 3B does not short/leak.

[Helmholtz]

Yes, 4A/4B are grounds.

What you tell about horizontal shorts is right (checked now)


1A+1B measures 1 Mohm on all three rocker sw. settings (-/N/+)
1A+2B measures infinite only in MINUS and NEUTRAL. It shows 1Mohm when it's in PLUS
2A+1B measures infinite only in MINUS and NEUTRAL. It shows 1Mohm when it's in PLUS

in the PLUS position 3A and 3B does not short/leak.

Looks ok except that I would have expected to see 2M instead of 1M. Reason is that between 1A and 1B there is a DC path through both vol pots in series. Does your amp have 500k vol pots instead of 1M ones?

[boroman]

Looks ok except that I would have expected to see 2M instead of 1M. Reason is that between 1A and 1B there is a DC path through both vol pots in series. Does your amp have 500k vol pots instead of 1M ones?

No, they are original, 1M's

Can I ask why you addressed Ultra Hi switch for potential problem and not the Midrange one?

[g1]

Because of the path through the pots you should measure something like 2M between 1A and 1B,

1A+1B measures 1 Mohm on all three rocker sw. settings (-/N/+)

Checked mine and reading is 1.9Meg so this is a good clue.

[Helmholtz]

No, they are original, 1M's

This would mean a leakage resistance of around 2M.

I recommend to disconnect the wire from either 1A or 1B and measure resistance between 1A and 1B again.

Can I ask why you addressed Ultra Hi switch for potential problem and not the Midrange one?

You described an asymmetrial interaction between the channel volumes. Both channels are mixed at the pin1 plates of V1 and V2. So after this point there is only one single channel and anything happening to the signal must influence both channels in identical manner.

[boroman]

This would mean a leakage resistance of around 2M.

I recommend to disconnect the wire from either 1A or 1B and measure resistance between 1A and 1B again.

You described an asymmetrial interaction between the channel volumes. Both channels are mixed at the pin1 plates of V1 and V2. So after this point there is only one single channel and anything happening to the signal must influence both channels in identical manner.

I'll measure that tomorrow. And about the Midrange is the same also for both channels, so it can be somewhere too?

[Helmholtz]

And about the Midrange is the same also for both channels, so it can be somewhere too?

The way you described your problem, the cause must be before the channel mixing point.

[boroman]

Allright. I finally found the solution. It wasn't jacks' fault. It wasn't ultra hi fault. Someone has cut two traces on the pcb near VR102, made a jumper with resistor and jumper to the reverb tubes. Whoever did that, it looks stock (solder joint etc), that's why I haven't noticed in before. Looking at other V4 guts I made shots of over the years, this jumper got my attention. I showed it to my friend and he said that's probably someone wanting the same amount of reverb regardless of the volume setting on the amp but "something went wrong". The jumper was desoldered along with that resistor, traces were jumpered and viola.

V4 is working great puts 100W RMS and no hum, noises, distortions... Thank you all for the help. Another lesson learned.

[Gaz]

Wow thanks so much for the last post/update because I had the same symptom and googled my way here. It turns out some jackass made the same Ill-fated mod to a V4 I just acquired and I completely missed it!