Ampeg V4 wierd channel 1 & 2 interacting

[g1]

I know these amps, I've been playing through V4B (the other one) for about 10 years now. they are overloading on about 11o'clock, but not a hint of distortion on lower volumes.

The V4B does not have the sensitivity switch so it has less gain in the first stage. If you have the sensitivity on the V4 set to -9db then you can compare the two models as far as the volume settings.

[boroman]

I give up..



I have changed two jacks as both of them were not shorting to ground when the instrument jack was released. I also changed one of the resistors which measured a little higher (7.5Mohm vs 5,6mohm). Having a bit of time, I also replaced diodes and added bias pot...

And I'm in the beginning. The same behaviour like in the first post. I don't get it. I have tried to measure all the caps and resistors near V1/V2 and all looks/measures close to its specs. No bad traces, no overlapping solder joints. I have got some photos of previous work on other V4's and it PCB/wiring looks the same. I have no idea what to do now. Maybe it'll be "spare parts" V4 for some new projects.

[boroman]

OKAY,

One more thing which I do not underrstand. In the attachment photo there's a shot of the original jack wiring. All of the lugs are shorted with each other, so ground is shorting with signal???



1 - solid wire jumper to 2 position
2 - negative (black) from pcb + resistor leg2
3 - positive (white) from pcb + resistor leg1
4 - tip (=2)

So from this picture everything is connected with everything. Resistor between 2 and 3, jumper wire between 1 and 2. So there's no way that we have separate ground here

Confused.

[Helmholtz]

All of the lugs are shorted with each other, so ground is shorting with signal???

Yes, that's the way it's supposed to be: Everything grounded as long as no plug is inserted.

[boroman]

Yes, that's the way it's supposed to be: Everything grounded as long as no plug is inserted.

If the plug is inserted, everything is shorted too.
That's why I dont get it.

All lugs are connected to each other, so there will be shorting with the jack inserted and without it too. Am I missing something?

[Helmholtz]

If the plug is inserted, everything is shorted too.
That's why I dont get it.

All lugs are connected to each other, so there will be shorting with the jack inserted and without it too. Am I missing something?

Seems you wired the jack wrong. Lug #3 (hot input) must not be connected to any other lug. I should be shorted to ground by the switch contact only. Inserting a (non-shortig) plug opens the switch and should allow you to measure the grid leak (5.6M) from lug #3 to ground.

[Helmholtz]

And I'm in the beginning. The same behaviour like in the first post. I don't get it. I have tried to measure all the caps and resistors near V1/V2 and all looks/measures close to its specs. No bad traces, no overlapping solder joints. I have got some photos of previous work on other V4's and it PCB/wiring looks the same. I have no idea what to do now. Maybe it'll be "spare parts" V4 for some new projects.

As already recommended, disassemble the Ultra Hi switch and look for internal shorts between the 2 channel contact rows. If you can't repair/replace it, you can always disengage it and hardwire your preferred setting. There is a good chance this will cure your problem.

[boroman]

Thanks for the tip with the Ultra-Hi. Will try to mess with it today.

About the jacks - this is a closer shot of factory (I mean, looks like...) wiring. You can see that all lugs are connected to each other. So they will short with each other in any in/out operation, right?

[Helmholtz]

You can see that all lugs are connected to each other. So they will short with each other in any in/out operation, right?

Lug 3 and lug 2 are not connected. There is a 5.6M resistor between.

[g1]

Am I missing something?

In your diagram you called lug #2 the tip. Lug #3 is the tip. #2 is the switch. So #3 should only measure grounded when no plug inserted.

[boroman]

OK, let's go ahead - it's not the input jack fault. Tested it right now and all seems OK.
What should I do to disconnect Ultra Hi switch leaving it on neutral setting? Looking at the schematic it seems it passes throught C3 (0,01uf), C102 (0,00012uf) and C101 (0,001uf). I thought neutral setting on this switch was not going through any cap (at least I remember it from V4B). It seems that the mid one from these caps is C101 so this is the neutral? (the same for CH2 = C104)

[Helmholtz]

What should I do to disconnect Ultra Hi switch leaving it on neutral setting?

As said you might have a short between CH1 and CH2 contacts inside the Ultra Hi switch. Without disconnecting the switch you might check with an Ohmmeter if there is low resistance between any of the CH1 contacts and the CH2 contacts. It's best to have both vol pots fully up for this test.

For further advice, please post close-up pictures of the switch and vol board to see the physical layout. We also need to agree on a way of numbering the switch terminals.

[g1]

I thought neutral setting on this switch was not going through any cap (at least I remember it from V4B). It seems that the mid one from these caps is C101 so this is the neutral? (the same for CH2 = C104)

It's a little different from the V4B here.
(+) setting puts C102 as bright cap across volume.
Neutral setting puts C101 & C102 in series but shorts the 2 ends together. However it is still connected to the wiper so could cause interference.
(-) setting puts C101 from vol. wiper to ground.

[boroman]

Thank you guys.

The story goes on:
I have disassembled and cleaned Ultra Hi and Midrange switches, cleaned contacts and now I see how it works another thing learned!
Inside switches is all OK, does not look strange at all. I have raised the two "contact elements" to have better contact with the points/lugs on the switch

speaking of Ultra Hi switch these are my measurements:

Pinout:

1A - 2A - 3A - 4A
1B - 2B - 3B - 4B

When MINUS is engaged: 3A shorts with 3B / 4A shorts with 4B
When NEUTRAL is engaged: 4A shorts with 4B
When PLUS in engaged: 4A shorts with 4B

Resistance between 1A and 4A is 175k
Resistance between 1B and 4B is 850k
No sound change between NEUTRAL and PLUS (usually when volue is up and treble is fully up going from minus by neutral to plus raises buzz and noise floor. plus seems the same as neutral)

Pots fully up (but these values not change when they are turned down)

IMAGES:

[Helmholtz]

Pinout:

1A - 2A - 3A - 4A
1B - 2B - 3B - 4B

When MINUS is engaged: 3A shorts with 3B / 4A shorts with 4B
When NEUTRAL is engaged: 4A shorts with 4B
When PLUS in engaged: 4A shorts with 4B

4A and 4B are ground (please verify) and are jumpered, so they will always be shorted. In MINUS position, the switch connects 3B with 4B and 3A with 4A, so all of them will be shorted with each other and grounded.
In position NEUTRAL the switch shorts 2A with 3A and 2B with 3B. In position PLUS the switch shorts 1A with 2A and 1B with 2B.

As row A and row B connect to different channels, it is essential that with the switch in MINUS position there is no leakage between either 1A or 2A and 1B or 2B. Because of the path through the pots you should measure something like 2M between 1A and 1B, but infinite resistance between 1A and 2B, between 1B and 2A and between 2A and 2B.

Also in PLUS position there should be no connection/leakage between 3A and 3B.

I hope I got all this right. If the switch was completely disconnected I just could have said: Make sure that none of the row A contacts leaks to any of the row B contacts.