1974 Twin Reverb Volume changing

[rustyadams]

I have a 1974 Twin Reverb Silver Face that volume jumps... gets loud then soft.. I could force it to happen by touching the volume knob, so I replaced the volume pot and it still happens. If I touch the shielded wire from the pot to pin 7 of V2B volume will change. My first question is where is the shield of this wire suppose to be attached.
thanks for any help

[mresistor]

I would look at pin 7 V1B and see where that shield wire is terminated.   Got any pictures of the area in question?  normally shield wires are terminated in those amps to a ground near the inputs or on the brass plate.
I would check all the ground/shield connections to that brass plate from the board  they have solder joints that have been known to crack and cause loss of continuity. They can be intermittant too like you have described.

[rustyadams]

 I compared wiring to V1 and found it was correct. Hot wire (white) of shield is going to pin 2 of the volume pot and the shield is going to pin 3. I've attached a photo, I really need to replace the white wires (quite messy). Other interesting thing I found channel 1 volume pin 3 is terminated directly to the brass plate. Channel 2 volume pin 3 goes indirectly thru some other connections to the brass plate. Thinking I should add a wire and terminate directly to the brass plate?

[Dave]

I'm thinking that there is an intermittent short between the signal wire and the grounded shield.


One way to easily test this, is to remove the ground connection. It is not necessary for functionality. It just lowers (maybe) the noise floor.


Remove the ground connection and see if it still has the bad behavior.


Dave

[rustyadams]

Good suggestion, but it did not make any difference. If I touch the volume knob the volume jumps or touching the chassis near the volume knob same result... frustrating! Also, checked the continuity of the signal wire it was good

[Latole]

I'm not surprised, you didn't have to replace the volume as requested in your other publication.

https://el34world.com/Forum/index.php?topic=31819.0

[mresistor]

Latole   wouldn't it be just as easy to scroll up a few replies instead of posting a link to this same thread.  Ok well the link is pretty quick to the beginning.

[Dave]

I think he mistakenly posted the wrong link. There is another thread about the same amp.


Dave

[sluckey]

I think he mistakenly posted the wrong link. There is another thread about the same amp.

Where is the other thread? Rusty has only made four posts and they are all in this thread.

[mresistor]

I see three posts in this thread   I think this is what Latole was trying to link to  https://el34world.com/Forum/index.php?topic=31822.msg350529#msg350529

[Dave]

Bingo.


Dave

[sluckey]

Sorry guys. Thanks for getting me straight.

[AlNewman]

I wonder if there's maybe a grounding issue in one of your jacks.  There's also the switches for the bright cap in the twin, which may be suspect...  Just spitballing. 

[rustyadams]

I'll take a look at the ground on the jacks and check the capacitor on the bright switch

[rustyadams]

Realize now that I should have asked the following question in this thread instead of starting a new one...


"I want to replace the volume pot. The schematics part number is 021832 1M-J.
and I want to replace some of the white wires, not sure what gauge (see photo)
Any recommendations on part vendors and best place to purchase pot and wire"

Should I delete the other thread?

 

[mresistor]

If you do then you will be deleting my answer to you suggesting where to get the replacement potentiometer and what it is.