2555 Issues

[Deftone]

My '89 2555 is having issues. Works fine until I pull the channel knob, then there is a severe volume drop and it sounds fuzzy. Sometimes the volume swells. It seems like it got better when I started switching the knob in and out.

Also, when I tap the top of the headbox with the amp turned on, I can here a ringing through the speakers.

AFAIK the tubes and caps are all original. I took it to a tech and he said he bench tested it and it was fine but the issues persist.

 

[FleshOnGear]

Maybe it’s the relay. The channel switch engages a relay, the signal doesn’t go through the switch on the knob.

 

[Kuga]

Works fine until I pull the channel knob,

Do You mean master volume knob?
I think original ones engage relay when you pull master volume knob. Reissue works backwards. When you pull master volume it don't engage the relay.

Could be a relay contact.

 

[dtier]

I would tap the individual tubes to try to locate the ringing if its a problem. Keep cycling the channel switcher, you might be able to mechanically clean an iffy relay contact if that's whats happening.

 

[Deftone]

Do You mean master volume knob?
I think original ones engage relay when you pull master volume knob. Reissue works backwards. When you pull master volume it don't engage the relay.

Could be a relay contact.

The "OUTPUT MASTER".....

I would tap the individual tubes to try to locate the ringing if its a problem. Keep cycling the channel switcher, you might be able to mechanically clean an iffy relay contact if that's whats happening.

Yea, It sounds microphonic when I smack the top of the headbox.

Maybe pull the chassis as well and spray the contacts with DeOxit?

 

[Jethro Rocker]

The "OUTPUT MASTER".....

View attachment 139757


Yea, It sounds microphonic when I smack the top of the headbox.

Maybe pull the chassis as well and spray the contacts with DeOxit?

Been there n done that with smacking top of amp. Microphonic preamp tube, likely V1 is my guess.
Although, all tubes are involved in both channels, the LED is activated in Lead Channel. So not sure but it was for mine.

 

[Deftone]

Been there n done that with smacking top of amp. Microphonic preamp tube, likely V1 is my guess.
Although, all tubes are involved in both channels, the LED is activated in Lead Channel. So not sure but it was for mine.

V1 is the little guy on the far left, correct?

 

[dtier]

The channel switching relay is sealed and would likely have to be replaced if it was the cause of your issue but not before testing it to confirm. There's no way to clean it. The first tube you have circled is V1. I would try substituting it with a known good tube regardless if it is microphonic or not to make sure it is not the issue.

 

[6StringMoFo]

I've had to do the relay swap in the past, solved my switching issue instantly.

If you have some tech skills it's not difficult, but take it to someone who knows about soldering if you don't. The traces on those boards are not so strong and are sensitive to prolonged heat (they can lift and or break).

 

[Deftone]

Just to follow up: I tried changing the V1, no change. Tapped the tubes and one of the EL34's sounded microphonic. I swapped it out with the only other loose EL34 I have and I didn't notice a difference. I order a quad of new EL34's and I have a number of tested 12AX7's on hand. I contacted the tech and he said to bring it back so as soon as the new EL34's arrive, I'll be bringing the amp back to him. He asked me to list the issues so I have that ready for him as well as some of the suggestions listed here. I appreciate the assistance.

FWIW this tech came highly recommended by Skip Simmons (a fairly well known Fender tech)

 

[Beryllium-9]

God damn Chinese amps... Who is the tech?

 

[Deftone]

God damn Chinese amps... Who is the tech?

"Patrick." like I said, Skip recommended him highly. He did a great job on my 2500 SL-X.