[Mark IV] FX loop died on me right before starting a gig!!

Arghh, had to play the whole set without effects and boost for solo. Disaster!
Not sure what happened though? When I switch the FX loop on I can hear the signal but it's very, very low. Sounds ok when I switch the FX loop off. A friend told me that there is a seperate valve which is responsible for the fx loop. Is this true? If so, where is it and how do I replace?

hi,

no tube assigned to FX loop

So what could've broken down?

My Mark 3 sometimes has connection problems that manifest like this. Being an 80's amp, it needs occasional maintainance of the physical connections.

I usually need to just clean the connectors well, retension parts that bend in the jack sockets to make sure they are snug and double check the link cable wiring and in the plugs.

After that you may want to get it checked though.

I am sorry for the stupid question, what do you use to clean the connectors?

Thank you

I use a generic 'contact cleaner' spray. Designed to clean switches, electronics and potentiometers. Can buy it on amazon. Deoxit is a popular brand.

If you are able to, you can open up the amp and see the sockets from the inside to clean them fully too. They can get full of dust over the years. Just make sure it completely unplugged and the capacitors have drained for a while before going inside it.

If cleaning and checking solder connections on cables does not fix it a tech may be needed.

Also, have you tried different fx to make sure they are not the culprits.

Just make sure the contact cleaner doesn't harm plastic parts (most of them don't these days, but still, better safe than sorry).
Anyhow, doesn't V2B take care of the FX Loop Return? I'd try replacing that tube and see what happens.
Good luck!

Thank you Promethean and Agustín!!

It does say in the manual that V2B is responsible for the FX Return. Will try to replace it first before I give it to a tech. Will let you know soon if that worked.

igfraso said:

Thank you Promethean and Agustín!!

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Any time!!

godisme said:

It does say in the manual that V2B is responsible for the FX Return. Will try to replace it first before I give it to a tech. Will let you know soon if that worked.

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Haha, I still don't know how I remembered that, hope that solves the problem!!

godisme said:

It does say in the manual that V2B is responsible for the FX Return. Will try to replace it first before I give it to a tech. Will let you know soon if that worked.

Click to expand...

yes and no fx loop signal too

So I replaced the V2 with a new Sovtek 12AX7 WC. The volume of the FX loop signal is back up again but now everything sounds a lot brighter when I switch it on. Tested on two seperate effect units: Nova and G-Major.

Another thing I noticed was distortion fading with time on channel 2 and 3 (after 5-10 minutes of playing). After that there is not much "juice" left, just a medium crunch type of tone insteade of a soaring lead on 3. Also, I've notice that the Lead Presence no longer works, it does completely nothing no matter if set 0 or 10.

What can still be wrong? Another pre-amp tube or something else?