MKV90 no audio in the 10 watt mode.

[KRichter]

Sorry if this was asked beforehand but have a little issue. My MKV 90, operates fine, but I never used the 10 watt setting. When I do, nothing happens, no sound. I just re-tubed it last January. Thanks guys!

 

[rarebitusa]

across all channels?

 

[KRichter]

Yes across all channels, one thing I thought of, possible cause I run EL34's in it?

 

[rarebitusa]

With el34s as long as the bias selection is set properly it should be ok. The 10W mode does use both V8 & V10 tubes so the likelihood of a single bad tube seems remote. If you set say ch1 = 90W, ch2 = 45W and Ch 3= 10W then ch1&2 have sound but Ch 3 is silent? Kinda thinking an underlying relay issue.

 

[KRichter]

I see there are 3 relays (one for each 10w switch), I couldn't imagine all 3 failing, if they were sticky I could see 1 going bad but not all 3.
So far, I switched the recto tube and the power tubes, using 6l6 and the el34's, same issue. I'll have to give them a buss tomm. In 10 years I never used it, possible is hasn't worked in 10 years? who knows. I just sent it to Mesa last Dec for a check up and everything thing was good. I'm not a tech, but know just enough to be dangerous!

 

[rarebitusa]

Bummer that the tube swap didn't reveal anything. I'd think Mesa would check everything if you had it in for a service. There appears to be 6 relays just in the power tube section.. either way I'd be interested in what the root cause ends up being.

 

[KRichter]

Well, just spoke to Rich at Mesa, he advised everything I did, says never really heard of that particular problem. Although he's not really a tech, he's usually the guy who issues the return issues. Say's because he would advise to have it looked at. I told him I don't know if it ever really worked, cause I never used it. I guess I'll just let it be. (until it's a problem).

 

[rarebitusa]

I do agree it's not a typical failure mode. It's got me thinking.. wonder if it's something to do with the mute circuit is being activated when in the 10W mode? Looks like FET J175M1 could do that, but not sure where it's gate gets driven from.

 

[KRichter]

I do agree it's not a typical failure mode. It's got me thinking.. wonder if it's something to do with the mute circuit is being activated when in the 10W mode? Looks like FET J175M1 could do that, but not sure where it's gate gets driven from.

Maybe next time I have to open it up I'll spray some cleaner on those? Who knows?

 

[KRichter]

I one thing I didn't to is swap out and preamp tubes because the amp functions fine except for that. I know the recto tube handles the 10w mode, any thoughts? Just making sure I didn't miss anything. THX!

 

[rarebitusa]

Just throwing things out here. Do you happen to use the footswitch? If so maybe disconnect that and try manipulating the controls from rear rotary knobs, also activate the mute from the from panel. Curious to see if any behavior changes.

 

[KRichter]

Just throwing things out here. Do you happen to use the footswitch? If so maybe disconnect that and try manipulating the controls from rear rotary knobs, also activate the mute from the from panel. Curious to see if any behavior changes.

Yep did all that. Everything functions as it should. Also manipulated all the toggles. I opened it up and sprayed some contact cleaner in there and nothing. Then remembered a had an issue with my 25 not switching from ch1 to 2. That was a loose ribbon, checked those on this and nothing. Swapped the ax7's, nothing. I don't mind but it's a de-valuing factor. It will function, (not the 10w) if I have 45 on one, 90 on the other.....but the 10 just won't work on any, It's gotta be across the board.

 

[rarebitusa]

Just throwing things out here. Do you happen to use the footswitch? If so maybe disconnect that and try manipulating the controls from rear rotary knobs, also activate the mute from the from panel. Curious to see if any behavior changes.

Yep did all that. Everything functions as it should. Also manipulated all the toggles. I opened it up and sprayed some contact cleaner in there and nothing. Then remembered a had an issue with my 25 not switching from ch1 to 2. That was a loose ribbon, checked those on this and nothing. Swapped the ax7's, nothing. I don't mind but it's a de-valuing factor. It will function, (not the 10w) if I have 45 on one, 90 on the other.....but the 10 just won't work on any, It's gotta be across the board.

Dang... well that beast has so many relays & control FETs, plus its packed in there... looks like someone would have to take it to the next level of debugging to sort this one out.

again grasping here... if you put an external signal in thru the Fx return and bypass the V preamps and rotate thru chans and power levels there's no change with the 10W? Also is there the expected power increase going from 45 to 90W... that would indicate all tubes are actually working.

 

[KRichter]

Sure is packed with **** in there and I think it's 2 boards down to even look at it, didn't try the fx cause everything else cycles thru fine, But I will later. And yep the difference 45-90 is there, so I'm good with that.

 

[KRichter]

Sure is packed with sh!t in there and I think it's 2 boards down to even look at it, didn't try the fx cause everything else cycles thru fine, But I will later. And yep the difference 45-90 is there, so I'm good with that.

Tried the FX & Bypass, all good there...no cure...

 

[rarebitusa]

well it sure seems you've given it the good ol college try... the available schematics are helpful but incomplete especially WRT the relay control matrix. The V:90 is not an easy beast to service... the main board takes alot of unhooking to gain access to bottom side (I know 1st hand) which would be necessary to replace a relay if that was the culprit.