Several faults in a three channel Dual Recto

[laurie]

G'day!

Long time user (have a few mesas) but first time poster... Hoping to get some help with a couple of faults in a recto I recently bought. I have tools and test gear and a fair bit of experience with fixing vintage tube amps, pedal repair and build, etc.

The amp is a three-channel dual rectifier solo head from the early 2000's.

There are two faults, I believe(?) unrelated.

EDIT: found a schematic and will do some fault-finding on the weekend.

Clean channel volume
Volume level on the clean channel is incredibly low. With the loop bypassed and the channel master on full and the gain on about half it sounds like a 5W or 10W amp. Channels 2 and 3 (orange and red) seem fine. Peel paint off the walls at that setting.

I have tried about 6 different tubes in V1 - Mesa Russian and chinese, GT, Fender, vintage westinghouse. All sound very similar and all produce the same low volume. Channel also has light distortion, unless you drive it very gently (single coil, guitar vol on 5)

I swapped out the phase splitter. No change.

Different power tubes (old - from Mark I reissue) and new power tubes (Mesa 6L6) and no change. Bold/spongy - no change. Tube/silicone rectifier - no change. Swapped V2/3/4 for thoroughness - no change.

Looking at the schematic, all of channel #1 (green) is separate from the other two channels until you get to the FX loop. So the orange and red channels peeling paint and the green channel being low is certainly possible.

Interestingly, I noticed that while there is no sign of butchery on the boards, the chassis has been in and out of the head many, many times. I suspect this fault has been there for a very long time.

My approach will be to check all continuity through channel 1 and check all component values. As the channel works somewhat but is a little distored and "grainy", I am suspecting the bias around V1b.

Has anyone else found a fault like this?

FX loop switching does not work FIXED - previous owner provided 5-pin lead, not 7=pin lead! This probably never worked....
With the FX "assign" set to FTSW the effects do not switch in and out. THe FX indicator on the footswitch changes. All other footswitch functions work...

Has anyone else experienced this before? According to the schematic, this should be a simple wiring fault. Because the FX switch works properly when "assign" is set to one of the channels, the FX circuit is all OK. It must be a break between the footswitch output DIN jack (control signal confirmed at that point) and the tip of the footswitch 1/4" jack.

 

[boss4]

Sounds like you need to take that amp to a decent technician! You can call mesa tech support though. I have a recto 3 ch too and no problems with the clean ch. Like you can expect is lower than the 2 and 3 ch (hi-gain ch's) but after tweaking first the clean channel and then the others two, I get a good balance between all of them.

 

[94Tremoverb]

Low volume and distortion on the clean channel, that isn't a tube, sounds like it could be a failed plate resistor - but it would have to be V1B since that's the only tube half that operates on the clean channel only. It could also possibly be a bad relay contact.

Non-operation of the FX loop switching is most likely the opto-coupler - there is one, even though the actual switching is JFET, but it can't be those or you would get no signal via the loop at all.

 

[laurie]

Low volume and distortion on the clean channel, that isn't a tube, sounds like it could be a failed plate resistor - but it would have to be V1B since that's the only tube half that operates on the clean channel only. It could also possibly be a bad relay contact.

Non-operation of the FX loop switching is most likely the opto-coupler - there is one, even though the actual switching is JFET, but it can't be those or you would get no signal via the loop at all.

Thanks! I was making similar edits to the initial post while your response was coming in! Appreciate the time and thought.

Agreed about the plate resistor - will chase that first.

Also agreed about the opto - it must work or the loop wouldn't work at all, so it really must be wiring or the foot-switch not putting out a voltage sufficiently close to "zero". Unfortunately I don't have another big-foot to try it with, but the NO FX footswitch jack will tell a tale.

Some starting points at least.

 

[laurie]

ONE DOWN

Stupid human tricks....

Question: why doesn't the FX button on the footswitch work?

Answer: the previous owner was using and provided to me a five (5) pin DIN lead!

 

[laurie]

Low volume and distortion on the clean channel, that isn't a tube, sounds like it could be a failed plate resistor - but it would have to be V1B since that's the only tube half that operates on the clean channel only. It could also possibly be a bad relay contact.

Checked all the resistors around V1B. Everything measures OK. Relay contacts all measure OK.

I'll dig the scope out of storage tomorrow and trace a signal.

 

[boogieman732]

My 3 channel Dual rec Seems to be the same way with the clean channel, In order to get decent volume on green channel I have to drive the gain to breakup and back my guitar volume down a bit, which still isn't a good clean, and still not close to as loud as the other channels.When I switch to orange or red it wants to break windows. I completely retubed the amp rectifiers and all but it still hasnt solved the volume issue although the amp sounds completely tits now. Ive been dealing with it since im mainly recording with it now and i can get good clean tones just at lower volumes, But when i rehearse with my band its an issue. Ive owned 2 channel rectos before but this is my first 3 channel so I wasnt sure If I was missing something But I wasnt really looking into the issue too deeply. Also my recto survived a bad car accident, and has had the chassis removed many times by the looks of the insulation on the bottom of the headshell.

 

[mastora]

Try putting your loop send/return pots at the back of the amp, at the 12oclock position

 

[DS-1]

Have you got the clean issue with the FX loop on?

My 3 channel Dual rec Seems to be the same way with the clean channel, In order to get decent volume on green channel I have to drive the gain to breakup and back my guitar volume down a bit, which still isn't a good clean, and still not close to as loud as the other channels.When I switch to orange or red it wants to break windows. I completely retubed the amp rectifiers and all but it still hasnt solved the volume issue although the amp sounds completely tits now. Ive been dealing with it since im mainly recording with it now and i can get good clean tones just at lower volumes, But when i rehearse with my band its an issue. Ive owned 2 channel rectos before but this is my first 3 channel so I wasnt sure If I was missing something But I wasnt really looking into the issue too deeply. Also my recto survived a bad car accident, and has had the chassis removed many times by the looks of the insulation on the bottom of the headshell.

 

[boogieman732]

With the fx loop on its a little better, I adjusted the mix level in back but it really made no big difference, But if I step on the loop button on the footswitch there is a noticeable increase in volume. I think I may have something going on because last night I was playing with it and i put the amp on standby and switched it from bold to spongy, and the amp shut off, It wouldnt turn back on until I switched it back to bold. Then I shut it down, switched it to spongy and attempted to turn it back on, and it fired right up, Its probably not a good Idea to change power options with It on but It still seemed strange, so again I made the switch with the amp on standby and it shut down, Ive noticed with the amp in bold with 6l6 tubes the top of the amp has been cooking, much hotter than any mesa ive ever owned previously, The tone im getting is amazing tho.

 

[boogieman732]

Sorry to OP for jacking thread. Fixed my problem today after tube failure. Brand new too Im done with jjs for good, I always used to use sovtek wxts and love them, JJs are easy to get but in my experience they are cheap crap, and Ive had nothing but problems with them.. WXTs are my favorite, cheap, sound amazing and are reliable, I use them in my peavey 5150 and get soo many people asking what I run cuz it sounds incredible, Went to my tech and he had a quad of 6l6wxts on hand and I snagged them along with a tung sol for v1. This recto is up and alive...Its like NAD all over again for me!

 

[twothemax]

good to hear. i always love my sovteks. for my 5150 they were great. saying that i have a brand new set of sovtek 6l6 wxt+ tubes. for sale. these are a matched quad. my new amp takes el34s so if anybody wants to do a trade shoot me a pm.
like i said a terrific power tube. the ones i have for sale are the PLUS power tubes. SOVTEK WXT+ Matched quad.

 

[boxcarmonument]

My DR had the same problem with the clean ch. I got around it by using the solo boost on the clean CH. It helped quite a bit.

 

[laurie]

My 3 channel Dual rec Seems to be the same way with the clean channel, In order to get decent volume on green channel I have to drive the gain to breakup and back my guitar volume down a bit, which still isn't a good clean, and still not close to as loud as the other channels.When I switch to orange or red it wants to break windows. I completely retubed the amp rectifiers and all but it still hasnt solved the volume issue although the amp sounds completely tits now. Ive been dealing with it since im mainly recording with it now and i can get good clean tones just at lower volumes, But when i rehearse with my band its an issue. Ive owned 2 channel rectos before but this is my first 3 channel so I wasnt sure If I was missing something But I wasnt really looking into the issue too deeply. Also my recto survived a bad car accident, and has had the chassis removed many times by the looks of the insulation on the bottom of the headshell.

I have tried two other identical amps at the local music store. HEAPS of volume on the clean channel. Not a Twin by any stretch, but at 2-3 on the master there is lots of sound. As much or more than I'm getting on 10. (all these test have been done with the loop defeated, so the loop master and solo level don't come into it)

I haven't dug the scope out of storage yet. Will do that this week. It has to be a loss of gain somewhere in V1B or even V1A - the gain stages after V1A on the orange and red channels would take care of the loss of volume if that's where it is.

Will keep you posted.

 

[laurie]

Second fault found. Took a while...

The j175 FET for muting the master volumes has failed. Was half-wave rectifying the pre-amp output.

I dont have a spare at the moment, but with the failed part pulled, the amp sounds.... AWESOME. Boffo!

 

[DS-1]

Second fault found. Took a while...

The j175 FET for muting the master volumes has failed. Was half-wave rectifying the pre-amp output.

I dont have a spare at the moment, but with the failed part pulled, the amp sounds.... AWESOME. Boffo!

What was the exact problem you had with this FET broken? Are you talking about the J175M3, near the send/return ones?

 

[boss4]

Second fault found. Took a while...

The j175 FET for muting the master volumes has failed. Was half-wave rectifying the pre-amp output.

I dont have a spare at the moment, but with the failed part pulled, the amp sounds.... AWESOME. Boffo!

Congrats & enjoy!

 

[laurie]

Second fault found. Took a while...

The j175 FET for muting the master volumes has failed. Was half-wave rectifying the pre-amp output.

I dont have a spare at the moment, but with the failed part pulled, the amp sounds.... AWESOME. Boffo!

What was the exact problem you had with this FET broken? Are you talking about the J175M3, near the send/return ones?

It was J175M1 - near V2. The clean channel was breaking up and very low volume. Sine wave appeared half-wave rectified on the scope.