MESA Maverick 2x12 (1993) Lead Channel Bleed to Rhythm Channel

[StringBob]

Group,

I have a mid 900's serial Maverick 2x12 combo, which for the second time in its life, has somehow blown the channel switching between the Lead and Clean (Rhythm) channel.

I've spent some time reviewing schematics found online and the signal path switching appears straight forward.

The last time this happened (still under warranty), the local shop / svce center claimed to replace an Opto Coupler IC which was stated to handle the lead / rhythm switching task. On the main board, I do see what appears to be a pair of Opto Coupler IC's, however they don't appear to have ever been touched with a soldering iron.

Typically I play this thing without the footswitch and usually in the clean channel mode. What I hear is lead channel gain somehow bleeding over to the clean channel. When on clean / rhythm, with the lead pre gain control full low, all is clean. However as the lead channel gain is increased a small amount of lead signal begins to pollute the rhythm channel in the background and the gain characteristics of the clean channel are somewhat impaired. It's like lead channel leakage and not full volume as one might expect when in lead mode. In lead mode, it plays fine as expected.

This amp has only a few hours of actual run time on it over the course of 14 years. It's not gigged and not beat up.

Anybody run into this issue before? Anybody know where I can obtain a circuit board parts list for this unit?

I have to imagine this is a typical problem since it has happened twice in my case.

I'm hoping to obtain enough info to make the repairs myself.

Any advice greatly appreciated.

Thanks.

[StringBob]

MESA Maverick 212 Opto Isoloator Part

Group,

I believe the opto isolator part replaced is as marked;

VTL SC9 8846 ASSM.IN RP

as this part shows evidence of solder work.

Since the schematics available online do not show this level of part detail, can anyone verify?

Also looking for manufacturer info and possible source detail.

Thanks.

[StringBob]

Vactec / Vactrol SC9

Group,

Guess I'm charting my own progress here. Apparently the Opto Isolator is a Vactec / Vactrol SC9, which by all research seems to be a defunct manufacturer, bought by EG&G, and subsequently Parker. All said, it appears this product is wildly obsolete.

Does anybody know what later Vactrol Opto Isolators might be substitued?

The aforementioned was the last serviced part (depot replacement).

Original stock on chassis (other parts) MSL 37V31 or 0885.

These all appear to be four lead (axial) optocoupler devices.

[Don Moose]

Assuming that
1) the LDR/Vactrol replaced was LDR1 at the entry to the lead channel,
and
2) you never, ever want to be able to use the lead channel,
you could cut LDR1 out of the circuit altogether.

That also assumes the schematic on schematicheaven.com is accurate.

Otherwise, Mesa should be willing to sell you a replacement part - your amp's out of warranty, right?

[StringBob]

Vactrol 5C9

Turns out I also need glasses. The input LDR was last warranty replaced with a 5C9 not an SC9 (reading error). Not sure why these keep popping (two in 14 years anyway) with low hourly usage.

I generally play through a Boss VF-2 and soild state stereo amp. I keep the Mesa around for when I want to get loud or off site jam somewhere.

I found a source for the 5C9; Allied Electronics $7 and change, and ordered a handfull to have on hand for the future.

Now the Vactrol 5C9 is an LED lamped device, so it should have great longevity, unless the bias resistor is too small and resulting current too high for the 5C9 LED/lamp. Considering this amp is circa 1993, the vactrols in production then should also have been LED lamped; thus, I'm still a bit puzzled as to why that particular part keeps failing.

I hate to keep replacing blown parts without getting to the root cause.
Is this a common ailment to Mesa amps?

I was even considering adding wire wrap pins or sip sockets for quick future replacment. Not sure how many times the PCB is going to tolerate desoldering LDR1.

From a construction standpoint, it's premium hand wired American product. A work of art.

[Don Moose]

The If_max on these should be on the datasheet (should be able to get that from your vendor).

The 5C1/5C2 datasheet I have says 40mA, with a derating factor of 0.9mA/degC above 30C - that's only 86F inside a tubes-down chassis before you have to start derating.

If you can find the series resistor value and the rail that drives it, you can get a good idea how close you are to the edge.

Hope this helps!

[StringBob]

Mesa Maverick 2x12 Switching Matrix Diagram

One thing that I do need is a switching matrix diagram for this. Not sure if anyone out there has seen one. All the schematics I've seen for this so far ar signal path, preamp, amp, and power supply.. However none of the control circuitry for the relays and LDRS (4) in this.

Also, the topside PCB mask is really scant on component designators, so I'm fumbling a tad to clearly identify LDR's 2, 3, and 4, although I have a pretty good guestimate based on the LED side voltage indications when toggling the rhythm / lead switch.

Not sure if anyone has a photofacts of this... I know I'm asking for everything at this point.. Though I'd throw it out there.

LDR1 appears is easy to locate as it's next in series with the instrument jack tip. Wierd thing is it's showing about 2Vdc on the LED side of the LDR in either rhythm or lead mode... no change... So that's what's leading me down the switching matrix diagram rabbit trail.

[StringBob]

Maverick 2x12 Schematic Set with Switching Matrix

Latest vendor schematics for the Maverick attached. Includes switching matrix.

Looking at the switching control and placement of LDR1, I don't see how this would have caused my problem last or this time around. LDR1 seems to be used as a pop reduction device possibly as it's always on and activates from rectified 6v filament supply voltage.

[StringBob]

Schematic update attachment

attached.

[Don Moose]

This is what I see with the switching setup:

In Rhythm mode, the FTSW SUPPLY is pulled to GND, turning on the 2 4N33 optoswitches. This engages RY1 and RY2; turns Q5 OFF, disengaging RY3 and LDR3. LDR5 is ON in this mode. So is LDR2 - since LDR2 is ON, its cathode is at GND and LDR1's cathode is at 3V*((20+39)/(100+20+39)) --> 1.11V

LDR1's anode would be at (3V - (47*(parallel If of LDR1 + LDR2)) - (Vf of LDR2)). I'm pretty sure LDR1 is going to be pretty much OFF in this mode.

In Lead mode, the FTSW SUPPLY is at 19V. The 4N33s will be OFF, as will LDR 5 and LDR 2. Since LDR2 isn't pulling down on LDR1's anode, LDR1 gets to turn ON. RY1 and RY2 would disengage. Q5 should be ON, engaging RY3 and LDR3. The 10uF cap is probably part of the anti-pop circuit, holding LDR3 ON and RY3 engaged for a bit when Q5 turns OFF. LDR4 is either drawn backwards or Mr. Smith is counting on the flyback currents from RY1 and RY2 to pulse it ON as another anti-pop measure.

All of that, if correct, implies that LDR1 cuts off signal to the lead circuit when in RY mode, leaving the 1M grid leak resistor on V2a to mute that path. Oh, and there's LDR5 - it also mutes that path at V3a - you may have a problem there.

Compare the ohms-to-gnd at V3a's grid (tests LDR5) in Lead and Rhythm modes as well as ohms from Input to V2a's grid (tests LDR1).

Be careful in there - have you read up on safety inside a running tube amp? These things have lethal voltages in them, so keep at least one hand in a pocket when the thing is open and plugged in. Also, keep your guitar in a position where you can't possibly touch both it and the amp at the same time (grounded strings!).

Hope this helps!

[StringBob]

Will check

Thanks Don. I'll check your reccomendations out.

A couple things I'll have to sit and study again.. I didn't see how LDR2 pulls down LDR1. I see the 39ohm voltage divider producing a 0vdc reference, then teh 20 ohm 100 ohm voltage divider to LDR1 cathode. the Vdiff between LDR1 Anode and Cathode looks like 2~2.5 vdc costant. Even when LDR2 is off, LDR1 still appers forward biased.. I'll have to review again.

One other point of puzzlement, is the location of the 5th LDR. I count only 4 LDRS on the topside of my mainboard. Unless there's one hiding on the tube socket side somewhere. When I say 5th LDR, I don't specificially mean LDR5. I haven't identified LDR2, 3, or 4 as of yet and unable to spot the 5th.

The sound artifact sounds more like a fuzz distortion artifact in the rhythm mode. I'm almost thinking it's LDR5 mute as that shorts the grid of valve 3A. Valve 3B is the output buffer stage of the FX loop. Assuming all the relays are working correctly, which I believe they are as the RHY channel path is loud and clear (when lead pregain at 0), i'm leaning towards LDR5.

The only possible way for signal to crosstalk is through the common part V3.

I'll digest and test your suggestions this evening. Thanks for the safety reminder. 300Vdc can leave a lasting impression.

[Don Moose]

I meant to say do the ohms check with the preamp tubes OUT.

Look for DC voltage on any of the nodes you're going to ohm between before you try to measure (both modes). If there's any DC (more than a couple tens of millivolts) DON'T try the ohms test. We'll have to think of something else.

About that 5th LDR - if could be buried, hidden or just not there. Mesa schematics are known to have deliberate errors and/or omissions.

More likely it's just in some odd location.

If you want to try some tube swappy before you bust this thing open, try your other 12AX7s in V3.

Hope this helps!

[StringBob]

Fixed

Don,

Thanks again for the assist.

After picking up a spare 12AX7 and cycling through the various preamp and buffer tubes, I ruled out a bad tube. Especially at VC3 socket.

Then I went and shotgunned all the LDR Vactrols with new 5C9's.

I hit pay dirt on the last one. The one closest to VC3 valve, which I belived to have been the lead mute LDR (named LDR5 by the schem.) wasn't bad.

In fact, I haven't the foggiest clue why the LDR I replaced fixed the audio issue I was experiencing (LDR3 or 4 parked next to a rly). All common sense pointed to LDR5.

Mesa PCB's have virtually no silk screening on them and the schematics obviously have subtle errors in them. Would have been nice if they'd labeled the relays and LDR's at least.

BTW, those rectifier caps take a while to bleed down. They seem to rest around 40 VDC for a good long while.

Cheers!

[Don Moose]

BTW, those rectifier caps take a while to bleed down. They seem to rest around 40 VDC for a good long while.

Ouchie!

Are there bleeder resistors across the main filters? (270k or so seems to be a good choice)

Hope this helps!