MKIII problem: losing Lead channel ...twice!

[edward]

Weird. I lost the Lead channel a bit over a month ago. Seems the NOS Mullard CV4024 (12at7), a reliable and great sounding tube that I love suddenly seemed to give up ...that's V3 BTW. When I went for the lead channel, all I got was clean tone and volume drop. Odd. So I figured it was a fluke. So I replaced that "dead" Mullard with another of the same (yes, from a reliable vendor).

OK, so same great sound for these weeks now and I am happy. Now just last night, poof. No Lead OD. Just clean tone coming out of my Lead channel, and the neither the Lead volume or gain knobs (the right-most two) make a bit of difference when you turn them. And the both times it went out, I came off standby from having been powered on for several minutes (so it's not like I played and all was fine, then poof) ...hadn't even hit played a lick yet; just "dead" OD right away after taking it off standby (could this be a clue of sorts??).

What am I blowing? Why? What to check and how (yeah, I'm handy, but need direction). Thoughts very much appreciated!

EDIT: some thoughts...
- could it be a just coinicidence that the I plugged in the same outlet of the same building where v3 blew the first time is where v3 blew this second time (though my DC5 never has had issue at the same location or outlet)?

- could my using a footswitch, a Mesa 2-button one which I modded to work so I could switch between R1/R2 as well, have caused stress and eventual failure in v3? (yet the switch works great and no probs with the amp otherwise, hour upon hour at a time).

- could using a 12at7 just not be a good idea on the v3 socket in the MKIII? My understanding was that an at7 is capable of higher current draw than an ax7, and is therefore a safe substitute. Not to mention that I've not had any issue whatsoevr with subbing an at7 in an ax7-spot ever, in either my DC5 or Fender (played many many hours), over that last couple of years.

I'm befuddled here, guys. MKIII and/or amp gurus help, much appreciated! Even thoughts or a direction in which to approach this would be great. I'm no tech (obviously) but know my way around a soldering iron and PCBs ...help!!

Edward

 

[Buddy]

It may not be your tubes . I had a similar problem and it turned out to be a lead pot . (I don't remember which one) . The back of the pot had become unattached and the wiper went intermittent . Due to impact for sure .
I'd also check all jack's and do a visual inspection of the board .

When is the last time you pulled the chassis and gave her a good spring cleaning ? It may be time to hang the 'do not disturb sign ' and unwrap 'er .
After a good cleaning , if the problem persist's, you may have to check voltages .
Be careful , she can and will bite ya .

 

[Restless Rocks]

Your foot switch mods could be suspect. Try unplugging all your ft sw's and see if the pull switches then work properly to change channels. If the Lead drive and Lead Masters do nothing then the problem would seem to be in the channel switching circuit.

 

[edward]

Buddy,
will check the pots and all connections there. But mine is not intermittant. The Lead channel failed and all I get is clean volume, and the Lead Master and Master (2 right-most knobs) do nothting, but the R1/R2 master volume works as it should.

Restless,

Yeah, the first thing I did was unplug the footswitches ...still no go from either the two aforementioned knobs. Something's dead (or is killing off my v3).

Thanks for the pointers, guys. More input welcome

Edward

 

[Boogiebabies]

If it is acting up, pull the chassis and get a chop stick.

When you switch to lead mode and it's still clean, tap the LDR's above the V3 and see if they kick in. Sometimes LED's get stuck and don't open fully.

 

[edward]

If it is acting up, pull the chassis and get a chop stick.

When you switch to lead mode and it's still clean, tap the LDR's above the V3 and see if they kick in. Sometimes LED's get stuck and don't open fully.

Ahhh, BB. Thanks for chiming in!!

"LDR" is/are??? Thanks! Will try tomorrow ...I really appreciate the tip

Ed

 

[Buddy]

LDR= a led and photocell in a small rectangular package . They are used in 'Boogs to switch channels . The leads are almost imaginary and the component is quite heat sensitive and fragile compared to say , a resistor .
They are located behind the pots , on the pcb and sometimes stacked one on another . Black plastic case with VTLxxx stamped on 'em .
BB !, zarking frood ! , you would'nt know of a supplier ?

 

[Boogiebabies]

LDR= a led and photocell in a small rectangular package . They are used in 'Boogs to switch channels . The leads are almost imaginary and the component is quite heat sensitive and fragile compared to say , a resistor .
They are located behind the pots , on the pcb and sometimes stacked one on another . Black plastic case with VTLxxx stamped on 'em .
BB !, zarking frood ! , you would'nt know of a supplier ?

I buy my LDR's from the Vogons or Allied Electronics depending on what galaxy I am in or if Zaphod hits the Infinite Improbability Drive. If I am on another ship and they use the Bistromathic drive, I can always get them from Mesa. If you are talking about the chop sticks, any good Italian restaurant will have them.

Time for a Pangalagtic Gargleblaster.

 

[Restless Rocks]

Hey Edward, it's unlikely that all the LDR's in your lead circuit would go south at the same time; more likely something amiss in the power supply to them. Here's a simple test: If you have a DC volt meter; push the lead ch knob switch IN. then plug a guitar cord into the ft sw jack and test for about 12 volts DC across the tip to ground. If this is not present then you have a possible loss of power supply.
If you do not have a a volt meter here's something else to try: Pull the lead ch knob out and while playing pull the treble shift knob in and out and you should hear a mid boost when you pull it out. Also the EQ AUTO sw should engage the EQ while the lead knob is pulled. If none of these work then you may need a tech. Possibly one of the LDR's had a shorted LED but if the Rhtym 2 vol still works properly check to see that it's not working all the time (even in R1) the R2 vol should only work when you're in R2. That's about all we can do without actually being able to get a meter inside the amp. There are several other things including zenor diodes and resistors that are possible culprits for which a tech would be your best bet.

 

[edward]

Buddy, BB, Restless, thank you so much for the specifics!!

OK gentlemen, so here's what I found (with the chassis still in the head cab ...haven't yanked it yet):

- cable in the footswitch jack yields 11.2 volts at tip/sleeve, so I guess that's checks ok.

- R2 vol pot affects ONLY the R2 and has no effect on R1 or Lead channel ...check.

- Activate Lead via pull pot and the GEQ (set on auto) does operate normally ...check.

So here's the kicker ...now the Lead channel WORKS again, but it doesn't have full gain (remember that beforehand, I got only clean tone and diminished volume with no effect from the Lead Drive or Lead Master knobs). I tried both the 12at7 that I had in there (the NOS Mullard CV4024) AND a known good Mesa ax7 in the V3 ...both sound similar, so I guess that I didn't really blow the Mullard.
So NOW what I get from the Lead channel is:

- the Lead channel is not full gain (maybe around 50-70%) and pretty loose/farty sounding.
- the Lead Drive knob now affects the OD as it did not before, but not by much.
- the Lead Master knob can now increase the channel's volume, but not to full power (I can go to 10 and play 2' in front of the amp!!), and setting "0" doesn't turn down the channel by much (and can't turn off the channel's volume whereas I sure it must have beforehand).

So there is definitely a problem, that now seems to be intermittant, and still faulty when it does "work." Shall I proceed to said LDRs? Let me know what to check for and I'll open up the chassis. And in case you haven't figured it out yet, I am trying to avoid having to go to a tech. Again, thanks, guys ...I really appreciate your help on this!!

Edward

 

[Restless Rocks]

Hey Edward, I guess it's time to get in there and check LDR 2 to see if it's passing signal into the lead circuit. I'ts true the leads on these things are very fragile and could even cause of the problem. I would do a visual check and carefully try to see if the thin wires on the LED side may be broken. The next steps would require testing the circuit with power on and I really can't recommend you do that unless you are a fairly well experienced tech. sorry!

 

[edward]

Thanks, Restless! Here are some shots ...perhaps may help you help me

These first two are of the Lead Drive and Lead Master pots. Hard to tell, but the LDRs and their respective leads "seem" to look ok:

These next two are general shots of the other LDRs. Again, no obvious indications of anything amiss.

So which is the LDR I'd be concerned with, perhaps the second one from the left is LDR2 ...I can't seem to find markings on the PCB near the Lead Drive/Master pots (except "LDR 5" that is clear on the other side of the board).

And is there a way to test the offending LDR? I understand electronics (though no EE by a long shot), so I understand the voltages and amperages = risk of death here. Alternatively, should I just order one and replace the (supposed) offending LDR in lieu of testing it? Guidance very much appreciated here!

Edward

 

[edward]

OK, update!

I thought about how you had said how fragile these LDR leads are, so I figured as a stab in the dark I'd freshen the solder joints at the leads in what I *think* was LDR2 (the second one from the far left). Results...it works!!! ...kind of

Good news is I got the Lead channel back up to real volume. The Lead Drive and Lead Master knobs both DO have effect now when turned, and can also crank up as she should.

Problem: it still doesn't have full gain and saturation. Maybe it's close to 80% gain/OD, but certainly not completely right. At least I can get good OD when I add R2 to the Lead channel, but that's not the point. I want the full glory of my Lead Channel back!

So ...is it possible that these LDRs can do a partial fail?? Or does the Lead channel utilize more than one LDR?? Thoughts and guidance here would be great. Thanks!!

Edward

 

[Restless Rocks]

Edward, LDR 2 is the lead input, LDR 4 is the lead output and LDR 5 is the lead Master Vol. Take a look at the MkIII schematic http://www.schematicheaven.com/ . Now look under the blue wire coming from the back of the Master Volume pot. There is a jumper wire someone has added at the output of LDR 5. I'd say this amp has experienced this problem before and someone has put a "bandaid" on it. Yes it's very likely that the LDR you heated up is bad and should be replaced. It does appear that someone with a soldering iron has been down this road already. Without experienced soldering skills it's possible to make a real mess and further damage the pads and tracks, I think that's already the case. It really would be best to seek the help of a tech who can acertain the reasoning for the jumper and perhaps put things back to stock.

 

[Restless Rocks]

Sorry I'm awake now and I meant to say LDR 4 has had a jumper wire added. LDR 2 is located just behind the Bass control and LDR 4 is located just to the left of the stacked LDR's (which concern R2) under the blue wire from the MV. You soldered on LDR 5 which is the Lead Master.

 

[edward]

Thanks, Restless! That site on schems is priceless ...I appreciate (and bookmarked) that!!

OK, so it looks as though at least one LDR is toast, and prob others. Interesting you mention that it has been worked on. I was, indeed, wondering whether some of those solder joints looked just gooked up with solder flux from someone's hack job, or it was just the decades-worth of age. I guess someone did some kind of work on her before me. Dang, I'm no engineer, but even my little R2 mod looks pretty "factory" if I do say so myself.

So is it worthwhile to just replace the LDRs? It seems like the PCB will come off the stand-offs (and the pots unscrew off the chassis) in a straightforward manner ...am I wrong in this assumption? I'd love to do this myself, but then you raised that issue of the jumper... Are you absolutely sure that doesn't belong there? I guess if I *gotta* go to a tech I will, but that's gonna cost me, bro.
Thanks for all your help, man!!

Edward

 

[Restless Rocks]

I am 1000% sure the jumper wire is non stock. Yes us techs need to make a livin too. But remember they're not making anymore MkIII's and it's very easy to do irrepairable damage to the circuit board by burning and pulling up pads and tracks.

 

[Buddy]

And how ! When you take into account what these amps have been doing for the last 20+ years you appreciate the beefyness inherent .

Even so , a couple extra seconds of heat from the iron will wreak havoc ! .....Havoc I say! Chaos ! Ah ha ha ha ha ! (Sorry).

An experienced tech has all the right tools , techniques and skills
to handle any problems as they appear . And even then , Sh!t goes wrong .

Ever try to repair plumbing in an old house ?

 

[Restless Rocks]

.And even then , Sh!t goes wrong

Ever try to repair plumbing in an old house ?

Ah so do you mean that literally?

 

[edward]

Alrighty, gentlemen, you've shamed/beaten me into submission

Frankly, that jumper wire is what concerns me, not the replacing of the LDRs which I am confident could do. Because if I replace them all, even perfectly, the question still remains as to why that jumper was put there?... OK, a tech it is. Thanks sooo much for the specifics and the advice. At least we got it narrowed down ...and that'll help keep the repair cost down. Take care, gents; I appreciate all the help and your sharing your expertise!!!


Edward