MarkIII Lead channel broken - UPDATE :)

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edward

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Hey all,

If you recall a few weeks ago, my MKIII Simul head lost the OD in the Lead channel. Volume was still there, but no OD. I thought I "fixed" it with a new tube in V3, but then it broke again, and for good. You kind folks here (Restless, et.al.) so kindly diagnosed the problem as a bad LDR. Well, I got it back from a tech just yesterday and she's back to her former glory ...wooohooo!!

The tech replaced 3 LDRs and two resistors ...one really large resistor, maybe a 5watt and the other maybe a 1watt. He also told me that whoever last worked on this amp butchered it (Hey Restless, you callled it right on ...that jumper wire you saw was a bandaid fix from the previous owner, ugh!). The tech had to fix a few PCB traces as some hack overheated the crap out of some areas of the PCB that I couldn't see from the top.

Kudos to the guys out in the Mesa Hollywood (sunset) store. I recall from some time ago that they didn't do any in-house work and sent gear out to Petaluma. But apparently they now have a tech who really seems both to know what he's doing, and whose shop rate is very reasonable to boot! And thanks to those of you here who helped diagnose the issue and give me an idea of what to expect (I hate going into a repair blind). You guys rock ...thanks!!!
:D

Edward
 
Hey thats great!

That gives me a lot of hope for my Mark IV. My amp is with my tech now because my lead channel lost its OD as well. He also suspects it is the LDRs.
 
Yeah, I was talking with the tech and he said the IIIs and IVs are the last holdouts using the LDRs for switching. Randall changed to relays in later amp series'. The LDRs seem to be the most common part failure in these older amps. And they offer no warning ...they just go. The upside is that they're cheap, easy to replace, and you're good for, what, another 20 years? :D

I wonder if the DCs used them ...I don't recall seeing them the last time I poked my head in my chassis. Hey Boogiebabies, you know the answer to this one??

Edward
 
I was reading the specs on LDRs and they last for up to 20,000hrs. If you play for about 3hrs a day every day they should last about 18 years. Not bad. Even relays will eventually fail. They can only switch so many times before they crap out. I don't know which is more reliable over time.
 
Hi Nick,

Thanks for that ...good to know I'm good for another couple of decades :)

Edward
 
edward said:
Yeah, I was talking with the tech and he said the IIIs and IVs are the last holdouts using the LDRs for switching. Randall changed to relays in later amp series'. The LDRs seem to be the most common part failure in these older amps. And they offer no warning ...they just go. The upside is that they're cheap, easy to replace, and you're good for, what, another 20 years? :D

I wonder if the DCs used them ...I don't recall seeing them the last time I poked my head in my chassis. Hey Boogiebabies, you know the answer to this one??

Edward
Click to expand...

The DC's use LDR's and relays. The schematics show three. The MK IV shows 27. :shock:
 
Boogiebabies, do you know where I can find a schematic for the Mark IV Rev B?
 
Thanks, BB ...I knew I could rely on you!

Dang, 27 LDRs in a MKIV?!! ...suddenly I'm feeling very fortunate with my "lowly" DC5 and MKIII. ;)

Edward
 
Edward - I just heard back from my mesa tech and I had a bad LDR in my lead preamp stage causing the channel to have no OD. He has it on order and should be in by the end of the week. I should hav my Mark IV back next week. :D

Just curious do you know how much LDR's cost? I know he order the replacement part direct from Mesa.
 
Hi Nick,

Cool, glad you got that diagnosed! IIRC, each LDR was billed at $13. I've had my MKIII back now for a week, and she is as good as new. Good to know they're not expensive ...I just hope LDRs will still be available 18years from now :D

Edward
 
LDR's are about 6-9 dollars a piece. I usually replace them all at the same time. Sometimes you may replace one only to have a couple more fail within days of each other. Get them all replaced.
 
My local Mesa tech could not fix my Mark IV, so he is shipping it back to Mesa today. It will probably be gone for a month but at least when it comes back it should be good as new.
 
I found a MKIII for $450 last summer in Houston. I bought it for the other guitar player in my band. I asked the seller why he was selling it so cheap, he said he hated it cause it sounded like a Marshall. I get it to my other guitarist house, we plug it in and it does sound like a Marshall, not much gain at all (I've owned 3 MKIII's) I kept telling him it souned like it was on Ch 2, but it couldn't be cause we had it on the lead channel. Well, we took it to a tech in Houston and sure enough, he says the thermo couplers (LDR's ) are bad and need replacing. Problem is that it took two weeks for him to get the LDRs. So if you have a MKIII I'd highly suggest buy a couple ldr's just to have when your go out. BTW, my .50Cal+ has LDR's too.
 
my Mark IV has the same problem as nick a I think.. But my tweed power is gone as well. And the amp is only 2 days old, bought new!
 

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