Mark III Behaving Strangely

[Third Age Amps]

Hello All,

Just purchased a MkIII and it has a couple issues:

1. The Volume pot gets scratchy turning up to 6-7 and turning higher, the volume drops.

2. the Bass pot is very similar to the Volume pot issue. Turn it up to 5, starts scratching, turn it up more, Bass and Volume drop.

I've t-shot all the related components in the tone stack, swapped the Volume pot, re-soldered all the pot connections, changed all the power supply caps, and swapped V1 with no change. Any ideas? Anyone experience something similar?

Thanks!

 

[fretout]

If the pot feels scratchy, then it's definitely the potentiometer that should be cleaned/replaced.

As far as the volume drop, I haven't heard of that before, but I had a volume drop issue with one of my Mark IVs, and it ended up being a bad 12AX7-A. I can't remember if replacing V1 or V5 fixed the volume/bass issue, but try swapping preamp tubes if you have some spares and see if the symptoms go away.

 

[Vigo1999]

Mine is behaving the same way!!! :?

plus whats happening on the bass pot is also happening on the treble pot with mine.

i will tear her apart and clean her pots and all to see if the problem goes away.

 

[Third Age Amps]

vigo1999 I saw your post and replied the same way! I tried replacing the Volume pot with no change. I also swapped every tube with a new one, also with no change. I noticed when I turn the Bass to 0, the problem with the Volume pot gets a bit better. I'm changing the Bass pot later today.

 

[Restless Rocks]

You have a leaking capacitor in the tone stack. Replacing the pots will not help.

 

[Third Age Amps]

Thanks Restless, I just figured that out myself.

vigo1999, change your tone stack caps! Since it's open, just change all four. Two ceramic and two film caps at the left bottom of the board right behind the Volume and Bass pots. It's kind of a ***** because of the grounding wire and the other wires, but just cut the tie-wraps and push them out of the way. May as well check your PS caps while you're there too. They probably need to be changed.

I changed mine tone stack caps to 250p, 500p, .02u, and .02u as I wanted more mid and mid/high punch. You can use the original values, of course, but it always seemed a bit too scooped for my taste.

Good luck! Now to do the Rhythm 2 pot mod, and build a proper 3 button footswitch!

 

[Vigo1999]

Thanks Restless, I just figured that out myself.

vigo1999, change your tone stack caps! Since it's open, just change all four. Two ceramic and two film caps at the left bottom of the board right behind the Volume and Bass pots. It's kind of a ***** because of the grounding wire and the other wires, but just cut the tie-wraps and push them out of the way. May as well check your PS caps while you're there too. They probably need to be changed.

I changed mine tone stack caps to 250p, 500p, .02u, and .02u as I wanted more mid and mid/high punch. You can use the original values, of course, but it always seemed a bit too scooped for my taste.

Good luck! Now to do the Rhythm 2 pot mod, and build a proper 3 button footswitch!

Man, really?? that sucks . . .well, will try too change'em then. wondering if those values will affect the bass tone. i dont wanna mess with that range, since i simply love my III's bass sound.

wondering where i can found those parts (the original ones or at least very good ones, dont wanna put some crappy chi%#se stuff on my mesas).

i neve wanted to do the rhy 2 mod on mine, i can manage the shared eqw tone stack and volumes (funny, cause i dont think mine have that drastical change in volume between channels.)

i have a 8 button custom switch 8) no tap dancing. absolutely!!

 

[Third Age Amps]

vigo1999 I get a discount at CE Distribution, so I can order them for you. They will be CDE Orange Drop 716P caps, same as OEM, but with higher voltage rating. They are only $1.15 each, so I can wrap them in bubble wrap and ship them in an envelope for almost nothing. Just PM me with your address and I'll get them.

 

[kdorsey]

...change your tone stack caps! Since it's open, just change all four. Two ceramic and two film caps at the left bottom of the board right behind the Volume and Bass pots.

While changing the tone circuit caps can be a nice modification, ceramic and metal film capacitors don't "leak", only electrolytic caps are susceptible to the dielectric core drying up or leaking over time. Not trying to be a jerk, but wanted to clarify since a specific comment was made above about looking for failed caps.

Personally, I'd send the amp to a technician to evaluate the issue. A 25+ year old amp needs some love when it starts acting up.

 

[Third Age Amps]

...change your tone stack caps! Since it's open, just change all four. Two ceramic and two film caps at the left bottom of the board right behind the Volume and Bass pots.

While changing the tone circuit caps can be a nice modification, ceramic and metal film capacitors don't "leak", only electrolytic caps are susceptible to the dielectric core drying up or leaking over time. Not trying to be a jerk, but wanted to clarify since a specific comment was made above about looking for failed caps.

Personally, I'd send the amp to a technician to evaluate the issue. A 25+ year old amp needs some love when it starts acting up.

I assume by leaking, Restless Rocks was talking about the cap "leaking" DC into the signal, which means it has broken down internally and failed, not that the cap was leaking it's dielectric. The caps are there not only to provide tone shaping, but to block DC from hitting the pots. That's what was causing the scratching/static noise when it was being turned.

I suggested changing all the caps because no matter what type, time and use will kill them. So, since it will be open and he will be changing the caps in that area, why not change two more for $3 in parts and 5 minutes?

And I am an amp tech. Not a genius, but I have repaired, modded, designed, and built amps for 6 years. I troubleshot high and low voltage issues on elevators for 18 years as well.

 

[McBarry]

Hi TAA,
what ended up being the cause of the volume drop??

Did U chase a signal through V1 - did it reveal anything?

Regs, DB

 

[b00g13mk2B]

I have a similar problem with my Mark IIA. The treble pot is fine until the pull shift is engaged. Then I have scratchiness and the tone seems to suffer. Looking at the schematic, the pull shift just switches a 750pf cap across the 250pf cap. So I ordered some silver micas (the original are ceramic). I had previously replaced all electrolytics as some were leaky after 34 years. Including the cathode bypass on the 12AX7 tubes.

I did not order the coupling caps, based on the observed behavior. Will be interested to hear the results on the Mark III.

Any other parts that one would consider replacing on on an old amp, or after things like power supply caps and screen resistors go?

 

[McBarry]

G'Day,
did U mean coupling caps were leaky (ie DC), or filter caps actually leaking dielectric?
In my MkIIA, the white end filter caps were bulging away nicely at the ends - in went some F&T's..
If it's just scratchy with the brightcap switched in, if it's not the actual cap, check there's no crud between tracks where it returns to the PCB - IF it does so - my memory of that corner of the board is rusty..

Re yr last question.. carbon comps often drift, and the relay contacts can fatigue. A little clean and tiny tiny bend and yr good for another 20yrs..
I go a bit mad cleaning the bejeesus outta all my vintage Boogies - I find all sorts of bent leads, parts nearly shorting together, crud between tracks etc etc..

Rarely have to replace all that much though..

 

[b00g13mk2B]

Several of my filter caps were leaking dielectric and bulging. There was an electrical burning smell, blue glow at the base of the power tubes and loud static when I hit the side of the amp. When I had the amp apart, I cleaned the boards. Can check again around the tone stack on the preamp board, but think it is good. Replacing a few caps is easy enough.

I did plug in a power tube the wrong way (or had a bad tube) and burnt a screen resistor. Changed both of those with metal parts. This was after replacing the filter caps.

My relay is doing fine so far.

Thanks for your tips.

 

[McBarry]

I found a pic of my 100W MkIIB..
Besides the fluff, it shows the bright cap nicely.
The only things likely to cause static are the cap itself, the internal switch contacts, or super slim chance of crud between contact(s) and pot metal chassis and thus ground (bloody long shot though..)
Easy to eliminate switch - unsolder the cap from switch and short it to the red wire manually.
Clean the lugs and pot body.
If it's still there, it's gotta be back towards V1, and it's CRO time I'd suggest..

But bloody hell, what on earth did prev owner do to get all that fluff inside.. no wonder I was cleaning for hours and hours...

 

[b00g13mk2B]

McBarry - Yeah, wondering how the inside got so funky. May have a clue from my recent experience.

I got some caps to try to get the scratchy out of the treble pot that only happened when pull/shift was engaged. Since ceramics don't normally go bad, I used Deoxit first. That did the trick. The Deoxit I used leaves things oily, maybe someone had given your amp the treatment before, but must have left the chassis exposed somehow.

 

[Third Age Amps]

Hi TAA,
what ended up being the cause of the volume drop??

Did U chase a signal through V1 - did it reveal anything?

Regs, DB

It was the Bass cap. It was allowing DC to pass (causing the scratchy pots) and causing the volume drop. I changed all four TS caps and she's back in the game! I completed the Rhythm 2 Pot Mod, and now want to tackle a Bias mod so I can dial in the power tubes properly.