Mark IIB Volume Swell

[zebpedersen]

Posted this issue a while back but I've had a chance to do a bit more investigating:

The volume on my Mark IIB has started fading and swelling in and out (at irregular intervals) and I want to get it sorted.

I've switched out all of the tubes and tested at low and high volume with the EQ switched in and out on both 60W and 100W modes, same symptom on all.

Any ideas?

 

[RR]

clean the potentiometers with Blue Shower or some electronic cleanser that comes in an aeosol can. Radio Shack used to sell this aeosol can stuff to clean TV rotary channel switcher (but ... who owns those TVs?), that stuff works.

Not sure if this will cure the problem but sometimes these pots get dirty and don't have good contacts. Just a suggestion.

 

[Boogiebabies]

Pull the amp out of the head shell or combo and play test it again. If and when you hear the volume
swell take a pencil and tap gently on the relay. In many cases the relay contacts are dirty and you can clean
them to make the issue go away. Be very careful removing the cap on the relay IT IS FRAGILE.

 

[zebpedersen]

Thanks for the suggestions.

BB: The relay is the light-blue plastic box with a clear cap located on the preamp board, right?

 

[zebpedersen]

BB - I think you might be right on the money.

I've got the chassis out and the amp plugged into a cab and I got the problem pretty much straight away. When i tapped with a pencil on the top of the relay the sound faded in and swelled up just like normal.

With regard to cleaning the relay - it's a pretty snug fit on the preamp board. I'm not a complete idiot when it comes to electronics, but at the same time I don't want to do any damage. Any tips / tricks for getting the cap off, and when it's off how and what to clean?

Cheers man this is very much appreciated.

 

[McBarry]

Zeb, my Mk IIA was volume swelling, but also "popping" like a...
lid comes off by GENTLY levering one of lids' long sides away from the blue plastic chassis . U might be able to see the tiny retaining lugs of the clear top which need to be freed of the base.

Clean between the long arms and the contacts underneath. BE SURE not to bend them - they're very thin metal.

A few months later I actually replaced mine. When off the board, I found 30yrs of crud under it, and was getting tiny values of stray DC between tracks. New relay was mounted approx 5mm off the board, with thicker leads for mechanical rigidity.

Didn't fix the popping, but swelling resolved.

 

[zebpedersen]

Thanks for the advice man. I'll give it a crack later on today - is isopropyl alcohol (or similar) okay for cleaning?

 

[zebpedersen]

Hmmm...this thing isn't budging. What tools/leverage are safe and effective to use?

 

[McBarry]

this is the relay intact

here's the lid partly off.
I used a fingernail, but when mounted on the board a THIN blade will work. Start at a corner, you can see in the prev pic the "rail" running on the blue chassis, which has a corresponding groove in the plastic lid that this "rail" fits into.

U can see the "arms" which have TWO tiny pads/contacts - one on the underside about 3mm back from the tip, and one on the topside which is right on the end/tip

U can see the top pad will hit the fly-over arm, while the lower pad will contact the lower mount. Notice my left arm is bent over a little too far. This was how I found it upon disassembly.

Hope this helps.

 

[zebpedersen]

That's amazing mate thank you, going to bust out a Stanley knife and have another crack at this one.

 

[zebpedersen]

Argh, I think this might be a bit beyond me.

When I used to work in IT repair I saw people try and 'DIY-it' and cock things up further time and time again - not sure I want to do the same thing in this situation!

Time for the pros, I think.

 

[Boogiebabies]

Not to say I am that dumb, but I had to use a razor blade to get between the cover and the base.
Once I got between them the cover lifted right off.

 

[McBarry]

Hi Ed, "dumb" is never a word I'd associate with Boogiebabies..even if tongue-in-cheek..
I only wish U were over here to cast a watchful eye over my Boogies..

I used a scalpel, but a long thin craft knife is also good.
To clean the pads I used same blade but with a tiny sliver of 2000grade emery paper.

Zeb, with steady hands and a gentle approach, I reckon you'd nail it.
And hey, if nothing else it will ensure a nicely functioning relay.

 

[zebpedersen]

Yeah I reckon the Stanley is a bit too thick...I'll try and find a scalpel, that's a really good idea.

I started another thread on this but no bites so far - if I do have to take it to a tech, what are the 'classic' Mark IIB mods? IIRC - noisy reverb, noisy FX loop, noisy EQ, popping footswitch...are there official 'book' mods to fix this stuff?

 

[McBarry]

Among others, Boogiebabies and Monstatone are guys who can really help U re mods.
I invested a STUPENDOUS amount of time researching the ch sw popping. Became a bit of a mission in the end..LOL
The causes are varied, but here's some main themes I kept finding on my MkIIA and Mk IIB, and from online research and email contact with knowledgable techs and Mesa themselves.
1> relay. Noise of the contacts themselves,
Also tiny eddy currents around the relay, often "fixed" or attenuated by capacitors and a diode on relay coil - often factory fitted
Further, crap on board under the relay, giving imperfect separation between tracks, allowing some DC crossover between tracks
2> Impedance difference between channels. Measured at each gain stage in the respective channel
3> Gain difference between channels. Combined with different impedance, a sudden difference in the signal chain manifests as audible "pop" as a sudden/sharp rise in signal travels through the amp. Also worsened if the signal has some "stray" DC superimposed on it. Especially noticeable going INTO the lead channel, and less noticeable going back to rhythm channel. But can be attenuated by haviing no signal at the instant of ch switching eg: mute strings for a second or so.

One guy suggested give the circuit to an engineering class as an assignment and see if they can diagnose the cause and then implement a solution.

Interestingly, LDR's were increasingly used from the IIC onwards - there's no doubt these work in attenuating popping.
But it'd be nice to definitively diagnose the cause, and design a solution. Naturally without sacrificing tone, and keeping things simple and cost-effective.
Yeah, I agree.. it's never gonna happen...

 

[zebpedersen]

Haha, no popping would be a nice bonus but it seems like a bit of a unicorn to me. Westside Distribution just recommended me the tech I was thinking of using anyway so I'll contact him and see what he says - as long as I can get a quiet GEQ, better FX loop and reverb I'll be happy. This will give me an excuse to get my Mark I worked on as well.

Cheers