Servicing Mark IIb coliseum

[user 41014]

Working on amps can electrocute you. I'm not advocating it.
I am happy to be told any of my assumptions are wrong.

I picked up K17x a while back and having got into the habit of pulling the chassis immediately found the 1st 30uF cap on the PS board bulging. In my very small sampling which does not pass scientific standards it seems this 1st cap which of course is before the choke takes the brunt of abuse.

I'm going to suggest checking the fuse every time you buy an amp. This yahoo seller had a 10A fuse to make sure the amp would fail to save the fuse.

I swapped all 3 electrolytics on the PS board. I'm told on good authority tho that some changes are needed from stock and I'll get to that shortly.

I finally got around to swapping the 220uF 8-bank. I use an 801 silicone for non-corrosive and neutral curing which is prob a bit over the top. I do need to get some one-side stick on weather stripping or whatever is used across the electrolytic-bank sticky enough to hold the wires down that traverse it.

Proceeding to a probing health check I found a couple things. At the very bottom of the SP-9A board in the 3rd pic you can see a disc cap that I'm getting continuity across roughly next to the mid pot and stuck to a orange .1uF 400v. Couple Q's on this disc cap:
1) Anyone familiar with this cap and its history? Is there any reason in the circuit I should be reading a short? I think it's shot and needs replacing except for Q2. It will need to be pulled to see what size it is.
2) Should it be there regardless of whether if it's stock?
I have 5 coli's open right now and the SP-8B doesn't have the disc cap there. The 2 Charlie Daniels coli's I picked up are also SP-9A's in the K230 range, a little later K's and one of them looks to have had the disc removed. You can see the silicone on the buddy orange cap from where it was. The forensics aren't as clear on his other coli but the cap is not there. He had trim pots installed at the PI buffer coupling caps obviously for bias so I know the amps are not bone stock.
The RP-8C (pic 4) has a disc in parallel with with a metal film R on the back of Master 1. It is shorted pushed in and when pulled acts like a cap. That would be the gain boost but after reading something here I wonder when the pull deep started in relation to the coli's or what pre amp. Looks like .003 uF 1KV || 33k ohms on M1 pulled.. You can see there is another disc cap on the board on the other side of the .1uF orange and maybe that serves the same purpose.

 

[user 41014]

Another short i found was the varistor. It's a GE V275LA20A. Looking at a Littelfuse LA series that suggests I'm looking at a 275V var with 450pF rating.

The blue one from mesa is described on their site as 150V domestic model. It's made by Sincera and the pF is rated higher but I hooked it to my cheap transistor checker and the tester came back thinking it was a cap at 1500pF.

To the people with much better EM experience than I have would you want the Vrating at 140-150V for 120-125VAC? It doesn't seem like you would want "headroom" on the V rating as you would be moving the clamping V further away from being effective protection but I'm not sure I've got varistors down.
With most electrical components raising the V rating of passive components greatly extends the usable life. In this case I'm wondering why a 275 was used unless the 150's were out and 275's for the exports were handy.

That said the SP-8B right next to me has a 275v varistor also. I haven't checked the other ones yet.

 

[user 41014]

Except for getting back to the PS board I still need to replace the purple 16V 470uF cap on the main board which I have ready and I've given up on finding NOS light blue mallory 50uF for the PS board. I have the 47uF from Mesa that they have been using.

Does someone on here have the loop mod details as it would pertain to the SP-9A? The obvious need is a pot and I have several linear and audio around here if someone can point me in the right direction.

I have a S with the loop mod so I can map out differences but will gladly be lazy if someone has the details.

 

[user 41014]

And speaking of I noticed all the amps Jeremy sent in came back with a PS board having 2 of the 30uF's, 2 40uF's or close to, and I think a 47uF plus one other that escapes me.

All the old K's came with 3x30uF plus the 50uF's. I see the double 40's on my C+ and KOB from '85. I don't have any B's later than the SP-9A, but would love a 9B coli!

Mike said the old ones need upgrading. Does anyone that has had factory service have the PS layout with a SP-9A? I could trace the later ones I have if it's all the same but the pre's are deff different with RP-11 and SOB compared to 9A. I know the 40's are post choke.

 

[user 41014]

I plan to check the diodes and that;s about all I can think of. The coupling caps at the PI have already been changed from 400V to 630V and I haven't heard of the need to do the same for other orange drops.

Are there any discs I should lift a leg and check?

I know Mesa rebuilds or at least used to rebuild the old pots that aren't sold anymore and I have some. Can anyone chime in on determining bad pots? Is it a feel thing when the wiper gets too loose? Does the carbon wear off and the R is out of spec just listening to it?

 

[user 41014]

As for the little disc cap on the board that was reading as a short- it is the same disc cap for the gain boost/pull deep circuit as on the back of the M1 pot. It does not read as a short with the knob pulled.
The only real Q there now is why did Charlie have these removed on his coli's?

 

[user 41014]

Mark IIB- no worries on hacking the thread- I'll hack it for you and it's all about servicing the IIB.

From M2B:
Hi There

I was reading your posts regarding servicing the mark IIB coliseums. I was interested as I have a 2B (60 watt) with the SP9A preamp which I'm having issues with.

I have lots of clear photos of the stock SP9A preamp..If you would like them/need them I can send them yo you?

I'm in the UK and the amp was serviced with all caps replaced (Mesa BMI caps)but there is a horrendous buzzing noise coming from the amp (preamp) even when nothing is plugged in.
It's not tube related and it's not the send/return jacks etc..
One thing that is definitely wrong with it is it has a bad lead drive push pull pot..It has a spot on the pot where the sound cuts out and it makes a terrible noise and even cleaning with deoxit does nothing for it.
I read that you have some spare pots and I was wondering if there was any possibility that you could maybe sell me one if at all possible?
If I record a clip of the amp misbehaving do you think you could take a quick look and see if you can maybe suggest what the issue could be.
It would be very much appreciated?

The 1st thing I would do is determine if this is power related. If you can post the sound that would help- are you 50Hz there?
Check for continuity (with amp unplugged) across the orange drop cap at the ground switch. While there, you might as well check to see if there is a short on that varistor too.
For the pot- do you have the # on the back or side? I'm used to seeing CTS pots so there should be a 137 in the numbers. Or does anyone know the taper and value of the LD pot?
I'm not the smartest boogiehead on here but I'm glad to help.

 

[user 41014]

And thanks for partially answering my Q about how to know pots are bad. A spot where the carbon is missing on the wipe is definitely one way to know it's bad!

And don't throw the bad pot out in case it's one of the non-made ones anymore. they can be rebuilt as Mesa does.

 

[woodbutcher65]

Just a note on electrolytic capacitors...you don't WANT NOS examples. Capacitors have a shelf life in addition to a service life. They'll go bad just sitting on a shelf after a certain number of years has passed. This is because the electrolyte in the capacitors is slightly corrosive and slowly eats the anodized layer on the plates. The same layer of anodized aluminum that is created when the capacitor is formed under proper applied DC voltages when the capacitor is first made.

Yes, they can be "Reformed" if you know how to do it and have the voltage and current regulated power supply required to do it. But even then, the electrolyte can only last so long as its chemical activity will eventually be used up no matter how perfectly the capacitor is treated.

NOS components are fine...but not electrolytic capacitors.

 

[user 41014]

Actually I do want NOS 50uF 75V (or the higher the V the better) light blue Mallory caps. They are soooo sexy! But 47 not 50 is a cap number so 50 is like buying an apple since 47 is one of those low brow standards you can't charge 3x what it's worth.

Of course everything you said is true. Use being the key. Caps are whores that like to be used as much as possible or they'll give up on you.

And ideally you would breadboard unused caps to charge (with a bleeder) occasionally for loose ones. I should really do that since I have some breadboards (carefully).

It's more than just theory but real life offers twists to the norm. These new snazzy dark blue BMI's look awesome. And the one on the SP-9A board needed to be replaced (I checked them all after removing). Of course the 1st on the power supply that was bulging needed changing for obvious reasons.

I wasn't going to comment until I could give a total health assessment but the 8 220 Mallory's I just removed on this coli have way lower, even exceptional ESR. My ESR meter is cheap and I don't have a leakage tester yet so this is not a definitive standing but the relative 4 decade ESR blows the new BMI's out of the water. The Mallory's won't be going into the garbage.

I'm not betting on another 4 decades and realize EOL could come quicker especially if it was expected to run on questionable mains that gigging can come with. But in spec is in spec and on whole what I pulled is closer to spec and cleaner tho leakage testing could change my view.

 

[woodbutcher65]

Your expectations are reasonable. And since apparently BMI has a supply issue right now and I'd rather eat bugs than put IC caps in an amp, I'm going to hold off on recapping my Mark IIB which is all original and running perfectly with no trace of any capacitor issues as of yet.

 

[user 41014]

I got to the 470uF cap at V3. It is rated 16V and the replacement mesa uses is the IC which I have on hand.
I also have the same IC series in a 25V which is the same form factor as the one getting replaced (the current 16V is smaller than the original).

I grabbed one of each and tested the 16V @ 483uF and the 25V @ 472uF almost dead on. More importantly the ESR was .15ohm vs .4 in favor of the 25V one so I upgraded and theoretically it will last much longer.

The pulled 470 tested 513uF and 1.3ohms ESR. That's 9x to the one that replaced it so it will make a difference. But looking at the sharp dressed chassis with a blue 470 that matches the BMI's who would really care about in spec and ESR?

 

[user 41014]

A note on the PSB- I tend to clean up pads before soldering with a handheld sucker. The leads on the 30uF/500V are force fit into the pads. I wouldn't and didn't. The leads are resting on the board covering the pads.
The 470uF leads are an easy fit but the underside clearance to the chassis is so shallow I cut the leads to top of board. I ripped up strips of cardboard for height so I can just barely get the lead to hold the hole. Once I get flow from the lead to the pad it's just a quick dab to prevent bridging a short to chassis.
I have a older and reliable Hakko FM-202. With newer models these are now dirt cheap if anyone is looking for a pro station at crazy cheap.

 

[user 41014]

I found another e-cap hiding on the front footswitch jack. It's a 3uF 12V that I don't have on hand. I'm sure it's filtering something (RF?). It would need a R for it to be a LPF.

 

[user 41014]

That should just about do it except the power board. For the power board I've looked at some IIC+ schematics and I don't see the way Mike is doing it now. Both my IIC+ coli and KOB use the dual 40uF caps and one or two smaller caps with 2 30uF ones, the way MB has said it should be if not.

I may have to trace those 2 examples. My thinking is that it goes choke --> 30uF (B+1) --> 30 uF (B+2) --> dual 40uF (B+3) with attendant carbon comps and not sure about the smaller e-caps. I'm not looking at it atm but I think the smaller ones are in addition to the bias caps. All my IIB's have 3 of the 30uF and I even saw that setup on someone's IIC+ schematic.

 

[user 41014]

I figured out the varistor (MOV). All of my originals are GE 240V which threw me.

For US mains it should be 120V x 1.2 = 144V, rounded up to the next available size, 150V and that is the max AC volts. That provides 20% headroom and is exactly what the originals and the mesa replacement are. It just happens that 150V max AC corresponds to 240V "varistor voltage" rating.

The mesa part is printed with "Sincera" but it looks like a Bourns and 20D241K is a Bourns part ID. The 20 on old and new varistors seems to translate to 6.5K max current surge. I've found 150VAC MOV's up to 15k A surge that on a small scale don't cost much more than the 6.5k A surge protection but this Mesa replacement is what I have now.

I also decided that even though someone's yellow plug on the power cord is solid and well done it's time to go back in black.

 

[user 41014]

Until I can find C14 jacks for these chassis' a normal black cord is on.

If you watch a video promoting a strain relief tool you can see how to squeeze the strain relief gripping the top and bottom on the inside of the chassis. Needle nose pliers work if you just slowly add pressure to the pliers while gently pushing the power cord out of the hole with the relief.

The hole for the aux mains is 1 15/64 (or 31mm?), mounting holes at 1.75". The hole for the strain relief/mains coming in is not round on the sides which are 9/16" wide. From top to bottom the hole is just barely less than 5/8", possibly 15mm.

 

[user 41014]

Everyone knows hockey is the only real sport but it's hard to miss that this has gotta be the most Broncos dressed coli I've ever seen. It was not intentional but it does look like ZZ Top should be singing sharp dressed chassis in the background.

 

[user 41014]

The little 3uF cap on the Lead/Rhythm jack was holding this project up. Some of my amps had 12V, others 50V. I've seen where Mike has suggested 2 or 3uF, 15V or higher with a question mark. It's easier these days to find higher V ones so I went with 3.3uF 63V Vishay.

 

[user 41014]

In case anyone wants to see the underside of a PS-1A board. You can see there is a trace cutting where one of the 50uF 75V caps spans.

One of the Mallory 50uF's was still really good, but the other was at the boundary of +20% so out they came. All the Carbon comps were within +/- 4%; awesome.

Two separate orders of the 47uF e-caps Mesa has were both way short on the leads and it wasn't worth trying to make them marginally work out.

Luckily I ordered some 50uF/100V caps off ebay. Someone had a company called Supertech in Taiwan batch some so I grabbed some 25/50's and these 50/100's. I was able to find a pair less than 2% of spec in the 1st few I grabbed and the ESR's were coming in relatively low making me think they are good caps at first blush.

The leads on the supertechs are super long and worked perfectly. As a bonus they are spec'd at 105 degree over the standard 85 degree. That and the upgraded V rating should extend the life.