Cap Maintenance

[Mongo1]

I'm not sure if this is the best place to post this since my question covers multiple components/areas, but it seemed better than multiple posts. Basically a cap job on a MKIVa, Studio Pre and a Fifty/Fifty.

I have a 92 MKIVa that saw regular use up until around 97 then went to sleep in my closet while I wandered off into digital confusion. I've since revisited my love for all things Boogie over the last couple of years and am putting the finishing touches on a rack build with a Studio Preamp and a soon to arrive Fifty/Fifty. My MKIV is just too loud for church even though we rock it pretty hard!

Ive just finished reading the 41 page, (600+) post MKIV sticky thread and several others specifically related to cap life. My MKIV does sound a bit harsh and flubby, considering the caps are all original and sat powered off for 13 years, I don't think they are up to any serious gigging. I plan on going though the tubes, but I think the caps should probably still be revisited.

My Studio Preamp sounds great, but it too has an abundance of years under its belt as does the soon to arrive Fifty/Fifty. I'm thinking wisdom would dictate addressing the caps on those too. I'm on the hunt for schematics to have on hand, but I figure this has been done by so many members, it should be clear what work I have ahead of me.

This is my first time working with caps in a tube amp. I'm very proficient with a soldering iron and should have no problem draining off any residual charges, cleanly removing the old caps and soldering in the new ones. I want 'a tone to die for', but have no desire to die for it... :wink: I guess I am looking for the 123 on exactly what to change for each of these, where to order quality replacements, what to be sure about regarding whether the caps arrive ready to install or need some type of pre-loading/forming to prep the innards...

I just looked in my Studio Pre since I have it out from rolling tubes.
Here's what I found in it.

Large sized blue Mesa caps:
(3) 30uf 500VDC

Med sized orange/black stripe caps:
(2) 220uf 63VDC

Small sized orange/black stripe caps:
(1) 1uf 50v
(1) 10uf 63v

Med sized brown cap (standing up):
(1) 4700uf 16v

There's the usual abundance of orange components (orange drops?) scattered around the board, but I don't think those are caps, or at least the ones I need to consider changing. I'll receive the Fifty/Fifyy next week and be able to post what I find in it.

Thanks in advance!

 

[Mongo1]

I've been doing more searching /reading and am always amazed by the abundance of knowledge here.
I found a link in one post for caps "http://tubedepot.com/electrolytic.html".

Would I be better off ordering what I can from Mesa and locating the rest at tubedepot?
Is there a benefit in getting Mesa caps or just order it all online?
Any preference of Sprague, F&T or Mallory for any specific application?

Should I also verify certain resister values that are known to drift?
If so, is there any reason to isolate them from the circuit to get a good reading on them?

I'm planning on setting aside a bunch of bench time to work though my gear. I'm just trying to gather the info I need to be thorough. Lots of posts I read had specific warnings for certain models, like not flipping a board too many times because brittle wires can break, how certain caps can be a bear to get out and over heating the pads is a common mistake, etc, etc... I'm hoping someone will jump in with some specifics on these models so I can prep what I need to to get this gear freshened up for another decade of great tone...

I'm gonna be working on (in this order):
* Studio Pre
* Fifty/Fifty
* Simul-Satellite
* MKIVa

I know I ask a LOT of questions, but I figure a job worth doing is worth doing well and its better to learn as much as possible before diving in, then to go back in and fix the things I should have taken the time to ask about...

Thanks again in advance.

 

[stokes]

Don,I sent you a PM.

 

[trycko]

Oboy! I was happy to find this thread since Im about to do the same thing with my .50 caliber+
Mongo1! did you get any more information about your recap in stokes mystical PM?

I would be glad to learn from your experience because I have the same questions that you had!

 

[Mongo1]

I had several recommendations about the caps. First and foremost is "DON'T ATTEMPT THIS IF YOU ARE UNAWARE OF THE DANGERS PRESENT!" Very good advice, as there are lethal voltages present after the power has been removed. Check, double check, and check again that they are ALL drained before reaching in to do any work. Definitely acquire schematics for your gear and verify that the circuits on the schematics are consistent with your gear. All of my schematics were different to a degree.

Regarding the caps themselves, the only ones in question are the electrolytic caps (round cans). The ones usually subject to replacement are the high voltage caps, as the low voltage ones are less subject to failure. There are different schools of thought regarding replacement. Whether they need it or not. I defer to the abundance of experience here on those matters as I make no claims to be qualified to be much more than proficient with a soldering iron.

I decided to replace all of them since the 50/50 and Simul-Sat were in storage for a loooooong time and sounded like poop with enough crackling going on to be useful for nothing more than a good campfire soundtrack. I had little success finding some of the odd spec smaller caps. A quick call and I discovered that Mesa had all of the caps in stock. I figured if I sent it in to have Mike B work on it, that's what he'd use. If its good enough for him, what can I say... If you decide to order from another source, Sprague and F&T caps were recommended for the primary high voltage caps because of their consistent quality. I would have done that if I didn't need to order some from Mesa anyway.

I addition to the caps, there are some other points to look into. I initially replaced some burnt grid resistors. The power caps are joined with some rectifier diodes and some resistors (150K) that should be checked. In addition the 100 ohm balance resistors for the heater voltage should be checked. If replaced, make sure they are matched as close as possible.

All of my parts are here, I just have not had time to get busy on it. I'll post a thread when I can.

 

[trycko]

Thanks for a quick response!

My amp is humming a lot and sometimes its making a thunder like crackling sound.
I switched tubes to new JJ's and even though the tone got better, the noise didn't go away. I opened it and found a cracked 100Ohm resistor and while clipping it away to take it to the store for a perfect match the one next to it cracked too.
I switched both but still noise so I figured I better get serious about it and do a recap.
Ill Probably buy F&T here http://www.watfordvalves.com/product_detail.asp?id=467
I Live in Sweden, so ordering from US makes me have to argue with the toll for some stupid fee

I wonder what those Mesa caps really are. My amp says mesa on the big blue ones and sprague on the orange/black ones, maybe they are all sprague, rebranded by mesa.
They will all go away anyhow.

Looking forward to hear about your progress.

 

[stoz]

I've been thinking about this too - my Mk IV is 12 years old, but I just thought I'd find a Boogie repair / authorised centre somewhere local and put it in for a service -

I imagine (perhaps wrongly) that valve & caps would be replaced and everything else checked for tolerance.

Am I wrong?

craig

 

[Mongo1]

there are lethal voltages present after the power has been removed. Check, double check, and check again that they are ALL drained before reaching in to do any work.

So how does one drain them?

The goal is to drain the power from the (+) lead of the caps to ground. It's best to do it through a resistor so you don't get a <<ZAP>> when it finds ground. The higher the resistor, the longer it will take to drain. I used a spare 2.7K resistor I had. I initially clip it onto the (+) side of a 30uf/500V cap. It seems like they all drain together, but I check them all.

They should be able to drain on their own going back through the two 150K resistors in the power section if you switch it to standby (unplugged obviously). This is assuming there are no problems in the circuit. I like to clip a drain lead on and triple check them all after a while. Better to be safe than dead...

I did find that there was always a very small amount of voltage present (1 or 2 volts) on the bigger caps for a long time. This is not a concern since it's a minor amount. Since caps are designed to store a charge and it seemed understandable that they would take longer to drain off the last bits. Like drips from a garden hose...

 

[Mongo1]

I've been thinking about this too - my Mk IV is 12 years old, but I just thought I'd find a Boogie repair / authorised centre somewhere local and put it in for a service -

I imagine (perhaps wrongly) that valve & caps would be replaced and everything else checked for tolerance.

Am I wrong?

craig

Craig,

I don't think its standard practice to replace all the caps. A good tech should know to check the common fault items (resistors, etc.) for a specific amp model, but I think you should confirm what they are going to do beforehand.

 

[stokes]

Just to clarify what Mongo said.It is common practice to change all the electrolytic caps,but not the non electros such as the coupling and tone caps.