1992 MKIV A....re-cap & spare parts

[El Rey]

Just acquired one of these.....needs a little work, but overall the amp is in fair shape and intact.

Can any original owners tell me how long they had theirs before it needed a cap job?

This thing looks original and doesn't appear to have ever had power supply caps replaced. Best I can tell though, they are in good shape with no bulging or leaking, and the amp sounds fine. No loud 60HZ noise, or ghost notes........YET. But, it's 27 years old!!!

Does any one know a source for the rubber chassis insulators and correct hardware? Mesa is slow to respond on this one. I'm betting they don't have these anymore. The idiot I bought this from had the wrong hardware in all four insulators.

Any help is sure appreciated!

 

[woodbutcher65]

You're talking about rubber insulators and correct hardware and I can probably steer you in the right direction if you give me a little more information.

Can you post clear photos of the parts you want to replace and where they go? Originals even if broken?

When I recapped and refurbished my Mark III I found that it was missing some rubber shock absorbing feet on the top surface of the chassis pan,
and I found the exact part from its manufacturer. It also had two broken standoffs holding up the power supply cap board which I replaced with nylon screws and standoffs, which is arguably a better way to do it than to use snap-in clips.

But show me what you need and I can probably tell you where to get the exact part.

Here are some clues: Manufacturers are likely to be Keystone and Panduit. Mouser Electronics probably carries them.


Now as for the capacitors...THEY'RE DUE. Replace them now before they degrade to the point that they cause real trouble and a high repair bill.

The capacitor manufacturers will tell you that aluminum electrolytic capacitors have a five year shelf life and a 10 year operating life. They recommend replacement of most capacitors at 10 years in service. That is a very conservative replacement policy and a lot of techs are a bit more relaxed about it. I DO recommend that the caps should be replaced at least every 15 years and yes, 10 is what I recommend for most amps most of the time.

Yes, I've seen amps running with no apparent problems with caps in them that haven't leaked or swollen that are 30 years old. Or even older. But I've seen amps that had caps in them that weren't near that old that had blown their guts all over the inside of the amp and caused additional damage, too.

Just days ago I got done recapping a nearly 30 year old Kendrick amp. All the caps in it looked fine. They even measured good but some had higher leakage than they should.

The owner got the amp back and plugged it in and began giving lessons on it. (That's this amp's usual job so it gets hours of use every day.) He called me that evening and said that the amp sounded fantastic. It hadn't sounded like that since he got it almost 30 years ago. So refreshing the caps can do a lot to restore the tone of the amp even if they aren't all that far off spec.

I would absolutely say that 20 years is as far as I'd let caps go unless they're really high spec super long life types. But I recommend a 10 to 15 year replacement interval.

I would not run an amp that had 30 year old caps in it. That one instantly goes to "waiting for parts" status. Those old caps are ticking time bombs.

As for where to get the caps, get them from Mesa. Mesa sources all the major power supply caps from BMI. BMI is the former Sprague Atom factory with former Sprague tooling run by former Sprague employees. They're first rate caps. And Mesa's pricing on them is absolutely reasonable. I'd even say they're selling them too cheap.

 

[stephen sawall]

I have seen amps where the caps are fine over fifty years.
If they go bad it can do a fair amount of damage. But there is no way to tell how long they will last.

My IV is from the first run in 1990. I have done a few small repairs. The caps test fine.

 

[woodbutcher65]

Have you checked them for capacitance, ESR, and leakage current? Out of circuit? If you haven't checked all those parameters, you haven't checked the caps.


It's your amp and your choice but caps are limited lifespan parts. Aluminum electrolytics WILL fail eventually. The sound and performance of your amp will slowly degrade as the caps do and when they fail they can do additional damage. If you want an amp you can rely on, you change the caps on a reasonable schedule before they go bad.

You say there's no way to know how long they'll last but that's simply not true. The manufacturers know, by having designed and tested their products, their failure rate at a given operating voltage, temperature, and duty cycle. Mesa has always used high quality capacitors in the power supply, and that certainly helps, but those caps were still not designed for a 30 year service lifespan. If your usage pattern is known, the engineers at the manufacturer of your capacitors will be able to tell you that X percent of those capacitors will be seriously degraded in performance at this point. And X percent will have already failed. It is a statistical certainty that 30 year old stock Mesa caps are not performing up to original specifications and performance standards.

You would not run your tires until they're through the belts and fail simply because you wore through them. Why would you treat your amp any differently?

You'll change tubes when the amp sounds weak and lifeless but replacing aged capacitors can make MORE of a tonal improvement than tube changes.


I don't get why people are resistant to having required periodic maintenance done. It's a sure bet that your amp will NOT sound WORSE after recapping, but it's very likely that it'll sound BETTER than it does now.

Change them. They're due. The amp's tone will improve. The first and most obvious change will be better sounding bass, especially when overdriven. It'll sound considerably smoother and more musical. Overall the tone will improve. Headroom will increase, gain and saturation will also increase. Clarity will improve.

 

[El Rey]

I have seen amps where the caps are fine over fifty years.
If they go bad it can do a fair amount of damage. But there is no way to tell how long they will last......

I have read and heard this also through the years, but the MKIV, I just acquired is the oldest amp I've ever had. Even though this amp sounds really good right now, I'm still leaning to re-capping it. Although I have the skills and tools to do it myself, I might just ship it to Mesa in Petaluma and have them do it. They are pretty reasonable, and will also go over the whole amp while it's there. Hopefully, Mike Bendinelli himself will work it. After working on aircraft all day, I just can't get excited about tearing apart guitar amps.

Mesa has great customer service. Just got off the phone with them today. They had the shock mounts (which I really didn't need) but had the chassis screws and large area washers that I do need. The idiot I bought it from gad the wrong hardware. $9 and it's on its way, should be here Saturday.

Have you checked them for capacitance, ESR, and leakage current? Out of circuit? If you haven't checked all those parameters, you haven't checked the caps.


It's your amp and your choice but caps are limited lifespan parts. Aluminum electrolytics WILL fail eventually. The sound and performance of your amp will slowly degrade as the caps do and when they fail they can do additional damage. If you want an amp you can rely on, you change the caps on a reasonable schedule before they go bad.

You say there's no way to know how long they'll last but that's simply not true. The manufacturers know, by having designed and tested their products, their failure rate at a given operating voltage, temperature, and duty cycle. Mesa has always used high quality capacitors in the power supply, and that certainly helps, but those caps were still not designed for a 30 year service lifespan. If your usage pattern is known, the engineers at the manufacturer of your capacitors will be able to tell you that X percent of those capacitors will be seriously degraded in performance at this point. And X percent will have already failed. It is a statistical certainty that 30 year old stock Mesa caps are not performing up to original specifications and performance standards.

You would not run your tires until they're through the belts and fail simply because you wore through them. Why would you treat your amp any differently?

You'll change tubes when the amp sounds weak and lifeless but replacing aged capacitors can make MORE of a tonal improvement than tube changes.


I don't get why people are resistant to having required periodic maintenance done. It's a sure bet that your amp will NOT sound WORSE after recapping, but it's very likely that it'll sound BETTER than it does now.

Change them. They're due. The amp's tone will improve. The first and most obvious change will be better sounding bass, especially when overdriven. It'll sound considerably smoother and more musical. Overall the tone will improve. Headroom will increase, gain and saturation will also increase. Clarity will improve.

Even though the amp sounds great to me right now, power supply capacitor replacement sounds like cheap insurance in the long run on this 27 year old MKIV A. I can't find any record of it ever being done on this amp. I hate dealing with it myself, I'll probably just send it back to Mesa. They did a MKII C+ for me several years ago, and they were very reasonable.......

Thanks for all the info all......

 

[stephen sawall]

Mesa does excellent work and a great idea to have them go over the amp.

 

[El Rey]

Mesa does excellent work and a great idea to have them go over the amp.

Thank you!!