Randall Commader cutting out

[Roly]

My intuition is that it's internal to the cap, possibly one of those rare manufacturing defects that slip by from time to time, but as with all intermittent faults only exhaustive testing will tell.

If it never shows up again it was (most likely) the solder joint.  If it farts just once more it wasn't; and yes, I'd certainly replace both caps while you are in there.

No, if the lug comes out of the filter cap it's busted.   

[ChewyNasalPrize]

Just jammed on it some more and still good.

Gonna give it several more days of periodic blasting just to make sure before I put it back together.

Be nice if it wasn't the filter cap but wouldn't hurt to replace them if it is. If they are original they are 35 years old!

Either way, I'm glad we seem to have zeroed in on the problem. 

Thanks!

[DrGonz78]

Nice Job! Hopefully the sucker stays a rocking!! :dbtu:

Yup Roly is totally on point about replacing the caps if it farts out on you once more. Those are definitely the original filter caps or at least I would bet money on it. Replacing the filter caps is like having a piece of mind feeling about the amp. Though it is always pleasant when filter caps remain healthy for a long time.

[ChewyNasalPrize]

Well, I jammed hard on it a few more times today and clean as a whistle.

Put it back together and jammed some more and still good.

Tested a total of about half/dozen times after solder job and no problems. Before it would have farted out every time, especially at the higher volumes I knew to be sure and test it at.

Also ran the preamp through the power amp of my Lab Series L5 and it really sounds good there too. Again I think better than the head to cab alone.

Anyway, I'm keeping my fingers crossed and will feel really good after a few weeks or so without incident.

This sure has been fun and educational for me. Thank you DrGonz and Roly for your help on this. I could not have done it without you! Bless you and bless this forum!

Now I gotta find something else that needs fixing....  :lmao:

Chewy

[Roly]

 :dbtu:

While that's good and pleasing and all, a note of caution is in order - we don't really know for certain that it was a dubious solder joint, it could still be an internal break in the cap that has been jammed back together by the work on the lug, and as Murphy's Law dictates, this will only reappear when everything is bolted down good and you are in the middle of a career make-or-break audition.

I want to stress that if it does it again, at all, then take both caps out and shoot them - no messing about, okay?

On the other hand if it's still solid after a few months of rocking out you can start to relax, it was the solder joint.   :tu:

[ChewyNasalPrize]

I heard that man!

At least for now, the Randall isn't my main gigging amp but that may change after it proves itself repaired because it really sounds great and especially sounds good with the pedals I have- at least compared to some of my other amps. And it's got that KILLER on-board tremolo!

My thinking right now is to have the Randall on hand as backup for the L5 or Peavey. If either of those go down, I can run the Randall through their speakers. That way I don't have two 212 combos to lug around but still have a good backup plan with plenty of juice and things stay fairly simple.

Of course I need to be sure it is going to work if I NEED a backup so a few more weeks of no probs and I'll be comfortable. 

Also, does replacing old filter caps that aren't necessarily "bad" have any potential positive effect on the tone of the amp? Sort of like a tone tune-up? Will it make the tone brighter/stronger/punchier/more resonant, etc... Or is it one of those things where it is either pretty much working or it's not?

I can't help but think after running through the power amps of the L5 and Peavey (which I had a local shop overhaul [before I found this forum]- has new filter caps and a few other bits) that it might benefit tonally from some new replacement parts? It might be worth it because it looks like a pretty straight-forward job that I could do myself (with your-all's guidance of course).

I don't know... Maybe I just I need a fixing fix.  :lmao:

Thanks!

[DrGonz78]

Changing the main filter caps will not change the tone. The subject comes up time and time again and it is worth researching replacing filter caps old vintage tube amps. They will claim that the sag in the power causes the amp to sound a certain way, or something or another... But with solid state amps I have yet to read of such stories. For me changing the filter caps is like replacing your old car battery. There are capacitors that are for the signal in the amp too and changing cap values in tone circuits alters the sound. There is even a debate about different brands and types of caps whether one is superior in tone. That debate is even harder to follow as it is very subjective and most of the time people can't agree, so they argue. Filter caps are there to smooth out the DC voltage and to remove AC ripple from the mains power. If the amp starts humming that can be a sign of a filter cap problem. Either bad solder joints at the terminals of filter caps or they are not filtering the AC, and you get hum. Personally, I would just change those caps if I was planning on keeping the amp for a while. Just take your time picking out the right parts before you order and ask questions to confirm.

[Roly]

Agree with the Doc.

When electrolytic caps fail they generally do so by losing capacitance, and in the power supply this will result in increased hum levels.  With supply decoupling caps down the signal chain it may result in instability, unwanted not-so-funny noises.

Electros in the signal path tend to have an easier life and are normally much less of a problem.  Naturally if you significantly change the values of caps in the tonestack or coupling between stages this will have some effect on the amp bandwidth, but before we get too excited we need to remember that electrolytic caps have a very wide tolerance on their nominal value, typically -25/+50% (which is one good reason to avoid them in the signal path); but an awful lot of horsefeathers gets talked about capacitors, their dielectrics, and its contribution to "tone".

There is a page somewhere where this topic is picked apart by some real testing of supposed "mojo" caps, "bumblebee", "paper in oil", etc., and concludes that if anything the revered "mojo" caps actually perform worse than new poly caps from Mundane Electronic Suppliers Inc.

When somebody says that they changed some caps for Das Wundercap$ and the tone was greatly improved, you can bet that a) the original cap(s) were actually faulty, or b) they are kidding themselves (and you).  And there is an awful lot of (b) about.

[ChewyNasalPrize]

More great info guys! Thanks!

There doesn't seem to be any significant hum or noise so I guess these caps are still pretty good. But I'll probably change them out anyway, especially since I plan to keep it.

Thanks again guys! You're the best!

[ChewyNasalPrize]

Welp- It was working good there for a while. Fired it up today and there was a lot of static and hum. Sounded OK for a few minutes but then went bad again but the hum was there much louder than I remember it ever being.

I'm assuming given the previous advice that new filter caps are in order.

I did run the preamp through another amp's power-in jack and there is no static. But there is still a fair amount of hum.

I opened it up and checked the resoldered joint but it didn't seem to be the problem. Nothing stopped the static once it started. No cutting in and out. Just once it starts it doesn't go away.

Any advice on what kind of caps to get and/or if I should replace anything else while I'm at it?

Thanks.

Chewy.

[DrGonz78]

Well I believe those are 6000uf 50v electrolytic caps(look on your caps and confirm). You will not find 6000uf caps as that is not a standard used today. Meaning you will have the choice of using 4700uf 5600uf or 6800uf caps. When I recapped a Randall RG80 I went with the 6800uf caps to add some extra filtering. I have read opinions that on filter caps it is better to go up rather than down, when picking an alternate farad rating. Either way it will work with 5600uf or 6800uf...

Here are the ones that I used...
http://www.mouser.com/ProductDetail/Nichicon/LGU1H682MELC/?qs=sGAEpiMZZMtZ1n0r9vR22ctH%2f%2fowniwfqbmxvr0mMJ4%3d

Edit: Meant to type 5600uf as the low figure...

[Roly]

I previously suggested that the cap you worked on may have actually had an intermittent internal connection and the fact that you now have hum and crackles still points in that direction, so I'd now do all five of those larger electros while you are at it - the two cans and the three biggies on the PCB.

[ChewyNasalPrize]

Thanks DrGonz.

[DrGonz78]

+1 to Roly! Yeah I also changed some of the other larger sized electrolytic caps while I had the amp open.

[ChewyNasalPrize]

thanks Roly and Dr. Gonz.

Ordered the big caps suggested above and picked up the three smaller ones at a local electronics supply store.

Fixin' to do some fixin'!  :dbtu: