Hot Rod Deluxe Caps - replace all at once?

[Kindly Killer]

I'm not much of an amp tech. I have Gerald Weber's DVD, but I'm still pretty weak on troubleshooting. I don't understand enough theory to really think through what's going on.

Let me tell you the amp's history so far. I bought the amp used at Guitar Center. It was super cheap and sounded AMAZING - someone had done some mods, but I think it was the first time they had been inside an amp. E.g. they had replaced C31 with a Sprague Atom, but they just clipped the legs of the old cap and soldered the new one to the old legs. Consequently, the solder joint between the old leg and the board failed and when the amp finally died on me, I located the problem by listening to the persistent tone coming from the cap (with no signal going into the amp).

So I replaced that cap with an F&T of similar value, plus the screen resistors, one of which had gone up in smoke. One of the tubes was also bad, and it bugged me that I don't know how to determine which failure was cause, and which was effect.

It just occurred to me that maybe I should have replaced all the caps? I read the test points on the big caps as follows:

C31 444v
C33 444v
C35 405v
C36 356v

I don't know what to make of that. It matches the general contour of relative values; does that say anything about the health of the caps?

The reason I'm worried about this is that the amp was acting kinda glitchy for a minute about a week and a half ago. I was doing my normal monday nite thing (hosting a jam - ughhhh) and in the opening set it started glitching and crackling. I flipped the power off and back on and it stopped acting up. I don't know what that does but I just know from experience that it stops that behavior sometimes.

So in general, if I replace one of those caps, should I replace them all? Are they like batteries, where one bad one takes the rest with them? At this point should I replace the one I put in a couple months ago? (Probably a couple hundred hours of playing time alongside the bad ones)

How can I determine what caused the glitch in my amp? How can I tell whether I'm right around the corner from another mid-show failure?

[52 Bill]

If you are trying to learn troubleshooting, you need to take a more reasoned approach to the problems. If the amp is making noises, try and figure out what part of the amp is causing the problem. Do the volume and tone controls change the sound of the noises? If you plug a guitar into the power amp in jack do the noises continue or do they stop?

You stated that the amp had been modified by a non professional tech. This is always the first thing to look into. Cold solder joints, broken pc traces, etc., can all be causes of noises. Cold solder joints from the factory are also possible causes, especially on the tube board.

As for the earlier problems, screen grid resistors are often burned when a power tube shorts. Except in the case of opens or shorts, the quality or condition of the filter caps cannot be easily judged by reading the dc voltages on them. If you read the ac voltage across them you may be able to judge how well they are filtering the power supply.

You replaced the one badly soldered cap. Was it bad or just badly soldered? Replacing all of the filter caps is rarely needed on a newer amp.

[The Dude]

Those amps are notorious for having cold solder on the output tube sockets. I'd check that first.

[stokes]

I've seen about a dozen of these Hot Rods,Devilles Blues Deluxe etc and all had at least one filter cap causing problems.If you know one cap is bad,its best to change them all,before the rest go one at a time.Actually I find clipping the old cap close to the body you can put a heat sink between the PCB and the connection,when you leave the original lead,if you do it neatly,it doesnt look like a hack job,and actually isnt.These crap PCB's they use dont take heat real well and those pads will lift with very little "excess heat".

[soundguruman]

"How can I determine what caused the glitch in my amp? How can I tell whether I'm right around the corner from another mid-show failure?"

Capacitors are the most UN-necessarily replaced item...
I don't understand WHY people think the caps need replacing...without any troubleshooting.

But in MOST cases, it's just that you need preamp tubes / output tubes.
OR in many cases, there's a bad solder connection...maybe on the tube sockets.

If the amp is old, and it starts HUMMING, and the hum gets louder...then you can suspect the capacitors.
BUT don't replace random parts...without KNOWING which part is really failing.

Random parts replacement...guessing....will generally get you NOwhere.

[Leo_Gnardo]

Capacitors are the most UN-necessarily replaced item...
I don't understand WHY people think the caps need replacing...without any troubleshooting.

If the amp is old, and it starts HUMMING, and the hum gets louder...then you can suspect the capacitors.

Also if the filter capacitors are leaking spuzz, heating up on their own, bulging, emitting smoke, exploding, or the leads are falling off.

FWIW I've seen filter caps as young as 18 years give up the ghost - SGM will like this - in Mesa/Boogies.

I've also seen filter caps continue working in amps 60+ years old.

[Kindly Killer]

Okay thanks for all info so far. Cold solder joints: if I can get the amp to act up on the workbench I can isolate the problem using the techniques on the Gerald Weber DVD. At that point I can just reflow all the solder in that area.

Is there another way to check solder joints? I could clip the meter to leg of a component on beep test, put the other lead on a PCB trace and wiggle the component, but that actually takes a lot more time than just reflowing every joint. But I can see the value in knowing exactly what happened

Can anyone recommend a video that focuses on just finding bad solder joints? Or can you describe the process. I need to watch the Weber DVD again, but I want to say that he only mentions looking for bad solder joints - he doesn't explain it. Also he mainly works on extremely antique amps - eyelet boards, turret boards, real relics...

Barring a miracle such the amp acting up exactly when I need it to, seems like my best option is reflow all the solder on the tube sockets. Is that right?

FWIW the cap I replaced was humming loudly - I don't mean causing the output of the circuit to contain hum - I mean the physical part was shaking vigorously enough to produce a tone from the part itself. I know that the joint to the PCB was bad, but at the time it never occurred to me that the cap itself might not be bad. Also I located the problem using the deductive methods from the Weber DVD - I isolated the problem to the power section by pulling the inverter tube and used the diagnostic skills I have available to me, which I am trying to improve by asking for help on this forum.

If only for furthering my understanding of how the amp works, I want to try reading AC voltage across the caps - I will have to find my test leads that are insulated all the way over the clips. When I get that going, what am I looking for, exactly? And just to be clear - I want to put the test leads literally across just the cap - not put the COM lead on the chassis like DC reading - is that right? What does AC voltage tell me, theory wise? Is that the main method for determining whether to keep/discard caps in the future?

[52 Bill]

If only for furthering my understanding of how the amp works, I want to try reading AC voltage across the caps - I will have to find my test leads that are insulated all the way over the clips. When I get that going, what am I looking for, exactly? And just to be clear - I want to put the test leads literally across just the cap - not put the COM lead on the chassis like DC reading - is that right? What does AC voltage tell me, theory wise? Is that the main method for determining whether to keep/discard caps in the future?

Because they only conduct on the positive half of the ac voltage, the rectifier diodes create a pulsed dc voltage. The filter caps job is to fill in the missing half of the voltage when the diode stops conducting. The better the cap the better it fills in mising half of the waveform. The difference between the two voltages is the ripple voltage. If your meter can correctly read it, you can get a feeling for how well the cap is filtering by reading the ac or ripple voltage that is riding on the dc voltage bus.

You can ground the common lead of your meter to the chassis or connect it directly to the negative side of the cap you are testing.

Cold solder joints can be found by visual inspection, mechanical stress, or temperature treatment. Any time I get one of these amps to work on I will just resolder all of the power tube sockets and clean and service the speaker jacks.

[Kindly Killer]

52 Bill, thank you very much. That helps me a lot. I took a good hard look at the power tube board and some of the joints looked oddly shaped and kinda dull, like maybe they were hastily done and didn't get hot enough first time 'round. I reflowed all power tube socket joints. Also - and I'm already ashamed to admit this so take it easy on me - the spade connector that goes to to the OT wasn't seated 100%, which was almost certainly my fault from the last time I was in there. OOPS! I pushed that down good and tight

Sounds great so far - no symptoms. Only time will tell now.

I put my meter on the caps and selected AC but it jumps all over the place and the meter can't keep up. It's an Extech 430. http://www.extech.com/instruments/re...s/EX430_UM.pdf It reads ACV from e.g. wall outlet or a wall wart AC transformer, but it doesn't seem to like reading the caps - acts like it is swamped with too much information. It will just flash a low number for just a flash, then a much higher number, then something else, and so on.

I'm not concerned about the caps being bad at the moment - just trying to build skills for future repairs.

[jeeptechfred]

The faulty 6L6 very likely damaged the screen resistor. Every time I am in one of these amps I measure the screen resistors making sure they haven't drifted.

Like others have said it is common to need to resolder tube socket pins. I will often do it as preventative maintenance on customers amps.

I agree with others that I prefer to only replace capacitors I can verify are bad. lots and lots get replaced for no reason. I test them with a capacitance meter, check their esr with an esr meter, check them for shorts with an ohm meter, and then visually inspect them. surprisingly some caps will test good but be leaking goo or badly bulging. I would call these bad.