Capacitor failure mode: the open circuit

[Rhodesplyr]

My own guitar amp, a Dean Markley CD-120, started exhibiting increasing background hum, so, I put it on the bench in a lull between jobs for clients. I checked the first stage 100uF/350V caps with my ESR meter and got good readings. Then, I checked the next three stages (22uF/500. 33uF/450. 33uF/450) and got off-the-scale readings (>100 Ohms) from all three. Further checking showed that all three electrolytics, Illinois Capacitor axials, had followed the failure mode of approximating open circuits. I couldn't get definite readings on any of them via my impedance bridge.

The only reason I'm posting this is that, while I've seen this happen to one capacitor in an amp or one section in a multi-can, I've never seen it happen to three discrete capacitors in a row--and not to the first filter stage, which gets more ripple, but to the stages after the choke.

The amp is from the 1980s; recapping it was somewhere on my To-Do list, but the power supply had appeared to be fine when I first resurrected it. It gets regular use, but never for extended periods, nor is it played hard.

These can be nice amps, by the way, if you 1) correct the original build problems and 2) get rid of the heavy-duty, but horrid-sounding original speaker.

[gbono]

High AC ripple current and heat are not friends of an alumunum electrolytic capacitor. If one cap opens up the others will share the increased ripple current and shorten their life as well. Loss of electrolyte (sometimes very slowly) will be the typical failure mechanism here. I remember military component specifications where stored electolytics were carefully weighed and their "shelf-life" was a function of component weight loss.

[Rhodesplyr]

High AC ripple current and heat are not friends of an alumunum electrolytic capacitor. If one cap opens up the others will share the increased ripple current and shorten their life as well. Loss of electrolyte (sometimes very slowly) will be the typical failure mechanism here. I remember military component specifications where stored electolytics were carefully weighed and their "shelf-life" was a function of component weight loss.

Good points, but the capacitors that see the highest ripple, the two 100uF, still measure good (though they will all be replaced). I though it interesting that the ones downstream from the choke would go first.

How does the electrolyte loss mechanism work in a multi-section can where only one section has failed in this way? I've seen this in Leslie filter cans.

[Gtr_tech]

If you've not replaced the bias trimmer, I'd suggest doing that while its on the bench. They are low quality and *will* fail. I had the one in my Signature 60 open up while it was running on the bench. Good thing I was keeping an eye on it.....I saw the redplating as it happened and shut it down.

I redid the bias supply to not go through the pot. I don't like that setup. I made it so the gnd ref is adjustable so in the event of a pot failure you don't lose bias.

But....I think if you used a good Bourns or similar multiturn trimmer in place of the stock POS you'd be ok.

[jrfrond]

Get rid of those IC caps. I've seen too many of them fail. Go over to F & T, and just recap the whole supply while you are at it.

Two things bug me here:

1) The voltage doubler circuit also "doubling" as a bias supply and bipolar 18V supply.

2) This is nitpicking, but WHY do people choose to draw simple schematics that have been done to death in a way that is initially difficult to see. It took me about 30sec. or so to realize that this is all garden-variety stuff in the PI/Power sections.

[Chuck H]

+1 on losing the IC caps. I've never liked them. Many amps I've seen with them ghost like crazy and they don't hold up well overall. Because of the extreme circumstances the following story isn't really a marker. But it does involve IC caps.

I was at Dean Markley's house about three years ago discussing the CD-60 reissue amp and Dean says "Hey, check this out. It's one of the last two I have". It was a CD-120, brand new and in the sealed original packaging. I said "Cool. But you might have problems firing it up, those filter caps have been sitting unused for twenty five years."... So he fired it up. It hummed real badly and after five minutes off standby, funny smell and smoke came out. I said, "See... I told you so. I can fix it though." He was so dissapointed that we didn't get to hear it that I said, "Let's see that other one." I asked him for an old lamp, a 100W light bulb and an extension cord. I made a current limiter and slow charged the second amp for about four hours. It hummed but it didn't blow up, so we got to hear it at least. We express shipped all new caps (including preamp and bias caps) and I recapped both amps two days later. And IMHO they are very good sounding amps. Some funny construction, but nothing any worse than anything else once you pick 'em apart.

Those speakers were made by Eminence to DMS specs and I actually didn't think they sounded bad. I don't know what he's loading into the new ones. I also don't know if he did away with the goofy effects loop on those amps. But I'd say that with the same big transformers and circuit as the original, a speaker upgrade and some minor circuit changes the new amps are probably really nice. They better be. Reissuing those amps is what he chose to do instead of building my designs.

[Rhodesplyr]

Gtr_tech: Thanks for the tip on the bias trimmer. I prefer these to be adjustable from the outside, so I may chassis-mount one. Frankly, I was looking at the power supply/output stage circuit board and thinking about replacing the whole thing with a turret board for greater flexibility in terms of component choices and implementing things like 3-terminal regulators for the +/- 18V supplies rather than zeners.

John: Yes, I think there were numerous choices made in this design that were attempts at "clever" cost-saving. F&Ts are already my choice for this one, and I'm going to recap the whole supply.

Chuck H: Interesting story. I heard that a number of these amps blew up in music stores when they were first marketed. I got this one for $50 because the owner had despaired of ever getting it to work right. It turned out to have a high frequency oscillation (~20V p-p) resulting from the preamp wiring's oversensitivity to lead dress. A couple of shielded wires to the tube sockets fixed that. I replaced a few of those tiny ceramic capacitors in the signal path with good results. I know that the original speaker was made by Eminence, but it never suited my taste, nor that of other guitarists who tried it. I felt like it had very edgy, ice-picky highs. It ended up going into a client's stage-rig Leslie Model 18 where it sounds great firing into a rotating foam drum. I've got a JBL D-120f in it now, but I've also played it through a pair of CTS Alnico 12s from some sort of organ.

Another thing I noticed about the original speaker was that it didn't sound good (to me) driven by any tube amp, but, for some reason (its impedance curve, perhaps), it seems to mate much better to solid state amps. I played a Peavey Musican 400 through it, and my client who bought it uses an Acoustic solid state head.

[jrfrond]

I tend to agree about the overall sound of Eminence speakers. They are good, utility-grade speakers, but they also tend to have an upper-mid peak that with some amps, can absolutely drive you nuts! On some other amps, they work just fine, but Eminence employs heavily-doped cloth surrounds across the board, which stiffens the whole motor assembly and emphasizes higher frequencies until they break in a bit. Paper or undoped cloth sounds much better. For the record, the only cloth-surround speakers that sound good to my ear for guitar are E-V and Peavey.

[Gtr_tech]

PV Scorpions? I can't stand those. I think they sound like cardboard with a VC attached. I guess they'd be alright for SS amps since they tend to be dead sounding, but the ones I've tried with tube amps had very little goin' on. I equate them to the Sovwreck "5881" output tubes. Tough, hard to kill, but nothin' special to listen to.....

[Chuck H]

I heard that a number of these amps blew up in music stores when they were first marketed.

I heard the big ass transformers would fall out because the mounting screws were too small and ripped through the thin chassis due to rough handling when shipped.