Power supply capacitors,, What's up?

[wdb]

Hi,

I'm new to this forum, but have been repairing hifi and guitar amplifiers for quite some time. I just finished working on a Fender Hot Rod DeVille and was somewhat astonished to find that in a 2002 build amp, which looked very clean, the power supply capacitors are going bad. I personally own an older DeVille which has all Illinois power supply capacitors and this one has worked great. But this newer one uses some other brand of capacitors and the two 22uf @ 500 volt ones are so bad that the preamp section goes into oscillation when ever you try to use the drive and or more drive switches. Are others out there finding power supply capacitor failures in amps this new ? In vintage amps, it is not uncommon to see the power supply capacitors still working fine. In fact the only reason I change them is that after 40 - 60 years, the probability of failure is high. But, I don't expect amps that are under 10 years old to have so many power supply capacitor issues. Are others experiencing this problem? I pulled the 22 uf capacitors in question and both read under 1uf. Not shorted, but way low on capacitance. Just seems strange to me. As I said, normally I work on vintage gear and I have just started taking in newer tube as well as solid state amps. Never thought I'd be doing a recap job on an amp that is under 10 years old. Scary.

[Chuck H]

Anyone can shoot a flyer in the group. This is probably the case. Just a bad run of capacitors that slipped through. It is true that there are some really crappy capacitors out there. Some that are common brands. But it's also true that capacitor technology overall has actually improved and there are some really good caps out there too. You'll never find them in a budget amp though.

I am curious how your discharging the caps such that you can get an accurate value read. Anytime I have tried this lingering electrons in the cap swell it back up to some (very) low voltage that prevents my meter from doing it's job.

I'm also curious how bad filters are causing oscillation at higher gain. And that you didn't say anything about the amp humming like a monk. Which it would if the filters were actually 1uf.

Chuck

[Enzo]

I have no problem with the oscillation. The plate node in the B+ will have some riple, but by the screen node it should be smooth DC. The remaining B+ filters downstream are there more for decoupling than for filtering out ripple. They keep any signal from riding out of one stage on the B+ and into another stage via the same route. If those decoupling caps dry out, there is no decouple, no isolation.

There were bad parts. I don't assume any ongoing phenomenon when I see something like that. COuld be a bad batch of parts, and all the amps they made that week had early cap failures. COuld be this particular amp was in someone's car trunk, parked in the open under the hot sun for a week or two and the poor things were baked at 150 degrees. How would we know?

[stokes]

I've pointed this fact out in a number of threads recently,I've seen an awful lot of just this type of cap failure on way too many newer Fender amps in the last few years.I've had at least a dozen and I would say about half of them exhibited oscillation issues and were solved by subbing one of the pre-amp caps.A friend of mine recently purchased a new Blues Deluxe and I advised him to change the caps immediately just on GP.We were both amazed at the improvement in an otherwise good sounding amp.The crap they are using these days just plain sucks.I keep hearing about how "cap production and technology has improved these days" an awful lot,but I have yet to see it in practical use.In my experience all the crap coming out of asia doesnt cut it.

[Chuck H]

I haven't had a build in a while but I plan to use Nichicon caps. The specs look good and they're becoming popular with the hi fi cork sniffers too. The ones I like (PW series) don't come in axial lead like I'm use to but that's fine. Atoms have always performed well for me but they don't spec as well as the PW's and they cost almost three times as much. What's that about???

I do agree that no major MFG is putting the better examples of cap improvement into practice. Quite the opposite. Production amps are being built as cheaply as possible for the market they target. I worked with some contenders in the biz not long ago and I was always p!$$ed off and allowing concessions for cost. If a Blues Deluxe sells for $750.00 then Fender spends about $200.00 making and shipping one unit. That includes the speaker, the little pack of swag that comes with the amp, the labor... That's total cost to Fender. When I learned that I thought "well f#@k me running, how am I going to do THAT?!?" So with the price of a good cap being four or five times what a $h!tty one costs, well... I don't like the slew of "disposable" tube amps on the market at all, So I guess it sucks to be me then 'cause there's a bunch of 'em. Worse yet is the trend toward buying these crappo amps and doctoring them up with a few select high end parts. And even worse than that, Many sound pretty good... Damn. At least until they pop a filter cap or a cheapo power tube.

Rant over

Chuck

[gbono]

Another possible answer to your component issue is that these are counterfit parts with degraded specs. I've seen this happen over the last 10 years in almost every industry (test equipment, computers, consumer electronics, etc). Not uncommon when products are built by CMs in asia. Most companies don't have the resources to do proper failure analysis on the parts to see if they are fakes - they just replace what's in the line and forget about what was shipped out the back door.

[Euthymia]

Stokes is on it.

I've been seeing it, too, with Fenders manufactured in the early '00's.

The Asian cap manufacturing industry suffered a counterfeiting/espionage problem around that time, and I first heard about it affecting computer motherboards, but I suspect the bogus electrolytics made their way into mass-produced guitar amps as well.

Here's an article about the phenomenon: IEEE Spectrum: Leaking Capacitors Muck up Motherboards

The brands that I have seen the premature failures in are Illinois and Lelon, both in Fenders. If I remember correctly, my Dell motherboard had fake Nichicons with brown crud coming out of the tops.

The good thing is that presumably the cap industry only used the bad electrolyte for a short period of time, so I suspect that new stock caps, even inexpensive Asian ones, will be fine, but the bad thing is that there are lots of amps out there with the crappy caps, and probably some surplus stock of crappy caps themselves.

[gbono]

The Asian cap manufacturing industry suffered a counterfeiting/espionage problem around that time, and I first heard about it affecting computer motherboards, but I suspect the bogus electrolytics made their way into mass-produced guitar amps as well.

Believe me it is still going on and when and if the economy picks up again - it will be an even bigger issue.

[Euthymia]

The remedy for it is for amp manufacturers to test and certify incoming parts, like we do with vacuum tubes.

We don't have to test each and every one individually, but take a random sampling of each shipment and hook them up to some test equipment and see if they are in spec and can handle their rated AC and DC voltage. This would go a long way.

Hook those things up to 500VDC and leave them there 24 hours a day for a week, then measure the capacitance again, and at least you'll sniff out a bad batch.

[Hub 2]

Hi,

I have a Fender Deville 2002 PCB, last week it started making a sound like a motor bike engine when I select the drive/ more drive channel. I have checked both sides of the PCB for bad joints etc, but the only thing I can find that is wrong are the 2 22uf @ 500 volt caps, they are both leaking, the positive side of these 2 caps are both test points on the schematic, so I checked the voltages, they are both 6 volts higher than the schematic reads, the other 4 large caps look ok.

Should I replace just the 2 leaking caps, or would it be better to replace all 6 ?

They all look like Illinois caps. Is it best to use a different brand ?

Many thanks,

Andy

[Enzo]

What do you mean by "leaking?" HAving 6 extra volts across a 500v cap doesn't mean anything. If you have 120v AC power and a 480v B+ filtered by those caps, remember that for each volt of change on the AC power, there will be a 4 volt change on that B+. SO if the mains goes from 119 to 123v - a not at all unusual occurence - there will be a 16v change in B+. 6v is nothing. The schematics even state that voltages are approximate.

Now if white goo is coming out the end of the cap, then yes, it is bad. "Leaky" in a cap means it can;t block the AC. Leahy filter caps allow hum.

[wdb]

Hi,

I have a Fender Deville 2002 PCB, last week it started making a sound like a motor bike engine when I select the drive/ more drive channel. I have checked both sides of the PCB for bad joints etc, but the only thing I can find that is wrong are the 2 22uf @ 500 volt caps, they are both leaking, the positive side of these 2 caps are both test points on the schematic, so I checked the voltages, they are both 6 volts higher than the schematic reads, the other 4 large caps look ok.

Should I replace just the 2 leaking caps, or would it be better to replace all 6 ?

They all look like Illinois caps. Is it best to use a different brand ?

Many thanks,

Andy

Hi Andy,

I would replace them all. Seems that if one fails, the rest seem to follow.
If you have pulled the circuit board, might as well do them all while you have it out.

As for another poster who mentioned that most of the AC ripple is taken out by the first couple of filter stages, that is correct. But,t he problem with the two 22uf caps in the preamp filter stages is that their most important function is decoupling between the plate circuits for the high gain preamp stages. If this decoupling function of the power supply is not working correctly, you get all kinds of feedback loops which can produce all kinds of issues like odd oscillations and motorboating as well as a lot of background noises. This is especially evident when you push one or both of the overdrive switches in the Hot Rod DeVille. So, if you have motorboating and odd oscillations with screwed up overdrive, suspect the two 22uf filter caps in the preamp section. If the amp just has a lot of 120Hz hum, it is probably the first filter stage, which, if memory serves, is a pair of capacitors in series. But, if any of these capacitors have failed, it would be prudent to change them all. While you are in there, the 100K plate feed resistors for the preamp 12AX7's should be upgraded to metal oxide or metal film 1 watt resistors if your amp is using 1/2 watt resistors. These amps look to be using metalized poly coupling capacitors. You can warm up the sound by replacing the ones between the phase splitter and output tubes with paper in oil types. The best I have used are the silver metal cased Russian surplus ones. You can generally find a vendor of these on Ebay. I generally get them from a seller in Odessa (Ukraine, not Texas). Shipping is not much more then a week.

[Amp Kat]

I remember awhile back I rigged up a leakage test rig I had got from Ken Gilbert to test for leakage and if it discharged to quickly thru the discharge resistor it was deemed bad for not holding it's charge vs time. A bad cap will discharge to quickly not being able to keep up with what is supplying it therefore pulsating instead of rippleing or stready DC. Not sure if you have ever popped open a Sprague Atom cap but there is a tiny aluminum wrap inside that casing that is no larger than the new Asian caps which are really quite small for a 22uf 500 cap like the IC one. The reason for the high price of Atoms is they are not making them anymore and are selling what is left. The demand for those just isn't great enough to make a batch of millions which would be required for a run so they hiked the price and called it an aluminum increase. Not sure which caps you guys use but have tested several and some are even marked polarity backwards and blow up. I would think the F&T are pretty good but how would you rank the caps for quality and price ?

[Enzo]

My concern here is that the original poster thinks his caps are "leaking" his extra 6 volts into the system. I could be misreading him, but that is what stuck in my mind.

[tedmich]

the "capacitor plague" started in early 2000 and still pops up every once in awhile with old caps, had to do with corporate espionage and several china/taiwan cap mfg. stealing a formulation which omitted a corrosion inhibitor. The caps outgassed and leaked popped like this:


replace the leaky bulging ones and all others that are same brand on PCB.