Power Caps bleed how fast? Does that indicate condition?

[Shontsy]

I was messing with my SF Champ last night trying to replicate that now elusive chirp, when out of curiosity I decided to see how fast the power caps leak down.

Every time I pull the chassis out of the cab and check the power cap voltage (out of curiosity, before I drain it via pin 6 on the preamp tube), it's only 5 V or so. What? Where is all that dangerous high voltage I keep reading about??

So I got to wondering how fast they leak down, since this amp is supposedly not designed to drain the caps by itself (or so I've read), and unfortunately for me, my electronics skills are not advanced enough to look a the schematic and know for sure if there is a bleed to ground or not. I think I see an obvious path to ground, but what do I know!

I plugged in the amp, checked the voltage on the caps, Yep, 300+ Volts.

Shut off the switch, and BAM it was down to about 12 V and leaking fast as soon as I cut the power.

So either this amp IS designed to drain the caps for safety, or maybe my power caps are in some sorry shape and need to be replaced??

Which is it?

This won't make me any less respectful of the caps, but if what I've read is true, it just seems to me I should seriously consider some new power caps!

Thanks in advance!

[52 Bill]

The circuit itself is constanly draining the caps, the only difference is that the power supply normally re-fills them when the amp is on.

When you turn off that particular amp the circuit is still connected to the power supply, unlike a Fender amp with a standby switch, which if not for the bleeder circuit, could leave the caps with a high voltage charge on them.

[jrfrond]

With no standby switch, the remaining filter cap charge bleeds through the speaker load through the power amp. If you shut the amp while playing, you can hear the volume get lower and distorted. No sound = No charge. In fact, that's the way I've been discharging caps for years prior to servicing them.

[Shontsy]

Yeah!! I'm NOT retarded! Good to know!

Next quick question is, with these caps going on 40 years, probably should replace them?

Can I still get Champ capacitor "cans" or do I have to buy individual now? If still available, where to buy the cap can?

And while I'm at it, where's the best place to get a replacement power switch? Marsh told me their "bright" slider switch barely sticks out of the control panel on a Champ. I've considered prying them both apart and putting the original plastic slider into the new switch, but I'd rather just get the right switch. The old one is just plain shot. Tried to fix it already, that is how I know I can get it apart to "steal" the slider. Plus it might not fit the new switch anyway.

Best Regards,
Eric Shonts

[Rob Mercure]

Eric,

Yeah you need the caps replaced and this would quite likely be the source of you chirp symptom - bad filters cause all sorts of mischief and it would take me days to describe the strange symptoms I've seen (heard) them cause over the years. Change 'em!

While can caps are not available the cost and trouble of replacing the can isn't worth the trouble. I used to work on televisions, etc., in the 1970s when these cans were common and replacing one really is a chore unless you've got some experience. It's much easier and cheaper to mount a terminal strip under the chassis and solder the discreet replacement caps to that - just make sure to remove the leads from the old cap terminals (it's tempting to use the old terminals as tie points for the new caps but this occasionally results in a minor explosion - not kidding!). Just leave the old can in place for up chassis visual continuity.

The slider switch you've got is so stinking common it isn't funny and was used on millions of USA manufactured products from WWII until the 1980s - and is probably still being used but not so ubiquitous. I haven't looked but I suspect that you can still find them at Rat Shack. But the are easily rebuilt if you've handy and most Champ switches really only need a shot of DeOxit or other good contact cleaner and perhaps tightening down the four tabs that hold the switch together. I don't know how bad your switch is but one old trick is to "turn the switch upside down" and use the normally unused contacts.


Rob

[jrfrond]

...the cost and trouble of replacing the can isn't worth the trouble. I used to work on televisions, etc., in the 1970s when these cans were common and replacing one really is a chore unless you've got some experience...

It is if you want to maintain originality in a vintage amp. In addition, the balance of the electrolytics should be replaced as well, aka "Cap Job".

The hardest part here are the cap tabs soldered to the chassis, for which a big ol' Weller solder gun will suffice.

[Shontsy]

Rat Shack Switch

Eric,

The slider switch... I suspect that you can still find them at Rat Shack. But the are easily rebuilt...but one old trick is to "turn the switch upside down" and use the normally unused contacts.

Rob

Yep, Rat Shack just might have it. I think this is the one.
SPST Heavy-Duty Slide Switch
Model: 275-401 | Catalog #: 275-401 Rated 6 amps at 125VAC.


I tried rehabbing the old one already, and now it doesn't slide so well! I think as some point the switch arc'd so bad it fried one of the contacts down so low it was beyond saving. Don't look at me, it was like that when I got it! I tried putting the "detent" feel back into it by taking it apart and bending the contact piece but all I accomplished was making it even worse. On my Champ, it's only a SP/ST, so it doesn't have the unused set of contacts like the typical Fender bright switch that is also used as a Champ power switch. Otherwise that would have been a sweet trick!

Anyway, I'll visit the Shack and pick up some new ones. 2 for $3, what a deal! Torres wants $5 each stating they are "very difficult to find!!". If these RS ones do not line up with the existing screw holes, I'm considering getting a Fender bat switch (better switch and $1 cheaper from most vendors than the cheapo slider!), and filing the sides flat to fit into the existing chassis slot, IF the pysical dims work out to allow it that is, which I think is a long shot. That would be a great upgrade, IMO, if it were to work out.

If all else fails, I'm probably going to tweedify this baby anyway, so I'll have 2 spare holes to put the bat switch into if I want to go that route. It would look a bit strange mounting the switch in the middle of the control panel though!

[52 Bill]

"If these RS ones do not line up with the existing screw holes, I'm considering getting a Fender bat switch (better switch and $1 cheaper from most vendors than the cheapo slider!), and filing the sides flat to fit into the existing chassis slot, IF the pysical dims work out to allow it that is, which I think is a long shot. That would be a great upgrade, IMO, if it were to work out."

I think that the switch will fit perfectly, but the mounting wings on the RS switch will be drilled and not tapped. You will have to use a couple of small nuts to mount the switch with the original screws.