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**Daver** · 08-30-2007, 09:38 PM

Are they axial, radial or can caps? Apex Jr has radial 5500uf at 200v caps for $3.95. Also, check ebay. Sometimes you can find something good.
Dave

**Enzo** · 08-30-2007, 10:19 PM

Digikey # P10659

4700/200v $15.80

or

# P13291

4700/200v $25.26

Or parallel a couple 2200uf, there are a lot more at that value than at 4800 in your voltage range.

**bob p** · 08-31-2007, 01:14 AM

Just for reference purposes, here are some photos that I found in a DIYAudio thread of the insides of a PM-1.5 amp. Although the amp isn't my M-1.5t, the internals are almost identical. There are 2 big blue screw-terminal sprague powerlytic 36DX caps in the foreground:

Never mind those black and blue caps in the back corner, that's somebody's ad-libbed repair.

Optimally, I'd like to use a similar grade of computer grade caps if at all possible, as these amps can really pound on the power supply caps. If possible, I'd also like to find something in direct replacement and/or a similar footprint. I'd rather not have to gut that side of the amp and fabricate a custom board for snap-ins if I can avoid it.

I'm also a bit worried about using snap-ins as a replacement, because their service life and their ripple current handling capability is typically a fraction of that of the computer grade caps.

**Enzo** · 08-31-2007, 03:16 AM

At the risk of sounding "basement," you need not fab a board for snap ins. Clamps and wires will do. After all, those screw term caps don't have a board.

From Allied:
507-0030
7400/200v $40.55 36DPowerlytic

613-0410
2200/160v $11.67 snap ons with 5A ripple, use two

862-5203
4300/200v $48.98 mmmm, maybe not

857-0300
5000/200v $35.45 sounds right

By the way, on the cobble job in the photos, you are aware that the original caps on the board were duals and CArver had a mod replacing the pair of duals with four caps on a small board with wires down to the original power supply board? That appears to be what this person was doing their own way.

**bob p** · 08-31-2007, 02:15 PM

duplicate post. sorry.

**bob p** · 08-31-2007, 02:17 PM

Thanks for your insights, Enzo. Your advice is always appreciated.

Yeah, I know that I don't *have* to fab a replacement board. I'm happy to drill a hole or two if I need to, and I'm not above resorting to the use of cable ties to hold things in place. If I go that route, I think I'd prefer to put the caps on a small board and mount the board on 4 standoffs. (I like modular repairs, so that the next time is easier)

Yes, I'm aware that the original dual caps are unobtainium, and I had just planned on buying sets of 4 2200uF 100V snap-ins for the low voltage rails. I'm thinking that I'd either drill counterbored holes 10mm apart so that I can snap mount them to the G10, and then wire them up PTP on the opposite side with leads running to the PSU board. I'd probably need to use a second G10 board as an insulator bolted between the cap board and the chassis for safety, and mount the cap board above it on a standoff. I could probably get by with mounting them on their sides using cable ties, but since I'll have to do this on more than one amp, its probably worth the effort to make a few copies of a simple replacement board. The Carver replacement board is hard to find and when its available its pricey because its out of production.

For the big caps, I'd still prefer to use similarly sized screw-terminal caps if I can find them affordably. Thanks for the help in locating them!

In your experience, how does an array of snap-ins compare to the computer grade screw terminal caps? I think that once you assemble a gang of snap-ins, the ripple current figures are similar to the higher capacitance computer grade caps. I'm also wondering about longevity.

On another technical note -- how safe is it to make a significant step up in terms of the total capacitance of the cap? If I step up to something like the 7400/200V Powerlytic instead of the 4800uF, do I need to worry about the narrower conduction angle stressing my bridge with increased current pulses over shorter durations? AFAIK the Carver amps stressed the rectifiers and triacs pretty hard with short intense pulses. IIRC these were a common failure point, and there was a TSB on one of the amps that mentioned increasing the current ratings on these parts from 3A to 6A.

it looks like the 857-0300 is a good choice.

thanks again!

**Enzo** · 08-31-2007, 11:04 PM

On another technical note -- how safe is it to make a significant step up in terms of the total capacitance of the cap? If I step up to something like the 7400/200V Powerlytic instead of the 4800uF, do I need to worry about the narrower conduction angle stressing my bridge with increased current pulses over shorter durations? AFAIK the Carver amps stressed the rectifiers and triacs pretty hard with short intense pulses. IIRC these were a common failure point, and there was a TSB on one of the amps that mentioned increasing the current ratings on these parts from 3A to 6A.

I don't know, you are over my head there. I am tempted to think that doubling the cap size means that much less rechanrging to do on each pulse. That is the rail holds up better between cycles. ???

I have not replaced very many caps in Carvers, but I have replaced quite a few 3A rectifier diodes. COnsidering the very large surge rating for even the 3A rectifiers, I would have thought that the caps were not the cause of their demise. COnsider the low rail voltage - 35? 40? - and then how much current the load demands of that rail when the signal is large, and I think that 3A was just marginal at best in the first place.

857-0300 looked good electrically, but I did not check the dimensions. You don't have a lot of room for different shapes.

**Steve Conner** · 09-01-2007, 09:42 AM

I'd be worried about increasing the capacitance much over stock, in case it messed up the magnetic field amplifier thingy.

**bob p** · 09-01-2007, 02:37 PM

Originally posted by Enzo View Post

I don't know, you are over my head there. I am tempted to think that doubling the cap size means that much less rechanrging to do on each pulse. That is the rail holds up better between cycles. ???

i was thinking that if the cap size is doubled, you'd still have the same amount of recharging to do on each pulse, since you have to replenish the same total amount of energy no matter how big or small the cap turns out to be. with a bigger cap, i was thinking that you'd have to replenish that same amount of energy over a shorter duration, or a narrower conduction angle. more capacitance in less time means higher current. i was wondering if the increased intensity of brief pulses would prematurely wear the rectifier, in an analogous way to how tube rectifiers can be ruined with big caps.

Steve?

**Tom Phillips** · 09-01-2007, 04:58 PM

Originally posted by bob p View Post

i was thinking that if the cap size is doubled, you'd still have the same amount of recharging to do on each pulse, since you have to replenish the same total amount of energy no matter how big or small the cap turns out to be. with a bigger cap, i was thinking that you'd have to replenish that same amount of energy over a shorter duration, or a narrower conduction angle. more capacitance in less time means higher current....

Bob,

A comment on the "total amount of energy" statement.

This may not make a significant difference but my thought process is...
If the capacitance is larger there will be less DC supply voltage sag. (Provided that the power transformer is not the limiting factor) With less sag the average voltage stays higher. Therefore, the amp can put out more power. If so, then the total amount of energy must be higher especially when the amp is pushed hard.

Like I said, this may not make a significant difference but there must be some effect. What do ya think?

IMHO the 3A rectifier diode rating is too low and a 6 amp upgrade is a good idea when you are in there.

Regards,
Tom

**Steve Conner** · 09-01-2007, 04:59 PM

Well I doubt it would blow the rectifier, but I think it would be likely to change feedback loop gains and phase margins. The power supply in these things has a feedback loop round it that regulates the triac firing angle, and you could make that unstable.

That's not likely, because from the diagrams you sent me, it has no integral gain and hence I'd expect a 90 degree phase margin no matter the size of the capacitor. But it's not impossible either: circuits with feedback have a way of biting back at you when you least expect it...

No harm in trying though: diodes and triacs are pretty tough, and you can easily see signs of instability by scoping the power rails while cranking "Back In Black"

If a kick drum hit sends them flapping up and down like a pogo stick, with your shop lights flickering and horrible noises coming from the transformer, you know you have a problem...

**Steve A.** · 09-01-2007, 08:31 PM

Well, I just hit the "Caps Lock" key on my computer kepyboard to get BIG caps. SMALL caps is more of a problem and I need to use Microsoft Word to get those.

HTH

Steve A.

**bob p** · 09-04-2007, 03:28 PM

Tom, your comments about total energy and sag make sense. I'm just not sure that I understand the regulation circuit well enough to opine on it, so I just thought I'd ask for other people's opinions. In my rudimentary understanding of the Carver PSU, if the voltage sags the supply responds with more current demand in an effort to avoid further voltage sag. The ability of the amp to tap more current on demand works to keep the voltage from sagging, and probably results in more total energy use. At least that's how I think of it. Of course, as Steve mentioned, these feedback circuits are complicated, and they're probably beyond my ability to predict their behavior. I guess another way to say that is that its always possible (or probable) that there's a deviation in real world performance between how the Carver PSU actually operates and how I think it should operate!

**bob p** · 09-04-2007, 03:29 PM

Originally posted by Steve A. View Post

Well, I just hit the "Caps Lock" key on my computer kepyboard to get BIG caps. SMALL caps is more of a problem and I need to use Microsoft Word to get those..

Hmmm... I never thought of the simple approach!

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