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Old 9th July 2007, 02:33 AM   #1
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Default Carver PM-1200 help

Hoping someone can help me repair this amp.
1) Amp started an intermittent "cutting in and out" last night at gig. It was an outdoor gig and was very hot and humid. The protection light was on. I contributed this to temperature. Both fans were operating at high speed. Quickly wired in spare 1200 to finish the gig (approximately 2 hours).
2) After the gig, I thought I'd check the other amp out in hopes it was a thermal issue and had cooled down enough. I re-wired it into the system and turned it on. No protection light! As soon a I sent a signal to it, same thing. Music cutting in and out and protection light on. The intermittent sound is almost like pulsing...about a second long for the in and out. Can't really tell if there is distortion, but I don't think there is.
3) I've owned three 1200s for years and always serviced them on a regular basis...check and adjust bias, oil motors, replace filters. Simple stuff. I did replace two Mabuchi motors several years ago, but other than that, I've had no problems what so ever with these amps until now.
4) I have a general knowledge of electronic repair and have worked on solid state amps in the past. I also have the service manual for these amps.

What I'm hoping for (long shot here) is that someone has experienced this problem before and would be generous enough to save me time and frustrations and share their knowledge! I'm thinking filter caps, and I will replace them while I'm in there. Any ideas will be greatly appreciated.
Tom
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Old 9th July 2007, 04:54 PM   #2
anatech is offline anatech  Canada
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Hi Tom,
More info is needed for some real help. My guess might run along the lines of cracked PCB traces. The filter caps may have failed, but I would expect some blown seals in that case.

These are complicated amplifiers, so you will need to be careful. I'm very happy to hear that you maintain them.

-Chris
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Old 20th November 2007, 03:56 PM   #3
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Hi Tom,
I'm also a muso, working in a duo. Our main PA system uses the PM1201. A slight distortion is now happening in both channels and, like you, are trying to chase a problem.
Would you have any schematics / service info on this model...if so is there any chance of a copy ? Perhaps then we may be able to throw some light on each others problem.....
regards
chas
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Old 13th December 2007, 10:32 AM   #4
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Hi I'm a carver expert.

The Carver PM series amps are a triple-rail Class-H design (called 'magnetic field amplifier' by carver because it stores a lot of energy the the large transformer core).

the most common fault on these amplifiers is caused by the power supply caps drying out so this is the first thing you should check when the amp starts to cycle into protect or sound a bit crap.

PM1200 / PM1.5 uses 3 voltage rails for the output stage (and the usual +/- 17 V for the input stage).
the rails are +/- 36V, +/-80V, +/- 125V, the protect circuit checks all the rails are correct and symmetrical. if a rail drops for too long, it will cycle the amp up and down.

Take a dual trace scope and compare each pair of rails for symmetry, firstly without a load*. then with a load.
the rails will wobble about a bit, but as long as they look similar you should be ok.

*with no load, the carver amps can get into a situation where they will not re-start. this is due to a tiny dc offset building up in the zobel network (that little yellow cap across the binding posts). connection of a test load will clear the fault.


usually, you find one rail has a lot of ripple on it - this shows you which cap is down. (its usually on the 36 or 80 V rails).

bypass the suspect cap with a 4700uF/80V on a bit of wire.. see if the amp is better.

If so, you've found the fault - now the tricky bit!
Carver PM amps use a custom double cap - you cant get them.. if you find some spares.. be wary because they will be as old as the one you are replacing!

a few companies make their own mod-board that you can fit (replaces both double caps).
I have my own mod which manages to fit a pair of phillips high-ripple caps into the space.

Please remember if you're working on these amps, that there are 3 sets of caps you need to discharge through a suitable power resistor if you dont want to loose your eyebrows.
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Old 13th December 2007, 01:23 PM   #5
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Thanks to all who responded. I traced the problem to a cold solder joint on one of the output boards. I'm going off of memory here, so....IIRC, it was a ceramic resistor, very low rating like one ohm or less, 5 or 10 watt. I suspect it was from the factory because every update in the manual had been performed on this amp, which I purchased used. The tech that had worked on it probably couldn't get the amp to malfunction, so updated everyting suggested in the manual in hopes of repairing it. I happened on the problem by leaving it on for a couple of hours and by gently pushing on all of the components with a wooden skewer and luckily I could make the protection circuit engage when I touched that particular resistor. A quick touch with a soldering iron and a little solder and I was back in business. These amps are real work horses. To the responder who needed a schematic...I got a nice photocopy of the service manual off ebay for around twenty bucks. IS there a souirce for the board to replace the obsolete dual capacitors. I've heard about them but have not been able to locate them.
Thanks,
Tom
Tomjammin(at)Bellsouth.net
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Old 5th January 2008, 10:01 PM   #6
anatech is offline anatech  Canada
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Hi Tom, AudioSystemsEng,
Solder joint connections on heavier components have always been an issue. Not just Carver amps either. There were service notes on resoldering the card connections also.

Remove the old solder first and clean the leads. This is important as it seems many "techs" feel that adding more solder should fix the problem. Nope. Use a little solder flux (for PC boards!!) and redo all the connections. There should be zero (0) balls of solder when you are done. Clean off your flux!! Inspect for problems.

Quote:
*with no load, the carver amps can get into a situation where they will not re-start. this is due to a tiny dc offset building up in the zobel network (that little yellow cap across the binding posts). connection of a test load will clear the fault.
I have never seen that happen. It's an AC signal.
Quote:
usually, you find one rail has a lot of ripple on it - this shows you which cap is down. (its usually on the 36 or 80 V rails).
Very common these days.
Quote:
a few companies make their own mod-board that you can fit (replaces both double caps).
Carver made them first and I think they are available again. Those "other companies" simply copied the Carver PCB (ripped off the design). I still have an original Carver PCB assembly with the new caps on it. I have made my own and even seen where some people have simply mounted radial caps and soldered them. BTW, Philips are not great quality, but they are cheap. The new PCB uses snap type caps on it. If you just tack the new parts in (not a reliable repair), at least glue them to the chassis. I'm tired of seeing terrible work.

-Chris
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Old 1st January 2011, 03:10 PM   #7
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Has anyone got a copy of the schematics for a PM900?
Just got one in with a couple of dead o/p devs, but i'm a bit baffled as to why the protection cct didn't shut the amp down.

The protection seem's a little different to the PM1.5's i usually work on as there is no protect LED on the from of the amp. In the 1.5's, all the fault detectors were OR'ed and fed through the Protect LED to drive the PSU shutdown signal.
Not exactly failsafe, because if the LED fails, so does the protection - haha!

Its possible the PM900 protection is identical to the PM1.5 minus the LED - but i'm not convinced... the more lights the better, right?
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Old 1st January 2011, 07:19 PM   #8
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Check thecarversite.com. They have most of schematics for dn load.
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Old 1st January 2011, 08:05 PM   #9
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yeah - i found that, but the pm900 only had the user manual.
thanks though, its good that carver and crown have put most of their discontinued schematics up for d/l :-)
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Old 1st January 2011, 08:21 PM   #10
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aha - i often google these things but this time i've actually found it on here...

http://thecarversite.com/manuals.htm

i'm just going to 'down 'em all' so if the site disappears forum members can PM me for a copy.
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