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Old 16th November 2007, 04:55 AM   #1
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Default Carver M500 Power Supply

I operated my Carver M500 in bridged mode as a subwoofer driver for about 5 years, till suddenly one of the main 6800 uF power supply capacitors went up in smoke popping its can from its base. Fortunatley I was close by and saved it (I think) by pulling the plug as fast as I could. I replaced it along with the other 5 main power supply caps, after learning that an amplifier this old (25 years?) should have all its electrolytics replaced.

The amplifier would turn on as normal but no signal was found on its outputs. I found that the power supply rails were operating much higher than they are supposed to, 98V and 55V, instead of the normal 74V and 40V, according to the schematic. I tried to reduce the voltages by adjusting the trimmer pot as indicated in the service manual but no such luck- could not get below 94V and 53V. There are a couple of 2-watt resistors on the main amplifier board that would run hotter than I would expect, probably due to the higher than normal supply voltages.

I've done the usual checking for shorts with a DMM across just about all power supply components including the triac, and the main amplifier transistors but found nothing shorted. No open transformer coils either.

I've learned that the diode thyristor on the Sealed Phase Network sub-board which sits on top of the main power supply board is prone to failure, so I removed the board (its small and has a plastic cap on top that is hard to remove, like on a relay). I did not find any shorts, but noticed that the leads on the thyristor were black (running too hot?). There was also a light colored hazy chemical-like substance on the bottom of the board so I lightly scrubbed it with some water and a toothbrush, dried it, and soldered it back in. I suspected that the thyristor is probably bad, but wanted to test it running, with the plastic cap removed.

Apparently I disturbed something because now the amplifier just blows fuses the instant I plug it in. All the connections on the sub board look ok, nothing broken. I don't think it was material from the capacitor exploding that I cleaned off the sub-board because none was found on the main board. Was it just harmless excess cleaning solution from the soldering process when it was manufactured? Should I just take the whole power supply apart, clean the boards, remove all components and verify their values, and then rebuild? I searched for a replacement for the thyristor but have not located one yet. Running out of ideas.... Thanks for any help anyone has to offer.
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Old 17th November 2007, 11:23 PM   #2
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Ok, I'm going on memory here (not a good thing!) but the last M500 I repaired about a year ago had a similar issue. If I remember correct, when the HV adjust pot goes bad and the res goes up, the PS voltages will also go higher. I will have to dig out the service manual and investigate, but in the meantime you may want to check that pot.
The fuse will usually go quick when the power supply goes open loop, or when the triac shorts.
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Old 18th November 2007, 03:24 AM   #3
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Default carverm500

how's the amp
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Old 22nd November 2007, 01:09 AM   #4
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toddyaudio,

I took your suggestions:

1. Verified that the pot is good- has adjustable range of about 51K to 212k in-circuit. According to the schematic it is a 200k pot.

2. Adjusted pot to the minimum and then maximum. Each time the fuse would just blow immediately.

I also double-checked every component for shorts and found none.

Engel dela pena,
I have not focused much attention on the amplifier board if that's what you mean by "amp", other than check the main transistors for shorts and any componenents that appeared visually burnt.
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Old 22nd November 2007, 03:26 AM   #5
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Ok, I pulled the cover on one of the M500's and this is what I measured: From center wiper terminal to the CCW terminal is 150min to 104Kmax ohms, the current setting for 70volts output is 25K. From the center wiper to the CW terminal is 30min to 224K ohms. The fact that you could only get down to 51Kmin seems to suggest that there is a problem. I will include a photo just to make sure the PS caps are oriented correctly.
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File Type: jpg m500potadj 055.jpg (67.9 KB, 210 views)
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Old 27th November 2007, 04:56 AM   #6
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toddyaudio-

I realized I was not referencing my pot resistance checks properly, overlooking the center terminal. From the center to the CW terminal I measured a range of 1.4k to 207k ohms. From center to the CCW terminal I measured a range of 8.18k to 118k ohms. I set the resistance to 25k as yours is, which happens to put the wiper back to its original position. From all this it seems that the pot is ok. I also verfiied that my capacitor orientations match yours from your photo. After all this I still suspect that the thyristor on the phase network sub-board is bad. It seems to be a known issue as pointed out in the service bulletin included with the schematic. I still have not had any luck tracking down a replacement. Thanks for your help.
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