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Old 21st May 2012, 08:51 AM   #1
pergo is offline pergo  Italy
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Default Carver PM1.5 problem

I buy a broken Carver PM1.5, that arrives with one channel in perfect working condition, and the other channel with 2 burned resistors and 1 broken transistor.

See schematic in attached. Transistor is Q20 (MPSU10) and resistors are R54 (910ohm) and R55 (91ohm).

After replacing those components, i try the amplifier.
I also replace Q13 for matched complementary pair.


The repaired channel, after 10 seconds from turn-on, re-burn R55 , and show 35V of DC offset at the output.

I check the R55's circuit zone, but i don't find anything strange.


Any advice?
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Old 21st May 2012, 12:49 PM   #2
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Your schematic didn't show up. Transistor amps tend to burn a lot more than one thing, often also burning lands off the PC board. Pull every component touching the bad parts. Make or acquire a DC power supply - I'm using the 18v peak voltage out of a 12 VDC battery charger I made. Measure the parts, even the capacitors, for leakage current in excess of what is necessary. You may also test the resistors for proper resistance. You may calibrate your leakage readings by checking transistor junctions with the 2000 ohms scale of a DVM. Forwards should be about 450-600 ohms, backwards 1999 ohms. However, stressed transistors and diodes sometimes leak at 18V when they don't at 2 v the meter puts out on ohms scale. Some transistor be junctions are not be rated to withstand 18v backwards, look at your datasheets before testing be. ce junctions should stand that if the b is shorted to e.
Sometimes the driver transistors were inadequately heat sinked. However, over 15 years, often leaking electrolytic caps initiate amp failures. I, and several amp shop owners who have posted here, replace every electrolytic cap before we start poking around on a burned amp with a meter. See this thread about finishing your job on a solid state amp. vintage amplifier repair/upgrade manual
Moderators may move this thread to the PA forum, as this amp is pretty big for home use.
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Last edited by indianajo; 21st May 2012 at 01:01 PM.
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Old 21st May 2012, 01:25 PM   #3
pergo is offline pergo  Italy
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here's the service manual: Carver PM-1.5a-PM1200 Service Schematics-1.pdf download - 2shared

hope now you can see it.

I've repaired thousands of amplifier (especially car amps), and some home amplifier.
I've all necessary equipment and skills

i'm stuck in this amp because it's a class H. Never work before on a classH. (in fact, 35V is the first rail)

I want analyze why that resistor continue to burn. If pass too much current, who cause that current?
The LTP and VAS are very simple.

Last edited by pergo; 21st May 2012 at 01:32 PM.
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Old 21st May 2012, 02:51 PM   #4
Tajzmaj is offline Tajzmaj  Slovenia
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Carver again.
So my advice since I have repaired many of those. Check everything. Take your time. Don't rush. There is a lot of different voltages there and one is +- 125V so please BE CAREFUL.
I will check schematic later and try to help.
Cheers, Taj
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Old 21st May 2012, 05:34 PM   #5
pergo is offline pergo  Italy
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fortunately one channel is good so i can check the differences.
I ask in case someone had similar trouble.
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Old 21st May 2012, 05:42 PM   #6
Tajzmaj is offline Tajzmaj  Slovenia
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It means that at lest psu works fine. I will check schematic now. I think there you have fried TO3 device. One of the comutators, outputs or driver.....at Pm1.5 there is 20 of them at 1.5A or 1200 you have 2 additional....
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Old 21st May 2012, 05:48 PM   #7
Tajzmaj is offline Tajzmaj  Slovenia
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Is your amp just 1.5 or 1.5A version?
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Old 21st May 2012, 06:15 PM   #8
pergo is offline pergo  Italy
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the difference is only the low-voltage output stage.
The "A" version has 2 pair of bjt.
The "non-A" version has only 1 pair.

Mine is the "non-A".

The other components are totally the same.
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Old 21st May 2012, 09:12 PM   #9
Tajzmaj is offline Tajzmaj  Slovenia
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And there is easier to repair non ,a, version because you can reach transistor socket contacts...in ,a, version there is quite messy...
My experience is that drivers never fail if there is everything ok with TO3 units.
Good replacement for mpsu ...are mje340/350. Outputs are MJ15024/25.
And don't forget to set idle.
Cheers
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Old 21st May 2012, 09:56 PM   #10
pergo is offline pergo  Italy
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for MPSU, i replace with MPSW42/92 in both channel.
The good one remains good.
The bad remains bad.
So, it isn't a equivalent's problem.

I check most component but they seems ok respect the working channel (same measure).
But some parts are difficult to measure due to the position.
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