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?
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?
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. http://www.diyaudio.com/forums/solid-state/136261-vintage-amplifier-repair-upgrade-manual.html
Moderators may move this thread to the PA forum, as this amp is pretty big for home use.
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. http://www.diyaudio.com/forums/solid-state/136261-vintage-amplifier-repair-upgrade-manual.html
Moderators may move this thread to the PA forum, as this amp is pretty big for home use.
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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.
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.
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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
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
If R55 is burning, then Q20 is most likely shorted (again) - remove it and test.
And from here forward use a dim-bulb. If you still have a short, the light will burn bright.
I would suspect Q15 as well, being as R54, R55, and Q20 would be the downstream weak links if it were to short.
Most likely Q15 went out, taking Q20 with it, then R55, then R54.
Just to be on the safe side, I would also check Q14, to make sure it is not a rail-to-rail short - it probably isn't, with a ground between the two, but nothing is lost by checking anyway. In the absence of that ground, you would essentially have 250volts between the two rails. But being as you did not mention any fault/burning with R23 or R24, this part of my reply is probably void.
And from here forward use a dim-bulb. If you still have a short, the light will burn bright.
I would suspect Q15 as well, being as R54, R55, and Q20 would be the downstream weak links if it were to short.
Most likely Q15 went out, taking Q20 with it, then R55, then R54.
Just to be on the safe side, I would also check Q14, to make sure it is not a rail-to-rail short - it probably isn't, with a ground between the two, but nothing is lost by checking anyway. In the absence of that ground, you would essentially have 250volts between the two rails. But being as you did not mention any fault/burning with R23 or R24, this part of my reply is probably void.
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You're welcome - whenever resistors are burning through, there is usually a short somewhere between it, and the voltage supply rail, and/or ground. With the amp turned off and unplugged (discharge the main caps), you should be able to determine the condition of the parts, from the ground between Q14 and Q 15, all the way to the point where R54 and R55 connect to the voltage rail.
Q15 collector to emitter should read open (both directions), Q15 emitter to ground should read 1.5K.
Q15 collector to emitter should read open (both directions), Q15 emitter to ground should read 1.5K.
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I think you misunderstood me - test these parts (Q14 and Q15) for an internal short, from emitter to collector, base to collector, etc.
Until your amplifier is stable, do not worry about offset, and until you find the short that is burning resistors, stop plugging the amplifier into power.
So you plan to remove them to see what the amp does without them?
Why not just test them to see if they are defective like I suggested?
Removing them from the circuit, regardless if they are good or bad, will not tell you if they are the problem. They need to be tested, just like any other suspicious component - and with the power OFF.
Why not just test them to see if they are defective like I suggested?
Removing them from the circuit, regardless if they are good or bad, will not tell you if they are the problem. They need to be tested, just like any other suspicious component - and with the power OFF.
I own 2 racks full of PM1.5,A,1200 amps. I use them to power stage wedges.....
Most of them were bought dead...or even uncomplete. I repaired all of them readjust everything and also replace all capacitors....
So here comes my advice.
Take your time. Check all components which are suspicious...and all others. 30V is not offset but there is open circuit somewhere. You have the lowest psu voltage present at the outputs.....
Once again. Check TO3 devices at the bottom of the amp. You have to remove bottom cover and than unscrew them.
If you want to repair it you should stop guessing and rather spend some time with multimeter ....
You should read ,,theory of operation,, chapter from service manual too.
And once again. Despite the fact that you have repaired ,,thousands,, of amps, be very carefull because the highest psu voltage is -+125V. And don't short anything with multimeter probes when adjusting idle and psu voltage....
All good
Cheers, Taj
Most of them were bought dead...or even uncomplete. I repaired all of them readjust everything and also replace all capacitors....
So here comes my advice.
Take your time. Check all components which are suspicious...and all others. 30V is not offset but there is open circuit somewhere. You have the lowest psu voltage present at the outputs.....
Once again. Check TO3 devices at the bottom of the amp. You have to remove bottom cover and than unscrew them.
If you want to repair it you should stop guessing and rather spend some time with multimeter ....
You should read ,,theory of operation,, chapter from service manual too.
And once again. Despite the fact that you have repaired ,,thousands,, of amps, be very carefull because the highest psu voltage is -+125V. And don't short anything with multimeter probes when adjusting idle and psu voltage....
All good
Cheers, Taj
DC at the outputs is not always caused by defective output devices. A quick diode test of these across the emitter of their perspective drivers, and the appropriate voltage rail, or across each device's emitter and collector, should suffice (power off).
Given the upstream problems, which he needs to discover first, it is probably just switching on the good rail, and sticking there. i.e. Q14 may in fact be working just fine, but without Q15, that's only half the signal - "DC". The +11.4V @ 1.14mA available via R15 is plenty to send Q14 into saturation, and leave it there, if not be borderline fatal in the process. Thereby making the amp all push, and no pull.
Given the upstream problems, which he needs to discover first, it is probably just switching on the good rail, and sticking there. i.e. Q14 may in fact be working just fine, but without Q15, that's only half the signal - "DC". The +11.4V @ 1.14mA available via R15 is plenty to send Q14 into saturation, and leave it there, if not be borderline fatal in the process. Thereby making the amp all push, and no pull.
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