Caver PM Repair

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Tweeeter21

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Joined 2015
#1
Dear Audio Masters,

I'm a slow and steady worker and usually solve my problems in the audio department because I like taking my time and solving the puzzle.

I was at the thrift store and scored a Carver PM 300. At first I was thrilled, now I'm asking myself the hard question- is it worth it?

As I perceive the problem there is 120 volts all the way into the transformer, but nothing coming out on the DC side. I checked the resistance of the leads and none are way out of the ballpark and continuity checks out. I even plugged it into 120 volts and it vibrated and seemed fine, although I chickened out 😱, because I didn't want to fry it totally.

If someone could walk me through this circuit I would totally appreciate it.

I'm stumped, I'm hopefully that someone could earn their stripes by helping out the new guy...😉

http://www.carvermanuals.com/manuals/Amplifiers/PM-300%20Owners.pdf
 
#5
Trust me.... If you do not have years of experience in the field of repairing switchmode power supplies, audio amplifiers, and do not have a dual-channel oscilloscope +the necessary theoretical background of the above, do not start repairing, because you will end up with a totally dead amp, in worse case a dead You..... Saing this after doing this kind of stuff for almost 40 years.
This is not the question of quick learning paired with self-confidence, here you need more than that, much more....
 
#6
Once I repaired an old M-400 (the "cube")... that was a nightmare. Things were blowing every half an hour, one after the other, I fixed one part, the other blew up. Maybe the newer ones are more reliable, but since then I am not touching a Carver amp even with a 3 feet pole.
 
#10
It's not the amplifier circuit that is tough, it's the audio-tracking triac-controlled power supply.

Check the diodes on the low voltage rail, if they're bad replace with 6A diodes.

Check all transistors for shorts.

Do not use a variac, it must be driven by its internal triac light-dimmer type circuit.

If the transformer looks like it has been on fire, it's scrap.
 
#11
I agree with DJK the amplifier itself is not that bad to troubleshoot, the power supply a PITA to troubleshoot. What makes all or most of the Carvers with that power supply hard to work on is the mechanical layout, every time you find something on the schematic that you want measure you can't get to it. The 400 is the worst, basically yank it all apart, test and replace everything you can, put it all back together and hope you found everything. Then come the alignments, very hard to get where you need to get. So without lots of experience AND test equipment the OP's amp is never going to work. He also started on the wrong foot by plugging the power xfmr straight to 120VAC, that's why it vibrated.

Another problem I always find in the amplifiers with TO-3 output and commutator xsistors is the commutator xsistors are always dead. They will measure fine using the diode function on a typical DMM but tested on a semiconductor analyzer they won't do anything, no leakage, no anything.

Craig
 
#12
Wow, some you you are tough to get through. Advisement appreciated. Is it worth it to send out?

I also agree bad idea to plug into the wall- exciting although temporary.

I ordered the service manual for it.

I tested the TRIAC seems to be fine. I have scope and a DMM. No signal generator.

I do not think the problem is very bad for some reason. But, I'm an eternal optimist and seem to be in the right crowd 🙂

Let me get this manual in the mail and then check some voltages in the Power Supply...

Many Thanks, I can tell this is a winter project- lots of wood in the yard so I should be fine.

Good Listening,
Luc
 
#13
djk said:
It's not the amplifier circuit that is tough, it's the audio-tracking triac-controlled power supply.

Check the diodes on the low voltage rail, if they're bad replace with 6A diodes.

Check all transistors for shorts.

Do not use a variac, it must be driven by its internal triac light-dimmer type circuit.

If the transformer looks like it has been on fire, it's scrap.
Click to expand...

Thanks for this detailed information. I really like this type of instruction. I'm going to pause us here until i perform the above task. Many thanks, Luc
 
#20
llwhtt said:
I agree with DJK the amplifier itself is not that bad to troubleshoot, the power supply a PITA to troubleshoot. What makes all or most of the Carvers with that power supply hard to work on is the mechanical layout, every time you find something on the schematic that you want measure you can't get to it. The 400 is the worst, basically yank it all apart, test and replace everything you can, put it all back together and hope you found everything. Then come the alignments, very hard to get where you need to get. So without lots of experience AND test equipment the OP's amp is never going to work. He also started on the wrong foot by plugging the power xfmr straight to 120VAC, that's why it vibrated.



Another problem I always find in the amplifiers with TO-3 output and commutator xsistors is the commutator xsistors are always dead. They will measure fine using the diode function on a typical DMM but tested on a semiconductor analyzer they won't do anything, no leakage, no anything.



Craig
Click to expand...



Craig, the TO-3 output and commutator resistors- where are they located? On the power supply?


Sent from my iPhone using Tapatalk
 
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