This amp had a piece of wire inside it that shorted out the leads on the positive rectifier.I replaced both rectifiers and all 4 50v 2200uf caps.It powered up fine.I benched it at 171watts RMS on both channels with a 4 ohm resister load.Then I tested it bridged into a 4ohm resistor load and it did 342 watts RMS(37 volts) right before clipping.I reduced the power to about 25 watts (sine wave at 1khz) and let it run for about 5 minutes,then ran it back to full power.After about 5 seconds the negative wave started clipping at about 50 watts.
Now I have +67volts and -21volts on the rails.There is a +DC voltage and a pulsed negative on the outputs of the rectifiers.So I thought the negative rectifier or new caps was at fault so replaced both rectfiers and all 4 caps again.
The amp tested good again.Until it got to about 140 degrees F,then it did the same thing again.+67 and - 21 volts with the negative half showing a pulse and the positive rectified at 67 before the coils.Rail voltage is still +67 and - 21.I replaced the parts again but this time it didn't help.
This would normally make me suspect a shorted winding in the transformer but I have good +/-53 volts and +/-15 volts in the rest of the amp.
I have been working on this thing for 8 days now.I have pulled every part on the rails but the driver boards and outputs to no avail and used about 15 feet of braid already.Im afraid that all the parts mounted to the heat sink are going to start breaking off the board from all the flipping over and over.
I am baffled at this point.Anybody got any ideas what I am missing?
Now I have +67volts and -21volts on the rails.There is a +DC voltage and a pulsed negative on the outputs of the rectifiers.So I thought the negative rectifier or new caps was at fault so replaced both rectfiers and all 4 caps again.
The amp tested good again.Until it got to about 140 degrees F,then it did the same thing again.+67 and - 21 volts with the negative half showing a pulse and the positive rectified at 67 before the coils.Rail voltage is still +67 and - 21.I replaced the parts again but this time it didn't help.
This would normally make me suspect a shorted winding in the transformer but I have good +/-53 volts and +/-15 volts in the rest of the amp.
I have been working on this thing for 8 days now.I have pulled every part on the rails but the driver boards and outputs to no avail and used about 15 feet of braid already.Im afraid that all the parts mounted to the heat sink are going to start breaking off the board from all the flipping over and over.
I am baffled at this point.Anybody got any ideas what I am missing?
Yes.The wave is the same.Its running at a very low duty cycle but they are both symmetrical (about 70 volts to center,140 P-P).Im glad you mentioned the inductors.They are separated by a black rubber silicone,but on a hunch I bypassed the one with the low voltage and as soon as I powered it up the amp pulled 20 amps.I removed the jumper and then pulled all the caps(little yellow ones) I could find on the neg rail.No help.
I tried to put a resistor on the 12 volt power wire to trouble shoot it with the inductor jumped but these JL's wont even fire up like that.I even took all the diodes out of the circuit on the small yellow transformer but all that did was burn out the fqp65no6 mosfet and the .1 ohm white resistor.
I pulled the inductors out and checked them over.Couldnt find anything wrong with them.
I tried to put a resistor on the 12 volt power wire to trouble shoot it with the inductor jumped but these JL's wont even fire up like that.I even took all the diodes out of the circuit on the small yellow transformer but all that did was burn out the fqp65no6 mosfet and the .1 ohm white resistor.
I pulled the inductors out and checked them over.Couldnt find anything wrong with them.
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Ok,Since you referred me back to the inductors I pulled them back out.I had epoxied them in,but got them back out fairly easy.I yanked a pair of output filter inductors from a Memphis 1000d and put them in place of the originals.They are only .3mh compared to the 3.6mh of the others but now I get +67volts and -62volts.
Much closer.There is a small voltage spike on the - rail compared to the + one.
I am now concerned about the 50 volt rating of the caps.Im not sure if that is the right ones.Someone before me replaced the 2 caps on the + rail but the - ones looked original.The negative power supply pulse is now gone,its more DC except for the spike.Im not really comfortable running it long like this or connecting speakers yet.
The idle current has dropped to about 1 amp now.
Any thoughts?
Much closer.There is a small voltage spike on the - rail compared to the + one.
I am now concerned about the 50 volt rating of the caps.Im not sure if that is the right ones.Someone before me replaced the 2 caps on the + rail but the - ones looked original.The negative power supply pulse is now gone,its more DC except for the spike.Im not really comfortable running it long like this or connecting speakers yet.
The idle current has dropped to about 1 amp now.
Any thoughts?
Well,I changed the caps to 63 volt Elna's,replace the inductors with similar ones and replaced the negative rectifier.I basically got the same results.This time I got a real bad ripple(ringing) on the negative rectifier.With the scope set on AC only,the trace is dead center on the positive rectifierl(after it settles)and I get +/- switching on the negative rectifier.So I removed the amp section from the circuit with just the caps,inductors and rectifiers on the transformer with a small resistive load and still have the same results.So I believe its the transformer even though I have a good +/-53 and +/- 15volts everywhere else on the board.
I guess its done for.
I guess its done for.
Its the voltage coming from the smaller power supply with the yellow transformer going to the last 2 pins on each driver board.
As with most amps the center tap on the secondary is referenced via the outer shield(ground) on the RCA input and isolated from the 12 volt input ground.But this amp has the 12 volt ground,secondary center tap,and the RCA preamp output shield all connected,and the RCA input shield is isolated.I assume it has balanced inputs.
So all voltages share the same ground regardless were I probe.
To me,it just seems that a shorted winding would cause major obvious problems.I mean,there should be some major current draw at idle even with no common ground RCA's plugged into it.
This is why I thought it was a bad cap somewhere on the rails,so I started pulling them.That didn't help.
The last time I worked on it I had the idle bias fully counterclockwise.As I started to increase the bias I could see the Negative rail voltage decrease just like the bias pot controlled the rail voltage.
Now Im thinking ok,maybe the Mosfets on the positive rail are open.But since the amp worked perfect till it got hot on 2 previous attempts I decided to isolate the power supply from the rails first.
That's when I checked the center pins on the rectifiers in AC mode.I get 0 volts on the positive rectifier and I see about 140 volts +/- 70 AC on the center pin of the negative rectifier.In DC mode the negative rectifier has a lot of ringing and if I push it hard against the heat sink with my finger the ringing almost disappears.Im sure it has a lot to do with the LCR circuit on it but the positive rectifier doesn't do that.And the more I increase the idle bias(which brings the voltage down)the more there is a dead spot like one of the diodes is open in the rectifier.Ive already replaced it 4 times prior to this test.Maybe I will change it one last time.
I don't know what else to do.I have been repairing audio gear for the last 30+ years,15 of which as my full time business.According to my records Ive done at least 4000 amps.
I wish I could put a different transformer in it,but of all the stacks of scrap amps I have nothing comes close to physically matching regardless of voltage.
Any and all suggestions/comments are welcome.
As with most amps the center tap on the secondary is referenced via the outer shield(ground) on the RCA input and isolated from the 12 volt input ground.But this amp has the 12 volt ground,secondary center tap,and the RCA preamp output shield all connected,and the RCA input shield is isolated.I assume it has balanced inputs.
So all voltages share the same ground regardless were I probe.
To me,it just seems that a shorted winding would cause major obvious problems.I mean,there should be some major current draw at idle even with no common ground RCA's plugged into it.
This is why I thought it was a bad cap somewhere on the rails,so I started pulling them.That didn't help.
The last time I worked on it I had the idle bias fully counterclockwise.As I started to increase the bias I could see the Negative rail voltage decrease just like the bias pot controlled the rail voltage.
Now Im thinking ok,maybe the Mosfets on the positive rail are open.But since the amp worked perfect till it got hot on 2 previous attempts I decided to isolate the power supply from the rails first.
That's when I checked the center pins on the rectifiers in AC mode.I get 0 volts on the positive rectifier and I see about 140 volts +/- 70 AC on the center pin of the negative rectifier.In DC mode the negative rectifier has a lot of ringing and if I push it hard against the heat sink with my finger the ringing almost disappears.Im sure it has a lot to do with the LCR circuit on it but the positive rectifier doesn't do that.And the more I increase the idle bias(which brings the voltage down)the more there is a dead spot like one of the diodes is open in the rectifier.Ive already replaced it 4 times prior to this test.Maybe I will change it one last time.
I don't know what else to do.I have been repairing audio gear for the last 30+ years,15 of which as my full time business.According to my records Ive done at least 4000 amps.
I wish I could put a different transformer in it,but of all the stacks of scrap amps I have nothing comes close to physically matching regardless of voltage.
Any and all suggestions/comments are welcome.
Good point.
All the rectifiers I installed were pulled from other amps.The original rectifier that shorted was defective and one of the others I replaced were defective(second batch).The last one I pulled before isolating the rails acted funny on one leg.It would start at .45 volts then go up to .7 then back down to .45 within about a second.So I put a different one in.I am not going to do anything with it for a few days.I got to backed up messing with it plus its labor day weekend.
I will put a new rectifier in it one more time before giving up on it.If it is frying rectifiers,what would cause this?
All the rectifiers I installed were pulled from other amps.The original rectifier that shorted was defective and one of the others I replaced were defective(second batch).The last one I pulled before isolating the rails acted funny on one leg.It would start at .45 volts then go up to .7 then back down to .45 within about a second.So I put a different one in.I am not going to do anything with it for a few days.I got to backed up messing with it plus its labor day weekend.
I will put a new rectifier in it one more time before giving up on it.If it is frying rectifiers,what would cause this?
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