Levinson No23 DC offset troubleshooting

johnsciolino · 30th October 2006, 11:17 PM

I have some DC offset in the ML 23 that trips the breaker intermittantly when ramping up with my variac. I believe it is in the input section after isolating the HV supply to output transistors. Has anyone had this problem with the No23? I already see a couple caps near some heat sources that need replacement however they are not shorted. This is a very difficult amp to take apart which requires that all the outputs be unsoldered and removed before the main board can be worked on. I would like to get it right the first time and was hoping someone on this forum has some experience with this amp.

Tube_Dude · 31st October 2006, 12:43 PM

Quote:

Originally posted by johnsciolino
I have some DC offset in the ML 23 that trips the breaker intermittantly when ramping up with my variac. I believe it is in the input section after isolating the HV supply to output transistors.

Many designs have DC offset , when they are feed with to low a voltage (depending of the circuit )...

Have you tried to feed the amp with the proper voltage?

johnsciolino · 31st October 2006, 05:46 PM

I should of specified 19v DC offset when bypassing the protection circuit.
I get 67,67,37v at differential pair but the transistors test OK. I think this amp uses FET at the input which I would suspect is the problem. A schematic that is legible would be nice if someone could kindly send one my way.
I would like to identify the FET on the board without reverse engineering everything.

Tube_Dude · 31st October 2006, 05:52 PM

Quote:

Originally posted by johnsciolino
I should of specified 19v DC offset when bypassing the protection circuit.

But this offset is with rails at full voltage , or at low voltage when you feed the amp progressively via the variac?

johnsciolino · 31st October 2006, 06:08 PM

Full voltage or or 3/4 voltage. Compared to the good channel as well. I think its the LM394 bad.

Tube_Dude · 31st October 2006, 06:15 PM

Quote:

Originally posted by johnsciolino
Full voltage or or 3/4 voltage. Compared to the good channel as well. I think its the LM394 bad.

Yes... probably the problem is in the input pair LTP, a schematic could be handy...

It don't have a nulling offset servo , like a integrator Op Amp..?

Tube_Dude · 31st October 2006, 06:33 PM

All I found...

http://marklev.com/No23/om/_4.html

http://marklev.com/No23/index.html

johnsciolino · 1st November 2006, 12:44 AM

The datasheet says direct replacement for the LM394. There seems to be some minor differences but looks like it may work. This is what the No23.5 uses according to some other posts I found.

30th October 2006, 11:17 PM	#1
johnsciolino diyAudio Member Join Date: Aug 2004 Location: florida	Levinson No23 DC offset troubleshooting I have some DC offset in the ML 23 that trips the breaker intermittantly when ramping up with my variac. I believe it is in the input section after isolating the HV supply to output transistors. Has anyone had this problem with the No23? I already see a couple caps near some heat sources that need replacement however they are not shorted. This is a very difficult amp to take apart which requires that all the outputs be unsoldered and removed before the main board can be worked on. I would like to get it right the first time and was hoping someone on this forum has some experience with this amp.

31st October 2006, 05:46 PM	#3
johnsciolino diyAudio Member Join Date: Aug 2004 Location: florida	Differential PAir-Schematic would be nice I should of specified 19v DC offset when bypassing the protection circuit. I get 67,67,37v at differential pair but the transistors test OK. I think this amp uses FET at the input which I would suspect is the problem. A schematic that is legible would be nice if someone could kindly send one my way. I would like to identify the FET on the board without reverse engineering everything.

31st October 2006, 06:08 PM	#5
johnsciolino diyAudio Member Join Date: Aug 2004 Location: florida	Either way.... Full voltage or or 3/4 voltage. Compared to the good channel as well. I think its the LM394 bad.

31st October 2006, 06:33 PM	#7
Tube_Dude diyAudio Member Join Date: Mar 2002 Location: Aveiro-Portugal	All I found... http://marklev.com/No23/om/_4.html http://marklev.com/No23/index.html __________________ Jorge

1st November 2006, 12:44 AM	#8
johnsciolino diyAudio Member Join Date: Aug 2004 Location: florida	MAT02 Direct replacement? The datasheet says direct replacement for the LM394. There seems to be some minor differences but looks like it may work. This is what the No23.5 uses according to some other posts I found.


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