Nytech Power amp (CPA602, I think)

JPhilips · 28th June 2006, 11:05 PM

Hi, has anyone any info/ schematic on this British power amp (MJ15003, MJ15004 output pair).
Right channel is fine but after replacing the left ouput pair & one driver (BD420) it worked for a short while then decided to short big time taking out both ouput transistors, both drivers, some of the smaller transistors (BC547) and a couple of the 100 ohm feed resistors.
The board has two adjustment pots, one is the bias adjust and the other is marked 'SYM:', could the failure be due to these being way out and how would you get an approximate adjustment without powering up.
Any help appreciated, as there's not much info about this model anywhere on the web.

jaycee · 29th June 2006, 09:55 AM

If the right channel is fine, you can take readings from that.

The SYM pot will be to adjust the DC offset. If its been implemented properly then this being mis-set wont do much damage. However, incorrectly set bias could blow the output stage.

You should have replaced both drivers even though only one came up faulty. The other could've been damaged enough to test ok but then fail when used.

JPhilips · 29th June 2006, 08:42 PM

I think you're right, the final death toll is MJ15003/4, BD419/420, BC547/557, 2x 1N4148 diodes, 100R & 150R.
I've replaced all (BD419/420 obsolete, BD829/830 used instead) exept the output pair which I've had to order from abroad as the availability of off-the-shelf semi's here in UK is awful.
The bias voltage on the good channel measured across two paralleled 0.39 ohm (0.195 ohm) resistors equates to 15mA which seems very low but that's untouched factory set.
Would the DC offset be measured at the output sockets with no input ?

forr · 29th June 2006, 09:27 PM

This is schematics of Ion-Nytech origin which may be quite similar to the one you are looking for.

forr · 29th June 2006, 09:31 PM

I think there already was a thread about Ion-Obelisk schematics on this forum. It may contain the component values of the above schematics.

28th June 2006, 11:05 PM	#1
JPhilips diyAudio Member Join Date: Jun 2006	Nytech Power amp (CPA602, I think) Hi, has anyone any info/ schematic on this British power amp (MJ15003, MJ15004 output pair). Right channel is fine but after replacing the left ouput pair & one driver (BD420) it worked for a short while then decided to short big time taking out both ouput transistors, both drivers, some of the smaller transistors (BC547) and a couple of the 100 ohm feed resistors. The board has two adjustment pots, one is the bias adjust and the other is marked 'SYM:', could the failure be due to these being way out and how would you get an approximate adjustment without powering up. Any help appreciated, as there's not much info about this model anywhere on the web.

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29th June 2006, 09:55 AM	#2
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	If the right channel is fine, you can take readings from that. The SYM pot will be to adjust the DC offset. If its been implemented properly then this being mis-set wont do much damage. However, incorrectly set bias could blow the output stage. You should have replaced both drivers even though only one came up faulty. The other could've been damaged enough to test ok but then fail when used.

29th June 2006, 08:42 PM	#3
JPhilips diyAudio Member Join Date: Jun 2006	I think you're right, the final death toll is MJ15003/4, BD419/420, BC547/557, 2x 1N4148 diodes, 100R & 150R. I've replaced all (BD419/420 obsolete, BD829/830 used instead) exept the output pair which I've had to order from abroad as the availability of off-the-shelf semi's here in UK is awful. The bias voltage on the good channel measured across two paralleled 0.39 ohm (0.195 ohm) resistors equates to 15mA which seems very low but that's untouched factory set. Would the DC offset be measured at the output sockets with no input ?

29th June 2006, 09:31 PM	#5
forr diyAudio Member Join Date: Dec 2004 Location: Next door	I think there already was a thread about Ion-Obelisk schematics on this forum. It may contain the component values of the above schematics.

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