0v Bias - Trimpot has no effect

b0red2tears

Member
2016-01-21 10:08 am
Hi,
I've had plenty of help so far with this repair but still no luck. Will try to keep this as brief as possible.
JVC A-X30 amplifier:

JVC A-X30 Manual - Stereo Integrated Amplifier - HiFi Engine

Attempting to set the bias voltage on this amp. The Left channel I am able to set to the recommended 11mV and it works fine (plays music without audible distortion) The right channel however is at 0v and the trimpot adjustment has no effect. It will play audio - i've only tried very quietly and the sound is not good: seems like the peaks are delayed giving a sort of pulsing effect.
Prior to this fault I replaced output transistors, replaced R658 (was burnt and open) and replaced Q618 and Q616 (open)

-I've since checked all other resistor values against the service manual - No faults. Also checked the trimpot - OK
-Checked all transistors as best I can with multimeter - None are open or shorted

Q618 and Q616 were substituted with BC546 and BC559 as I could not get the original toshiba's. But I've since swapped these to the left channel to eliminate the possibility that they are incompatible and they work fine in the left channel.

Help Please!! I don't know what else to try
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Well, you aren't the first to run up against unobtanium proprietary components in old design amps. FWIW, Destroyer X has been playing with Super A clone adaptor boards for years now. Why not check out his threads and try a little DIY enterprise: http://www.diyaudio.com/forums/solid-state/214633-dx-super-module-coupled-dx-amplifiers.html .

Whilst I'm sure you'll get somewhere just by losing the Super A feature as Sakis suggests, I'd be surprised if the amplifier sounded much good without further clever tinkering.
 

b0red2tears

Member
2016-01-21 10:08 am
OK, so the method to lose Super A as sakis described works well. A 1.5k resistor in series with the trimmer brought it into range to set the bias. The sound is good and I would be happy to leave it at this except there is a problem with the thermal compensation. When the heat sink reaches its maximum temperature (about 58 degrees C) the bias voltage falls to an unacceptable level and some distortion becomes apparent.

I assume there would be a method of calculating a different value of thermistor to fix this?