NJD PM100 amp module

[jbmdisco]

Evening,
i have come across this old amp module of which i used to have 2 of. I have lost the notes that told me what the connections where. Hoping that someone can enlighten me.
Marketed as NJD PM-100 from about 1988.
I have come across this module before in another amp manufactured by another company so can only assume that it was quite a common fitment.

Have attached a couple of photos one of the top and one of the track-side.

I know that the connections where soldered onto the track-side (power,input+output)

[nickb]

Here my take on it. It would be nice if someone else could double check. Before you hook the speaker up make sure the speaker out DCV is within 100mV or so of 0v ( AC CT/SPKR 0v). Also, it looks like a bit of track might have been cut away near the 0v track and AC In - it would be good to know what is going on there.

EDIT: Added i/p connections. Without working out the schematic, I'm not so sure about these. I think the cut track was to allow the input ground reference to come from the preamp ( for noise reasons probably).

[MarkusBass]

Yes, I agree. The input stage ground was cut from the amp's ground to avoid hum. The ground for the input stage is provided from the preamp, or maybe rather from the power supply.

Mark

[mozwell]

There still needs to be a 0V reference, maybe solder a 1k across the cut track

[jbmdisco]

Thanks for replies guys. I managed to get a hold of the other amp module i previously fitted. Attached a picture of the connections on solder side.

I have fitted the amp, in question, to a VOX AGA150 accoustic amp. On the previously fitted board I had soldered a wire over the broken track which works really well (it is used in a keyboard amp). I have a slight problem with it now fitted to the VOX. When played through I get distortion (which has been traced to the module). If I fit a 1k resistor across the missing track the distortion is lessened but i get a hum. I have tried 1.5k and 0.5k resistors to monitor the results. Larger value gives no distortion but a louder hum and lesser value gives distortion but no hum.

The original fault with the AGA was that the power/amp smps board had fried and the replacement from vox did a similar thing once fitted (only took them 6 months to finally supply and after returning blown board was told it would be at least another 6 months!!!) I have since fitted a seperate power supply for the pre-amp (a small toroidal) and a larger toroidal for the amp side (both are well away from each other and due to where the power amp had to be fitted etc 2 power supplies where ideal as 2 different voltages are required)

When the distortion was first apparent I thought that maybe 25vac was a little bit shy for the amp so tried a 40vac transformer instead but this was not the case. I have also swapped the power caps. Now I am thinking that the power resistors (22ohm) may be the problem and are either breaking down or of the wrong value. I never had the original transformer from the NJD unit so the correct voltage is a mystery although amps of a similar age and power I have had usually are north of 50vac.

Am sure I am missing something so simple here. Any Ideas ???

[nickb]

My guess is that the DC conditions are wrong, quite possibly something to do with the input grounding. Since the input stage was uncertain I would proceed by drawing a schematic from the board.

Also I assume you have the CT of the transformer connected to OV.
Is the transformer 20-02- or 40-0-40?