[Colourstar]

Out of curiosity I thought I would spend some time on this tatty and neglected record player. It dates from around 1964 and has a transistor amp comprising two OC75s acting as pre-amp and driver for the AC128 push-pull output stage.

The amp works to an extent but is distorted. Taking some voltages provided readings that were all over the place. The first OC75 (pre-amp) had in the past been replaced by a BC154, along with two outsize resistors, all tacked onto the print side of the pcb. I removed all of these and remounted the transistor correctly, along with two more suitably sized resistors. I've changed the electrolytics, which I thought would be past their best, with the exception of C5, for which I don't have a replacement to hand.

Mike's excellent KB Museum website has a copy of the official manual for the player and the link below is provided with his kind permission, although if this flouts any forum rules then of course it should be removed.

file:///C:/Users/760/Downloads/STC_PC1_2_3_4_crts.pdf

The 24v supply reads a shade high at 28v, after which the transistor voltages are all over the place. I've put the actual reading in brackets after the figures from the service info:

TX1 Pre-amp OC75 (replaced with a BC154)
C 11.6v (5.3v)
B 4.9v (2.0v)
E 4.8v (4.1v)

TX2 Driver OC75
C 14.2v (6.5v)
B 2.3v (0.8v)
E 2.2v (0.7v)

And finally the output pair, for which no voltages are noted on the circuit

TX3 Output AC128
C (28.4v)
B (13.2v)
E (13.8v)

TX4 Output AC128
C 12v? (13.8v)
B (0.1v)
E (0v)

I'm not overly familiar with transistor circuits, beyond knowing that the base should be about 0.6v higher than the emitter, which suggests that TX2 might show some promise. Beyond that I'm a little lost, so any help would be appreciated!

Steve