Topward 6303A repair

[mrpelikan]

About time I contributed something back to this forum.

Last week I was given a Topward 6303A (has moving-coil meters for output current & voltage) in non-working condition. The slave channel only worked in parallel mode. In series or individual mode the slave had no output.
The circuitry for mode switching is complicated but for good reason - when going from one mode to another a break-before-make is necessary to avoid a momentary short-circuit on the three relays immediately behind front panel. These relays switch the power supply outputs to independent, series or parallel. Fortunately its less likely there's a problem in this section as removing these boards from behind the front panel is a real mission.

A previous contributor kindly posted a pdf of the 6303D and its mostly relevant for the 6303A. The 6303A doesn't have the 7805-regulated supplies at top of the master & slave drawings (are probably for the LED displays) and those areas are unpopulated on the 6303A boards. Also control of the transformer secondary relays L1, L2, L6, L7 is different on my unit.
The screen printing on all boards had different component numbering to the 6303D. Also the white screen printing on yellowish laminate can be difficult to read if lighting isn't just right...

For anyone experiencing the same fault, on the slave board check Q23 and Q24. Q23 is in the voltage control circuit, Q24 in the current limiting circuit.
Q23 (NPN, type 2SC1815 in my unit) collector-emitter was around 3kohm
Q24 (PNP, 2SA1015)  collector-emitter was less than 0.5 ohm

Unfortunately these devices seem to be less available now, especially in single-digit quantities. Also the pinout of both has base (left) and collector (centre) transposed compared with many potential equivalents. I ended up using BC547 & BC557 respectively from my junk box and carefully "crossed their legs" on base & collector. On power up the unit behaved as it should and was satisfying to know that a supply in otherwise good condition has been kept out of landfill.

Hope this helps someone somewhere.