Tony G0MQG Tips

Repairs to a BNOS power supply

A club member brought a power supply for repair with no output. (a Bnos) a search on the internet provided a schematic for a smaller version which uses the same control circuitry.

A check with a multimeter showed that there was a short circuit across the output and the schematic showed that it uses “crowbar” over voltage protection so I removed the crowbar Thyristor, the short circuit disappeared and the output voltage reappeared though slightly high at over 15 volts but was adjustable.

Clearly the control i.c. was ok as was the output transistors so I obtained and fitted a replacement driver transistor (a TIP121 darlington) this got the output correct and adjustable to 13.8 volts leaving just the crowbar circuit to sort out.

Question:- how to test the crowbar circuit without placing a short across the output.

Answer :- put something in series with the thyristor anode to limit the current, I used a small 12 volt bulb so that I had a visual indication of the thyristor firing.

I replaced the thyristor with a BT152 but although the short circuit had disappeared the thyristor wouldn’t fire even with the output voltage set at maximum (about 16.5 volts).

The crowbar circuit is controlled by an MC3423 crowbar driver IC. which I replaced and now the thyristor fired fine at 16.1 volts which seemed to fit nicely with the component values fitted by the manufacturer.

Now just need to load test it, I use high power rheostats for this and found that all was ok (output voltage 13.7 volts at 18 amps load).

So I refitted the cover and passed it back to the owner

Tony g0mqg

Christmas 2017