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

NARC Training 2019

Training is very important to us as we realise that is how new people can come into the hobby and attain their Foundation, Intermediate and Advance Amateur Radio licenses.

The Syllabus and Exams for all levels is changing significantly from August 2019 and so like many other clubs we need to take a break from running courses from August to give us time to research, prepare and plan courses based on all three levels for 2020. However we will be running three courses before August; A Foundation course in April, an Intermediate course from March - May and another Foundation course in June - July. So if you would like to take either of these exams with NARC in 2019 you need to book your place as soon as possible by emailing Exam Secretary David G7URP: This email address is being protected from spambots. You need JavaScript enabled to view it.