RS 813-979 27V 2A power supply schematic?

[G6Tanuki]

Some may say this is not 'vintage', but I've had it for over 20 years and it's been really useful when working on various bits of mobile two-way and military radio-stuff that's designed to run off a 24/28V DC supply.

It's misbehaving - shutting-down at random and the series-pass transistor on the back getting really hot. I'm thinking that this could be a 'crowbar' circuit sensing overvoltage and so shorting the output, at the expense of the output transistor.

A circuit-diagram/schematic would be really helpful in my problem-determination.

[turretslug]

I've the 13.5V/4A version of this in the exact same box, yes, it's a handy and dependable beastie and in circuit thoroughness and implementation it's a cut above the nasty cheap rig-killer equivalents from the Far East. It's pretty much an "application note" 723-based job and does indeed have a 15V Zener/C106 thyristor crowbar set-up across the output. Hopefully, the twice volts/half current one you have is simply a scaled-component counterpart- I don't have the circuit for mine, but it's a classic case of picturing the circuit in the head from the component values and layout. Good luck with it.

[Biggles]

Could it be a random spike occurring and triggering the crowbar due either to the type of load you have on it, disconnection transients or even RF getting in somewhere, possibly because the decoupling on the crowbar sense has deteriated or failed altogether? We used to have a particularly nasty problem with the crowbar tripping on 30A 24V site power units caused by adjusting the volts output preset pot which turned out to be noisy and intermittent. Changing the pot from an old open construction carbon skeleton preset to a newer equivalent did the trick. It caused a lot of trouble on a lot of the units and until you adjusted the pot on routine PPMs you never knew whether you would get away with it or hear the fizzle and pop which meant a struggle with a monster weight 19" rack mount job.
Alan.

[turretslug]

Biggles raises two good points- the overvoltage crowbar is the classic simple circuit with the input to the gate of the thyristor decoupled with a small average-quality electrolytic which could quite conceivably have gone duff over what might well have been 30-40 years. Having praised the PSU as decent quality and not a rig-killer, etc., one design oversight I found, at least with this 13.5V version, was that the upper end of the feedback divider was a small preset variable, so any contact iffiness here could result in the output going high intermittently. I replaced this preset with a decent quality Bourns 2-turn wirewound (NOS work clearout!) front panel control, giving output adjustment over 10-15V for a more useful piece of test gear, and wired with one end to the wiper, so that any wiper intermittency won't make the output go higher than 15V. I also fitted one of those Chinese 0-100V/0-10A LED monitoring modules (ISTR £1.60 delivered!), it consumes no more than the original panel LED, and an earth continuity terminal.