Rectifier Unit No5 for HRO.

[Colinaps]

Just tidying up the rough edges on the PSU which came with my HRO, the British kind that is an almost perfect cube, not the "doghouse" style.

I notice the HT is about 300V when the radio is drawing 75 odd mA. That would seem to be a bit enthusiastic to me, when the radio works just fine at anything over about 150V and 55mA. The mains supply here is pretty steady between 240 and 242V, the input taps are set according to the markings and there are no smells or scorch marks to suggest shorted turns on the primary.

Were all these rectifier units the same or was there a lower voltage version?

I feel the need to fit a dropper - maybe just a series 10W Zener, for want of imagination.. any ideas?

I'm in the throes of knocking up another psu for a similar set, aiming for maybe 200V (ish) B+ with a regulated line for the VFO.

Colin mm1aps.

[Herald1360]

A 47V 10W zener would knock off a useful bit without getting desperately steamy.

Are the heater voltages OK or are they high also? If they're high too going up a tap if there is one would do it wattlessly.

You could buck the rectifier HT feed using a couple of small 50V secondary transformers for a bit of fun.

[G6Tanuki]

I'm not familiar with the workings of the particular PSU you mention, but the idea of using a VR-tube and associated resistor across it and using the regulated voltage to feed the HF osc/BFO is entirely sane.

[In previous times I ran a HRO5 on the end of a student-halls-grade power supply. With lots of 3Kw room-heaters and kettles switching on and off voltage-regulation of the supply to the HF osc and BFO was essential]

[turretslug]

I'm not familiar with this unit but is it possible that a different rectifier with somewhat less forward voltage loss has been fitted? An indirectly heated substitute often has substantially less forward drop than a directly heated original, e.g. 5Z4 and 5Y3 and so on, also later versions of the same rectifier often improved in efficiency. 'Thirties smoothing chokes had appreciable winding resistance, about post-war this also seemed to reduce somewhat- could this component be different from original spec., or has someone bumped up the reservoir capacitor value unreasonably?

With that sort of headroom, I'd knock up a "standard" reference/comparator/series pass regulator in HV silicon (no objection to valves, though, apart from space and heater isolation).

[Colinaps]

You may have a point on the reservoir cap - it's a replacement so might be a bit over value. I shall have a quick look. There are two chokes and the rectifier's a 5z4.

[turretslug]

Musing further, and given that a comms. receiver with class A output won't go very low in minimum current (taking AGC action on signal valves into account), I'd try disconnecting the reservoir capacitor and see what the resultant on-load HT was- you might find that choke-input working gives good results with sensible HT and mains transformer will run cooler, with rectifier also having an easier life. A straightforward shunt-regulated supply for oscillators/mixer screen alone would then be the order of the day.

[Colinaps]

Nice one - taking the first e'lytic out of circuit drops the on-load voltage to about 200V with no audible hum. It was the right size - 8uF. I guess they were maybe made with very dirty mains in mind - mains input filter caps, two AF chokes, one RF choke and three electrolytics. If not expecting really iffy quality of supply, that would seem to be overkill in what really were austere times.

A silicon regulator sounds doable - a couple of resistors and a zener inside the radio would probably do - but I have a slight IF oscillation issue to fix first.

[turretslug]

That's encouraging- but I still find it odd that an apparently "as intended" PSU should have such a huge output discrepancy! From limited idle tinkering with this sort of thing, I might have expected a maximum of about 25-30V difference with rectifier types and a bit more with going from "sensible" to "ludicrous" reservoir values in pi-filtering but a 33% excess output is quite something. I wonder if this "British" unit was intended as choke-input but someone between design and factory-floor decided that reservoir had been omitted? Some odd things do happen. It might mean that quite a few HROs have been getting quite hot for quite a long time...

[turretslug]

My maths was also an "odd thing"... For "33%", read "50%"

[Colinaps]

Ah - WFTW volume3 has the info - it should have a nominal output of 250V, at 50mA (which barely seems adequate, but the transformer if quite a big lump, as are the chokes so maybe it's a typo) and the transformer tags should be 250-0-250.

I'll have another look tomorrow night.

[Alistair D]

The circuit shown here has some potential for a low heat sold state rectifier/regulator arrangement with a minimum of components. The operation of this circuit does not introduce any switching transients that could cause RF interference. There would still be some drift due to the temperature co-efficient of the zener. Some experimentation would be required.

https://www.vintage-radio.net/forum/...822#post643822

Scroll down for the circuit.

Al

[Colinaps]

Cue sound effect of rummaging for thyristors!

I think there would be room to include that in the existing PSU as well as whatever I put together myself..

I saw something in an old ARRL handbook about putting the chokes in the HT return line rather to remove most of the stress on the insulation therein, but that's probably another topic.

Colin.

[Colinaps]

And right enough, the transformer output is 250-0-250 on the nose..