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Old 5th Nov 2019, 6:37 pm   #1
G3PIJpeter
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Default Roband Electronics M34 PSU

A long shot - there's nothing on the Web although Roband is still going after 60 years.

I purchased a very grubby rack panel at the recent VMARS auction and discovered inside it a Roband M34 stabilised PSU. Hefty Partridge transformer, block paper capacitors, GZ34 rectifier, EL34 series regulator plus a couple of 12AX7s and an 85A2.

Does anyone have a circuit diagram to share or more accurate details about the outputs? The two preset pots have only a limited effect on the output voltage, varying it from around 200 to 220 volts. The unstabilised voltage at the smoothing capacitors is a whopping 540 volts, although that measurement was taken under no-load conditions.

Photos can be provided if that will help.

Peter G3PIJ
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Old 5th Nov 2019, 7:54 pm   #2
HamishBoxer
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Default Re: Roband Electronics M34 PSU

If no circuit available I would check the reses and caps carefully around the control circuitry.You may well find high reses have gone even higher.
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Old 6th Nov 2019, 3:36 pm   #3
G3PIJpeter
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Default Re: Roband Electronics M34 PSU

The PSU works OK so none of the passive components can be that far out. My main question is about the specification: 200-210 VDC out - but at what maximum current?

The EL34 is used as a series-pass element and its maximum anode current is given as 91 mA. From this, is it OK to assume that the PSU is rated at a nominal 100 mA? The EL34 maximum anode dissipation is 25 W: it can drop 250 V at 100 mA before exceeding this dissipation, which allows the unregulated voltage to be 220 + 250 = 450 V. I have measured 540 V unregulated but that was at zero load. . . . . . perhaps I am answering my own question here!

It would still be nice to have sight of a circuit diagram to save tracing it out. Then I can replace the EL34 with an S11E12 and sell the EL34 to the audio brigade. Also - why does it have 2 x 12AX7 instead of the much simpler EF91 or similar? None of the standard circuits that I have seen uses a pair of double triodes.

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Old 7th Nov 2019, 11:56 am   #4
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Arrow Re: Roband Electronics M34 PSU

Quote:
Originally Posted by G3PIJpeter View Post
Why does it have 2 x 12AX7 instead of the much simpler EF91 or similar? None of the standard circuits that I have seen uses a pair of double triodes.
It might use one - or both - of those twin triodes in the cascode config. A cct. diag. will confirm - but I understand that is not available. I do realise that tracing out the cct. 'by inspection' is not exactly an easy task, but sometimes needs must, to quote a phrase.

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