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Old 10th Feb 2018, 10:44 pm   #1
Station X
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Default National HRO PSU. Capacitor values.

The attached circuit is taken from Wireless For The Warrior and is the only version I've come across showing C10 and C11.

One of these caps, probably C10, was definitely present in my PSU. It was connected to the open circuit smoothing choke and encapsulated in pitch along with the choke. 0.01uF and judging by its size probably 1000V. The other cap was probably in there too, as the choke can had more than the expected two wires emerging from the pitch, but they'd been snipped off.

Anyway I've dug out the contents of the choke can and fitted a new choke in there.

Has anyone any idea what the value of the other cap C11 may have been?

I'm aware that the smoothing caps are all 8uF.

Thanks.
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Old 10th Feb 2018, 11:18 pm   #2
ms660
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Default Re: National HRO PSU. Capacitor values.

If it's a 697 dog house then probably a 0.01uF @ 350 VDC, 1st part of table 111:

https://www.prismnet.com/~nielw/nat_ps.htm#tab3

I had one of the above but the mains transformer frizzled.

Lawrence.
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Old 11th Feb 2018, 3:15 am   #3
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Default Re: National HRO PSU. Capacitor values.

The transformer in my HRO PSU also frizzled about 35 years ago.
But the choke was OK. I don't recall finding the capacitors shown on the diagram between HT and heaters or to the mains input.
But I do remember my replacement transformer having only a 4-V heater winding and 250-0-250-V HT windings and room for enough turns of ordinary 3-A rated PVC-insulated wire to give around 6.7V (to allow for loses in the umbilical, as per the manual) and using silicon rectifiers in choke-input configuration to get the HT down to something reasonable without sweaty resistors.
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Old 11th Feb 2018, 3:23 am   #4
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Default Re: National HRO PSU. Capacitor values.

And a nip from the break-in terminals on the back when the other hand is on the case is not easily forgotten!
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Old 11th Feb 2018, 10:47 am   #5
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Default Re: National HRO PSU. Capacitor values.

In my view the transformer in the doghouse PSU was probably designed to operate at 60Hz, with no 'meat on the bone'. As a result they at best run hot and often frizzle.
I recall reading somewhere that at Bletchley Park the operators would put the PSU on the floor and warm their feet on it!
Andy
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Old 11th Feb 2018, 1:48 pm   #6
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Default Re: National HRO PSU. Capacitor values.

Station X

In the "National_power_units_instruction_manual" these are listed Mlca : 0.005mfd +/- 10%, 300 V DC W
Confusingly they are then stamped as "not used", certainly my "dog house" does not have them.
Confusingly the British version the "square one" does, plus a few other bits.

Hope that helps

TYJ
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Old 11th Feb 2018, 3:48 pm   #7
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Default Re: National HRO PSU. Capacitor values.

Quote:
Originally Posted by M0FYA Andy View Post
In my view the transformer in the doghouse PSU was probably designed to operate at 60Hz, with no 'meat on the bone'. As a result they at best run hot and often frizzle.
I recall reading somewhere that at Bletchley Park the operators would put the PSU on the floor and warm their feet on it!
Andy
Interesting point- I've encountered a few items of US-made professional kit having transformers with dual 115V primaries that despite apparently generously ratings would run fiercely hot and be really brown and crisped after a few years service. I think if I ever had an HRO power supply, I'd be tempted to fit a 12V 5W bulb or the like in series with the primary and use it as a softly-glowing "power on" indicator, just to take the edge off things. Naturally, generous heater wiring and a good sample of rectifier would minimise secondary-side compromise.

4.7nF 1kV ceramic capacitors don't cost too much, also whilst "Y" rated 4.7nF ceramic components aren't necessary here, they would have the voltage margin and are pretty plentiful,

Colin
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Old 11th Feb 2018, 4:05 pm   #8
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Default Re: National HRO PSU. Capacitor values.

Thanks for all you replies. I had no idea that my question would generate so much interest.

I had been looking for an EMER or W.H.Y. with a parts list, but had no joy. You've come up with the goods. Thanks.

My dog kennel unit is actually labelled 5886 and rated at 180VDC 70mA. It delivers this fine into a dummy load, despite the rectifier valve being low, but I haven't checked the transformer temperature over a long period. There is no mains voltage selector switch and there is only one primary winding.

As I said, one or perhaps two, of the caps in question were inside the pitch filled choke can. I had to mechanically remove the pitch to get the o/c choke out. The cap I recovered intact was 0.01uF and 32mm square by 8mm thick, which made me think it was a high voltage type.
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Old 11th Feb 2018, 4:22 pm   #9
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Default Re: National HRO PSU. Capacitor values.

Looking through my component stocks I found I had some 10nF 2kV ceramics, so I'll probably fit those.
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Old 16th Feb 2018, 10:06 pm   #10
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Default Re: National HRO PSU. Capacitor values.

A couple of things to keep in mind:
The PSU, if USA made, likely only used enough iron core to make it run acceptable hot here. When you drop to 50 Cy they xformer will get much hotter.
The other thing is that the line voltage here at the time was 110VAC. Likely you will have to reduce input voltage somewhat to compensate for the higher line voltages.
Many old radios I see here have fried the power transformers, mainly due to the higher line voltages, and the person who installs the wrong rectifier.
If you need a rectifier, just let me know. I may have a spare or 2. Yours for the postage.
I have not yet gotten around to restoring my HRO.
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