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Old 30th Jul 2021, 5:14 pm   #1
unitelex
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Default Farnell E350/S27 HV PSU usefulness

I purchased one of these via Auction about 4 months ago.
It had been at a university in the Philippines, and probably had been there since it was made in the early 80’s.

I had been considering getting one of these for a while, my Variac and isolation transformer are useful but sometimes a DC source helps.
There are two versions, a 100mA and a 200mA version (/S27)
An EL34 is used for series pass regulation, with two EL34’s fitted to the 200mA variant. There are other threads on this site discussing the differences in more detail.
It has HT output variable to 350V and also some LV AC heater outputs

The 100mA version, capable of 30W is sufficient for powering smaller sets where the heaters are powered from a separate LV feed, however AC/DC sets need more current at high voltage for the heater chain.
I am often working on AC/DC sets or stereo sets which require more power so I went for the 200mA version.
The seller sent me a video of him powering up the unit (unloaded) before he sent it.
It came via DHL from the Philippines, I followed its route using the DHL tracker. It arrived on the specified date despite some bad weather at the time in Europe.
It was well packed.
Rather than plug it in right away, I opened the case to check internal condition. I usually do this for every used item I receive.
I found that one of the Mullard EL34 valves had lost its vacuum, white getter, although there was no sign of broken glass envelope.
I removed it and checked its heater for continuity. the heater is still intact despite the loss of vacuum. So I concluded it lost its vacuum during transportation from Philippines to my UK location, otherwise had it been like this in the Philippines then the filament would have burned out when it was powered up for their demo video.
I removed the remaining good Mullard EL34 and replaced both with a couple of Russian EL34’s I obtained previously from a forum member.
I had in mind to keep the Mullard one for audio use, and wondered how the Soviet one would perform in this application.
After some more checks, mains plug, fuse etc and smoothing cap check I powered up the unit. The voltages came up after the warmup period, about 10 to 15 seconds later. I found the current was low and not achieving anywhere near 200mA even with the rear mounted current limit pot set at maximum.
After some investigation it turned out that the current limit pot, a wire wound Colvern type was the cause. The back came off the pot without much persuasion and I found the wiper not making proper contact with the wire wound track. This was re-tensioned and degreased and the psu could then deliver the full 200mA at 350V Great! So the soviet EM34 seems to be perfectly capable of performing sufficiently in this application.

I have used the PSU more than I ever thought I would.
I use it for
1. Reforming caps
2. Powering HT on sets before the heaters are powered to check decoupling cap leakage or shorts etc.
3. Bring HT up slowly on sets while monitoring the current consumption
4. Conveniently powering 110V AC/DC sets
5. Checking power consumption and efficiency of modern equipment with switched mode power supplies, including GU10 LED lamps etc
6. Most usefully I use it for helping to diagnose hum problems. If there is still hum when the HT is fed from the regulated E350 then the cause is not the rectifier/choke/Smoothing capacitors in the set.

I have also used the 6.3V AC output (5A) to power an AC set via the heater wiring, without any external Mains voltage or HV DC supplied. Basically the 6.3V from the E350 powers the heaters directly and the sets transformer up converts this to nearly the full HT voltage. I can’t see any reason why this would cause any issues for the set provided the 6.3V is injected at or near the transformer’s 6.3v terminals. Maybe some forum member might have a concern, I would be interested to hear.

I have also found advantage in being able to disconnect heaters from the transformer and drive them from the E350. This allows the heaters to remain powered during work on the set eg component replacement while the HT is switched off. This saves time not having to wait for heaters to warm up and reduces stress on the filaments. This is particularly useful when replacing capacitors one by one as the set can be quickly powered and checked after each replacement.

So in summary I am very pleased with my acquisition of the E350/S27 and it gets daily use and brings some test advantages not possible with a Variac.

As a matter of historic interest, I have a family member who worked at Farnell Instruments in the 1980’s, he told me that this particular PSU was very popular and remained in production for a long time, partly due to its popularity and partly because the MD at the time had originally designed this PSU and he believed nobody dare suggest it be made obsolete.

Chris
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Old 31st Jul 2021, 7:49 am   #2
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Default Re: Farnell E350/S27 HV PSU usefulness

Blimey Chris, shipping can't have been cheap, still nice PSU. I use my HV PSU all the time for prototyping valve amps, testing valves, testing caps etc etc. Mine's a bit more flexible having a higher voltage & current spec - 500v @ 3A so can be used for low voltage high current work too, but I suppose it's what you work on on. 350v would be fine for radio work but for some valve amps a tad too low. Still, you could always use a fixed 100v PSU to "boost" voltage if needed.

Nice acquisition, Andy.
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Old 31st Jul 2021, 8:31 am   #3
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Default Re: Farnell E350/S27 HV PSU usefulness

I have one of these too and find it very useful. It’s the 2 EL34 version. It let off smoke and gave the familiar smell of the mains filter Wima blowing up but managed to find a replacement and all is well again.
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Old 31st Jul 2021, 8:36 am   #4
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Default Re: Farnell E350/S27 HV PSU usefulness

Something similar happened with my E350, but it was a capacitor across the secondary winding of the transformer which failed:-

https://www.vintage-radio.net/forum/...ad.php?t=42853
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Old 31st Jul 2021, 10:02 am   #5
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Default Re: Farnell E350/S27 HV PSU usefulness

Yes you're absolutely right Graham. My recollection was wrong. It was C7 across the secondary that let go on mine.
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Old 31st Jul 2021, 1:58 pm   #6
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Default Re: Farnell E350/S27 HV PSU usefulness

Only have the 100mA version, but still find it very useful, it's the older version with the black front panel.

There is also a 100mA version like yours at work, I've used it occasionally for repairing motor inverter boards that have had major faults on a bigger 300V 9A supply. Very useful to have a lower current supply for this, as board traces can get vaporised by the big supply if the faulty parts get missed.
The site it came from must have used it a lot more, as the markings on the controls are very worn.
I expect it will end up in a skip soon as they want to clear the storeroom it's in, I've already asked about trying to save stuff but they have refused to sell anything.

David
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Old 2nd Aug 2021, 2:59 pm   #7
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Default Re: Farnell E350/S27 HV PSU usefulness

An HV power supply can also be leveraged to test/diagnose mains SMPSs. Just feed ~350V DC to the 'live side' reservoir capacitor ( after disconnecting the DUT from the mains and ensuring any subsequently exposed mains plugs / IEC sockets etc are safely isolated ). Many SMPS will operate at lower voltage / higher current, maybe going as low as 100-150V DC.

As a disclaimer, in no way can this be considered a 'safe' procedure but at least you can limit the energy available to vaporise you and your components!

dc
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Old 2nd Aug 2021, 3:31 pm   #8
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Default Re: Farnell E350/S27 HV PSU usefulness

I can't see how doing that is any more dangerous than running the chopper circuit from the bridge rectifier of the SMPSU as normal with an isolating transformer on the mains input. It's certainly safer than doing it without an isolating transformer. Of course working on SMPSUs has its dangers, but then so do a lot of things.

And surely if you apply DC to the mains smoothing capacitor, the input rectifier diodes of the SMPSU would be reverse-biased so the mains input connector, etc will not be live. Probably better not to risk it but I wouldn't think that was a major hazard.

I once had to repair one of those infernal Boschert 2-stage SMPSUs and I was lucky in that I could make use of a very nice HV power supply where I was working at the time, It went up to 300V 5A or something equally lethal, but it had good current limiting. Of course I was careful as 300V at any current could be fatal, and I did blow another set of chopper transistors during the repair but at least I didn't melt any more PCB tracks or have transistors on the ceiling.
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Old 3rd Aug 2021, 1:49 am   #9
unitelex
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Default Re: Farnell E350/S27 HV PSU usefulness

Thanks for the interesting comments.
I meant to mention a couple of *minor* drawbacks I found with the E350:

1. The current limit pot is on the back of the unit so not very accessible, have left it at max current
2. The front panel output terminals are those push button wire gripping types, not so convenient with my 4mm test leads. This is easily resolved with short adapter leads of course.

I don't want to modify the unit, so decided to live with these minor drawbacks.

Last edited by unitelex; 3rd Aug 2021 at 1:49 am. Reason: typo
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Old 3rd Aug 2021, 7:21 am   #10
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Default Re: Farnell E350/S27 HV PSU usefulness

mine has standard 4mm sockets not seen one with those push connectors are they original ??
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Old 3rd Aug 2021, 12:22 pm   #11
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Default Re: Farnell E350/S27 HV PSU usefulness

And mine look to be even older, and has two different types of 4mm sockets. Your control knobs don't look original either, so perhaps somebody has updated the front panel at some point?

Handy piece of kit though, enjoy?

Cheers
Chris
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Old 3rd Aug 2021, 12:25 pm   #12
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Default Re: Farnell E350/S27 HV PSU usefulness

Crispy57's is identical to mine.
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Old 3rd Aug 2021, 1:39 pm   #13
unitelex
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Default Re: Farnell E350/S27 HV PSU usefulness

Right, yes checked various images online and it looks like mine has the wrong terminals.
I will be more than happy to replace the push grip terminals with something like the original type. Ideally I would also like to replace the knobs for the original type if I can find some
Thanks for the heads up!
Chris
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Old 3rd Aug 2021, 3:26 pm   #14
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Default Re: Farnell E350/S27 HV PSU usefulness

Mine has the push terminals as well. I get the impression that these were used near the end of production as a cost saving exercise and not a particularly safe one at that.

Al
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Old 3rd Aug 2021, 7:53 pm   #15
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Default Re: Farnell E350/S27 HV PSU usefulness

Mine has a push terminal for the ground connection and different sockets for the heater supplies.
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The OP's later one has knobs that match the style found on later supplies, such as the TOPS triple output PSU, TSV70 Mk2. & probably others.
The grey E350 at work has some different knobs to all those shown in this thread so far, can't take a picture as we are not allowed.

Maybe they used whatever they could get hold of at the time.

David
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Old 3rd Aug 2021, 8:15 pm   #16
unitelex
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Default Re: Farnell E350/S27 HV PSU usefulness

Quote:
Originally Posted by factory View Post
Mine has a push terminal for the ground connection and different sockets for the heater supplies.
Attachment 238772

The OP's later one has knobs that match the style found on later supplies, such as the TOPS triple output PSU, TSV70 Mk2. & probably others.
The grey E350 at work has some different knobs to all those shown in this thread so far, can't take a picture as we are not allowed.

Maybe they used whatever they could get hold of at the time.

David
Yes this makes sense, the unit I have has the university PO number on a sticker which suggests 1985 as the purchase date. So that would suggest a late unit as you say.
I'm still going to change the push grip terminals for some quality 4mm conventional terminals
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