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Old 8th Jan 2022, 12:12 pm   #1
geeoboeh2s
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Default Elpac ELV Power Supply

I originally placed this thread in the wanted section but that wasn't the appropriate place.

I have a fully regulated variable voltage Elpac ELV125 bench power supply. This has two meters on the front panel, 0-30Vdc and 0-10Adc. A substantial heatsink is mounted on the rear panel containing 4 off 2N3055 power transistors, one acting as driver for the remainder in addition to two power diodes feeding a choke followed by a 22,000uF capacitor.
Off load the transformer provides 23-0-23Vac. In addition to the full wave rectifier circuit a second half wave supply is provided to a 6,800uF capacitor on the control board.

The pictures probably provide a better explanation. The terminals on the front panel are not original.

I acquired this unit about 20 years ago and it likely that it was modified by a previous owner. When first used the output voltage was variable over the range 6-16Vdc.using the front panel control.
Regulation is provided by an LM305AH integrated circuit and a 2N2905A transistor.

I last used the supply about 5 years ago and it worked within the constraints above. A couple of days ago I had an urgent need for a supply of around 15Vdc at 5-6A. It is my standard practice to reform electrolytic capacitors that have not been used for some time. On this occasion I broke the rule although I don't think that this the cause of what happened next.
Power was applied with no load connected and immediately magic smoke appeared. Power was removed within a few seconds.
On examination a resistor on the control board was seen to be badly burnt and had scorched the board (see centre picture). The resistor measured 2.7 ohms but the marked value was not readable.
In order to repair the unit I will need to determine the circuit, not a difficult task but laborious.

Can anyone help with a manual or circuit diagram and control board layout.
I would also like to know the original specification. Markings inside the unit say ELV125 12. A google search suggests that the number 12 implies 12 volts at 12 amps, but this is not consistent with the front panel meters.

Comments appreciated.

Chris
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Old 8th Jan 2022, 12:53 pm   #2
PETERg0rsq
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Default Re: Elpac ELV Power Supply

I do not know if it is just the pictures, but that is one of the worst assembled PSU's I have seen.

I presume the previouse owner has gone through it, as it couldnt have left the factory like that!

Two options in my opinion:-

Work out the current circuit, and fully restore what you have, replacing all necessary components, and re-assemble much more professional and safe assembly.

Completely re-build the regulator using modern components and upgrading its circuit (improve function and safety), only keeping transformers and meters.

Third solution is of course to replace it, but thats not what this discussion is about
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Old 8th Jan 2022, 12:59 pm   #3
M0FYA Andy
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Default Re: Elpac ELV Power Supply

Looking at picture 3, the edges of the holes for the meters look rather rough, have the meters been added at some time?

Andy
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Old 8th Jan 2022, 1:32 pm   #4
geeoboeh2s
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Default Re: Elpac ELV Power Supply

Thank you both for your replies.

I agree the front panel appears a modification. Originally it was probably a 12Vdc, 12Adc stabilisd supply.

A quick calculation suggests that I will probably achieve around 18Vdc on the large capacitor from a choke input filter so a 15Vdc output should be available with some component changes. If this is to be variable then the lower voltage may have to be limited to 12Vdc to keep the dissipation in the series pass transistors to a safe limit.

I would still like a circuit diagram and layout of the control board for the original unit.

Chris
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Old 8th Jan 2022, 3:52 pm   #5
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Default Re: Elpac ELV Power Supply

The mains transformer seems to be based on rather coarse laminations. It looks to be of very low quality and the big electrolytic will be getting iffy by now.I'm assuming one of those TO-99 cans is an LM723? The 2N3055 could be genuine, but considering the care over the rest of it, they may be early fakes.

It's not of commercial quality. Home made? id so by someone more in the know about metalwork than electronics.

That transformer could have come from a battery charger.

David
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Old 8th Jan 2022, 4:14 pm   #6
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Default Re: Elpac ELV Power Supply

Elpac seem to make basic PSU modules intended to fit inside equipment. So maybe it started out as a much simpler PSU and has been added to to create an adjustable and regulated supply.

EDIT: an alpaca elv 125-5 seems to be a 5v version. Clearly meant for mounting inside something. I wonder if it's as said, the front panel has replaced the original input output panel.

Last edited by duncanlowe; 8th Jan 2022 at 4:20 pm.
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Old 10th Jan 2022, 12:31 pm   #7
geeoboeh2s
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Default Re: Elpac ELV Power Supply

Duncan,

I think that you have it completely correct, an original Elpac ELV125 12 fixed 12Vdc, 12Adc modified for variable voltage operation.
There will be more than 12V available as 2 of the terminals on the heatsink assembly are almost certainly for remote sensing.

David,

Both big electrolytics (one is not visible in the pictures) measure very poor esr and will need to be replaced, at a cost.
The TO99 can is a LM305AH somewhat earlier than the 723 series but if it is working I am quite happy to leave it.
Agreed the transformer does look a bit marginal but as it is feeding a choke input filter both it and the associated components are less stressed.
I will check the 2N3055s and if neccessary will replace with 2N6258s.

Chris
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Old 10th Jan 2022, 12:57 pm   #8
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Default Re: Elpac ELV Power Supply

I don't like the look of that white axial capacitor on the PCB.

It remind me of paper or low value electrolytics which tend to be unreliable.

The mains transformer may not be as bad as it looks.
Instead of assembling the laminations E-I-E-I etc some manufacturers assemble them EE-II-EE-II etc which gives a rougher appearance.

Last edited by Silicon; 10th Jan 2022 at 1:04 pm. Reason: more info
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Old 10th Jan 2022, 1:37 pm   #9
geeoboeh2s
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Default Re: Elpac ELV Power Supply

Silicon,

The 2000uF, 15V white capacitor is definately to be replaced.

I have no experience of transformer design so I can't comment on the construction. However Elpac seems to have been well known in the USA and quite high prices are being asked for some of their items on ebay.com

I will have to check for a date code on the large electrolytics but I am guessing the original unit is from the late seventies, early eighties, and could well have been aimed at the computer market.

Chris
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