Farnell 30/10S Power Supply - Help

[rovernut]

Hi
Some time ago I purchased second hand Farnell 30/10S power supply. I thought about using this as a good 24V bench power supply for my Clansman Radios

The unit I have has been modified to give a nominal 12V output which can be adjusted slightly.

On inspection the voltage control board has been modified, one resistor replace and another partially removed. I have tried with these back in place and the output was a fixed 21.8V and could not be adjusted.
Has anyone any experience of this power supply and is there a circuit diagram/manual available (I could not fond one on the internet)

Any help would be very much appreciated
Richard

[Refugee]

Have a look at the transformer wiring.
I am guessing that the secondary windings have been rewired as well as the voltage sensing resistors that you have already put back.

[mark_in_manc]

I can just about see a label 'feedback' on one of your photos. I have a Farnell supply - a variable 5amp 0-30v thing - which has external feedback terminals - the links (to the actual supply terminals, used for general applications where you don't need to worry about cabling losses to a distant load) were missing on mine and it behaved very oddly - I thought all sorts of things were wrong with it! Well, just a suggestion.

[Alistair D]

Not quite sure if this manual covers all the versions it claims to but it gave me all the information I needed to repair my B30/20.

As I went to attach the file I realised that at 5.5Mb it is way to big to post here. PM me an email address and send you a copy.

I will put reducing the file size on my to do list so it can be posted on the forum

Al

[Diabolical Artificer]

Those two variable resistors have been hacked about by the phantom twiddler, candidates for checking and replacement also looks like a resistor with one end floating = R34.

Andy.

[rovernut]

Hi
Thanks for the information and thoughts, I will have a closer look later today.
Alistair D - thanks for the offer of the manual - PM sent
Cheers
Richard

[Silicon]

Some of these Farnell power supplies have several secondary windings.
There is often a third winding to power the PCB circuit.

The connections can be varied to reduce or increase the amount of secondary voltage going to the rectifier.

If the power supply is running at a fixed voltage, the windings can be optimised to reduce the heat dissipated in the pass transistors.

The previous user may have also modified the winding connections.

[Refugee]

I said about the secondary windings back in post#2.

[rovernut]

Hi
Looks like just 1 secondary winding 36-0-36 (Transformer is a Cumbernauld TR288 - can't find any info of this)
The manual the Alistair sent me is helpful, the "conrol board" functions in a similar way but all the pin connections are different. Looks like I will have draw out the circuit diagram from the board.
Cheers
Richard

[Refugee]

The 36-0-36 winding will be power for the control board.
The windings you need to measure are the big ones with thick wires.
It looks like two are wired out of four.

[ronbryan]

The high current windings are available on terminals 1 to 8. What is the AC voltage between the thick black wires? The position of the black wires on the terminal posts may have been altered during the modification, as Refugee has said.

* post crossed with Refugee

Ron

[Alistair D]

Your power supply is not a B30-10. The B30 is a fully variable 0-30 volt supply with a range switch covering 0-6, 6-12 etc and a variable giving a range of 6V. I have sent you the wrong manual. Yours is a preset supply so B30-10 and 30-10s are different beasts.

As far as I can remember the manual for yours shows how to re-configure terminals 1-8 on the transformer and control board changes to cover various voltage ranges. Alas I do not have a manual for yours.

Sorry about that.

Al

[Alistair D]

I am pretty sure that the manual you are looking for is this one.

http://air-ministry.uk/farnellS.pdf

Al

[rovernut]

Thanks for the helpful suggestions…

The voltage across the output of the high current windings is 17.9VAC this equates to appx 12.5V when fully rectified, however the rectifier circuit appears to be a voltage doubler as 23.4VDC is seen across the large smoothing capacitor (link between 7&2, output between 1&4). Changing the tapping on the transformer; I can get 23.1VAC which full wave rectified with doubler will give appx 32.6VDC (not tried this in anger yet). I am a bit puzzled how the secondary high current windings are wired as 1&2 are independent, whilst 3-8 are all connected (I assume that they are in series, but haven’t quite worked which order they are in)
I started monitoring the control voltage on the base of one of the “stacked” 2N3055’s whilst doing this the voltage dropped to zero and since then the control board appears to be dead. With the control board removed, I have supplied he base of the “stacked” 2N3055’s from a bench power supply and this does control the output voltage as expected.
I will need to look at the control board a bit more closely, the power for this board is from the low current 36-0-36 tapping on the transformer
Will keep you posted
Cheers
Richard
Posted this before I had seen Alistair's post

[rovernut]

Originally Posted by Alistair D

I am pretty sure that the manual you are looking for is this one.

http://air-ministry.uk/farnellS.pdf

Al

Many thanks for finding that - that looks like the one,

[Diabolical Artificer]

The voltage across the output of the high current windings is 17.9VAC this equates to appx 12.5V when fully rectified

17.9v AC x 1.414 = 25v

To get the board up and running try powering it with just one 3055 or even better a similar Q out of the junk box, might save tears later.

Andy.

[rovernut]

Hi
Thanks for all your help
All now working (certainly helps having the manual - Thanks Alistair)
Cheers
Richard