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Old 6th Apr 2023, 10:38 pm   #1
Philips210
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Default Farnell L30A Power Supply

Hello once again.

About ten years ago I picked up a Farnell L30A bench power supply from a local car boot sale. I've only recently got around to having a look at it.
Half the problem with Farnell power supplies is finding the right circuit diagram as there were many different versions of power supplies but with only a few generic diagrams.

The L30A is specifically 0 to 50V and rated at 500mA. Please see the attached pics.
It's in a fair condition considering that it's probably been stored in damp conditions. The front panel and top need come cosmetic attention but it is generally a good prospect for repair and should clean up well.

On opening it, the first thing I found was the quite strong smell of PVC from the wiring but it looks to be in excellent order and is still very flexible. I've noticed the same smell on mains leads on a Telequipment 'scope and on my Philips TVette. I think there must be some type of degradation going on as I don't really detect anything similar in other equipment wiring. I'm thinking of taking the drastic step of replacing the wiring, which won't be a quick job.

The overall construction looks really good with decent accesibility. The Plessey 1000uF 100V reservoir capacitor has a date code of 6946. I tried reforming it with mixed results, although the leakage current is respectable its capacitance is about 870uF so a little on the low side. I expect it's dried out somewhat so I'll probably replace it.

The control circuit PCB looks to have had some attention in the past. The main concern is a diode has been added, cyan encircled, in pic 4. By the look at the way it has been fitted this is unlikely to have been done by Farnell.
Also R12, orange encircled, which is shown in the circuit as S.O.T which I assume is Select On Test. It's a carbon composition type which is a surprise as the others are either metal or carbon film types. There doesn't seem to be any mention on selecting the value of R12 in the service information.
Also zener diodes Z1 and Z2 look to have been replaced.
I had intended to draw the circuit diagram specific to the L30A but as the unit has been got at it won't be straightforward.

What I'd like to ask is if any member owns the same model power supply if they can confirm the following component values?

The value of R12 (it's 3M3 on my board),

Zener diode Z1,

Zener diode Z2.

Also, whether the encircled diode is fitted to your panel?


Regards,
Symon
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Old 6th Apr 2023, 10:49 pm   #2
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Default Re: Farnell L30A Power Supply

Quote:
Originally Posted by Philips210 View Post

What I'd like to ask is if any member owns the same model power supply if they can confirm the following component values?

The value of R12 (it's 3M3 on my board),


Regards,
Symon
Symon,

just a quick note on R12. The silk screen says "R12 SOT". Generally "SOT" means "select on test". So the value will be chosen from a given range during manufacture of the psu.

You should thus assume that the value you have is correct for your particular psu. If someone else comes up with some other value, it doesn't mean you should necessarily change yours to their value.

If you find a manual, that may give a procedure for selecting R12, which is probably related to the gain of one of the transistors.


Richard
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Old 6th Apr 2023, 11:19 pm   #3
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Default Re: Farnell L30A Power Supply

The circuit diagrams are given elsewhere on the forum:

https://www.vintage-radio.net/forum/...d.php?t=177887

Steve.
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Old 6th Apr 2023, 11:27 pm   #4
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Default Re: Farnell L30A Power Supply

Thanks for your reply Richard.

That's a good point that you make regarding R12. I agree that it will vary from one model to another. It's just that the way it has been fitted and it being a composition type leads me to think it's been altered by someone. Having an idea from other working units would be helpful at least as a guide.
I doubt that Farnell would have fitted R12 in that way.
I'm surprised that Farnell doesn't mention a typical range that R12 should lie between.

Regards,
Symon
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Old 6th Apr 2023, 11:33 pm   #5
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Default Re: Farnell L30A Power Supply

Quote:
Originally Posted by fetteler View Post
The circuit diagrams are given elsewhere on the forum:

https://www.vintage-radio.net/forum/...d.php?t=177887

Steve.
Thanks for the link Steve, I already have those circuits. The actual circuit for the L30A is not shown in those manuals. It would have been helpful if Farnell provided the exact circuit for each model variation rather than a generic diagram. One obvious difference is the mains transformer itself. In my L30A the main secondary winding is 0-64V but not shown in any of their circuits.
Thanks anyway for trying to help.

Regards,
Symon
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Old 7th Apr 2023, 12:15 am   #6
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Default Re: Farnell L30A Power Supply

You've probably already found this but just in case.

Alan
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Old 7th Apr 2023, 12:27 am   #7
Philips210
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Default Re: Farnell L30A Power Supply

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Originally Posted by ajgriff View Post
You've probably already found this but just in case.

Alan
Hello Alan.

I didn't actually have that particular one and I've noticed in my poor copy D10 is shown reversed. Your linked diagram agrees with my board. I'll have a good look at the circuit you provided. Thanks very much.

Regards,
Symon
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Old 8th May 2023, 3:16 pm   #8
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Default Re: Farnell L30A Power Supply

Hello again.

I've now completed the repairs to my power supply which was in poor cosmetic condition with a corroded front panel and case. A number of components needed replacing and I've also replaced all of the wiring including a new mains lead due to a strong smell from the PVC insulation. Attached are a few pics of before and after.

The front panel was going to be tricky to repair and it would have been awkward applying transfers to obtain a good finish. I decided to remove all switches and sockets and gave the bare panel a good clean. It was then scanned at 600dpi resolution in a flat bed scanner. I then 'repaired' the lettering using MS Paint. The repaired artwork was then printed on plain paper. It was a matter of trial and error to obtain the exact size whilst maintaining the correct aspect ratio. A covering of clear self adhesive plastic film was then applied to the paper legend. The various cut outs were then carefully done with a scalpel. In the case of the wirewound pots, enough clearance was made for the hex nuts so that when the nut is tightened it safely clears the new label. The collet knobs, once fitted, hide the gap in the paper legend. Once the cutouts were correct, the new paper panel was stuck to the metal panel by double sided adhesive tape. The overall result is not perfect but quite reasonable.

The top of the cover to the power supply was in poor order with bubbled paint though no signs of rust. It looks like the steel is plated with zinc or cadmium. I had to scrape off the old paint for a reasonable repair so did this in the garden and wearing a mask just in case it was cadmium plated.
I found a very similar colour paint to the original, it is Rustins ASAP Grey which is slightly darker than the original. I applied three coats which goes on very well with a decent soft brush.

Internally, the wiring was completely replaced which thankfully removed the horrible plastic smell.

The rocker switches on the front panel were dismantled and cleaned and tested for operation prior to reassembly. Similarly, the wirewound pots were cleaned to ensure reliable operation.
I managed to reform the 1000uF 100V reservoir capacitor resulting in a very low leakage current so was fit for service. I changed the other two electrolytic caps due to doubtful leakage.
A few other parts on the PCB were replaced including VT4, BC182L which was low gain.
The small grommet protecting the three wires going to the 2N3055 power transistors was perished so was replaced. The mystery extra diode was removed from the PCB. I had no problems setting up the four presets as per Farnell's instruction in the user manual. I was quite surprised how little adjustment was needed and the good as ever Sifam meter was really accurate.
Overall I'm pleased to have overhauled this decent quality power supply which should hopefully give many more years of trouble free service.
A few more pics to follow.

Regards,
Symon
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Old 8th May 2023, 3:20 pm   #9
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Default Re: Farnell L30A Power Supply

A few more pics.

It's been a good project!

Regards,
Symon
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Old 8th May 2023, 4:13 pm   #10
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Default Re: Farnell L30A Power Supply

What a nice job - there's some time in that!
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Old 8th May 2023, 4:17 pm   #11
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Default Re: Farnell L30A Power Supply

Thanks Mark, it's taken about a month on and off between domestic duties etc.

Regards,
Symon
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Old 8th May 2023, 5:23 pm   #12
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Default Re: Farnell L30A Power Supply

I forgot to mention that I'll draw a clear copy of the circuit diagram in due course.

Regards,
Symon
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Old 8th May 2023, 7:35 pm   #13
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Default Re: Farnell L30A Power Supply

Very nice Symon, and I can see its big brother lurking in the background in a couple of your photos. Grand old work-horses, I've used mine regularly for many years with no hint of a problem.

Cheers
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Old 8th May 2023, 7:58 pm   #14
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Default Re: Farnell L30A Power Supply

Thanks Chris, yes they're nice reliable power supplies. The only problems I've had is twitchy pots and the the meter switch becoming intermittent. Infact only yesterday, I tackled a problem meter switch on my LT30-1. These are made by C&K and often the contacts become tarnished over time.

I have successfully stripped these switches and polished the metal rocker plates and six fixed contacts. This makes it reliable. Unfortunately the switch housing cracked as I was crimping back the four metal tabs. The switch was ruined. In my box of switches I found a normal C&K DPDT toggle switch which had the same series number as the Farnell paddle type switch. I stripped both switches and transferred the plastic body over to the original switch. Despite the same series number the toggle switch was different to the original but would nevertheless fit the metal housing. Reassembling it and soldering the wires back it thankfully worked OK. It's easy enough to do as removing the three screws under the power supply, then the whole front section can be lowered for easy access. That's what I like about Farnell power supplies, they're great for servicing.

Regards,
Symon
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Old 9th May 2023, 5:52 pm   #15
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Default Re: Farnell L30A Power Supply

Quote:
Originally Posted by Philips210 View Post
The mystery extra diode was removed from the PCB.
The diode was probably fitted as a later modification either to provide reverse voltage protection or protection against a voltage spike. This thread is worth reading: https://www.vintage-radio.net/forum/...d.php?t=139936.

Alan

Last edited by ajgriff; 9th May 2023 at 6:18 pm.
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Old 9th May 2023, 6:56 pm   #16
Philips210
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Default Re: Farnell L30A Power Supply

Hello Alan.

I had wondered that. I noticed later versions had an extra diode fitted. I think damage can occur if say a battery or large value charged electrolytic is connected across the output and the mains to the PSU is switched off.

I had compared the updated circuit with my added diode and it didn't agree with what I expected. It looked to be an unofficial mod, perhaps incorrectly done. When I get the time I'm going to draw a clear copy of the circuit with the correct component values for this 50V 500mA supply and try to see what the unofficial diode was for.
I think it would be useful to add the extra diode in its correct position something that I will look into.

Thanks for the link, I hadn't seen that thread.

Regards,
Symon
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Old 9th May 2023, 8:26 pm   #17
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Default Re: Farnell L30A Power Supply

One other useful addition I'm thinking about would be to add an LED and associated circuit to indicate the onset of current limiting as in later models.

Regards,
Symon
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