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Old 5th Sep 2017, 10:39 pm   #1
MrBungle
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Default Are Farnell L30 and TOPS units reasonable power supplies?

My old Pl310 is misbehaving and requires a new voltage pot as it's getting dicky and the newer one is missing two LED segments now. I've kind of had enough of them now. Also not a big fan of digital metering now I look back. I spent the afternoon cooing over new Keysight supplies and then came to my senses when I saw the price tag of a couple of U8001 units.

Decided to look at something simpler from a more maintainable era. Farnell supplies seem to be reasonable on paper. The question is, are they any good in reality?

Ultimately I'd probably go for a TOPS unit for dual rail/5v typical stuff and a single L30. Op amps and analogue metering are the limit of complexity I'd like on the bench now. Then I need a 12-15v 20A ish unit.

I fully intend to refurb them on receipt rather than expect something perfect out of the box.

Primarily some questions:

1. Are they reasonably reliable with slightly more standard parts then the PL supplies?
2. Do they have good transient response and overshoot characteristics?
3. Any bad revisions/models I need to avoid?
4. Are the meters reasonable quality?

Any input appreciated.

I really would grab an HP6236B but the things are impossibly rare in the UK.
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Old 10th Sep 2017, 4:32 pm   #2
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

Hi Mr Bungle,

I've repaired dozens of L30 and their ilk, and can highly recommend them for general use - especially the dual-output versions, like the LT30/2. These like most Farnell PSUs use Sifam meter movements, quite top-notch! Some of the older supplies are also worth laying hands on, but older (60's and 70's) had germanium transistors throughout, and while substitutes are relatively easy to find, my experience is that faulty transistors tend to have further reaching damage consequences.
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Old 11th Sep 2017, 11:54 am   #3
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

I remember that in the 1970's Farnell ran an advert demonstrating the reliability of their power supplies consisting of a photo of one of their early models of L30 (plain screw terminals without integral 4mm sockets), and a caption saying something like "Our biggest competitor".
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Old 11th Sep 2017, 12:19 pm   #4
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

Excellent. Thanks both.

I actually managed to get my hands on an unmodified, unrepaired, original E30/2 since I posted that and it's rather nice. Made from surprisingly good parts (sprague powerlytic filter cap and sifam meter). I was quite surprise when I set it to 5v and checked the output on my DMM and it was 5.00v.

I'll grab a matching L30 for circuit development as a current limit will be useful there (keeps the magic smoke in!)

They are a lot smaller than my PL supplies which is a big win.
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Old 11th Sep 2017, 12:27 pm   #5
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

I have an L30/T which seems to be reliable although I'm slightly surprised when checking that it seems quite old!
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Old 11th Sep 2017, 12:41 pm   #6
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

The Farnell L30 is better built than the Thurlby equivalents, using better quality casework that is easier to work on, but they are much the same electrically, so can suffer from the same sorts of problems. Performance is very similar. No digital metering to worry about, I suppose.

The E series is good, but no variable current limiting. Also - importantly - they are vulnerable to damage caused by externally applied voltages. I've modified all mine (2 additional diodes) which seems to remove the problem. Details will be on the forum somewhere.

A friend of mine discovered this the hard way when using one of them to charge a battery. The power went, the battery damaged the supply, and when the mains came back the full rectified DC from the transformer/rectifier/smoother combination was connected straight to the battery with the inevitable result. Luckily, this was outdoors!
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Old 11th Sep 2017, 12:58 pm   #7
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

Ah yes I've built a power supply that blew up in exactly the same way

I've actually got a Farnell order (the irony) arriving today that contains a few big silicon rectifiers and a load of replacement capacitors for it. I had noticed the lack of protection having been fearful of that failure mode. The only thing that isn't being replaced is the main filter cap as it is a nice sprague one and I've got older ones that are all good of similar age.

The thing I wasn't too happy about was I found a RIFA X class capacitor between -V and Earth that is cracked. I'll replace that with a Y1 from the Farnell order. Also an 18v zener is literally on the board and has roasted it a bit so I'll replace that with another one mounted 5mm or so off the board.
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Old 11th Sep 2017, 1:53 pm   #8
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

Why not re-case the old ones and use analogue meters? A fun project, for me anyway.
 
Old 13th Sep 2017, 7:14 pm   #9
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

Quote:
Originally Posted by mhennessy View Post

A friend of mine discovered this the hard way when using one of them to charge a battery. The power went, the battery damaged the supply, and when the mains came back the full rectified DC from the transformer/rectifier/smoother combination was connected straight to the battery with the inevitable result. Luckily, this was outdoors!
I saw a similar failure. It was a glassman SMPSU being used to keep a car battery topped up while a car audio system was being tested. No problem at all with voltage and current limit set appropriately. The only problem is the supply has crowbar protection, which should be fine if you set it well above any voltage you will encounter. However as the poor chap found out if you knock the remote setting plug out of the back, it sets the crowbar voltage to zero. The FET used for the crowbar, and the associated tracks did not fare very well! I did manage to repair it though!
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Old 13th Sep 2017, 7:31 pm   #10
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Default Re: Are Farnell L30 and TOPS units reasonable power supplies?

I love crowbars. They help you sleep at night. Anecdotally, when I was at university, one of my lab colleagues thought that because the fuse in a 10A logic power supply kept blowing that they should just jam some folded up stripped mains wire in the fuse holder. Smoke came out of his breadboard, the remains of the power supply and the circuit breaker on the bench popped after a hilarious 20 seconds of panic. Turned out one of the pass transistor was C-E short and the crowbar was kicking in and blowing the fuse. They made him fix it after lab. Last time I spoke to him he was assembling satellites so must have learned something!

Farnell arrived today. I will be refurbishing the E30/2 over the weekend. I'll create another thread for this as I'm sure someone else will come across one in the future.
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