Farnell L30 dual PSU - UK Vintage Radio Repair and Restoration Discussion Forum

BulgingCap · 26th Jul 2017, 11:00 am

I have had a Farnell L30 dual 30v 1A bench PSU for many years- so long that I no longer recall where it came from.
Yesterday I was using it and discovered a fault, quite by accident, when I connected the scope ground to a piece of equipment that was being powered by the supply. On investigation I realised that the left channel of the PSU had the -ve terminal connected to PE (ground.)
I delved inside and discovered that the 2700uF 63V res cap on the faulty channel had a pop rivet end pushing through the plastic sleeve of the can. The clip holding this cap for each channel is pop riveted to a plate that mounts to the top of the transformer.
The caps have 'fluted' sides, and just by chance the larger diameter on this cap was sitting against the rivet end. The cap on the other channel was mounted with a 'depression' next to the rivets.
I was tempted to just rotate the cap, but for the sake of removing a few self-tappers, I drilled out the rivets and refitted shorter ones.
These PSUs, and the single channel version, are very common in our world as they were used in colleges and uni etc. so if anyone has one it may be worth a check between the -ve terminal and the ground terminal, or take off the cover and have a look- I'm sure that mine wasn't the only one assembled that Friday afternoon.
BC

Refugee · 26th Jul 2017, 12:51 pm

I would have been tempted to just file the rivets down.
I would measure the insulation with a Megger.

turretslug · 26th Jul 2017, 2:27 pm

I've often worked with or encountered kit with floating-from-chassis or voltage doubler etc. supplies that relied solely on capacitor sleeving for isolation to high constant or possible potential- whilst this sleeving may have high dielectric strength per se, it's pretty thin and larger reservoir capacitors are frequently retained with sheet metal clamps that often have stamped, burred edges and/or get overtightened. Something of an Achilles heel really, and seemingly overlooked as such- I like to use e.g. heatshrink sleeve cushioning, or simply sturdy plastic clamps and always watch for problems with rivets or excess thread length.

26th Jul 2017, 3:05 pm

Quote:

I would have been tempted to just file the rivets down.

I would bash them with a hammer and drift, no chance of filings causing more problems.

BulgingCap · 26th Jul 2017, 3:21 pm

All valid comments, thanks- I posted as I know that there are a lot of this model and its half size brother the single 30V 1A PSU (can't recall the model)
On reflection, and listening to Turretslug, I should have replaced the steel clamps with some kind of plastic clip, or fitted an insulating sleeve of some sort over the clamp arms. - I have used a form of plastic cradle with either a rubber square band or tie wraps to mount large caps in the past, but obviously never 'borrowed' any as I don't have any stock in the shack.
I checked the other cap and the -ve terminal is connected to the case, as is normal, but I'm sure that not all do and some cans are isolated. I may revisit this as I still haven't refitted the case.
BC

short wave · 27th Jul 2017, 3:41 pm

Hi "Bulging Cap"

As a quick fix to insulate the cap what about a few turns/layers of insulating tape round the cap(s) where the cap is held in the clip? Your unit has transformers I haven't seen before.

Yes "Cumbernauld" were owned by Farnell... might be an early type. By any chance does your unit have any date codes on the main caps or output transistors?

Regards, S-W.

Biggles · 27th Jul 2017, 9:27 pm

A word of caution. Many years ago at work I managed to blow one of these up by connecting a 12V SLA battery to one of the outputs. I can't remember if the PSU was switched on at the time but it caused damage which I quickly managed to repair fortunately before the boss found out.
Alan.

26th Jul 2017, 11:00 am	#1
BulgingCap Pentode Join Date: Apr 2017 Location: South Coast, Western Australia. Posts: 129	Farnell L30 dual PSU I have had a Farnell L30 dual 30v 1A bench PSU for many years- so long that I no longer recall where it came from. Yesterday I was using it and discovered a fault, quite by accident, when I connected the scope ground to a piece of equipment that was being powered by the supply. On investigation I realised that the left channel of the PSU had the -ve terminal connected to PE (ground.) I delved inside and discovered that the 2700uF 63V res cap on the faulty channel had a pop rivet end pushing through the plastic sleeve of the can. The clip holding this cap for each channel is pop riveted to a plate that mounts to the top of the transformer. The caps have 'fluted' sides, and just by chance the larger diameter on this cap was sitting against the rivet end. The cap on the other channel was mounted with a 'depression' next to the rivets. I was tempted to just rotate the cap, but for the sake of removing a few self-tappers, I drilled out the rivets and refitted shorter ones. These PSUs, and the single channel version, are very common in our world as they were used in colleges and uni etc. so if anyone has one it may be worth a check between the -ve terminal and the ground terminal, or take off the cover and have a look- I'm sure that mine wasn't the only one assembled that Friday afternoon. BC Attached Thumbnails

26th Jul 2017, 12:51 pm	#2
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Farnell L30 dual PSU I would have been tempted to just file the rivets down. I would measure the insulation with a Megger.

26th Jul 2017, 2:27 pm	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,394	Re: Farnell L30 dual PSU I've often worked with or encountered kit with floating-from-chassis or voltage doubler etc. supplies that relied solely on capacitor sleeving for isolation to high constant or possible potential- whilst this sleeving may have high dielectric strength per se, it's pretty thin and larger reservoir capacitors are frequently retained with sheet metal clamps that often have stamped, burred edges and/or get overtightened. Something of an Achilles heel really, and seemingly overlooked as such- I like to use e.g. heatshrink sleeve cushioning, or simply sturdy plastic clamps and always watch for problems with rivets or excess thread length.

26th Jul 2017, 3:21 pm	#5
BulgingCap Pentode Join Date: Apr 2017 Location: South Coast, Western Australia. Posts: 129	Re: Farnell L30 dual PSU All valid comments, thanks- I posted as I know that there are a lot of this model and its half size brother the single 30V 1A PSU (can't recall the model) On reflection, and listening to Turretslug, I should have replaced the steel clamps with some kind of plastic clip, or fitted an insulating sleeve of some sort over the clamp arms. - I have used a form of plastic cradle with either a rubber square band or tie wraps to mount large caps in the past, but obviously never 'borrowed' any as I don't have any stock in the shack. I checked the other cap and the -ve terminal is connected to the case, as is normal, but I'm sure that not all do and some cans are isolated. I may revisit this as I still haven't refitted the case. BC

27th Jul 2017, 3:41 pm	#6
short wave Pentode Join Date: Mar 2013 Location: Leeds, West Yorkshire, UK. Posts: 246	Re: Farnell L30 dual PSU Hi "Bulging Cap" As a quick fix to insulate the cap what about a few turns/layers of insulating tape round the cap(s) where the cap is held in the clip? Your unit has transformers I haven't seen before. Yes "Cumbernauld" were owned by Farnell... might be an early type. By any chance does your unit have any date codes on the main caps or output transistors? Regards, S-W.

27th Jul 2017, 9:27 pm	#7
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Farnell L30 dual PSU A word of caution. Many years ago at work I managed to blow one of these up by connecting a 12V SLA battery to one of the outputs. I can't remember if the PSU was switched on at the time but it caused damage which I quickly managed to repair fortunately before the boss found out. Alan.