One half of my LT30-2 made smoke

[firegazer]

Hello Forum,
   I'm new here, and looking for some advice on how/whether to repair my (very old) Farnell LT30-2/BT Bench PSU.

I had been using it again after a few years in the loft, and the right side was working fine; 12v for a transistor oscillator and VCA I was hoping to use as a Morse code practice oscillator.

The other day I switched it on again and the left-hand meter went to about 4 volts and made a nasty buzzing sound, and then a small bit of smoke came out of the left rear vent on the top of the case...

I switched it off quickly, as I live in a (largely) wooden house :-)

I took off the case and had a peep inside, but nothing was obviously melted. I can provide images, if that would help?

So, my options seem to be:

1) get a new, more modern PSU, and stop using this old one just because I've had it for 40 years, and the analogue dials look cool;

2) disconnect the left-hand side and just use it as a single PSU;

3) do some fault-finding and see whether it can be mended (and maybe serviced a bit, if that's a thing?)

I don't usually play with anything involving mains voltages, so if I go (3), I'll have to be super careful.  I do have a Fluke 79 which is happy to do mains volts, but has two blown fuses (don't ask) for current measurements.

Is it sensible to take the case off again (it's done up and abandoned in my office right now), power it up, and see which bit makes the smoke, or could that make a repairable problem into a bigger (and maybe not repairable) problem?

Any advice appreciated.

John

[firegazer]

Images...

[Ian.M]

Are you sure you don't mean "... out of the right rear vent ...", as I can see suspicious possible burn marks  on the right side PCB, which appear to originate from D3, one of the diodes in the bridge rectifier supplying the output pass transistors with unregulated DC.

The buzz would have come from that side's mains transformer and would be characteristic of a bridge rectifier diode failing shorted, resulting in the transformer being overloaded with DC in the secondary, or one diode in the bridge failing open, with a  short on the bridge output, again resulting in an overload with DC in the secondary.

Test D1-D4 to see if any of them are open or shorted.  You may have to disconnect the transformer secondary feeding the bridge to get unambiguous readings.  I believe that's the pink wires on terminal pins 11 & 13 near the rear board edge - you only need to remove ONE wire. 

If you are lucky replacing that diode may fix it.  If you are unlucky, something is pulling the unreg DC rail down, maybe a failed reservoir capacitor or short from the pass transistor collector(s) to the heatsink, and the new diode will smoke the same way.

Chances are you may need to measure current at some stage in this repair.  I'm not going to ask about your Fluke fuse mishaps (though I'm sure the story is some combo of   and possibly ), but if you post photos of the DMM with the back off (preferably with a ruler next to it, or on a sheet of graph paper for scale to get the actual fuse sizes), and of the fuse ratings on the back of it, we may be able to suggest where to source replacements without paying through the nose.  Properly rated multimeter fuses aren't cheap, but there's no need to get ripped off!

[firegazer]

Ian,
   thanks for your very helpful reply.

I agree that the diodes on the right-hand side look a bit cooked, but it was definitely the left-hand side that had the problem. I didn't actuall measure the output voltage on the right-hand side, though. Maybe that's why my oscillator was working one minute and then stopped...? :-O

I didn't realise (until reading the Fluke 79 manual on-line) that it was even possible to measure components whilst they were still fitted in the PCB, but I guess it depends on the local circuit around them?

I measured (using the DMM on diode forward voltage mode) the four main diodes on each side. I only had ten mins before going out, so I'll do it again properly tomorrow morning, but two of the left hand diodes (D1 and D3?) seemed to be short-circuit in at least one direction. D1 to D4 on the right-hand side all say 5.2v ish (I think).

I'll double-check tomorrow, and let you know what happens.

I'll send you an image of the inside of the Fluke, too. It's a "79 series ii", which says it needs a 1A and a 15A (FAST reacting or something).  It would be great to find out how to replace them for less than 10 quid each...  There seem to be a lot of 11A fuses on eBay, but I guess they are too low value for this?

Best regards,

John

[Andy Watson]

... depends on the local circuit around them?

The big red capacitor in the middle of the PCB is dated "74" - so I think this is the nearest service manual that is available (there are others - Farnell seem to have gone through at least ten revisions of the "L series"!)

I

[floobydust]

You can see the PCB has extra/bigger holes to accommodate bigger diodes like 1N5404, although C10 0.22uF 160V uses up one hole. The diodes conduct in pairs so one shorted can take out another.
The main filter capacitor C1 2,500uF 64V I would check it too.

What I find is recapping and repairing old PSU's puts you in a better place than buying new cheap chinese stuff that is low quality and needing constant repair.
Compared to old gear that needs love every 50 years lol.

There are only a handful of electrolytic capacitors (C1, C2, C4, C6, C5) in the channel, I'd just replace them all. C4 4.7uF 63V date code is 1974 
I think C6 1,500uF makes the PSU slow and seems odd. (edit: C6 is 1,500pF)

LT30/1 schematic snippet from the above PDF.

[Ian.M]

A short circuit anywhere downstream of the reservoir capacitor would probably blow F2, the low voltage fuse, so, unless a previous idiot has wrapped that in tin foil (or popped in a wrong, higher rating one), the odds of the fault being due to a bridge rectifier diode failing, or C1 or C10 breaking down, are pretty high, and the odds of damage elsewhere much lower.

[firegazer]

OK. More data and images...

Using the DMM set to Diode mode, I get the following:

.	Left	Right
D1	0v both ways!	0.53v slow
D2	0.50v	0.52v fast
D3	0v both ways!	0.53v slow
D4	0.50v	0.52v fast

So it looks like the left D1 and D3 are now pieces of wire, rather than diodes.

The fast/slow on the right was how long the DMM took to measure the result, in-circuit of course. I guess the D1/D3 diodes have some capacitor near them that causes the DMM to have to charge them up before a reading happens??

D5 and D6 were fine (0.52v) on both sides.

The big smoothing caps looked fine on inspection. They both have weird dents in the cylinder case, but I guess that's by design (to give them more rigidity?) as they both looked identical.

I tried to measure them in-circuit with the DMM but it said "Discharge", then "OL" then "4224", "5xxx", "7xxx", "9xxx", then "OL" and stopped, so I guess they are too big to measure with this DMM??

I've included images of the caps, and of the DMM with its missing fuses. I might have the original fuses somewhere, but they weren't in the 'safe' place where I thought they'd be, so that could be a long search.

I'll order some 1N4003s.

The "change all the electrolytics" advice sounds good if you have some wall drawers filled with them, but I don't. The big ones seem to be 50 quid each (??) so I'll probably swap the rectifier diodes first and see how I go. Does that sound sensible, or are you all shaking your heads and smiling? :-)

Thanks for all your help.

John

[Andy Watson]

I'll order some 1N4003s.

I think the 1N400x are marginal for a 2A supply. If there is space on the PCB, I would use 1N540x.

[Ian.M]

Yes, and from the photos, D3 on the right PCB is slightly different to the others so has probably been previously replaced.

You need to know the last digit of the 1N400x and 1N540x part numbers codes the reverse voltage rating and for rectifier use that generally should be at least double the peak of the AC voltage (see specifc datasheet for details - the codes don't line up).
 
It gets away with 1A diodes because rectifier diodes are generally rated in terms of max. average DC current through them, and in a bridge, each pair of diodes is off for half the time so the average for them is half the average output current.   However that's pushing them to the limit with no safety margin so replacing them with >1.5A rated silicon rectifier diodes would be desirable. 
The problem with using 1N540x series 3A diodes is their legs are slightly thicker,  + the body is longer, close to double that of a 1n400x 1A diode.  Fortunately the PCB is designed to accommodate that with extra larger hole for each diode at each end a bit further out.  Currently the green capacitor is using the extra holes for D1 and D4, but that could be moved over to the holes those diodes right leads are currently in and the 1n540x diodes fitted with their right legs jumping over the capacitor legs.

Personally I'd try to scrounge up a pair of 1N4003 - 1N4007 from a scrap board to see if it fixes it, and if so, order a pack of 10 1N5402 to replace all four bridge rectifier diodes on both boards + leave you a couple of spare 3A diodes which are generally useful to have around, especially if you ever need to charge batteries from it without risking damaging reverse current flow.  If you decide to do an exact 1N4003 replacement please replace all four in the affected bridge as the other two have certainly been overloaded.

[firegazer]

OK. I'll go with that plan. Thanks.

I first ordered a box of different 1A diodes, including the 1N4003, intending to try to swap just D1 and D3, but then Andy's message came in, so I also ordered some 1N5402s, so now I can follow the complete plan :-)

I hope I have some solder braid, or I'll be making my third Amazon purchase in a day!

I may be a good source of rectifier diodes for a while, if anyone needs any :-)

Might be a few days, as I didn't want to pay for any delivery...

Best regards,

John

[factory]

I'll order some 1N4003s.

I think the 1N400x are marginal for a 2A supply. If there is space on the PCB, I would use 1N540x.

The thread title is wrong, the pictured PSU is a LT30/BT, a twin 30V 1A PSU. The 2A twin version has ten 2N3055 transistors in total.

I think C6 1,500uF makes the PSU slow and seems odd.

LT30/1 schematic snippet from the above PDF.

C6 is 1500pF.

The "change all the electrolytics" advice sounds good if you have some wall drawers filled with them, but I don't. The big ones seem to be 50 quid each (??) so I'll probably swap the rectifier diodes first and see how I go. Does that sound sensible, or are you all shaking your heads and smiling? :-)

Modern descendant of the Mullard/Philips 2200uF 63V can capacitor, with solder lugs, not sure why you've seen them at £50.  They may or may not be fine, failure tended to be open or short circuit in stuff at work.
https://uk.farnell.com/vishay/mal205018222e3/cap-2200-f-63v-alu-elec-solder/dp/1166586?st=2200%2063v

David

[wasedadoc]

The dents in that electrolytic were intentional.  By themselves not a reason to replace it.

[floobydust]

At the very least, while you are in there, check the value of the smaller electrolytics C2 47uF 63V, C5 100uF (possibly located in hell at the binding posts?), C4 4.7uF 63V (wet tantalum?) if you can.
If a cap dries out and goes low value, the PSU can misbehave or oscillate. Most of the time, the load is worth more $$ than the cost of replacing them, so I'll just do it for the few dollars. They don't last forever and I find rubber bungs dry out and crack at 50 years, end of life. These seem to have epoxy as well.

C6 is 1500pF.

Yes, fixed. The schematic default is uF but the scan is a bit fuzzy C4 seems to show no polarity on the schematic.

[factory]

C4 is shown in the pictures, it's a Siemens MKL, seems to be a metallized lacquer film type, according to this databook.

https://hallmanlabs.com/wp-content/uploads/2020/01/SiemensMetallizedPlasticCapacitorsDataBook1978-79.pdf

C4 is shown as non-polar on the diagram of the 2A version too.

David

[firegazer]

OK, more data and images...

I clipped out D1 and D3, and tacked the new 1N4003s that arrived today onto the back of the PCB.

I powered on and got smoke, again! It came from D3.

Then I double-checked and found I'd tacked the new D3 diode on in parallel with the existing D4, which faces the other way, and made a piece of wire.

That new D3 was now the very old and cracking apart D3, so was binned. D4 is probably also past its best, now.

Luckily, I have 2 kgs of 1N4003s, now, so I tried again (just replaced D3) and got a gentle mains hum instead of smoke :-)

The output gives an accurate 0 - 30.0v on the DMM, so it looks like the rest of the circuit is working OK.

The bridge rectifier diodes will next be replaced (at least on the left side) for beefier 1N5402s, and I'll move the capacitor as suggested.

I guess the 1N5402s aren't as necessary as Ian suggested, as I'd mis-named the thread, thinking LT30-2 meant "LT30 dual version", instead of denoting the max output current. This one is only 1A max each side, so the rectifier diodes are getting about 0.5 amps (? is that right, probably a factor of 0.707 in there somewhere!). If I can fit them in, I guess it can't hurt to go large.

I tried to measure C2, C4, and C5, but without much success. C2 seemed to be 116uF (LH) and 108uF (RH) in-circuit, but that's probably not very useful.

I'll solder out one pin of each, tomorrow, when I try and do the diodes, and do a proper measurement of them.

I also managed to find the Fluke fuses and have included an image of them. Any advice on getting proper replacements would be appreciated.

Best regards, and thanks for all the help.

John

PS - image of my D3/D4 balls-up attached by general laughs! :-)

[Ian.M]

Yes, if yours is a 1A psu then the 1N4003 diodes are well within their ratings, so replace like with like.  I'd still replace all four in the side that blew, but no need to touch the other side at this time, and no need to upgrade the diodes.

Unfortunately it looks like your Fluke isn't one of the ones with cheaper fuses.  I'd hoped we could at least have got your 300mA range back for not too much.   Maybe one of the UK members can suggest a supplier that's reasonable for the fuses you need - they're still going to cost, but hopefully not an arm and a leg!

[firegazer]

OK, four new 1N4003s are in and the left-hand side is now working.

I still need to get some contact cleaner for the voltage pots, which are quite gunked up and stiff, and I need to replace the right-hand side regulator diodes, too, at some point, as well as replace the electrolytic capacitors before they fail.

But for now, I'm back in business with a working PSU, without getting electrocuted or the house burning down :-)

Thank you all for your help. I've saved having to buy an expensive, modern PSU, and given this one some more life. I wouldn't have had the oomph to do that without your support and help.

I haven't used the 1N5402s I ordered, but they are massive compared to the 1N4003s!

Best regards,

John