Newbie.. I service machines which use 2 x 1980's Vintage Linear PSU's (one 15vdc & one 5vdc) by Lambda, Coutant, Power One, Int Power, Farnell. Eventually the electrolytic capacitors swell and the PSU packs in. I've refurbished all the above make 15v and ALL of the above make 5V PSU's WITH THE EXCEPTION OF the Lambda 5v. I've tried fixing up four of them by replacing the cap's but it only shoves out 2.5 volts. Adjusting the v-adj pot has no effect at all. Any ideas please??

 

Without a schematic or even photos of what the supply looks like, it'll be pretty difficult to make recommendations.

However, there are usually a few power transistors mounted on the sides of the old Lambda linear supplies; you might find that one or more has opened up.

I have an older Lambda, but it's a variable output (~0.25v-50v) - so quite different from what you're trying to fix.

 

Thanks for the suggestion. As usual, the power trainsistors appear to be fine (checked out of circuit on multimeter transistor setting) 3 x FBN-L198, 1 x L60V5, 2 x Diodes 1N1202A. Other than the electrolytics components include 9 x resistors, 1 x disc cap, 1 x LAS723B Transitor, 2 x rotary pots (volts & current limiter) and of course the transformer. I cant find a datasheet but will load some pics up soon

 

<snip> 1 x LAS723B Transistor <snip>

Wait a minute - that's a 150mA positive regulator IC, not a transistor!
Datasheet:
http://www.datasheetarchive.com/dl/Datasheets-111/DSAP0033069.pdf

If that link doesn't work, click here:
http://www.datasheetarchive.com/LAS723-datasheet.html#
and click on the small image of the PDF about halfway down the page to download it.

You'll have a hard time finding them in the round metal package anymore; those were very expensive to make/purchase, and usually only used for very high-reliability or military applications. However, several manufacturers offer the 723 regulator in other packages. You might have some 'daughterboards' made in order to be able to plug the different packages into the board holes for the old regulator.

It would probably be less expensive than replacing these good old linear supplies.

 

Excuse the transistor typo. With time on my hands a couple of years ago I removed the metal can and soldered 10 leads in its place, the other end of which I soldered to a dual in line socket (after figuring out the connections) into which I popped a dual in line 723 and still no luck. If you're sure it could be the regulator is there any way of testing it out of / or in circuit? (my skills are more mechanical than electrcal). Thanks again for your input.

 

pin 4 is the voltage ref pin there should be a resistor connected to it check to see if that resistor is the correct value

 

also check the voltage at pin 6 this voltage drives the base of a pass transistor which acts as an amplifier to increase the current. as the load increases the voltage of the wiper of the pot for voltage drops. when this occurs the 723 increases the voltage at pin 6 driving the pass transistor harder. thus we have regulation.

 

Sorry for the delay in getting back. 2 resistors (out of circuit check) connected to the regulator are OK. Pin voltages on the cyl can regulator (with -ve probe to psu ground) are: p1=1.5v p2=2.6v p3=5v p4=5v p5=2.6v p6=1.9v p7&8=23v p9=1.7v p10=2.3v. The solder joints on the Power Transitors are lightly soldered (in case I have to remove them again) so prior to reassemby I noted the underside transitor connections and made good one open circuit on the topside (where the solder didn't run through). Does this help?

 

PS PSU output still down by 50%

 

Does the output drop significantly under load? If so I'd look at the driving / output circuit not the regulation circuit.

 

The L60v5 is a 6A overvoltage protector for 5v; if the voltage across pin 2 (+in) and the case exceeds ~6.6v ±200mV, then pin 2 is shorted to the case until the current falls to 0 and the temp is below 71°C.
Datasheets are available from this page:
http://www.datasheets.org.uk/L60V5-datasheet.html

If the L60V5 is shorted, that would kill your output.

If it's bad, it might be replaced with a thyristor, a Zener diode and a couple of resistors.

The 1N1202A is a standard silicon 200v 12a rectifier:
http://www.datasheetcatalog.com/datasheets_pdf/1/N/1/2/1N1202A.shtml

What would really help is if you could document the circuit as best as you can. Create a schematic, filling in the part numbers and/or values, where applicable.

 

It's down by 50%? Then I'd look at the transformer secondary windings and the two 12A rectifiers, as a failure of either would explain that.