lexicon pcm-xx power supply repair

for all of you guys who had trouble with lexicon pcm-90/91/80/81

i bought a pcm-91 with a faulty power supply, needless  to say for a small amount of money ;-)

thought this couldn't be so complicated to fix. i wasn't so easy in the end....

at first i noticed the hi voltage cap which has a bulge, it has a voltage about 270 V dc (main ac is 230 v here in germany, so it has to 310 or sth. like that).
so i changed it; wasn't so easy to get a replacement because the height of the 100µ/400V el-cap is only 2,5 cm. finally i found a replacement....
because of known problem with electrolytic caps and smps i changed all other el-caps to 105° low esr types with same or greater voltage (red dotted)

next change was the switching mos, a IRF830 which has a low resistance from one pin to another, don't ask me which one...

after that and some looking at the other components i thought the ps should be able to work now. unfortunately it doesn't....

there are 310 V dc at the bulk cap but it seems like the copper-circuit doesn't start :-(

i changed the 2 small driver transistors, no success

after few days break i checked the rectifier diodes at the output side which were ok

at last i measured every resistor and capacitor on the hi voltage / mains side.

i found the r2, a 750kOhm / 0,5W has no resistance.

after replacing this beast my lex awake to new life
i saved 50 bucks a new smps would cost from the german lex service ;-)

maybe this helps some of you guys with faulty lexicon device...

thanks a lot to svart for the tc wizard ps & general switch mode power supply thread!!!

now i have a tc power supply with a more complicated failure on the bench :-/

Thanks! Very help full indeed.
Might come in handy one day although I hope not 

I realize this is an old topic but I also have the same problem. I'm hopeful it's as simple as replacing a resistor but I can't seem to locate R2 on the Astec LPT45 power supply for my PCM 80.

It's a long shot but any chance someone can confirm this is the same power supply and where R2 is located. thanks!

In a switchmode supply, there is normally a high value resistor, normally at least a 1 watt resistor, between the rectified d.c. input and the power supply terminal in the switcher i.c. These often go high. Also the filter cap on the switcher i.c. often loses value and hence the supply stops.

@robomix
yesterday evening my PCM 81 decided to play a trick on me after turning it on (after a few months that it was off) I heard a leak and immediately after smoke and a burning smell !!
so I turned everything off and decided to open it to understand what had happened. I immediately noticed that the capacitor C3 had exploded and I wanted to ask you if it was a normal 47nF capacitor X2 (I am attaching some photos)

Audioman said:

a normal 47nF capacitor X2

Click to expand...

Looks like that to me.

PME285 - "EMI suppressor, class X2, metallized paper".

Yeah, stay away from Rifa metallized paper X2 caps, they're all bound to fail, sooner or later. Replaced it with whatever X2 Mylar/PET/PP cap you can find. 100nF is a more widespread (and possibly easier to find) value - just make sure the pin pitch matches the original (and the thickness, if it's in a very tight spot).

Khron said:

Looks like that to me.

PME285 - "EMI suppressor, class X2, metallized paper".

Yeah, stay away from Rifa metallized paper X2 caps, they're all bound to fail, sooner or later. Replaced it with whatever X2 Mylar/PET/PP cap you can find. 100nF is a more widespread (and possibly easier to find) value - just make sure the pin pitch matches the original (and the thickness, if it's in a very tight spot).

Click to expand...

Hi Khron,
Do you advise me to change all the electrolytics since the power supply is a bit old?

P.S. what does It mean the writing on the exploded capacitor PME 285 MB 40/56/110 B ?

Unless it otherwise works, i'm not sure i would - those look like Rubycon for the most part, which is one of the top brands, and one of the very few reliable ones left nowadays. For futureproofing, maaaaaaybe change the little 22uF one on the primary side, but i wouldn't necessarily bother with the rest.

PME 285 is the series; not sure about the MB, and the numbers may or may not be some code of a standard or whatever.

@Khron
I forgot, I noticed a component called RT1 on the power supply (photo attached) which appears to have a damaged casing.
What is it?
and is it time to replace it?

Especially if that blown Rifa cap was "downstream" of this NTC thermistor, then yes, that very much needs replacing too.

Khron said:

Especially if that blown Rifa cap was "downstream" of this NTC thermistor, then yes, that very much needs replacing too.

Click to expand...

how do you understand the value of the thermistor? It's covered in glue and there doesn't seem to be any writing

It's usually marked on one side, but considering this isn't a very high-power thing, something like a 10 ohm one of a similar diameter should be just fine.

https://en.wikipedia.org/wiki/Thermistor#NTC
(Second bullet-point in that list)

Last question i promise
I misure the BR2325 battery that Is soldered on the board, since i must change It Is better solder a socket for batteries instead of a battery with solder pins ?

It's probably easier to change later down the line if it's in a socket (assuming you can find one that fits the pin spacing of the, i'm assuming, solder-tab version of battery).

So, somewhere along the line, Lexicon went from Linear Regulated PSU to SMPS...





An interesting project would be to replace those SMPSes in the newer Lexicons by Linear Regs PSUs.

@Khron
thermistor and capacitor taken at the local electronics store, for batteries it is more difficult but I do not give up.

@Khron I noticed that the 100uF 400 volt capacitor (photo attached) on the primary has a hole on one side (the one facing the capacitor that exploded) I think it needs to be replaced right?

It's Jamicon, which is a "second tier" brand (imho) - normally the 400v cap isn't in *that* stressful of a position, but especially if it's now been physically damaged, yes, it would be quite wise to replace that too.

Khron said:

It's Jamicon, which is a "second tier" brand (imho) - normally the 400v cap isn't in *that* stressful of a position, but especially if it's now been physically damaged, yes, it would be quite wise to replace that too.

Click to expand...

@Khron What brand of capacitors do you suggest?

Panasonic, Nichicon, Rubycon, United Chemi-Con, Elna.

Sure, there's also stuff from Vishay, but i'm a bit cynical about companies that make ALL sorts of components; the Japanese ones at the beginning more or less specialize in capacitors (ok, except Panasonic - so sue me).