yam spx 90

[kepeb]

Hey,
looking at another spx90 psu.
this is the one with the SMPS but your probably gonna know that already anyway if you have any idea what i'm going on about below.

plugging it in I get a reading of 240v on the + at the board input ( right where the mains wires come in )but the neutral side reads 0v.
iv'e checked the negative power lead continuity from mains and followed continuity through to D1 (DF06 737M) with no power applied. where is it going???

there is a strange thing i noticed too. after assuming the large primary side cap (c11 - 68uf 400v on UK version) was probably in need of a renewal anyway.  (I have no esr meter but capacitance checked ok, but still i will replace, it's old) only...
on the schem it appears it's oriented so that the positive side is connected to the bridges negative output via r2 (6.8 5w) why?

hope its ok to link here  :
user-enzo has kindly posted a schem about halfway down the page so you can see..
http://music-electronics-forum.com/t18634/

 

[PRR]

"yam"?? The root vegetable?

> if you have any idea what i'm going on about

What is the PROBLEM? Tastes bad? Thick smoke? Lays there dead? I'd hate to write-up my recipe for stale yams if your real issue is fire in the oven.

> it appears it's oriented so that the positive side

Read the diode arrows.

They disagree with the +- marks.

One or the other must be wrong.

Since reverse voltage would explode C11 quickly, the +- is wrong.

> I get a reading of 240v on the + at the board input

AC or DC?? There's no "+" at the mains input.

Compared to WHAT?? All voltages are across _2_ points, what's your other point? Line neutral? Chassis? Just someplace?

> right where the mains wires come in )but the neutral side reads 0v.

If your meter's other lead is on chassis, AND your mains socket is wired correctly, you SHOULD find zero volts AC on the other lead.

Forget that. Do you have 320 Volts _DC_ across C11 (not C11 to chassis!)? Then you are OK to there. If 200 Volts DC and no big smoke, then C11 is open.

ESR is NOT a big issue here. Either C11 holds charge from moment to moment, without heating, or not.

If C11 is not getting any charge, check switch, fuse, and L1 L3 for continuity as shown. If open, no juice. Each of the two windings per L could be temporarily shorted to bypass a fault.... they are there only to prevent RF/CPU crap from leaking to/from the mains. Similar Ls may be extracted from dead PC supplies.

In general, I would NOT attempt to mess with a line-side switcher. It is as dangerous as sticking your fingers in the wall socket. None of the internal voltages make much sense. Everything is inter-related so the part which may seem to be a wrong reading is probably NOT the problem.

It is "just" a +/-18V and +5V supply, plus PC2 which simply tells something (a CPU?) when the line feed has gone dead. You can cobble an equivalent fairly easily.

 

[kepeb]

apologies, i have gotten this off to an offensively ignorant and bad start.
please let me try to clarify in a les profoundly stupid way  :-[

'yam', sorry Yamaha, although I've never tried a yam and I'm interested in the recipe for sure

problem is no power out from the PSU.

Okay, I'll ignore the misprint on the schematic then, I did wonder. cheers!

yes, again apologies I was referring to the AC mains input points and inaccurately called the 'NEUTRAL' side '-'

this was referenced to mains 'ground' / 'Earth' which is wired directly to the chassis upon entry, but ok i got this now.

good to know about the importance/job of L1, L3 and ESR here, thank you for this.

I will check those points out. carefully of course, so you would suggest dumping this and making a new PSU from scratch?

 

[Walrus]

All of the SPX90 power supply faults I've seen have been caused by C19 - C24 drying out and going low value.

 

[kepeb]

hey, well I routinely replace electrolytic's in gear of any significant age.
this is actually my second spx and the power supplies were doomed from the start.

I got lucky with the last one, after removing all the now conductive glue residue, changing all the electro's and re plugging all the connectors it worked fine.

that one had only minor damage but the glue has literally eaten parts I didn't see on this one.

also, does the ESR matter on the secondary side in this PSU? I was under the Impression its very important it be low in all SMPS.
I don't think I will ever understand switch mode. but i'm trying  :-\

 

[reanimatorstudio]

I don't think I will ever understand switch mode. but i'm trying  Undecided

almost nobody will. The thing is they take out parts  and modify to make it cheaper. You end up with something very critical and incomprehensible

 

[MagnetoSound]

All of the SPX90 power supply faults I've seen have been caused by C19 - C24 drying out and going low value.

I don't have the SPX circuit to refer to, but I have repaired two SMPS, one in my G4 and one in a Allen & Heath mixer and both times it was the timing caps connected to the PWM controller that had dried up.

HTH,

 

[s2udio]

This may help....
http://www.morpheustechnology.com/eXe_Projects/Power_Supplies/switch_mode_power_supplies.html

SMPS are basicaly a spin of of the TV industry,CRT days using flyback chopper cicuits,Ahh those days as Mr TV repair man  :-\ !
Repaired shedloads to date...sky boxes ,freesat,behringer,tc electronics etc.
Always test run them up on a variac or series bulb....saves that flash bang wholop stuf !
Always replace ALL the electros.They are nearly all "CHEAP " chinese ones..say no more.
Check ALL diodes and bridges....nearly all use Ultra fast semies and Transient voltage protectors nowadays .
Most fatal faults cause shorts/opens , across diodes,TVP's, bridges . If in doubt REPLACE !
Some use Fusible Resistors (as in the SPX,i think), these go open and prevent excess current flow just like a fuse..again replace with the "Correct" part.
Hot running resistors ,2 or more watts go OC ,Had a bunch of Lexicon MPX verbs , that had  faulty 2w resistors....nice little earner that one.
If a severe fatal fault has occured then the PWM chip will probaly need replacing as well....if it dont oscillate after replacing most of everything else thats
The next place to look.
The transformers are usualy robust but can fail and replacements are very hard to find..!! its scrap !!
Dont be frightend by them ..But do watch for the 3 to 400 dc volts on the bulk DC rectifier stage......ouch !!!!
And.....CHECK and RECHECK all your WORK carefully .
Happy fault finding

 

[kepeb]

hey,
cheers for the link
yea, I didnt update but turns out it was simple enough.
after replacing everything that was eaten by the hidden corrosive glue and a good clean it was good as new.
in case anyone does stumble across this thread in future tho, do be careful.

 

[solderspongebob]

I did a similar fix for my brother-in-law on his Sky box PSU. Replaced all of the electrolytics with nice Panasonics from my component box. Can't say I could identify the manufacturer of any of the original caps that came out...

Funny thing is, he swears the unit gives a clearer picture and sounds better than before! Maybe he was just being nice to me 

 

[s2udio]

Funny thing is, he swears the unit gives a clearer picture and sounds better than before! Maybe he was just being nice to me 

Well it certainly revives them to "Like New" status