Lexicon MPX-1 problem

[MLM]

hi there DIY chaps,
i'm trying to work out what's wrong with a friend's Lexicon MPX-1 fx unit that won't power up - i'm guessing the problem lies somewhere in the power supply pcb, as the on-board fuse there blows immediatley when the unit is switched on, also there are small "burn-out" marks around 2 large (2W) resistors on this board... just wondering if anyone has experience with this unit and knows of any common problems/things to look at first, maybe someone might even have a link to the schematics? any help would be much appreciated!!
thanks very much in advance!!
Matt L.

 

[Michael Krusch]

I guess you don't have a schematic, but a photo will help a little bit to give you some advice.

 

[tony dB]

i would start looking for some broken pcb.resistor or ps joint.
Happens often.

good luck,
Tony.

 

[pucho812]

hmmmmm? For starters I would replace the resisters and see what happens then. If that doesn't work you could check out in the support pages of lexicons website. http://www.lexiconpro.com and you might be able to find helpful info. Lexicon does not give out schematics you have to buy them and they are costly but it's much cheaper then to send it in for a repair. They have a flat fee fo $500.00 U.S. for repairs regardless of problem. Money covers parts, labor and shipping back to you. You have to get it there. I am running into a problem with my lexicon 480L reverb and I refuse to send it back for repair when I know where the problem lies. I can fix it myself it is much cheaper.
hope that helps some. Best of luck on fixing the issue.

 

[Marik]

Something draws too much current. To find out what, first, disconnect the PSU board from the main circuit to rule the last out. If the fuse still blows, check shorted electrolytics with ohmmeter. Then we will see.

 

[CJ]

unhook the pwr supply from the rest of circuit and refuse/reboot.
if the supply works fine, you have a problem on the board.
never been inside one, but i bet it is full of proprietary chips, which could mean a real fight to get it going again.
get a magnifying glass and go over the whole board. you might have to remove a lot of stuff to do this, but 90 percent of all problems can be spotted visually. probably a cap, or a dial lamp wire or something. chips don't usually break. maybe the pwr supply filter caps have gone south. put an ohmmeter on the pwr lines that feed the board with the pwr supply disconnected and see what you get. if you are really frustrated, rich, and about to give up and toss the thing, a dangerous last resort is to jumper the fuse and plug it in for 5 seconds.wear safty glasses! then feel around for heat. be warned this is a last ditch efort, but i do it all the time to find hot chips. and i know i will catch a lot of crap for this.
cj
oh,
keep the fire extinguisher handy!
(note my signature!)
:razz:

 

[SonsOfThunder]

[quote author="cjenrick"]a dangerous last resort is to jumper the fuse and plug it in for 5 seconds.wear safty glasses! then feel around for heat. be warned this is a last ditch efort, but i do it all the time to find hot chips. and i know i will catch a lot of crap for this.[/quote]
I will do this sometimes at my day job in order to find a problem. I like to say: Don't turn it off 'til the smoke stops rolling! That way you know you will be able to see where the problem is. Being in R&D, its okay if I break something or totally fry it, though...YMMV!

keep the fire extinguisher handy!

Always...this is how I got my nickname..."Charcoal Charlie":razz:

Peace!
Charcoal

 

[MLM]

thanks very much for the help! :thumb: :thumb: found a broken track joining the 2 resistors, so i sorted that out, put another fuse in and powered up, now the fuse doesnt blow at all - with or without the main board connected, but there's no voltage out of the P/S. any ideas?
thanks again :grin: :grin:

 

[CJ]

start with the transformer.

Charcoal Charlie! I love it!

A more sophisticated technique on the smoke test would be to hook it up to a variac and put an infrared camera on it, but who has access to one of those expensive gadgets?

 

[SonsOfThunder]

[quote author="cjenrick"]Charcoal Charlie! I love it![/quote] :grin:

A more sophisticated technique on the smoke test would be to hook it up to a variac and put an infrared camera on it, but who has access to one of those expensive gadgets?

How about just using your finger? Slightly wetted of course.

Peace!
Charlie

 

[Category 5]

[quote author="cjenrick"]start with the transformer.

Charcoal Charlie! I love it!

A more sophisticated technique on the smoke test would be to hook it up to a variac and put an infrared camera on it, but who has access to one of those expensive gadgets?[/quote]

Any Sony Handycam with "Nightshot" will automatically pass infrared while filtering out all other light.

Shane

 

[CJ]

Cool! I have an old 8 mm.