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Electronics => Repair => Topic started by: mckenzie on September 13, 2015, 02:07:59 pm

Title: Help identifying these old SMD compponents, Lexicon PCM80 repair.
Post by: mckenzie on September 13, 2015, 02:07:59 pm
Newb to this forum so hello, nice to be here!

Hoping someone is good with SMD component identification. My Lexicon PCM80 Digital Effects Processor (made in 1995) VFD display has failed due to a bad cap leaking everywhere.  The unit still works but with no display. I've cleaned things up & buzzed out the tracks which miraculously are all ok. Problem im having now is identifying the SMD components that test bad.

ZD1 - has the identifier '30' printed on it, obviously some kind of zener but what value?
D1 - some kind of diode, it has D3 printed on it ?
Also Q1 could be iffy, this has just got LG as an identifier on top?
Q2 looks like some kind of regulator & im going to assume its ok. This just has M Y printed on top?

There are no schematics for this module hence the guess work. Also finding & fitting a new display module would be very difficult/costly.
Thanks for any help
Title: Re: Help identifying these old SMD compponents, Lexicon PCM80 repair.
Post by: Kjelt on September 13, 2015, 06:50:26 pm
Did you look here?
http://www.marsport.org.uk/smd/mainframe.htm (http://www.marsport.org.uk/smd/mainframe.htm)

There are many more smd code sources, just have to see which package it is match the code, download the datasheet and check if the connections are right.
For zeners double check the voltage with a small current on lab psu and take the highest wattage available in that package if there are more versions available.
Title: Re: Help identifying these old SMD compponents, Lexicon PCM80 repair.
Post by: Len on September 13, 2015, 08:41:34 pm
Q2 looks like some kind of regulator & im going to assume its ok. This just has M Y printed on top?

It looks to me like Q2 is a MOSFET for the switching power supply. A regulator would properly be labeled "U" not "Q".
Title: Re: Help identifying these old SMD compponents, Lexicon PCM80 repair.
Post by: mckenzie on September 14, 2015, 02:25:10 pm
Thanks for the replies & thanks for correcting my mistakes. I was a bit lazy initially with this as i didn't bother to try to understand how the circuit functions as my knowledge is very limited (obviously!) , I thought i'd just try to replace the components that were bad. So anyway yes, after having a closer look at VFD's in general this part of the circuit looks like a switching power supply for the VFD with Q2 doing the pulsing.

The components are all SOT23 apart from Q2. I think I can be fairly sure ZD1 is a 30v zener. D3 looks to be a switching diode (1SS187). Q1 is tricky to identify though, some sources say its a voltage detector (RT9819) & others say it's a standard BJT ? If Q1 was a voltage detector pin 3 would be VDD which is connected to the smd fuse & V+. If it were a transistor pin 3 would be it's base.

Sorry for my poor understanding of this, this post should probably be in the beginners section.

Thanks
Title: Re: Help identifying these old SMD compponents, Lexicon PCM80 repair.
Post by: marshallh on September 15, 2015, 07:35:36 pm
Some more pics here https://www.usbid.com/parts/M202SD01H (https://www.usbid.com/parts/M202SD01H)