Lexicon PCM70 Repair

[Siegfried Meier]

Hey guys.

I have a PCM70 that doesn't seem to have any input, or output.  When I fire it up, the meters peak to full (as is common with these series of units) so I know the meters are working, and all other controls and functions appear to be working, but I can't get any audio in and out.  Anyone know where to start looking, or is there a common issue with these that anyone is aware of?

Thanks for any help or advice you can give.
Sig

 

[QUEEF BAG]

have you looked for voltage on the power pins of the chips?

wait...what does the display look like?
functioning normally or garbage in the display?
if the display is filled with odd characters, you may only need to
reload the program.  though that would wipe out yer registers.

if display is working or totally dark, it's probably a mission inside

you can download service info from lexicon at no charge!

go to lexicon ftp site

ftp://ftp.hmgsl.com/LEXICON/legacy/

user name: customer

password: service

 

[Siegfried Meier]

Display totally works...

Thanks for the link, I'll do some diggin'!

Any advice anyone else has would be awesome!

Thanks,
Sig

 

[Siegfried Meier]

So, still can't seem to figure out this thing, but I noticed today that when I put my finger on the back of the input pot, and somehow shorted it temporarily, I saw the level spike on the meter and I heard a burst of lush, sweet, juicy reverb come out of the speakers.  This leads me to believe that my output stage is not cooked, since I can hear rev.  Something on the input is not jiving.  Do these input pots ever go bad?  I wouldn't think so...maybe I'll try resoldering all the connections?

 

[Siegfried Meier]

Ok, so I get 0.6v at TP18.  I also have 13.58v at TP14 and nothing at TP20, which according to this indicates there's an issue with the VCA (U14).

U14 is just an old dbx 2150a - any problems with putting a newer THAT 2150a in here?  Or some form of them?

Let me know if I'm off base on this info please, before I rip the entire thing apart...

Thanks,
Sig

 

[Siegfried Meier]

Well, I removed the 2150a, soldered a socket in place now, stuck in the 2181C that I had here but it didn't help...

Any other ideas?

Thanks,
Sig

 

[QUEEF BAG]

are you sure this is not an electro-mechanical issue?

if you can push on the input pot and get sound, (even occasionally)
it may be just a bad solder joint or connector issue.

got a non-conductive thingy to push and prod with?

i am unable speak to the VCA issue

 

[Siegfried Meier]

Well, I'm not getting any of the INPUT sound, so to speak.  I'm shorting out one of the pins on the input pot to get "hum", and therefore I'm able to confirm the output is actually reverberating.

I've resoldered the input pot, as well as the input 1/4" TRS connector, still the same thing...

 

[Siegfried Meier]

Well, I lied...

Now that I resoldered the input pot connections, when I short it out with my finger, I get input sound + reverb...!

So, what does this mean?  The input pot is pooched?

 

[Siegfried Meier]

Doh!!!  Upon closer inspection, the circuit board on the pot is cracked!!  I think I've found my problem...


70 manual states part #200-02616 - it's a 10k ALPS pot.  But I highly doubt that Lexicon stocks those anymore after all these years...?

 

[wtmnmf]

This is really - really common. 

The lesson here may be to check the simple before complex.  Is it on?, Does the signal get from the input to the circuit board?, etc. I find that complex systems lead people to think of complex failures, but the complexity usually means that a simple failure will take it down before a complex one has a chance.  Designers also tend to ignore the simple failure modes, and thus solder a weak pot to a circuit board, where it will surely fail if it receives a blow, and likely fail in time, due to solder joints flexing. So much time and energy put into the development of an elegant and beautiful product, only to be let down by a poor choice on the mechanical front. The pot is actually supporting the board here! On the other hand, simple systems, when they fail simply, are repaired without a second thought; but when we are required to think about it, the failure is often of the complex sort that we had formerly reserved (in our minds) for the complex systems.

I realize that it may seem odd to focus on the psychological aspects of troubleshooting, but when we approach a problem which is outside of our routine experience, the psychological aspects present a substantial barrier to adapting to the problem at hand and focusing quickly on the solution. I also think that the thrill of solving a complex puzzle leads people to be habitualized to look for solutions along those lines.  Doh!! and a slap on the forehead can be just as rewarding  though 

One thing that I like to do is to get a cotton swab and clean the boards and components.  It does nothing for the electronics, but it allows me to relax and take in the physical aspects of the thing, and also get familiar with the layout. Chances are good that I'll see a cracked pot or a cold solder joint. It also looks nice when you're done

Good for you for getting another PCM70 going.  Long live the PCM70.

 

[Siegfried Meier]

Found a part# that works from another thread here.  Had to bend the leads a bit and solder from the top, but it works perfect - panasonic pot - and the PCM70 is alive!!

Not bad for $200 and a $1.36 part!

Thanks for the help,
Sig

 

[QUEEF BAG]

nice job...sometimes ya gotta look for a while

love the pcm 70!

one guy i worked for for a while said 80% of audio problems
were electromechanical.

sometimes i think he was right.