PCM70 problem,please help.

[lydmann]

I have an old Lexicon PCM70 that is acting up.
The LED display lits up all the way,even with no input.
The unit passes dry audio when I set the wet/dry mix to 50%.
It looks like all the menus and display is funtioning properly,but no effect comes through.
When I set the MST LVL (master level) to less then -16, the LEDs turn off,and the meter shows the audio input as normal.
The voltages seems normal,+15V,-15V and +5V are within normal limits( about 2% off at the most).
Tried to replace the lytics, but no change...
The backup battery seems OK (3V).
I have also done the reset procedure,with no luck.

Has anyone had this problem?
Can it be the bypass capacitors in the digital section? They are little green ones...
Or is it more likely that some of the IC`s are broken?
I have read that the IC sokets in old Lexicons oxidize, what cleaning solutions are recommended?

Does anyone here have the schematics? (please PM me if you do)

Any help is really appreciated!

Kåre

 

[lydmann]

Update:

After getting the schematics :grin: I did some reading.
It looks like the ARU unit is sending an overload signal to the LED meter.
Now I wonder what causes this.....

I see there is a lot of people checking in on this post,but no replies.
Does anybody out there have any knowledge on digital reverb troubleshooting and construction?
Any input will be appreciated.


Kåre

 

[dale116dot7]

A bad DRAM chip can do this. I have a PCM60 that just had this problem. Remember, the guts are very static sensitive so make sure you work in an ESD-safe manner and in that kind of area. Does it make any 'rasping' digital noises? I can suggest a few things to 'scope except that I don't have PCM70 schematics, just PCM60.

-Dale

 

[lydmann]

The sound I get through, only dry, is without any noises of any kind.
The wet signal is totally silent,except when I load a program. Then I can hear a little "pop" with what sounds like a reverb tail.

Do you have the PCM60 schematic in .pdf? I would love to have that one too :grin:
I can send you the PCM70 schematic, just PM me your email adress... It is about 16MB including service manual.
Would be nice to compare the PCM60 to the PCM70 topology.
The service manual is very good for troubleshooting, and explains the circuits pretty well.

Guess the DRAM chips are hard to find now? Looks like HM50464, schematic says 4064 (64KX4, 120 ns).
Havent found any decent data sheet on these yet.


Thank you for responding! :grin:
I think it is worth a little effort to keep these machines running. Great stuff!



Kåre

 

[SSLtech]

Schematics in electronic form???

-Can I get a copy?

Keith

 

[mata_haze]

I'd LOVE a copy too...

mattia.

 

[Walrus]

*polite cough*
I wouldn't mind one too....

 

[lydmann]

Solved. It was the comparator LM211 that was faulty.

I had some great help from someone at the forum. :grin:
Thank you again!!

A very helpful person with great knowledge.

I hope this post will help someone get their old Lexicon working again.
They are great machines.

Kåre

 

[gyraf]

And - as always, when you have interesting stuff in electronic form - please remember to forward a copy to:

groupdiy (at) gmail.com

:razz:

Jakob E.