Dead Lexicon PCM70, need some help.

[radardoug]

I am working on a Lexicon PCM70, it has no reverb, otherwise everything functions. I bought an HP signature analyser and went through it with that, all chips give right signatures except two, but replacing those chips does nothing. I am starting to suspect two programmed chips in there, U48 and U49. I do have a reader, but need a working unit to read the chips in. Can anyone help?

 

[cavedata]

I am working on a Lexicon PCM70, it has no reverb, otherwise everything functions. I bought an HP signature analyser and went through it with that, all chips give right signatures except two, but replacing those chips does nothing. I am starting to suspect two programmed chips in there, U48 and U49. I do have a reader, but need a working unit to read the chips in. Can anyone help?

 

[radardoug]

Awesome Cavedata! I was thinking about this unit this morning and thinking I might have to declare it unfixable. Back to work! Thank you so much.

 

[radardoug]

An update on this one. The 74s288 has one byte wrong out of 32. I will get another one burnt and update.

 

[Ivan Blajer]

G'day any update on this? I have PCM70 with teh same problem no wet signal. Also the level meters pegs to max after you load a program.

 

[radardoug]

I have someone testing the CMU chip, but it will be a while. Sounds like your problem may be different. Get the manual and spend many happy hours working on it!

 

[amoreno]

Hello, I have a unit with no wet audio too. Please could you tell me which programmer you have? if you put W8 jumper in s-t do you have signal on tp1/12? because I have signal, so maybe a high speed processor problem and u48/49 are very hot. I try to replace but need a programmer to dump the files that are in the post. I swap memory and nothing change. BR, Antonio.

 

[radardoug]

I get the GALs programmed by a guy on Ebay. His email is [email protected]. He does this as a service. You need to provide him with the GAL files. But its more likely the CMU is faulty.

 

[radardoug]

I have just done a board which will replace the CMU chip. I'm getting them made, and will post if they work.

 

[shortbread]

I have just done a board which will replace the CMU chip. I'm getting them made, and will post if they work.

Hi radardoug,

Did you have any success with the board you made to replace to the CMU?
I've got a PCM60, with a CMU that holds DAB15/D15 low at all times and have been looking for a way to replace it with a modern solution.
Unfortunately I have no idea what the CMU is actually doing, as I'm fairly new to digital audio - and the only mention of the CMU I've been able to find is on 10000cows:

"The CMU consists of two registers. One register system is a 16-bit successive approximation register. This uses the (single) PCM-53 DAC as an A-D converter. Those chips are expensive so you use one converter for both outputs and the input!!!. The SAR, comparator, and DAC have a nonlinear settling time, so the program provides timing pulses for the sequence, as well as DAC and sample+hold timing. Clever. For writing, the CMU contains a DAC output register which is sent to the DAC. I need to look at the specific timing of this to determine if it is a simple transceiver or a latch.... I expect a latch. Possibly it's the same latch that performs the SAR function."

This is a little over my head at this stage, so I'm off to investigate ADC/DAC registers and see if I can wrap my head around it.

Good luck with the project!