PCM 70 no wet output

[geezbill]

I've done the usual things resetting unit, replaced battery, I changed the compataror LM211P. reseated memory, DAC, etc. All electrolytics new and voltages correct. I get no audio when wet is set to 100%. The service manual states to run a signal generator and check signal from TP7. I have scoured the mainboard many times and cannot find a TP7 anywhere. TP2 and TP13 both have no output. Any help would be appreciated.

 

[radardoug]

I have been working on a PCM70 with the same problem. I have spent a lot of time on it. I did find a one byte error on one of the proms, but that did not fix it. I have concluded it is the CMU chip, which is unobtainable. I have yet to absolutely verify that, but I am pretty sure. Does anyone know what circuitry is in the CMU?

 

[geezbill]

I have been working on a PCM70 with the same problem. I have spent a lot of time on it. I did find a one byte error on one of the proms, but that did not fix it. I have concluded it is the CMU chip, which is unobtainable. I have yet to absolutely verify that, but I am pretty sure. Does anyone know what circuitry is in the CMU?

you wouldn't happen to know the location of TP7 on the board?

 

[radardoug]

Most of the test points are on the left hand side, study the schematic and look close to the ics by TP7.

 

[geezbill]

Most of the test points are on the left hand side, study the schematic and look close to the ics by TP7.

believe me i looked multiple times even with a magnifier. i think TP7 doesn't exist on my board. i swapped around all the DRAM chips and on startup briefly get a wet signal then it goes away. I might order new DRAMs

 

[geezbill]

this board has some tantulum caps i think i'm going to replace them with electolytics. 35 year old tantulums might go bad

 

[geezbill]

no change with the tantulums replaced

 

[zamproject]

I have scoured the mainboard many times and cannot find a TP7 anywhere

Schemo sheet 9/10 -analog motherboard-
rev 3 description:
...deleted TP7 and TP8...

 

[geezbill]

Schemo sheet 9/10 -analog motherboard-
rev 3 description:
...deleted TP7 and TP8...

thanks i tried searching the pdf but it wasn't searchable (was in image format).

 

[zamproject]

I read it from the image...

 

[geezbill]

ordered replacement drams from china. not paying $10 apiece on ebay for them so i gotta wait for next steps

 

[geezbill]

replaced the drams, the AD converter, the comparator, replaced the Z80's , replaced the WCS rams. all caps replaced, replaced firmware chips to 3.0 . . unit still has no wet output. fml

 

[radardoug]

You may have the same problem I have, the Lexicon custom CMU chip. If you find some, let me know.

 

[pucho812]

Cmu chip was mentioned here.
It is a Harris ic chip
https://groupdiy.com/threads/looking-for-a-lexicon-pcm60-or-pcm-70-cmu-40-pin-ic.80496/