Lexicon PCM70 repair help

[beatnik]

i have bought a Lexicon PCM 70 as non working with the intention of repairing it, or at least wasting some time trying to understand obsolete 8 bit digital circuits...

i have managed to get the analog path working. all the power supply rails are fine.

the main problem is in the digital circuit. the processor is basically not turning on properly and when the unit is powered on the display is not lighting up

after reading the very useful troubleshooting procedure in the service manual i have scoped out the master processor section and found out there is problem with data line 7 (pin13 on the z80 cpu) which is not displaying data and is stuck high.

the remaining data lines and all the address line are showing some data on the scope so it seems the processor section is at least partially working

i have tried removing ROM, RAM and ADC chips to see if they were keeping the data line 7 high , but they don't.

The rest of the i.c. are not socketed. I have removed a couple of the logic chips that are on the data lines and I am waiting to put fresh ones in and see if it makes a difference.

There are more i.c. that I could try swapping out but it's kind of a shot in the dark

Just thought I might ask some help here and see if there is someone around that knows these kind of units

 

[beatnik]

removed one more of the logic ic  and i am now seeing activity on data line 7

i am going to stick new ics in and see if the system starts up properly

 

[beatnik]

after replacing U61 all the cpu data lines are displaying data. the address lines seem fine as well, at least on the scope

before replacing U61 i had the pushbutton leds lighting up randomly when the unit powered up. this issue is now gone so i am thinking at least i made some improvement but the processor is still not starting up

service manual says check pin 11 of U27, U39, U38 and U51 for clocking of display data. if there is no clocking, U59 might be the cause

i have +5V here instead of a clocking signal and unfortunately I have already replaced U59 to no avail

i need to find out where the clock signal is being lost, but things are starting to get very complicated

the manual only lists U59 as a possible problem for a missing clock but actually this same signal is going to many places in the circuit.

I don't understand what part of the circuit is actually generating the clock, if only I knew that I could try disconnecting things in order to isolate the problem, maybe someone can suggest this ?

service manual can be downloaded here

https://www.manualslib.com/manual/1003004/Lexicon-Pcm-70.html

schematics are page 75 and troubleshooting guide page 54

 

[Evertide]

Do  you have 13 MHz at pin 3 of U50?

 

[beatnik]

thanks for your reply

I have just checked and there is 13MHz on the quartz oscillator

the following net labeled MC is also measuring 13MHz. is this actually the master clock signal ?

I am going to try follow this signal and see if I can spot any problems along the line, but to be honest I am quite lost at this point, and any suggestion would be very helpful

 

[Evertide]

See page 96 right bottom , the 13 MHz is the master clock.

See page 77 left top ,  the U 50...

 

[beatnik]

right so at least we know the master clock is fine

in fact i had previously checked the clock on the Z80 cpu and that measured OK

 

[beatnik]

I should add that I had to replace the display with a new one because it was cracked, so I am sure the display is fine

The problem with this unit is that someone has been messing with it before. The analog section was completely wrecked, with ics removed very poorly and damages to the soldering pads. I have managed to get the analog stages working, and luckily it seems that they didn't touch the digital section, but there are definitely problems here as well

 

[beatnik]

done some more fault finding and noticed that pin 20 of the z80 master cpu was at +5V . this is the I/O request signal

i have substituted the master cpu with the slave and started seeing some kind of data on pin 20, that goes nicely through U59 as well as display logic and driver ics

the display finally started lighting up as well, but showing just an incomprehensible mess

i will try a new z80 and hopefully will be able to boot power up diagnostics at least

 

[beatnik]

after replacing the duff z80 cpu and doing the memory and register reset procedures i can now get the device to boot up properly and the display working nice, programs load and i can edit the parameters

there are still some problems though, all the leds on the input meter are stuck on and i cannot hear fx signal on the output

i will have to do some more debugging but i am glad at least the main computer section seems to be working fine now, hopefully the fault is somewhere in the analog stage or at least not too deep in the conversion stages

 

[ijzerpro]

Check R80 , 5.1E . That is the connection between DGND  and AGND.

 

[beatnik]

R80 checks ok

i have started scoping out the analog section (service manual page 84) and found that the input signal interrupts at vca U14 pin1 input

there is signal on the other leg of R90

do you think the vca i.c. is defective or there could be something else pulling the signal down ?

 

[beatnik]

i have pulled U14 out and tried a new 2150 vca i.c. but the signal at the input is still missing.

without the i.c. in the circuit there is signal on its input pad so this means there is something else bringing it down.

if i remove U22 which is the digital/analog converter and i put U14 back in then I have signal at the vca input, although all the leds on the level meter are still stuck on

 

[ijzerpro]

Check U19 on point 1 ( TP80 ) , and check  the VCA  if there is some voltage to open the  VCA anyway. ( check de datasheet ) You can also check that by TP18 . The board have TestPoints  for easy troubleshooting.

 

[beatnik]

thanks for your reply 

I have 0V on TP20 (U19 pin1) and -4.85V on TP18 - I suppose this is the voltage that opens the VCA ?

I also have -0.65V on TP15

 

[beatnik]

I have traced the signal back to U15  converter i.c. and on output pin 21 there is some signal that reflects the input level, although it seems a bit strange that the waveform is not symmetrical and the level increases only in the upper portion of the waveform

on TP3 and TP6 i have no signal. I am trying to trace the signal and find a potential problem but I cannot really understand this part of the circuit

 

[dave.n]

after replacing the duff z80 cpu and doing the memory and register reset procedures i can now get the device to boot up properly and the display working nice, programs load and i can edit the parameters

there are still some problems though, all the leds on the input meter are stuck on and i cannot hear fx signal on the output

i will have to do some more debugging but i am glad at least the main computer section seems to be working fine now, hopefully the fault is somewhere in the analog stage or at least not too deep in the conversion stages

i have a dead slave processor error on my PCM70. where did you buy your replacement z80 cpu?

 

[charlesaudio]

i have a dead slave processor error on my PCM70. where did you buy your replacement z80 cpu?

Seems to be still available on digikey :
https://www.digikey.ca/en/products/...DaIATAGwCcIAugL6OEVkgBacALAMIAMgqgAUAogHEGjIA