Lexicon PCM 60 troubleshooting

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kosty

Well-known member
Joined
Dec 8, 2009
Messages
114
Location
Vienna, AUSTRIA, EU
Hi guys,
I recently got a broken PCM 60 (with v3.0 board) and decided to give it a try.

The symptoms :

- On both Plate and Room settings the meters are hot on most size and decay settings. This happens randomly and not always but more often on some settings. (Usually larger size with longer decay. Small size and short decay is always OK).
- When the meter are overloaded, the outputs emit a high pitched noise, reminding of a feedback loop (mic in front of monitor). The main signal passes thru but with a significant lower level than the noise.
- Bypass is working fine
- Room reverb seems to work fine on working settings (very few)
- Plate setting has some kind of weird delay (see next 2 points)
- When the Mix pot is set to Wet, the stereo image shifts to the right size. The wetter the signal, the higher the output is on the right side. I think this is caused by the pot itself, since this behaviour changes by wiggling the pot. I may exchange the Mix pot with the Output Pot since both are the same model (10K x2, 6-pin).
- When 100% Wet and set to PLATE, I can hear a dry signal (not 100% sure if Dry or processed) on the right output and a processed signal with a significant delay on the left input. It sounds like a ping-pong delay effect.

What I have done so far (with proper ESD precautions):

- The previous owner apparently already changed 3 out of the 4 DRAM chips. I replaced the position of the DRAM and the behavior didn't change.
- I have reseated the 4 Roms
- I have set Jumper W22 to test mode to bypass the digital circuit. AD/DA seems to be OK and no meter overload shown.
- TP6 and TP7 shows 0V AC/DC with no input signal
- I have removed all 4 DRAMs -> meter doesn't show any problems.
- With only 2 DRAMs inserted in the sockets on the right (U23 + U26) the meter shows no problems.
- With only 1 DRAM inserted in the second socket on the left (U21) the meter problems appears but only shows level to -12 or -6dB.
- With only 1 DRAM inserted in the first socket (U18) the meter shows full overload.
- I have tried different DRAMs in both sockets with no difference, meaning that probably the DRAM chips are OK.
- It doesn't matter if there are other DRAMs in the sockets U23 and U26.
- When the first 2 DRAM are removed, there is some kind of white / pink noise on the outputs + signal (can't say if dry or wet signal)

What I think where the problem is :

On page 8/22 of the schematics, you can see that the 4 DRAMs (U18, U21, U23, U26) are connected :
- to the MMU (U29) with the traces YA0 to YA7
- to the ARU (U24) (Schematics page 1) with the traces DAB0 - DAB15

The traces to the ARU affecting the meter overload coming from the DRAM sockets (U18 and U21) are D8-D11 on the second DRAM and D12-D15 on the first DRAM.

My questions for you now :

1.) Can the problem be caused by ARU oder MMU ?
2.) Can the Roms be causing this ? (4 chips with the inverse room are on the way to me)
3.) Any ideas how to fix this or what test points I should measure ?

I have the following tools to my disposition : Multimeter, Digital Oscilloscope, Signal Generator, Fuzzmeasure Software)

I hope someone can help me (maybe Dale ?) and I would be really thankful to any ideas / hints and I'm looking forward to learn something new !
Cheers
 

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