Repairing my Lexicon PCM81 (Fixed & Video)

  • Thread starter Thread starter glpg80
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glpg80

glpg80

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So I recently scored a NOS Lexicon PCM81 made in 2000. It still had the VFD screen protector over it. The unit has a common input gain adjustment problem where it peaks the rails of the outputs anytime the input level is adjusted. But when the input knob is left alone, the peak noise and popping will settle and the unit will operate as intended.

Lexicon buffers the input level with two op-amps in stereo, and sends the signal to a stereo dual gang potentiometer. The output of that is then amplified again in stereo before going to a custom lexicon ADC. I suspect the potentiometer is bad and ordered a replacement ALPS 10k dual gang pot. For anyone that wants to know, the 15A is not for the amperage rating on the pot, but the friction measurement. Lexicon originally spec'd ALPS potentiometers, and sometime later they stopped using them and switched to another type.

Here's it's current state. I'll update once I get it back together and repaired to see if my hunch was correct:


IMG_3739.jpg

IMG_3741.jpg
 
Nice score!

I recently replaced the volume pots in my 1996 Carvin F150 for the same issue. Cleaning them helped, but it didn't cure the problem completely. The pot in your pic looks nearly identical to the Carvin pots I replaced. Replacing the pots solved the problem. My PCM70 peaks higher, depending on the preset. I've often wondered if this has anything to do with the pot?
 
Does the PCM70 have a digital gain adjustment? The 81 has both analog gain and digital gain. The service manual discussed how to set both in general for a troubleshooting suggestion on decreased noise.
 
I haven't had the PCM70 for very long. A couple of months at the most. I've looked in the manual, but I haven't found anything that directs me to internal volume level settings. Effects levels only. +/- effects vs dry. It doesn't audibly clip when it goes to red, so maybe it's normal for this box? The level increases when running chorus and delay together. I need to spend a full day with it. I tried to MIDI switch it, but I guess it has to be setup for that.
 
A lot of guys will run an old school 160A compressor/limiter just before their PCM70 to prevent something like this happening I believe. The older the gear, the more sensitive the inputs are to level changes. The input gain can be tapered, but the change in amplitude can cause some aliasing to occur. I think the PCM70's have 16 bits right?
 
I believe it is a 16 bit device. I nearly purchased a Rockman compressor last week, but decided against it at the last second of bidding. My Rockman head (B rig) is quite compressed, so I'd like to avoid compressing the Marshall side. The PCM70 clips visually, but the sound is spectacular. No pops or fizzies at all. I especially love the tiled room and circular delay presets.
 
TrueTone500":2w8270c3 said:
I believe it is a 16 bit device. I nearly purchased a Rockman compressor last week, but decided against it at the last second of bidding. My Rockman head (B rig) is quite compressed, so I'd like to avoid compressing the Marshall side. The PCM70 clips visually, but the sound is spectacular. No pops or fizzies at all. I especially love the tiled room and circular delay presets.
Click to expand...

Fun fact, the tiled room runs a randomized reverberation algorithm on that preset and that preset only for the PCM70. It's the same algorithm one of the original lexicon engineers created when it was founded.
 
glpg80":3ml98ssn said:
TrueTone500":3ml98ssn said:
I believe it is a 16 bit device. I nearly purchased a Rockman compressor last week, but decided against it at the last second of bidding. My Rockman head (B rig) is quite compressed, so I'd like to avoid compressing the Marshall side. The PCM70 clips visually, but the sound is spectacular. No pops or fizzies at all. I especially love the tiled room and circular delay presets.
Click to expand...

Fun fact, the tiled room runs a randomized reverberation algorithm on that preset and that preset only for the PCM70. It's the same algorithm one of the original lexicon engineers created when it was founded.
Click to expand...
That's very cool. Can the later Lexicon boxes load the same settings to achieve the same results? If yes, I'll write down the settings. The only other Lexicon boxes I've owned are MPX1, MPX G2, and the PCM42. My PCM70 is in near-mint condition. I waited until I could find a good one.

RYFuHEX.jpg
 
The only other unit that is known to run that algorithm, other than the 224, is the lexicon model 300 which runs it on everything. However, the 300 doesn't quite have the same warmth as the 60, 70, and 80/81. The tiled room is the only reverb patch in the early PCM series to run it. To get the same warmth and also have the algorithm on all reverbs, you'd have to find a 224XL.
 
So far, I'm liking the PCM70 more than any delay I've owned. Even more than my TC2290. The TC was cool with its switcher system and all, but it just seemed to lack the warmth of the PCM42 and PCM70. Maybe I didn't have it setup properly, but I spent a lot of time with it. I really liked the flanger. For a digital flanger, it sounded great. Took me a long time to let go of it, but I honestly do not miss it. Oh... I had a PCM60 as well. Excellent reverb!
 
I've never been a fan of the 2290. It just thins everything out way too much and makes it seem sterile. I'd rather have a simple carbon copy pedal delay over the 2290 any day. It's hard to beat what a lexicon reverb brings to the table. I did a lot of comparisons with eventide stuff and even strymon. For me, I really liked how the PCM kept the dynamics, warmth, and overall detail of whatever it was you were playing. The pedals just don't even come close IMO.
 
Only fx unit I've ever heard that sounded better than a PCM70 was a 480L.
Absolutely stellar sounding.
I'd have to check the manual, but I believe only version 3 of the pcm70
responds to program change messages. On version 1 and 2, the Midi was for sysex backup and preset loading. Also, only version 2 had the tiled room preset, but version 3 will load the preset if you have the sysex data ( which I do ).
My v3 pcm 70 is one of my most cherished pieces of gear. Had it for about 10 years. You should hear it on vocal recordings. Outstanding.
Sounds great on everything.
Congrats on finding a nice one at this point in time. They are becoming very hard to find in good working order.

Edit: Sorry, didn't mean to derail the thread. Nice job on the pcm81!! Hope you get it sorted.
 
Spent the rainy night tonight putting it back together and was able to get it fixed!
 
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