Lexicon PCM70 no reverb

[radardoug]

Any Lexicon PCM70 experts on here? I have one which outputs clean audio but no reverb. Seems to be stuck in some sort of straight through mode.
I can pull all the rams in the WCS and still audio. I can pull all the audio memory rams and still audio.
Anyone got any clues?

 

[RoadrunnerOZ]

Is the Bypass switch LED on?

 

[radardoug]

No, its not in bypass, but I just did a double check, the wet was at 50%, turning that to 100% I got no audio.

 

[RoadrunnerOZ]

No, its not in bypass, but I just did a double check, the wet was at 50%, turning that to 100% I got no audio.

Ok cool. I’ll have a look at the schematic later - I just did one recently with a similar problem so I’ll have a look at my service notes as well. Just out onsite at the moment.

 

[RoadrunnerOZ]

Section pages 5-5, 5-6 of the service manual outlines problems and tests for no wet signal. The DAC for wet is U15 and there should be output on pin 21 - if no wet both left and right it’s likely the DAC or the comparator U16 or the associated circuitry controlling these.

 

[Tubetec]

You could try a hard reset just in case the memory is scrambled ,
https://www.sweetwater.com/sweetcare/articles/hard-reset-for-lexicon-pcm70/

 

[radardoug]

No, I have done that, first thing I did.

 

[CJ]

OT; how does the bypass jack switch work on these, is it just a 1/4 inch phone jack, coax cable out to a on / off push button switch? thanks!

 

[RoadrunnerOZ]

OT; how does the bypass jack switch work on these, is it just a 1/4 inch phone jack, coax cable out to a on / off push button switch? thanks!

The bypass switch on front is momentary latching but the bypass jack socket requires the switch to lock on or off like a stomp box switch

 

[CJ]

Cool! i want our lead singer to be able to switch that on for the middle of Venus by Shocking Blue when Meriska is going aahhhhh ahhhhhhhh aahhhhhhh, etc

 

[RoadrunnerOZ]

The bypass LED should light up when the foot switch is used - it just grounds the tip to the sleeve and goes through a buffer IC 74HCT244

 

[RoadrunnerOZ]

The service manual indicates a locking switch but I’d try a momentary switch or touch bridge tip and sleeve on a jack plug just to test and make sure it’s not self latching

 

[RoadrunnerOZ]

Standard tip and sleeve 1/4”

 

[radardoug]

Well, made some progress on this unit. Outputs were low, and it turned out to be the muting fets. The output circuit is a real mess. Output from the last opamp goes through a 10K resistor to the level switch. There a 680 is switched to ground or not for the pad.So the unit has 10K output impedance!
There is also crosscoupling of the tip normalling contact that means with only one jack in, the outputs are connected together, and then there are protection diodes to the rails. All crap that does not need to be there.
Then the DAC that controls the wet/dry was faulty, only on the wet side. So now I have that all working.
The fault is still no reverb, but TIP: If you want to know if the WCS is going, just take out the WCS memory chips. Output will turn to white noise!
So the WCS works. I'm waiting on an ARU chip, probably not that. CMU chips look rarer than hens teeth.

 

[RoadrunnerOZ]

Output from the last opamp goes through a 10K resistor to the level switch. There a 680 is switched to ground or not for the pad.So the unit has 10K output impedance!

The 620Ω (as shown in my SM anyway - maybe 680 in your unit) goes in parallel across the 10K in non-padded mode by the look of the schematic, so more like 600Ω-odd output impedance

There is also crosscoupling of the tip normalling contact that means with only one jack in, the outputs are connected together, and then there are protection diodes to the rails. All crap that does not need to be there.

The tip normalling line is so the left out or right out by itself is mono, usually for live use (or if you just don’t want stereo).

 

[radardoug]

Ah yes, you are right. Missed that. If you are using it mono, why mix the two sides? What does happen is when a mute fet shorts, it can stop both outputs.

 

[RoadrunnerOZ]

It’s just the way the system works - live sound is largely mono and stereo effects would be half correct - like a ping-pong delay would only have half the echoes so they sum the outputs to mono unless a second jack is plugged in - same as digital pianos, synths and keyboards of any kind which normally have left/mono jack which normalling is broken by inserting the right jack.

 

[RoadrunnerOZ]

This unit is mono in, mono or stereo out - unlike units that are multi in multi out with each channel being discrete or linked in pairs (like for example AMS) where you have routing options.

 

[radardoug]

Live sound is mono?

 

[RoadrunnerOZ]

Live sound is mono?

Largely due to audience placement - if in true stereo one side of a large auditorium employing a dual stack PA would be missing half the sound. However with a lot of venues using multiple speaker arrays with focused sound this has changed to a degree over the years. In small venues however stereo sound works. In some of the larger venues a mix of mono and narrow width stereo is applied. It gets pretty complex as every seat needs to have full sound. Narrow panning, so front of stage listener gets the effect of instrument placement is common - as the band only occupies a small angle of front view of the audience, wide panning would put the sound outside of the player apparent position when the PA stack is far either side of stage. Focused arrays can send both sides of a PA to a fairly wide angle - but stereo only really visually benefits those close enough to a stage to perceive position of players - so for example front centre target zone may be narrow stereo and rest of hall in mono. A lot of mixing desks have left, right and mono outs, or you can use group assignments to create mono channels for zone feeders.
Imagine trying to get a decent sound in full stereo to 60,000 people spread around an arena. Best of luck!!
Some effects and indeed some synth sounds (like from Access Virus) will collapse in mono and these will be in stereo - any effects that employ motion will suffer or disappear in mono.