Lexicon PCM70 repair

[salomonander]

hey
i will just be lazy and copy my gearslutz thread maybe someone here has an idea:

so my pcm70 has massive amounts of white noise on the wet signal. the dry one is fine. the reverb also seems to be working - its just super, super noisy. the noise seems to change with the programm that is selected. its white noise and distortion to my ears.
i did change all caps (except for that odd .22uf tantalum i found hidden today) and made sure that every ic sits tightly in its socket.

and today i wrote:

ok - so cleaning and reseating all chips did not help either.... search continous
supply voltages measure fine. except for the ac line. it should carry 7-9vac but does carry 12,9vac. weired - i dont see how this could be changed - maybe im measuring something wrong. the line seems to feed the meter only and that runs without a problem.

i did the diagnostic as described in the service manual for noisy or distorted wet signal (section 5-6) and it seems as if the problem sits in the high speed procssor or wcs according to the manual. when in self test mode there was no reverb but clean audio in 100% wet. i also tried swapping the rams around as described in the next section of the manual but no change either.

another thing that might help is the input pot. when in 100% dry the pot acts smooth from quiet to full volume. when in 100% wet its sounds as if there was a loose contact. the wall of noise can not be turned up smoothly. it breaks up in the first quarter of the pot (due to little level i guess) and then works properly. same when i hit the input pad - the noise kind of cuts out like a bad connection.

the input level shown on the frontplates leds varies with the programm selected. i guess this should not be the case.... especial on the preset "major chord" it totally craps out in the reds and the sound gets even worse (if thats even possible

still, all effects work as they should (behind the wall of noise) - and when the unit is in self test mode the wet signal is clean (but has no effect any more)


here is a link to the service manual:
http://www.orner-stepp.com/wp-conten...viceManual.pdf
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[rock soderstrom]

Hi, the link to the service manual doesnt work. Cheers

 

[Rob Flinn]

Look towards the bottom of this page

http://www.orner-stepp.com/bill/lostmanuals/

 

[rock soderstrom]

Thanks! 

 

[beatnik]

maybe the problem is in the analog section and not in the digital one

as you say the potentiometer is acting weird i would definitely try to replace that as also it's easy and cheap to try

 

[salomonander]

maybe the problem is in the analog section and not in the digital one

as you say the potentiometer is acting weird i would definitely try to replace that as also it's easy and cheap to try

thanks, but if i read the service manual correctly it cant sit in the analog section since the unit performs well in self test mode. as far as i understand the self test mode, it uses the analog sections, AD and DA, as well as the mix sections. so i guess they are all fine.
since the pot acts normal in self test also i assume its not the issue either. but i might have missunderstood the manual. correct me if im wrong please....

 

[beatnik]

i am not a digital expert but first thing i would do is to be sure the power supply is 100% correct. i fixed some pcm and that was the problem. the fact that you have a out of spec voltage on Vfil is a bit suspicious, i would try correct that first, even if it's just powering another part of the circuit, maybe the meter can inject noise into audio ?? just guessing here

did you try to reset the memory as displayed in 5.3.1.0 ?

another possible cause of noise is a bad u67 socket as described on the field bulletin at the end of the manual

hope you get this sorted

 

[salomonander]

thanks beatnik

the idea about the meter possibly injecting noise is very interesting. ill give it a try. i will check that power line again and see what is wrong. i have not recapped the psu but measured the caps for uf and esr. they measured great. but maybe they still cause funkyness...

 

[beatnik]

i had dodgy power supplies and just replaced them. lexicon still supplies replacement, i think i have paid something like 40-50 euros.

 

[salomonander]

havent resolved the issue in the psu yet. there are basically two parts an both seem to be fine. but its tough reading the schematic correctly there.

what i just found out is that the unit works flawless on really low levels. there is a certain threshold that will trigger the white noise. but when i keep the input level very low the unit does just what it shoud....  when i approach the threshold slowly with the pot i can hear the white noise creeping in - i can then still back off and it will go down. once i go over the threshold the white noise is triggered and stays regardless of how the input pot is set at a very loud volume. i can still trigger the wet signal with the pot and hear it working but its covered up at that point. maybe this helps somehow.

 

[prulup]

havent resolved the issue in the psu yet. there are basically two parts an both seem to be fine. but its tough reading the schematic correctly there.

what i just found out is that the unit works flawless on really low levels. there is a certain threshold that will trigger the white noise. but when i keep the input level very low the unit does just what it shoud.... when i approach the threshold slowly with the pot i can hear the white noise creeping in - i can then still back off and it will go down. once i go over the threshold the white noise is triggered and stays regardless of how the input pot is set at a very loud volume. i can still trigger the wet signal with the pot and hear it working but its covered up at that point. maybe this helps somehow.

Hi, I have the very same problem, did you sorted it somehow? Thanks!

 

[radardoug]

Are you testing this by itself on the bench or through a mixer? Did you do a hard reset?