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repair advice: sequential prophet 2002 (voice problem) wanted! Drum Machines & Samplers
Old 27th October 2014
  #1
Here for the gear
 

repair advice: sequential prophet 2002 (voice problem) wanted!

Hi all,

I just bought an prophet 2002 sampler that supposed to work great,
but when i got to test it, i found out that voice8 sounds about -50% in volume of what it should be. (half as loud as the other 7 voices.)

the seller is kind of being an asshole about giving me a refund or buy it back
i know.. i know.. i should have tested it when i picked it up, but the guy looked super stressed out and had an appointment with an imported client or something :S.. so now i need to fix it.

anyway!

so i went to swap cems, also with my prophet 2000 and they all work fine!
so this is not a cem problem. my guess..


i tried to do some measurements on the CEM3379 's
in 200V ac on my cheap multimeter i measured on:

pin 10 (ground) and pin 12 (GAIN C.V)

- 06.8 on the first 4 cems

- 09.1 on the last 4 cems

this in all modes.. when playing the same sample over all cems.

weird thing is that the first 4 cems sound equal in volume,
5 is a bit louder..
6 is a bit louder than 5.. then 7 is the loudest of them all..
and then voice8 that is at about -50% of what it should be..
so voice8 is the big problem here.

this is not what i measure from the cems. i measured big difference between the first 4 and last 4 cems.

it also does some other weird stuff like when in the top half of the presets,
when playing individual samples. it sudently becomes a 4 voice.. so it only uses the first 4 cems.

because of that weird behaviour i thought, could this be as simple as an system eprom problem? or ribbon cables?


could anyone give me simple instruction on how to measure the
control voltage/control signal of the volume from cem8/voice8 ?

or any advice on where to go from now ?

did i make correct measurements to check if the vontrol voltage reaches the cems ?

or would you think it's an op amp problem that happens after the cem path/circuit.. ?

I did think this because of the measurements i did, but i dont know mutch

although it's wierd that the first 4 measure 06.8 and the last 4 voices 09.1.

any id on what part may be broken

i first want to do some research myself before checking if i have enough money to have someone certified have a look at this.

thank you.

p.s. i will upload the control voltage schematics.
if you need more from the service manual i can upload that to.. and if you need pictures i will take them !
Attached Thumbnails
repair advice: sequential prophet 2002 (voice problem) wanted!-2000-service-manual-p047.jpg   repair advice: sequential prophet 2002 (voice problem) wanted!-2000-service-manual-p065-1.jpg  
Old 27th October 2014
  #2
Here for the gear
 

here are the voice schematics!
Attached Thumbnails
repair advice: sequential prophet 2002 (voice problem) wanted!-2000-service-manual-p048.jpg   repair advice: sequential prophet 2002 (voice problem) wanted!-2000-service-manual-p049.jpg   repair advice: sequential prophet 2002 (voice problem) wanted!-2000-service-manual-p050.jpg  
Old 31st October 2014
  #3
Here for the gear
 

Bump!
Old 1st November 2014
  #4
Lives for gear
 
acreil's Avatar
 

First, does it have some sort of auto-calibration mode?

Second, if you have an oscilloscope, look at the output of the CV DAC (TP1).
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