Roland SH-101 analog synth repair

[ricothetroll]

Hi !
I have this non working Roland SH-101 a friend of mine gave to me. It seems like there is a problem with the VCO : the noise generator is working, along with the VCF, envelope generator, etc... but no VCO sound.

The VCO is a CEM3340, whose datasheet is here :
http://curtiselectromusic.com/uploads/CEM_3340-3345_Long.pdf

Here is the service manual for the SH-101 :
http://privat.bahnhof.se/wb447909/dinsync/service_manuals/SH-101.pdf

I checked pin 8 (ramp) and pin 4 (pulse) with an oscilloscope and I found out the chip is oscillating but very slowly (~1Hz). Then I started to look for a problem in the frequency control circuit. The frequency control in is pin 15 of the CEM3340. It appears to me that here multiple signals are summed : RANGE WIDTH, VCO internal TUNE, CV from Bender board (portamento), VCO user TUNE (cf schematic pg.7 of service manual).
Looking at the block diagram on pg.2, I see that the Keyboard control voltage that goes to the VCO first passes through the PORTAMENTO circuit. It first goes out from IC10 pin1 -> IC9B -> CVout/CVin connectors -> IC4B -> PORTAMENTO -> CEM3340 pin15.

I traced this signal : at pin 4 of IC10 I see ramp signal with an offset of about -7,5V. The frequency of that ramp signal is changes when I play notes on the keyboard. That signal disappears after IC9B.
Should I replace IC9B ? Or am I missing something ? I must admit I have no idea how this ramp signal, whose frequency seems to define the frequency of the VCO, is then converted into a CV...
Best regards.
Eric

[frequencycentral]

You'd be best posting on electro-music.com: http://electro-music.com/forum/index.php?f=112

I have some knowledge of electronics, and have scratch built a modular synth or two, but I've just trusted the repair of my whiteface ARP Odyssey MkI to a pro: http://www.synthrestore.co.uk/

BTW, that CEM3340 is a rare and valuable chip - be really careful there.

[ricothetroll]

Thanx a lot for your answer ! I actually didn't know this forum. I'll post my question there.
Best regards.
Eric

[thedefog]

Hey ricothetroll,

PM me. I've repaired 3 of these before, so I know them in and out.