Discussion about the Korg PolySix synthesizer

Unfortunately, both the calibration problem and MG bleedthrough are
happening on the new clone board, not an old damaged board. It's not the
first time I've seen new boards that wouldn't calibrate, either. Generally,
the 4 and 8 lights come on and you can change the 8 to 7, 6, 5, but you
can't make the 4 move down. I tried a couple of different types of DAC08 and
even an original spec 1408 but it made no difference. I also tried a 4560 in
place of the TL072.

 

I did lift one end of R9 to verify that the MG leak was on this trace from
CN06-2 to R9. The only way I can see this happening on a new board is if
there is contamination on the bottom of CN06 between pins 1 and 2 that needs
to be cleaned up, since the old Molex connector was moved from the old to
new board and the area around CN06 is a real mess on the old board. I know
it's a pain to remove these headers, but for a new board it makes more sense
than cutting the trace.

 

I think all Mike's flattery was an attempt to bribe me into not mentioning
the other problems found on his new board. Ha, I warned you. This is for
"educational purposes" for the rest of us trying to build new boards.

 

1)       Blown fuses - solder bridge between C6 and C7. Although these two
capacitors each have one pin connected to ground, the pins closest together
are not ground, they are +15v and -15v. It might seem like a bad design to
have these two pins so extremely close together but I guess it's the best
way to get +15/-15 rails down to all the op-amps and comparators.

2)       D4-D7 unstable, no control from sub octave switch - No solder on
ground pin of IC29.

3)       No MG at all, no Effects Speed control - Several pins not soldered
on IC16.

4)       From Tape not working - C4 had a 47nF (47000pF) cap in it instead
of 470pF.

 

Missing some IC socket pins while soldering is an easy thing to do with all
those shiny pins on a shiny new board and is not as easy to find as you
might think. I didn't notice these until I got it under a big lighted
magnifier.

 

Don B.

 

  _____  

From: PolySix@yahoogroups.com [mailto:PolySix@yahoogroups.com] On Behalf Of
klosmon
Sent: Thursday, October 13, 2011 2:47 PM
To: PolySix@yahoogroups.com
Subject: Re: [PolySix] Don Backshall is a great guy!!

 

  

Nice guy, indeed.

Two things:
first, I've encountered many old 367 boards that were incapable of D/A 
calibration (as specified in the service documents) that still worked 
fine -- this is probably not an issue.

Second -- regarding the MG bleedthrough...
I've encountered this on many battery-damaged boards; it has to do with 
damaged traces bleeding through to each other.
A fix I've found that works 99% of the time: cut the trace on the 
bottom of the board at CN06-2, and also the trace that goes to R9 (near 
CN05). Cutting the trace at both ends eliminates any possibility of the 
mod signal being affected by adjacent traces. The mod bleedthrough 
should disappear. Now solder a jumper between CN06-2 and the end of R9, 
and hopefully the mod function will work, without distortion or other 
problems.

~GMM

Mike wrote:
> 
>
> Don Backshall solved all of my PolySix issues and did it for basically 
> a handshake. Furthermore, he mailed my KLM-367 board back to me in 
> good faith before my PayPal payment even cleared!!! I sent him as much 
> extra money as I could because he really deserved it. Here's the 
> correspondence bleow:
>
> If the e-mail below can't restore your faith in blind internet 
> transactions, I don't know what will.
>
> (Don's Response October 6)
>
> Okay, the boards are on their way back. Priority Mail with delivery 
> conf # 03103490000077720492.
> Still three problems that I am aware of:
> 1) D/A calibration - Can't get the right lights to come on, but seems 
> to be in spec anyway.
> 2) MG leak - I'm assuming this is caused by some crud between pin 1 
> and 2 on the bottom of CN06 header. I know these are a pain to remove, 
> but I think it needs to be done. Sounds really bad when it's cold. 
> Gets a little better when it warms up.
> 3) MG delay - Known problem with 2N3904 in Q5 position. Using the 
> (2SC945) transistor from the old board seems to fix this, but the pins 
> are different. You can compare what's connected to what between old 
> and new boards to see what pin goes where. One of my old boards had a 
> 2SC2785.
>
> $8.60 postage
> $1.60 fuses
> $0.15 capacitor
> -----------------------
> $10.35 grand total - You can send it to this Paypal acct 
> backshall1@bellsouth.net <mailto:backshall1%40bellsouth.net>
<mailto:backshall1%40bellsouth.net>
>
> I'm pretty sure you can get this board sounding just like it should, 
> even if you can't get the right calibration lights to come on.
> Have fun,
> Don Backshall
>
> _______________________________
>
> (my response to him solving my problems October 05)
>
> That's great!! It looks like I'm a decimal doofus! Ha Ha. What's the 
> bill? I can send you a PayPal payment today or tomorrow. I'll praise 
> your expertise on the Korg PolySix forum too if you're intersted.
>
> Thanks again,
>
> -Mike
>
> 

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