[sdiy] Korg Polysix major problems - Update

[Karl Ekdahl]

Tom

> That said, pretty much every problem I've seen on a
> Polysix that  
> wasn't obviously mechanical could be traced to either
> IC30 or 31 (the  
> two 74LS08 latches) on the main processor board. They
> control so much  
> of what the synth does that all kinds of weird effects are
> the result  
> of decayed traces near these chips as a result of battery
> acid  
> damage. There are traces and through-holes under the chips
> too, if I  
> remember correctly. I've even had other weird faults
> reappear years  
> later on a synth that I cleaned up and fixed once. It
> turned out I  
> wasn't thorough enough and a through-hole had failed.
> In my  
> experience, battery damage around IC30 and IC31is by far
> the  
> commonest source of faults and should be checked over with
> a  
> magnifying glass very closely before you start trying to
> determine  
> whether anything else is wrong.

I measured all the connections to the chips and they all work, but i also noticed a few weird things; it seems like the service manual is incorrect at a few places, like pin 4 on IC30/IC31 being tied to +5V when in reality it's in fact pin 5. Also i noticed that IC34 is in fact labeled IC24 on my PCB - any other errors in the service manual?

> It seems very unlikely to me that a CPU would be the cause
> of the  
> problem, although of course it is possible. However, you
> report  that  
> you've tried another set of boards and got the same
> result. That's  
> too big a coincidence to be a coincidence! Instead, that
> suggests to  
> me that the problem doesn't lie on those boards, but
> rather in the  
> circuitry to which they connect. If you've tried
> swapping the CPUs  
> over and got no improvement, I'd be fairly certain that
> it isn't CPUs  
> that are the problem. It seems highly unlikely that
> you've got two  
> dead CPUs. One would be unlucky, two is just not likely.

The thing is that i still get a weird signal on Pin 24 of the 8049 with both CPUs. This pin is tied to the clock input of the flip-flops that generates the Gate ON/OFFs and i would expect a clean signal, but instead of the fairly low frequency clean digital signal i get on all other digital lines in the synth i get a very fuzzy high-frequency signal that is so faint on my scope i didn't even notice it at first. It also has a very low amplitude sawtooth in the same signal! If disconnecting this leg and i start poking with it "free hand" i start to get random Gate signals!

Question: does the 8049 handle the arpeggiator and everything regarding what notes to play? I'm just wondering maybe it stuck in some kind of mode that i cannot get it out of.
 
> Someone suggested that you test the SRAM. Again, the main
> 8048 uP  
> talks to the SRAM address lines via the two latches IC30
> and IC31,  
> and also IC24 and IC26, a pair of 14042s. Rather than look
> to see if  
> the SRAM is dead, check these chips - any failure here
> would give  
> similar symptoms to a dead SRAM.

Hmm well there's no battery in the circuit at all right now and the memory 

> Replacing IC30 and IC31 is a pain, but not impossible. I
> cut the legs  
> of the old chips with side cutters and take the bodies
> away. You can  
> then heat each leg with the soldering iron before pulling
> it out. If  
> I had a vacuum desoldering station I probably wouldn't
> need to bother  
> with this, but I don't have one, and this is low tech
> and works. Once  
> you've got the chips off you can check the traces
> underneath. Be sure  
> (really sure) that the tracks are ok before you put new
> chips in again.

Well i replaced IC31 as it had acid-damage, IC30 measured just fine to start with so i didn't both with it.
 
I'm still confused... :(

Thanks!

Karl


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