Vintage Synth Repair

As many of you know these are often riddled with issues from the battery leaking etc. I have an expander version here that had a few bad buss interface chips... learned some new tricks there... but still no MIDI or Patch load happening. Bad comparator, I think the patch load is ok now.

But..the MIDI. Once you run a sequence to it it will at some random time reset to patch 00 but no voices ever trigger. The only way I can think that is happening is data is getting corrupted but it randomly catches what it thinks is legit data which happens to be a patch change to -00.

Anyway traced all lines to the 68B50 uart and all looks well. Data in..ok. Ummm... changed the chip. No difference. The unit seems solid in terms of the cpu continually being ready to change patches. I see patch data changing on the dac buffer out.

Well I knocked out two and I've begun to just write a troubleshooting guide for these things since I get sick of forgetting which pins to test etc. every time. hehe. But HOW do you include stuff like THIS in a troubleshooting chart?

Ok... so I had one working in my van studio for a while and it quit. Blank display or just locked on zero's. I found at one time that resoldering some stuff seemed to help and this time around I notice that DB0 is feeble looking and low..like it's not getting pulled up right. ODDLY though...hmmm... I measure 5.00K from the 5v line to pin 2 on IC 3 which had already been replaced with a new chip and socketed. I pull the chip..no effect..still 5.00K Obviously with that kind of reading you'd be thinking 'ahh there are two 10K pull ups in parallel on that one for some reason.

Yet... all others of the 8 pull ups are absolutely fine at 10.03K ish. wow. Ok so ... I find that hmm it is indeed the line I'd previously messed with and found some chip that I removed, socketed and put in and it all worked. IC17.... WELL it TURNS OUT that the stinking thing SHORTED THROUGH BUT LISTEN.

With that chip still untouched in it's socket... the machine had started to work briefly a few times AFTER I just cleared some flux and possible solder dust between some pins. lol. Totally faked me out making me thing there was some intermittent fine hair of conductive something floating around changing the game.... TWICE Including in front of witness of Jim a guy I let stay here sometimes.... I got a reading of 10,02K instead of the 5.00K that resulted from the parallel pull up resistance that joined in when the freaking LS377 was shorted through. I'm speechless. This is something so odd I had to document it. I Pulled the chip and sure enough those pins measure a dead short. There is nothing even microscopic between the pins. I scraped. Nothing. It's internally shorted. An..intermittent internal short? ON AN LS SERIES CHIP! Whahaaaa?

Oh well anyway I managed to get the analog board issues done on yet another one that I'd previously gotten going swapping out the IC3 tranceiver...*IT* appears to have a revisitation of another peculiar problem from the past. I find that some sounds transfer just fine from my DK600 which sounds always lovely. HOwever when I pound some high velocity into a sound I might hear this strange phantom modulation! Or some sounds just..do it all the time! Sounds that sound clean on the DK600. I tranfer them and they sound similar but adopt some gnashy aspect.

Before this type of thing I recall was happening because there wasn't enough 5V level I think and current was bleeding back off of sample and hold cells when they were supposed to be HOLDING... creating a nasty modulation...a waveform with of course a decaying ramp then sudden recharge for a sample time and then back to decaying..

Hopefully that'll be easy to resolve. These sure are beautiful units when it's all working right. But good GRIEF you can spend a lot of TIME getting them right!