Vintage Synth Repair

Fixed the hopeless switch matrix :o) there was a ribbon cable plugged 
in the wrong way round into the common analogue board! Large repairer 
error #5.....

So now all the button functions work properly and I have Oscillator 3 
back!... although only 4 voices working. 

I also changed 4 of the hc138's to LS138's and the filter has now 
started tracking the keyboard properly...I discovered two HC377's on 
the digital board too..they should be LS variants again...

So basically im left with OSC 1,2 and 3 not working on the same voice 
card(3 dead cem3340's on the same card surely not? I hope) OSC 3 dead 
on one other card. Hopeless autotune '0 tuned' all out of tune with 
each other even on the same cards...

Oh and the weird pot problem: when you 
move the pots the numbers in the alpha display move up and down which 
is correct, however say you turn one pot and move to another, 
well the first pots status doesn't clear from the display, it's 
stuck there, you can see it flicker as you move the other pot but 
it's stuck there unless you go back and zero the original pot. Only 
a few pots seem to cause this problem.

I reckon swapping out those wrong HC ic's will cure that..maybe..

Im sure I'll find more problems too. It's amazing how 'previous 
repairer error' can completely disable a synth that probably only had 
a couple of faults to start with.

Getting there...



--- In vintagesynthrepair@yahoogroups.com, "eggwheatis" 
<eggwheatis@...> wrote:
>
> Well ive now got sound out of it :o) this is going to be fun 
> though...yet again someones been in this before me that looks like 
> they weren't on the ball. All the IC are in sockets...but most of 
> them have been replaced sometimes with the wrong variant.
> 
> No sound was indeed caused by the PSU....an electrolytic capacitor 
in 
> the +15 circuit on the DMUX board was put in backwards. Repairer 
> error #1.
> 
> I thought I'd start by fixing the LED's on the left hand control
(LHC) 
> board and the missing digit in the alpha display so I could see 
what 
> was going on better...
> 
> The dead/stuck on LED's were caused by one of the legs of the ls378 
> at U8 on the LHC board being bent under itself in the socket. 
> Repairer error #2.
> 
> The dead digit in the alpha display was caused by a dead output on 
> the LS273 at U1 on the LHC board.
> 
> Anyway so now I can see and hear whats going on I have a whole host 
> of new problems :o) 
> 
> I have no Oscillator 3 at all across the board, Osc 1 and 2 also 
dont 
> work on one voice. None of the oscillators will auto-tune, i get 
> the '0 tuned message'. The filter self oscillates but wont track 
the 
> keyboard.
> 
> The switch matrix is hopeless on the LHC board, buttons activate 
> wrong functions basically, the appegiator doesnt work nor does the 
> hold function...
> 
> Things I can see right off the bat but need to get spares to fix 
are: 
> All the 74ls138's on the digital board have been replaced with HC 
> Cmos variants...not a good idea, these are not compatible with TTL 
in 
> most cases, so i would think they are causing a lot of problems
> (fingers crossed). You need LS in there or HCT at least.. Repairer 
> Error #3
> 
> Also there is a TL071 missing out of a socket on the DMUX 
board..this 
> is connected to the auto tune circuit..buffers one of the outputs I 
> think. Repairer Error #4
> 
> All in all a lot of fun!
> 
> Phil.
> 
> 
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, Philip <eggwheatis@> 
> wrote:
> >
> > I just got a broken Memorymoog. It's really clean, near mint in 
> fact, but not working. Half the reason I bought it is because I 
like 
> a challenge :o) ..I haven't even opened it up yet so I thought I'd 
> post up the problems to see if anyone has an idea.
> > Â 
> > It boots up ok, but makes very little sound the most you get out 
of 
> it is a thump or slight squeak when you hit a key, the same pitch 
> wherever you hit. The digital side seem to be mostly working.
> > Â 
> > If you hit autotune it comes back with '0 tuned' so it can't tune 
> any of the oscillators probably because it cant 'hear' them so to 
> speak.
> > Â 
> > First thoughts are some kind of DAC problem.
> > Â 
> > 2: All the LED's on the left hand side don't work at all but the 
> buttons do. A few are just stuck on all the time, again the buttons 
> work.
> > Â 
> > 3: one digit on the alpha/numeric display is barely visible.
> > Â 
> > 4: this is a slightly weird one which is hard to describe, when 
you 
> move the pots the numbers in the alpha display move up and down 
which 
> is correct., however say you turn one pot and move to another, 
> well the first pots status doesn't clear from the display, it's 
> stuck there, you can see it flicker as you move the other pot but 
> it's stuck there unless you go back and zero the original pot. 
Only 
> a few pots seem to cause this problem.
> > Â 
> > Any ideas would be helpful. Obviously the first thing im going to 
> check is the PSU when I open it up later.
> > Â 
> > Thanks, Phil.
> > Â 
> > 
> > Send instant messages to your online friends 
> http://uk.messenger.yahoo.com
> >
>