Vintage Synth Repair

A bit more fixed..:o)

well actually diagnosed..

The reason all the voices were dead on one card was because a 
CEM3340 VCO chip was shorted, this pulled down the 7905 regulator 
that sits on the edge of each voice card killing the supply to the 
other CEM's. I found that a CEM3360 VCA chip was also dead on the 
same card. So after swapping a few CEM's around I know have just one 
Oscillator 3 down. 

After that I noticed the VCF adsr wasnt working on one voice. A 
quick look at the schems revealed it is generated by a CEM3310 on 
the contour generator board. Sure enough I found the 3310 was dead.

So that, on the face of it, appears to be the voices sorted out. Im 
sure I'll find more problems.

The synth is still totally out of tune, so it will need full 
calibration. I'm not going to try that until I have located all the 
CEM chips I need.

I also swapped all the HC ic's out for the correct LS versions and 
anything that isnt specified in the parts list. For instance a lot 
of the op amps are 1458's, the schematics and parts list specify 
4558. The manual also has an addition saying you should swap any 
found to 4558. The 4558 is apparently superior.

I still have the problem with the displayed pot values, I talked 
about in an earlier post(below). It drove me mad..I checked out the 
whole of the POTPOS circuit, the MUX circuit..the display 
circuits....eventually after forgetting theory, schematics and just 
randomly testing things, I found CR1 to be open on the DMUX board! 
It's a 6.2 Zener..I avent changed it yet as I was going out..but 
after checking the schems I reckon I might have solved it..it's not 
mentioned in the service manual, and it's right on the edge of the 
schems..

Thats it for now..Phil.








--- In vintagesynthrepair@yahoogroups.com, "eggwheatis" 
<eggwheatis@...> wrote:
>
> 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
> > >
> >
>