Vintage Synth Repair

Alexis has it right -- tantalums were used because they pack a lot of capacitance into a very small package.  (Low ESR is good too.)  Tantalums are used a lot in cell phones now for the same reason.  Now electrolytics in the 1-22 uF range can also be made very small, which was not the case in 1980.

I have seen lots of early 80's units in which the power supply was damaged because of a shorted tantalum cap.  When I work on a synthesizer of this vintage, replacing all tantalums with electrolytics is the first thing I do.  Pay attention to polarity -- both tantalums and electrolytics are polarized, so make sure the (+) end of the new electrolytic goes where the (+) leg of the tantalum was.

I wouldn't bother adding capacitors across the power inputs of IC's.

Regarding multiple capacitors -- as far as I know, multiple capacitors (i.e. multiple capacitors in one can) aren't made anymore, so I would wager that any you find are old and no longer "fresh."  Whenever I've had to replace a multiple capacitor, I replace the single multiple cap with individual capacitors of the correct voltages and values.

If space is a problem, get creative -- you can place the new caps somewhere near the original cap location and run wires from their legs to the holes in the PCB where they connect.  (Obviously, the shorter the wires, the better.)  Insulate the solder joints on the cap legs with electrical tape or, better, heat-shrink tubing.  Zip ties or hot glue are useful for holding capacitors in place when they aren't soldered directly to the board.

Remember when replacing electrolytic capacitors that it's always OK to replace them with same value, higher voltage rating.  Also, since most electrolytics are used as ripple/spike filters, their capacitance value is not critical above a certain minimum, meaning that you can replace them with higher value, same voltage rating caps.

--
David Shuman

--- In vintagesynthrepair@yahoogroups.com, Daniel Forró <dan.for@...> wrote:
>
> Thanks for your explanation and the link. So can I take it as a rule  
> of thumb and change all tantalum (in PSU, logic circuits or audio  
> circuits) for low ESR electrolytic? Or is there some case where to  
> change just old tantalum for new one? Could for example using of  
> tantalum rated for higher voltage serve as a sort of better protection  
> against possible fail?
> 
> I had this fire problem with two ceramic disk caps in the PSU of one  
> old Russian instrument which was not in use for few years. Fortunately  
> I could see the fire, it started immediately after switching on, so I  
> just switched off. It was enough to change those caps and instrument  
> is OK, but I will do complete recapping.
> 
> Daniel Forro
> 
> 
> On 8 Apr, 2013, at 11:32 PM, Alexis V. Rogers wrote:
> 
> >
> >
> > Tantalums were used due to size and lower ESR. You can get low ESR  
> > electrolytics these days. The problem with tantalums is that they  
> > close (short) when they fail. When used in a power supply situation,  
> > I've seen tantalums burst into flame and end up looking like burnt  
> > marshmallows.
> >
> > I've never added capacitors to the IC power lines unless I was  
> > replacing with a different IC whose datasheet called for it.
> >
> > And, I get my can caps through http://www.tubesandmore.com/.
>