Vintage Synth Repair

I'm restoring an old (late 50s early 60s?) Univox and I've noticed something odd about the wiring of the capacitors in the thyratron circuit.

From what I can gather from the schematic, it's using a thyratron as a simple AR envelope generator. The timing capacitors are switched in from a rotary switch on the front panel marked Circuit 1 2 3 4. Each 'circuit' has a different value capacitor switched in: 1, 2, and 4 are: open, .47uF, and 16uF. Circuit 3 though puzzles me, as it has 2 electrolytics wired in series - fair enough if they modded it to lower the capacitance, but they have their positive terminals connected together - inverse series. It looks like the original wiring - no sign of recent bodges - the caps are old Hunts cans - half of a 16uF dual can wired in series with a single 8uF axial. They have DC across them - the same as the other caps on 2 and 4.

To add to the confusion, both measure as 18uF on my meter, summing to 9uF in series. These haven't been powered up in years, maybe decades, so maybe there's some strange effect here caused by the drying, or have they somehow reformed themselves to the same value by being connected back to back?

I'm proposing to just replaced them with a single 4.7uF and see how it sounds.

Any suggestions as to what's going on?

Thanks
Susie

To further

--- In vintagesynthrepair@yahoogroups.com, "Susie" <gardens@...> wrote:
>
> I'm restoring an old (late 50s early 60s?) Univox and I've noticed something odd about the wiring of the capacitors in the thyratron circuit.
> 
> From what I can gather from the schematic, it's using a thyratron as a simple AR envelope generator. The timing capacitors are switched in from a rotary switch on the front panel marked Circuit 1 2 3 4. Each 'circuit' has a different value capacitor switched in: 1, 2, and 4 are: open, .47uF, and 16uF. Circuit 3 though puzzles me, as it has 2 electrolytics wired in series - fair enough if they modded it to lower the capacitance, but they have their positive terminals connected together - inverse series. It looks like the original wiring - no sign of recent bodges - the caps are old Hunts cans - half of a 16uF dual can wired in series with a single 8uF axial. They have DC across them - the same as the other caps on 2 and 4.
> 
> To add to the confusion, both measure as 18uF on my meter, summing to 9uF in series. These haven't been powered up in years, maybe decades, so maybe there's some strange effect here caused by the drying, or have they somehow reformed themselves to the same value by being connected back to back?
> 
> I'm proposing to just replaced them with a single 4.7uF and see how it sounds.
> 
> Any suggestions as to what's going on?
> 
> Thanks
> Susie
> 
> To further
>
WOW...That's a rarity

If it were my restore I would put a bi-polar 10 uf in that place.
or a couple of 18uf in series and make sure the rated voltage is as much or better.

Scott in Vermont

Scott, the schematic calls for a 8uF so maybe I'll use that value instead, although the schematic has caps 3 and 4 the other way round - I think a misprint as I'd expect each click of the rotary to increase the capacitance. I think I'd need a value closer to .47 than 16 to give a good spread of attack times, if indeed that's what it's controlling.

A thought occurred to me that if the one cap was shorted out, then according to the values on the cans I'd have something close to what the schematic says. Maybe it was bodged in the factory when they did their testing? That doesn't explain why the 8uF is the wrong way round though. It isn't obvious from the 8uF body which way round it goes as there's an indent at both ends, but there is a + printed on it and the other end is connected to the can so it's definitely reversed according to the schematic.

Quite why an 8uF is showing as 18uF on my meter is puzzling. I suspect it would be different with 100V across it, if it survived the shock. Maybe as the oxide layer thins the capacitance increases?

630V 8uF bipolars are available locally, so maybe that's my choice.
Thanks
Susie

ps I'm really looking forward to hearing what this beast can do!

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

>
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, "Susie" <gardens@> wrote:
> >
> > I'm restoring an old (late 50s early 60s?) Univox and I've noticed something odd about the wiring of the capacitors in the thyratron circuit.
> > 
> > From what I can gather from the schematic, it's using a thyratron as a simple AR envelope generator. The timing capacitors are switched in from a rotary switch on the front panel marked Circuit 1 2 3 4. Each 'circuit' has a different value capacitor switched in: 1, 2, and 4 are: open, .47uF, and 16uF. Circuit 3 though puzzles me, as it has 2 electrolytics wired in series - fair enough if they modded it to lower the capacitance, but they have their positive terminals connected together - inverse series. It looks like the original wiring - no sign of recent bodges - the caps are old Hunts cans - half of a 16uF dual can wired in series with a single 8uF axial. They have DC across them - the same as the other caps on 2 and 4.
> > 
> > To add to the confusion, both measure as 18uF on my meter, summing to 9uF in series. These haven't been powered up in years, maybe decades, so maybe there's some strange effect here caused by the drying, or have they somehow reformed themselves to the same value by being connected back to back?
> > 
> > I'm proposing to just replaced them with a single 4.7uF and see how it sounds.
> > 
> > Any suggestions as to what's going on?
> > 
> > Thanks
> > Susie
> > 
> > To further
> >
> WOW...That's a rarity
> 
> If it were my restore I would put a bi-polar 10 uf in that place.
> or a couple of 18uf in series and make sure the rated voltage is as much or better.
> 
> Scott in Vermont
>