Polivoks synths

We currently have 2 Polivoks synths here at the moment.

They have a reputation for being built like a T34, but as Gordon Reid stated in his Sound on Sound article on this synth a few months ago, this opinion is only based on the impressive bulk of the instrument, the aluminium chassis is very thin and the endless plastic mouldings are made from a very brittle styrene based polymer. The Internet folklore is that the Pots are very intermittent and scratchy and this is unfortunately true. This is a little surprising as the Pots are large, sealed and very solidly constructed, rather like the Allen-Bradley pots used in the Minimoog.
The bad news is that the contrary behaviour of the Pots can render the synth almost useless and certainly stifle any creative flow.
The good news is that by dismantling these Pots and thorough careful cleaning with Isopropanol and a tiny drop of De-Oxit on the wiper contacts can bring them back to perfect working order.
There seems little point in just cleaning one or two intermittent pots when the entire front panel PCB has to be taken out, bite the bullet and do them all. We cannot say how long this maintenance will last, but as the synth is a quarter of a century old, and has not had the Pots cleaned before, we reckon on a decade of knob twiddling good times before any further attention should be necessary.
The filter cutoff frequency pots seem to get a lot of use and abuse, and on both Polivoks here they are a write off and will have to be replaced. This particular Pot is a 100 kiloOhm linear Pot, but finding a suitable form factor replacement is tricky as the very large pots used back in the day are not available from the usual electronic component suppliers and the modern pots available are far too small to sit well on the PCB and to take the front panel knob correctly. We haven’t yet found the perfect solution to this issue and enquiries are ongoing.

The screws that hold the synth together are fairly horrible flat head (sometimes called cheesehead) screws which are easily damaged and fiddly to reattach, we’ll probably replace them with crosshead 4 millimeter screws.
As for the circuitry, it is good and easy to service, all the main elememts are on separate PCB’s, the OpAmps used throughout most of the synth are supposed to be replaceable by 741 type IC’s, the exceptions being the filter chips and the VCA chips and the 2 14 pin IC’s used in the VCO’s. Recent experience has cast some doubts on the internet versions of the Russian IC compatability list. Replacing the filter frequency control driver chip (which is supposed to be 741 compatible) with a 741 led to complete loss of frequency control and massive and raucous oscillations. On the other Polivoks, the LFO sample and hold buffer chip had been replaced by a 741, and the hold drooped so fast it sounded like a 70’s Syndrum. Putting in a CA3140 Mosfet input op-amp cured that, (this chip would be the weapon of choice in this application by most synth designers past and present). Also look out for dry solder joints on the connectors as we have seen this before.
All in all though, this is a good sounding and versatile synth, not sure if it is worth the mad prices it is currently commanding on Ebay auctions.

As a kind of addendum to the above experience, please for the love of all that’s holy, don’t try to remove the slide switches on the panel circuit boards! The screws that hold these switches physically to the front panel PCB’s are just about impossible to remove, the yellow paint that secures the threads prevents easy dismantling, and the heads of the screws are so soft that any force used will destroy them, requiring the use of a bolt cutter to remove them. It appears that Isopropanol dribbled into the switches combined with some vigorous switch excercise is enough to restore them to perfection. Also having had to use a bolt cutter to get a switch out, stripped it down and examined it, the amount of tarnish on the contacts of the switch is slight and easily cleaned with Isopropanol.

As stated above, the Pots do not look too bad most of the time, yet when you take one out of the synth, and measure its resistance responses, it can be totally trashed in terms of being a useful control, and dismantling and cleaning does seem to make a huge difference.

Having just today finished the dismantling and cleaning of all the Pots in a Polivoks, and tested it afterwards, we have some idea of how a Polivoks is supposed to be, and it will certainly help with the other one here at this time, and with future Voks’s which we know are on their way.