PX-1100 Reverberator

[untune]

[Reposted from the Dynamics Processors section]

I recently posted in the black market about tracking down a fuse holder from a Ukranian-produced effects unit. It's a digital chorus/delay unit that was manufactured for a short time in the early 1990s, and they are often referred to as an "Estradin PX-1100" online (due to the factory they were produced at.) These don't come up often in the UK and I missed out on one a few years ago, so I took a punt this one when I saw it. My original intention was to test it without doing anything too intrusive on the provision I could send it back if it wasn't working. The fuse holder cap was missing and also it uses a non-standard 2-pin power connector (which was also slightly damaged). The power cable was long gone, the parts are very old and specific therefore in order to test it safely I replaced both the fuse holder and power plug—I fashioned a plastic plate to hold the new pieces as they didn't quite match the case dimensions of the originals. This modification essentially means that I've committed to keeping it.

Bearing in mind I had to break two plastic seals to get inside the unit (which were stamped with the manufacture date, November '91) it's safe to say that there has been no maintenance at any point in the last thirty years. Everything looks clean inside, no obvious bad caps etc. I cleaned the pots and re-glued a few plastic button surrounds that had come loose with age. Powering it up, the unit does work, but it clearly has some issues.

Firstly, it sounds quiet. Even with the output volume on maximum, the input clips without the output getting to a decent level. I ran a synth through it, adjusted the input/output to various degrees but there was no change. There also seems to be considerable low-frequency loss. I thought it may be an impedance issue, but testing it with a passive bass resulted in a quiet and heavily distorted output, sounding like it was going through a distortion pedal. I'm not 100% sure but I think input impedance is ~220K and output is ~2K (according to the manual specs) and should be happy with most things.

Secondly, engaging the delay results in spitty, harsh and distorted repeats. I'm hoping that issue one is the root cause of issue two, considering that all the ICs appear to be esoteric Russian chips that are likely to be almost impossible to replace. Other than that, everything seems to function correctly (as in, all controls engage and disengage and effect the signal as they are supposed to.)

Anyone have ideas on where to start troubleshooting? Current/power supply issue perhaps? I can get access to most of the board without too much hassle and parts should be easy enough to replace. I've managed to find some literature, including schematic/manual but they aren't in English. I've attached the block diagram with a rough translation (can't vouch for accuracy but I think it's near enough to understand); not sure what some of those blocks in the middle translate to but I expect they're filter stages for clock noise or something similar.

Any help would be greatly appreciated. Thanks!

 

[untune]

And just to follow up since the original post, I did some measurements on the PSU board last night. Transformer secondary measures 28.5v, and the points after rectification that should be +10v/-10v according to the schematic measure at +9.25v and -9.05 v—I expect these are close enough? In the last ten mins of snooping I noticed a slight bulge on one of the power supply caps that I missed first time round

 

[untune]

Bit of a bulge on C7 here. Could that be the cause of the issue?

 

[Khron]

Bulge or not, ~30yo soviet era caps oughta get the boot, out of principle. Lack of low end may well also be caused by dried-out caps (but not necessarily the power supply ones).

 

[untune]

Thanks Khron, yeah I agree—replacements for the two psu electrolyticsare already en route, easy access so shouldn't be any trouble to swap them. After that is done, I'll test and if there's no improvement, I'll start at the smaller caps near the input (specifically a couple near the first gain stage)

 

[untune]

Just updating in case anyone else needs the info in future; complete re-cap cured the issues. Not so much in the power suply, but the small silver coloured K-50 electrolytics on the main PCB seem to have been the root cause of most of the problems. I swapped all the yellow 47uFs out first and saw little improvement, but once I started swapping out the 22uFs things drastically changed for the better with the signal level and low frequencies.

There are three 5uF caps that are paired with comparator ICs and replacing those cleared up the distorted repeats. I've only swapped two out (C33/C34) at the moment and the problems there seem to have gone. I also did a 1uf (C12) and a 2uF (C40) at the same time, not sure if those had any bearing on that.

Nice that it all works again though.

 

[Whoops]

Nice that you solved the issue, that seems to be quite an interesting unit, would love to try one out

 

[untune]

Thanks Whoops, yeah they certainly are; I had the chance to buy one of these maybe a decade ago (I forget if it was this one, or a similar unit called a PX-1000) but the guy I was talking to sold it unexpectedly mid-communication. Never seen one in the UK since, and this came up recently although I spotted the missing fuse cap at the last minute and managed to negotiate it down a little due to the fact that it needed work to even test if it would power up.

There are videos around but it's hard to find anything that gives a good demo of it. I really like it though, has a unique character which is the thing I love about units such as these. I was doing a little reading about the Estradin factory in Zhytomyr, which is amidst the atrocities being committed in Ukraine right now. A good article on what (was) happening at the former plant Zhytomyr: how the Electrovymiriuvach is being revitalised • Ukraїner

There are a couple of other Estradin effects in existence, one of which is a vactrol-based phaser box which I'd love to get my hands on someday.