Symetrix 501 noisy

[phantombox]

There are 3 trimpots in the circuit, 2 are relatively close to the VCA but none is directly or indirectly connected to the VCA output. The 3rd trimpot controls the sidechain somehow.

 

[Freq Band]

The "IRC" 16pin chip is most likely a "network resistor" (several resistors in chip form), from "International Resistance Company", and the value of the resistors is 103 (aka, 10 with 3 zeros = 10,000 ohms each).
Not sure how they're connected inside the chip though.
Glossary of Electronic Resistor Networks

 

[Script]

Just opened a 501. Two broken pots (50KB). The left one came off when loosening the washer. At least one other pot, although still in one piece, does not connect properly.

 

[Script]

And broken solder joints on the input and output jacks. Also, the internal contacts of the jack don't really connect properly any more cos of corrosion.

But no illusions over here, it's most likely a dead 101 VCA.

 

[Script]

Push button switches show fragile to no connectivity most of the time. Basically all mechanical parts would need swapping out before testing of VCA can even begin...

 

[Newmarket]

For the NP caps - bodge in 2xPolarised electrolytics (twice the required value) "back to back".

 

[Script]

I wire-jumped the switches, brute-force 'fixed' the jacks, and inserted functioning pots from the trash drawer. Signal thru is stable.

There are no NP caps in the one over here. Just polarized 100uf on the output. Radial NP would fit, if leads are bent.

At least one IC was popped.

Sidechain seems to work now, all pots change behaviour on output as expected...

Yet output sound is thin and all sizzled up. In a hummy kind of way. So it's either still the jacks (ground), yet another popped dual op amp, or the ECG-101 malfunctioning.

I don't spend much time on this.

 

[Script]

Houston, we hear you loud and clear.

Input and output jacks didn't connect internally, so I wire-jumped to underside of PCB.

Limiter works. Comp section doesn't any more. But makes me think, the ECG-101 might be ok.

I had desoldered and measured the four transistors (2N4001, 2N4003) and then reinserted.

 

[Script]

The whole thing works as expected now.

The comp section didn't work cos of a cold solder joint or a short caused by desolder pump debris on PCB underside.

Except for the caps (new, but inserted provisionally), I have used trash items (including the 5532 SMDs) so far -- nothing to loose.

The blue box is the Allison Research ECG-101 VCA..The black box is part of the SC detector. Both are canned and fully sealed on the underside.

Failure was due mainly to mechanical parts.

 

[Script]

Here's the 'finished' unit back to life.

Unit is serial number 3019 from 1983-5-10 ('Towa Engineering Co Ltd' says the label).

The look is a bit toylike. Coloured buttons are original, but arranged differently. Pushbutton caps were grey and smaller but impossible to remove, so I simply inserted new old pushbuttons including their red caps from a decommissioned Midi interface , which gives the unit bit of a Xmas feel.

With fixed and/or upgraded CHEAP gear I tend to swap at least one LED for a blue one. Here it's the two threshold indicators.

Dual op amps were all LF353. I kept the sidechain untouched for that 'vintage' sound (SC plus 101 VCA).

Upgraded the audio path with recycled SMD 5532 and C0G caps. Fully elco recapped.

The fix was cheap. The power caps, new power switch (old one was broken), op amp sockets (old ones were flaky) and the two Nichicon Muse BP caps in audio path sum up the expensive parts.

Couldn't find used jacks that would fit, so fixed the old ones by running wires. Might revisit if I can find suitable jacks.

LED Meter needs recalibration and so does everything else, I assume. Might do, or not. It's a 'colour' box after all with a somewhat raw feed-forward plus peak limit sound.