Partially working Symetrix 501 - help!

[nevetslab]

Those are standard DPDT and 4PDT pushbutton switches, horizontal mtg, very common parts. You'll recognize them once you get to the parts selection at Mouser, Digikey, Farnell/Newark and others. I have had some of these become intermittent over the years, and have been able to seep contact cleaner down the sides of the switch shaft, which has the tiny fitted contacts that are captive in the molded plastic shaft that keeps those captive. Exercising them with the contact cleaner, but replacing them yields the best solution.

[g1]

Related question: what do you call this type of dpdt switch (in order to identify a suitable replacement)?

Extinct.
It's a DPDT push switch. Maybe someone is making them like that again. I know for a time they were not available. If you can't find a source for the same type, you may have to use something similar and run wires to the board.

Someone else here might know a source. It's the 6pin switch shown in post #1.

(edit: missed Nevets post above, that's great if same type is available)

[g1]

This C&K F2UEE at digikey looks like it might be a match, check dimensions and pin spacings, and whether knob will fit.
https://www.digikey.com/en/products/...k/F2UEE/417517

[nevetslab]

g1 just beat me to that. the 4PDT is a C & K F4UEE, digi P/N 401-1226-ND $4.11ea

The photos of the two switches look like the standard pin spacing and button shaft. The data sheet will reveal all that, which should be there on the Digikey form just to check

[focusbob]

Thanks so much to both of you!

[focusbob]

Haven't had a chance to desolder the switch in question from the pcb, but I disassembled the pcb from the chassis this morning to take a look at the solder side of the pcb. I was surprised to see that the pins of the "offending" side of the dpdt in question are bridged (see attached pic)! It doesn't look to me like there has been any post-production work anywhere on the pcb. Is it possible that there is a trace between the pins of the offending side of the dpdt? I struggle to think of why symetrix would do this, though maybe to maintain the uniformity of appearance of the front panel switches (assuming a spdt switch was not available in the same format)? I know the way to tell is to desolder the switch but now I am somewhat afraid of lifting a trace that may be between the pins... edit: in case it helps, the top pin of the offending side of the switch runs to the middle pin of the "ratio" potentiometer.

[nevetslab]

That one side looks like the three solder pads ARE part of the foil pattern. Only way to know for sure would be to desolder it and look. I'd use solder wick and 700 deg temp on the tip, and don't doddle in the process. You're looking to see if the bridge IS in the foil pattern. If you're replacing the switch, then you could do that then. I'd be ordering the replacement switches as it's old enough to warrant that.

[g1]

What is the name of that switches function? It's possible they are only using half of the DPDT.

[focusbob]

What is the name of that switches function? It's possible they are only using half of the DPDT.

That is the "compressor in" switch. Given the (lack of) connections from that side of the switch to the switch pins, I'm having a hard time envisioning what it could be used for (looks "unused")...

[g1]

edit: in case it helps, the top pin of the offending side of the switch runs to the middle pin of the "ratio" potentiometer.

Where does the other end of that trace go?
Judging by the 'wrong' schematic, they only use one side of the switch (SW1A). They will have used a DPDT where they could have used a SPDT to limit the parts count, it's not that unusual.
You said threshold light works, if it is set up like the later unit, most of it must be ok. Just that line running from the ratio wiper may not be getting to the "U7" stage (whatever equivalent in this version).
On available schematic that would be: ratio wiper > D16 > R93 > U7A

[nevetslab]

If that is the unused side of the switch, I'm surprised the PCB layout guy didn't tie those contacts to the other side of the switch for redundancy. Adds long-term reliability to the switch.

[focusbob]

Where does the other end of that trace go?
Judging by the 'wrong' schematic, they only use one side of the switch (SW1A). They will have used a DPDT where they could have used a SPDT to limit the parts count, it's not that unusual.
You said threshold light works, if it is set up like the later unit, most of it must be ok. Just that line running from the ratio wiper may not be getting to the "U7" stage (whatever equivalent in this version).
On available schematic that would be: ratio wiper > D16 > R93 > U7A

I have spent an embarrassing amount of time attempting to "trace" the connections from the "ratio" potentiometer. The circuit topology appears very similar to what is on the newer schematic. However, there are some combination of notable differences and my unfamiliarity going from multi-page schematics to the circuit itself that is leaving me confused. For example, here is a picture of what I "traced" from the diode labeled "D16" on the schematic I posted. I have checked my tracings several times for accuracy and still don't understand how what I am seeing corresponds with the schematic. Any thoughts on my poorly drawn effort, and/or more importantly on what to try next?

[focusbob]

Also, more board pics!

[g1]

Here's a pic of what I think are D16 and U7A. Check that the diode is good. Check resistance from anode to wiper of ratio pot. Looks like there is a 4K3 resistor along the trace so I expect you should measure 4.3K.



Also, here is another pic showing what I think is damage from a screw. Maybe there is a screw beneath that is too long and cutting into the board. Anyway, there may have been enough pressure to crack a trace nearby, or crack the wafer of the ratio pot. Make sure the pot is ok.

[focusbob]

Here's a pic of what I think are D16 and U7A. Check that the diode is good. Check resistance from anode to wiper of ratio pot. Looks like there is a 4K3 resistor along the trace so I expect you should measure 4.3K.



Also, here is another pic showing what I think is damage from a screw. Maybe there is a screw beneath that is too long and cutting into the board. Anyway, there may have been enough pressure to crack a trace nearby, or crack the wafer of the ratio pot. Make sure the pot is ok.

Thank you, again! You are singlehandedly keeping me going on this project