Mystery Capicator, DOD Spring Reverb

[jumunius]

In the midst of trying to a) learn some basic electronics and b) spruce up some cheap, expendable old gear, I removed a polarized cap from my DOD R-845 Spring Reverb without first marking the polarity. Worse yet, I have now lost the cap (I thought I wrote the value down). However, when I had it, I did try it both ways and found no noticeable difference in sound.

I can't find a schematic or service manual online, so I wonder if anyone can tell which direction the polarity should go if I simply describe what is happening in that part of the circuit. Rather than draw a schematic, I have tried to describe what is happening around the NE570N chip below.

So here is a description of the placement of the mystery electrolytic. Side 1 of the cap connects to Pin 2 and 3 of the NE570N. Side 2 of the cap connects to:

a) the positive side of a 1uf cap which connects to Pin 14
b) Pin 12 of the NE570N by way of a 100k (or 700k, can't read) resistor and 220pf cap that run parallel.

Any ideas about which direction to place a cap, and what value to place there? Thanks in advance.

In case you want to know more of what is happening with the NE570N

Pin 1 goes to + of a 1uf electrolytic to ground
Pin 4 goes to positive rail
Pin 5 is connected to Pin 7 via a 4.7k resistor in parallel with a 220pf cap (and the 7 also connects to a 4558 pin 3 via a 3300pf cap)
Pin 6 is unconnected
Pin 8 connects to ground via 120pf
Pin 9 connects to another 4558's pins 3 and 5 via 120pf
Pin 11 is unconnected
Pin 13 goes to negative rail
Pin 15 goes to 20M (?) resistor etc
Pin 16 ?????

[Rodgre]

I have one of these. I can pop mine open and let you know where it goes later today.

Roger

[jumunius]

I have one of these. I can pop mine open and let you know where it goes later today.

Excellent, thanks! And the value please. I don't have it in front of me but I believe the cap in question is seated by the "bottom" side of the NE570, as in next to the 8th and 9th pins.

Curiously, is your signal noisy? Now that I've recapped it, I was thinking of following some advice from a thread elsewhere on the site and reseating the transformer to find the least noisy position (assuming that the noise is related to the spring tank picking up interference from the placement of the transformer). Any other ideas would be appreciated.

[ashcat_lt]

Can you not just measure DC voltage at each end?

[Scodiddly]

Can you not just measure DC voltage at each end?

I think you'd be safe leaving it out while measuring, no problem.

[jumunius]

Can you not just measure DC voltage at each end?

Well, yeah, I guess. Since I'm new at this, what would I be looking for and what would I do with that information?

Thanks,
Jim

[ashcat_lt]

Put the black probe on a convenient ground point and touch the red to each of the points where cap should go. Measure DCV at each point. Whichever of these is less positive (more negative) gets the - leg, and I bet you can guess where the other goes.

[jumunius]

Put the black probe on a convenient ground point and touch the red to each of the points where cap should go. Measure DCV at each point. Whichever of these is less positive (more negative) gets the - leg, and I bet you can guess where the other goes.

Ok, thanks. That won't help me with the value then (Rodgre, did you get a chance to open yours?), but if I can find the cap I can at least place it.

Cheers,
Jim