Old Nichicon cap polarity confusion - help please!

[mcfate]

Hi All,  I'm attempting to recap a sony CP-3B power supply for a C-37A mic.  On the bottom of the original 20/20 multi-part caps, there are 2 adjacent lugs on one side and 1 lug on the opposite side. This is one that I removed from the circuit:

I would assume the single lug is the negative lead, but the can is marked "negative" on the 2 lug side - and to make things more confusing, there is an arrow marked "input" pointing to 1 of the 2 adjacent lugs.

I have scoured online for a schematic, and the closest I could find is a drawing for what I think is an earlier version, the CP-2

Some of the values are different from my CP-3B, but it looks like pretty much the same thing.  In the drawing, all 6 of the 20mF caps have one side tied to ground, in my unit all of the single lugs share a ground trace.  Am I correct that this is the negative side? - this would be opposite the side marked negative on the can??!  The can itself does not show continuity with any of the lugs, and does not appear to be grounded in any way.

I had a japanese friend translate these markings-

apparently they mean "special characteristics".  He suggested that it is very possible that whoever was in charge of printing english onto nichicon caps in the 50's could have got the translation of negative and positive mixed up, who knows!

There are 3 additional 2000mF caps which I think are marked the same weird way (negative side opposite the negative terminal), since the drawing shows the positive side connected to the large 45 ohm power resistor - is that correct?  In my unit this resistor has a different value, but the layout is otherwise identical as far as I can tell.

Is there a way to test this unit with new caps installed without blowing anything up, especially the mic which is my favorite for vocals these days?  I do not have a capacitance meter, but I do have a multimeter and access to an oscilloscope.  Thank you for any advice!

 

[mcfate]

Here is the circuit board of the CP-3B, I drew a rectangle around the single lugs of the 20/20 caps in question.

 

[Mike Cleaver]

Chuck that sucker and replace all the power supply caps with new ones.
Easy fix and it will solve any hum problems.
Those caps have got to be at least 30 years old.
If you want multi-section cans, you can find them at Antique Radio Supply but check the physical size first.
I'd just go with some single high quality electrolytics.

 

[zebra50]

What Mike said!

From the PCB, the negative is clearly the lug that is by itself - it looks like they will only fit in one way.

 

[mcfate]

Thanks for the reply guys - my goal here is to replace the old caps.  I should have been more clear that the pictures above are of the original caps which I have removed from the circuit board.  I will be replacing them with modern single radial caps.  The problem is that the markings on the old caps don't seem to jibe with what makes sense based on the schematic and circuit board - and I don't want to install the new caps backwards.  I will tie 2 of the new caps together on the negative side and make them fit in the old negative slot just as soon as I can correctly determine the polarity scheme.

 

[Gus]

Are you sure the caps are bad?  Why are you recapping it something you read on the WEB?

To do a proper repair you should own a cap tester.

Funny thing I have measured decades old caps that still test good.   Value, ESR ,DA, Leakage at working voltage

The cap in the picture looks like it still might be good I do not see any leakage around the bottom seal

Don't believe the stuff on the web about recapping test it yourself.

Funny to read this you are working on a microphone system and no cap test gear?

What caps are you going to use?  

 

[gemini86]

This is your ground trace, you can see the mounting screw going right through it...

 

[PRR]

> Are you sure the caps are bad?

Look at the rectifier diodes he has, and what the model before used. This is VERY early 1960s, right? Japanese cap quality was just starting to get decent. Sony was still an inexpensive brand (with rising aspirations). Most cap companies were lucky to get 20 years life those days.

> I have measured decades old caps that still test good

So have I. But the ones that went bad were tossed-out decades ago.

I'm not convinced it is good or bad. I do think, if I were to depend on this unit, I would seriously consider replacing those caps EVEN IF they "measured good".

> might be good I do not see any leakage

Proves nothing. The old-old Japanese caps would often dry-out with no external clue. Also welds break. Remember: Japan's radio industry bootstrapped itself from ruin. In 40 years they were best-in-world, but this is less than halfway there.

And what does four caps cost against the value of a fine mike? (The capsule is perhaps much better than the cost-cut electrics.)

____________________________________________________

> determine the polarity scheme

It can easily be deduced with board in hand, but nothing beats direct observation.

Disconnect the microphone. Pull the caps. Use a battery-powered DMM or a passive analog multimeter. CHECK the meter's polarity against a well-marked battery (so that you don't have black-red leads swopped, internal fault, brain-fade, etc). Dry floor, dry shoes, well-insulated probes, children playing in street so they don't interrupt, good stance so you don't slip. Power-up. Probe each "pair" of cap holes. The polarity you see with caps oout will be SAME as with caps in (they don't do anything tricky here). Make notes.

The voltages won't be right without the caps, probably 3/4 of normal.

 

[mcfate]

Thanks for the replies everyone, and PRR thanks for the practical test - I'm reading 261 volts with the negative probe on the ground trace - so I will proceed and no longer fret about the markings on the old cans.  Is this a valid test for the 2000mF caps as well?