View Full Version : Roll your own center tap poteniometer?


blhagstrom
12-09-2010, 09:48 AM
SO, I was just finishing a Sansui 5050 rescue and went to listen one more time and the volume pot broke. :cry:

SO, I'm checking out dual gang 250K pots and find out I have a center tap pot and they aren't around anymore. :cry:

SO, I see a post elsewhere that mentions building a voltage divider from the two hot legs for the center tap. :scratch2:

OK, I understand what that is all about but I'm a noob. Can someone explain what I need and what I need to do?

I think its a resistor from each hot leg to a point that will be the center tap and then a resistor to ground? :yes:? :no:? What values?

Something tells me it should be simple. Something tells me its a common problem to solve, I've seen center tap pots before. My Akai has one with a broken shaft.


Finding a donor is an option but that could be a while. The unit is not terribly valuable but it is all working and pretty and it seems a shame to scrap/part it.

Sam Cogley
12-09-2010, 01:15 PM
That was probably my post. I would have more to report if I had ever managed to get that stupid preamp working at all.

westend
12-09-2010, 01:35 PM
These guys (http://www.potentiometers.com/SP50B.cfm), maybe?

blhagstrom
12-09-2010, 04:19 PM
Hmm, I think I was envisioning this backward. Half tap made me think of transformers half tap.

After looking at the schematic and finding a Chinese supply page that showed a half tap and said it was "laudness" I thought of Loudness. That's what the half tap appears to be used for. SO, if I don't rig it up, I loose the loudness function, I think.

I'm wondering if I could hook it to the live side with, say, a 125K resistor and get some loudness function back. The would emulate the current connection from the half tap to the feed tap. But, that would put an effective 250K on the signal path of that line. OR, would it be OK to just run the half tap line to the feed tap directly and end up with only an extra 125K on the line.

blhagstrom
12-09-2010, 07:13 PM
Mmm, looking at the schematic again....I think a dual gang 125K with point to point wiring and 125K resistors on the feed and tagging the half tap direct to the feed pin would give me an equivelent circuit. I would loose the upper 1/2 volume. Probably not a big deal. This ain't no power house.

Wonder if they make 125K duals.....

cubby01
12-09-2010, 10:08 PM
I don't know what the inside construction of your dual gang 250K center tap pot looks like but can you rebuild it with 250k wafer disk from other pots?

blhagstrom
12-09-2010, 10:33 PM
I don't know what the inside construction of your dual gang 250K center tap pot looks like but can you rebuild it with 250k wafer disk from other pots?

Good idea, its a Nobel pot. I'm not sure how compatible the disk is with other pots. All the pieces are there and IF I could pick a really good glue, I may be able to rebuild it that way too. I was looking at it as far as re-assembly goes and it seems strange, in that, I'm not real sure how it was assembled in the first place, and I'm really good at puzzles.

A friend has (had?) a bone pile that I need to go hunting through.

Oh, the price of education....

nerdorama
12-09-2010, 10:50 PM
If you could fit a stepped attenuator in place of the pot, you could put the loudness tap wherever you needed it in the resistive ladder.

John

Dmannnnn
12-10-2010, 04:58 AM
If you could fit a stepped attenuator in place of the pot, you could put the loudness tap wherever you needed it in the resistive ladder.

John

I am considering this option for my tone controls. There are many different types of 2 pole, 10 or 11 position switches on ebay. Many of them eastern block surplus. It would be relatively simple, just wiring all the contacts together with a resistor string. And like john says, you could tap it anywhere you want. The channels would track perfectly as well.

My question on this arrangement would be switch type. I would think a shorting type (make before break) would be the way to go. Although, I've never seen stepped tone controls in an amp. Maybe there is an obvious reason?

The sweet whispers attenuators that came with my Bottlehead preamp were 12 position rotary switches wired in this configuration and they weren't any bigger than a regualr pot.

blhagstrom
12-10-2010, 11:11 AM
:scratch2:

very interesting

nerdorama
12-11-2010, 01:41 AM
A shorting type switch is correct. Without some resistance to ground in the circuit, you'll likely get a nice pop whenever you change switch position. I have a small attenuator that uses a microprocessor and switches small relays so only a few resistors are in the signal path at any position. It has a regular potentiometer which feeds an analog signal to the cpu. I think it was a trial balloon by Goldpoint and Bent Audio.that never went into production. Bought it at a tube audio show and now use it in my line stage. It definately added some clarity to the sound.

John

w1jim
12-11-2010, 09:06 AM
What broke on the pot; did the shaft snap off?
If so then you might try to reattach it. I've had some success doing this. It won't be as strong as new but it it will work. Here's a picture of one I did on my Pioneer SX-5580 (black SX-1050).
The shaft came from a donor pot and I filed the pieces for a tight fit.


http://www.audiokarma.org/forums/attachment.php?attachmentid=216797&stc=1&d=1277251990

http://www.audiokarma.org/forums/attachment.php?attachmentid=216798&stc=1&d=1277251990

blhagstrom
12-11-2010, 12:03 PM
The plastic disk holding the wiper in the outside pot shattered.
It looks like they stake it on the end of the shaft and probably put a little to much stain on it. The stopper pin is on that disk so maybe someone cranked on it too much.

The pieces are there. IF I tried a good glue that worked well, I could probably get it back together. Trouble is, plastic can be a pain to glue.

I was hoping there was some quick solution. Looks like it may be a longer term thing. Ah, the price of learning new tricks...