[markmokris]

I replaced a bad volume control in a Philco 610 with a 500 audio taper potentiometer. Now I find that the set is at almost ear piercing volume at just a mere 10% travel.

http://www.nostalgiaair.org/pagesbymodel...013797.pdf

Do these radios NOT use an audio taper? Is that my problem?

I bought some 500K audio taper potentiometers on eBay. I assume I was sold audio taper pots.

Mark

[morzh]

Take a meter and see if it is linear by chance.
Or if it has continuous scale from 0 to 500K without gaps.

[Radioroslyn]

Or take the original and measure the resistance from the wiper to the right terminal with the control set at the center. Linear taper should read 250k on the other hand an audio taper will read some what higher as most of the resistance is in  the first half of the rotation. In the second half there won't be much of a change in the resistance.

[markmokris]

Thanks for the suggestions.

I did do some careful measuring on both pots. I think both the vintage pot and the new pot are audio taper. However the slope of the first 50% is steeper one control the the other. I think this accounts for the difference I am experiencing.

I ran into a similar situation on an Airline tombstone a while back. I replaced the vintage control with one of the more modern ones. Same problem. The audio seemed to get too loud to quickly.

Either the pots I am buying in EBay are strange, or maybe modern audio taper pots are just different?

Mark

[Radioroslyn]

Only your ohm meter knows for sure!

[Nathan Slingerland]

From the 610 service bulletins (#217 and #217B) I see that the original volume control was part number 33-5106.

According to the 1946 IRC catalog, the replacement part is IRC part D11-133.

   

   

It's 500Kohm and that part has (A) taper - which is linear:

   

[markmokris]

That must be it.  I measured the vintage control, and I was expecting an audio logarithmic taper.  So that is what I thought I saw.  But I believe you are right!  Certainly that explains the different response.

Thanks Nathan!

Mark K8KZ

[markmokris]

So I do have a question. This is a fairly typical volume control circuit in the Philco 610. Isn't it? Why would it need a linear tape pot and not a more typical audio taper pot?

Mark

[morzh]

They could be trying to work with the already present non-linearity of the 75 triode section.
Just a guess.

[Radioroslyn]

Guess #2 is that an linear taper volume control makes the set sound like it can play really loud as most of the gain is in the beginning of the rotation. Personally I think it's marketing trick to give a buyer the impression that a given set has a lot of volume when it doesn't.

[markmokris]

Guess #2 is that an audio tapper volume control makes the set sound like it can play really loud as most of the gain is in the beginning of the rotation. Personally I think it's marketing trick to give a buyer the impression that a given set has a lot of volume when it doesn't.

Terry,

I think you are right.

At this point, I believe the more modern volume control that I used as a replacement has a bit more advanced slope than the older control I replaced.  I wired in the old control again to evaluate the difference.  I believe that they are both audio taper.

The older control advances the volume pretty quickly,  just not as quickly as the newer control.  So I am chalking it up to a difference between the controls in the first 50% of the audio taper curve.

The only reason I replaced the vintage control (It is not an original Philco part) was because the shaft was goofy.  It had flatness on both sides, and I couldn't get the Philco knob to grip.  I figured out a way to make the knob work, so I put the vintage control back in.  It works better.

Mark K8KZ