Pointer Dial Philharmonic Selectivity Trimmers

[Don Jackson]

Can someone verify the correct orientation of the selectivity trimmers in an early pointer dial Philharmonic? In my set the trimmer closest to the front of the set is meshed when the selectivity / fidelity control is fully counter clockwise. In this position, from front to back the trimmers are meshed, unmeshed, meshed and unmeshed. Is this correct?

The volume control has been replaced and I wounder if someone removed the fidelity control during the process and messed things up. The set was recapped at some point in the past and plays, but could be better. The set only works when the fidelity control is near the full clockwise position, when you turn the control to the left it dies out. I assume it is way out of alignment but want to check everything before attempting to align. I am planning to replace the volume control with a tapped unit so this would be a good opportunity to check operation of the selectivity / fidelity control or repair.

[Scott Seickel]

Thank you Don. 

[Don Jackson]

Here is a copy of the 500K pots offered by IRC and a taper chart from the catalog. It appears they did offer some with 25 k and 50 K taps. Any of the tapped audio pots I recall seeing had taps at 100k or more, as you noted. I have a couple of newer IRC 13-133 pots but they are 7/8" diameter but they  don't look hopeful for installing taps.  

[Scott Seickel]

Thanks Don.  Looks like it is easier to do with one of these pots that were designed to accept taps.  I couldn't find a catalog online. 

[Don Jackson]

Scott

The potentiometer is an IRC 13-133. It is listed in an IRC catalog which is available online if you don't have one. In the catalog is is noted as a "left handed logarithmic taper for audio circuit control" with a 3 (C) taper. It was easy to install the lug, there is already a hole there which has to be drilled out a bit larger. I used a 4-40 nut and bolt which fits well. Applying the epoxy was a bit more difficult than I thought, I did 2 and the second one turned out better than the first. The second one has a short shaft so I attached a dial with a 0 - 100 scale and recorded the results. 0=0 ohms, 10=18 ohms, 20=1.4k, 

30=5k, 40=25k, 50=46k. Beyond half scale the resistance goes up quickly to 540 k at the maximum.

I had to trim the epoxy a bit to provide clearance for the wiper but you can shave the epoxy a bit just as it is starting to harden. In the end it measures 21.3 k which is not bad.  The resistance of this pot comes up fairly slowly so it should be better than the pot which is in the set, which comes on really fast and is annoying. Consider this an experiment, if I did another one could do better. The first one ended up at 31k, the resistances changes very fast around the 20k point so it is hard to get the epoxy in the right spot. It is very easy to make a mess with this stuff. I might add that the 40 k point is only about 1/8" past the nut, so the same pot could be modified for a 40k tap. I have lots of various 500k pots, this type seemed like the best prospect, it is 1 1/4" diameter.

[Don Jackson]

Scott

Thanks for clarifying the fidelity caps. I made a copy of the early schematic from the archives and made an enlarged copy and have made note of the values of the resistors and capacitors. There are some fuzzy symbols next to the caps on the schematic which I could not make out. But I see now that C37 is noted at 5000 pf and C32 as 2500 pf. A bit confusing but now it makes sense! And the early schematic shows the switch in the maximum selectivity position. The is a lot going on in the Selectivity/Fidelity circuit so it took a while to figure it out. Also lots of things that could get messed up.

[Don Jackson]

David

Yes, good catch on the capacitors. One could easily assume they are mica capacitors. I checked the C divider and it is fine, but noticed another capacitor that appears to be the same species close to it. I am going to remove the diode assemblies and check them, a bit of a pain but a necessary task.

[Don Jackson]

Mike

Nice boxes! Both of mine are gone, the one for the electrolytics was replaced with an FP capacitor. I made a copy of the dimension and might try making some myself. As for the bass tone assembly, I will probably replace the caps and hope for the best but this is good information to have which I have added to my files for future use.

[mike hadley]

Refering to your bass tone assembly I reworked mine to suit the 50hz mains supply over here in the UK,

there is a mention here

https://ehscott.ning.com/forum/topics/philly-bol-schematic?id=39258...

you can do this

you can run this on the bench as a stand alone filter, using a signal generator through a resistor, someware around 10k will do,

(eliminating the low output impedance of the generator). Place the generator across the whole stack and a scope on one of the taps you can then get the whole thing to resonate. This is wht I did with mine to get 50hz resonance to match the UK supply

Mike

[David C. Poland]

good that you addressed the step tap caps because they are in a high voltage situation off a tube plate circuit.

And do pay attention that the -C voltage divider is ok.

Replacing the caps inside the 3 diode assemblies is a pain, but do it and check the inside resistors, too.

[Scott Seickel]

"I am going to remove the bass tones assembly and see if the caps have been replaced. I have read other posts here that stress the importance of having pairs of caps as close to .02 and .025 as possible."

I went thru this 10 years ago.  I measured the chokes from 4 sets and found that they varied in inductance by a notable amount.  I modeled the circuit, which is a 6 pole filter,  in LT Spice and didn't get the expected results due to the variation in the chokes.  They are low tolerance parts. 

Best approach is to sweep the assembly out of circuit and adjust the capacitor values for the deepest 60 Hz notch. 

Of course, this is optional.  Most just replace the capacitors and move on.  Only person I know of who did this during their restoration is Mike Hadley who is on this forum.  I don't recall what his final capacitor values were. 

I need to find some of those older IRC potentiometers.  Is there a part number on them?