[dr.delco]

I'm just finishing up on my Model 66 chassis. Full electronic repair, all caps, out of spec resistors, alignment etc.

What I've noticed is a noticeable 120hz hum coming from the speaker.Of course the electrolytics were replaced with exact OE values, and I've checked them to be sure they are not faulty.

I've discovered that I can just about eliminate the hum by changing the value of the 8uf cap connected in the screen circuit to 30uf or so.

I could just do that (change the capacitance value that is) but I'm wondering why the hum is occurring with the correct value?

Any help or suggestions would be appreciated.
Thanks,
Al

[morzh]

1. Cap #51. Has it been replaced?
2. Tiedot missing from the right #50 cap crossing the horiz. bus (sch mistake). When you connected the cap, was this connection established? (Though symptoms would be different).
3. #49 electr. cap, replaced?
4. Rectifier tube, is it good?

It is ok increasing value of the right cap. But f8nding the reason is better.

[klondike98]

Welcome to the Phorum!


The misprint and the electrolytic caps had me confused on my 66B restore. Here’s a link to some of the thread https://philcoradio.com/phorum/showthrea...#pid146845

Hope it helps.

[morzh]

Riders has this tiedot mistake on several radios.

[David]

   
Having never worked on a model 66 I do not know if Beitmans is any better but have at it . David

[Nathan Slingerland]

If you're looking for the official Model 66 service bulletin (#197) we have that in the Philco Library:

Go here and type `66` into the search box.
https://philcoradio.com/library/index.ph...bulletins/

Or here's a direct link:
https://philcoradio.com/library/download...%20197.pdf

Thanks to Terry (radioroslyn) we've made a few fixes to these long-standing bugs in the service info.

In the case of the Model 66 we haven't gotten to fix the missing tie point yet - but I will update our service bulletin this weekend.

[morzh]

David

Same mistake there.

[dr.delco]

Fellow Phorum members, thanks for all the input so far.

To answer some questions. Yes, C49 and C51 were replaced. I know about the missing tie point on the schematic.

I can say that I went over all connections in the power supply circuit to make sure all connections were correct and had good connections.

I have restored a lot of vintage radios, Philco and others, and have never had this level of hum with all necessary components replaced.

As I stated, I could add extra capacitance to the circuit which practically eliminate the hum, but it bothers me why it cannot function properly with the 8uf as designed.

[morzh]

Dumb question,

Do you see any signs of rework on your speaker?
I am implying the possible humbucking coil inversion.

Also:

Is the filament winding if the power xfmr grounded at centerpoint as per sch?

Is your AC plug plugged into outlet or via isolation xfmr/Variac?
What happens if you invert your AC plug polarity while plugged in the wall outlet?

[Chas]

60hz hum? Should be 120hz. If actually 60 hz could be the 80 is out of balance... Were any of the wires to the output transformer swapped? That would defeat the hum bucking system.. There could be heater cathode leakage in the 75...

GL

Chas

[Radioroslyn]

Could be something simple like running the 75 tube's grid lead outside of the tube shield.

[morzh]

Terry

In most cases this would cause howling/motorboating.....but worth a look at.

Chas

Swapping wires to output transformer does not defeat humbucking coil. Better yet, swapping output (Secondary) winding doesn't do it either. The only way to defeat humbucker is to disconnect it from voice coil and then swap either humbucker or voice coil leads when reconnecting them in series.

[Nathan Slingerland]

Philco Library version of the service bulletin now includes the fixes from Bob's restoration link.

[dr.delco]

Chas is correct, it would be a 120Hz hum being a full wave rectifier. I used the term 60Hz because that's a common term for this kind of problem that most can relate to.

Anyway, I forgot to mention that when I started the repair, I found that the original filter choke, #48, PN 32-7018, was open.

I only had one spare filter choke available, a Stancor C-1709. The specs for the C-1709 are 8.0 Henries @ 85 Ma DC, DC resistance 250 ohms + 15%

This may or may not be the best replacement for the OE Philco part, but I have not been able to find any specs on the original PN 32-7018 choke.

Does anyone have the Philco 32-7018 choke specs? Or can anyone suggest a better alternative choke than the one I used? does anyone have a spare 32-7018 choke?

 Again, it was the only one I had and it did get the radio working so I could repair any other issues. But, considering that the choke is a filtering component in the power supply,

my hum issue might be a result of not using the correct replacement choke.

[morzh]

No your choke is much smaller: 32-7018 is 70H and 2,300 Ohms.

But then #49 cap I mentioned before is a better one to increase than #50 8uF one.

Also you could serialize a resistor with the choke to bring it up to 2,300 as otherewise some of your voltages are higher, plus it will help filtering. The current is below 10mA so the dissipation is minimal and mere 1W resistor should suffice. Then the 49 cap might icrease just a tad.

Again, whatever works.