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Philco 77 volume control - Printable Version

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Philco 77 volume control - kenlew0405 - 04-21-2012

Hi everyone,
I recently got a partly refurbished Philco 77, but the volume rheostat/ potentiometer doesn’t work (it sparks and smokes). Can anyone tell me what kind of rheostat/ potentiometer I need to replace it (a Mouser number would be great)? I don’t quite understand what the schematic is indicating.

Thanks a bunch
Ken


Schematics can be found here: http://www.nostalgiaair.org/PagesByModel/838/M0013838.pdf


RE: Philco 77 volume control - murf - 04-21-2012

(04-21-2012, 09:55 PM)kenlew0405 Wrote:  Hi everyone,
I recently got a partly refurbished Philco 77, but the volume rheostat/ potentiometer doesn’t work (it sparks and smokes). Can anyone tell me what kind of rheostat/ potentiometer I need to replace it (a Mouser number would be great)? I don’t quite understand what the schematic is indicating.

Thanks a bunch
Ken


Schematics can be found here: http://www.nostalgiaair.org/PagesByModel/838/M0013838.pdf

I would contact Mark Oppat on that one.
I am sure he would have the switch for you.
murf

RE: Philco 77 volume control - kenlew0405 - 04-21-2012

Thanks murf. I'll contact him and see if he can help.

(04-21-2012, 10:14 PM)murf Wrote:  
(04-21-2012, 09:55 PM)kenlew0405 Wrote:  Hi everyone,
I recently got a partly refurbished Philco 77, but the volume rheostat/ potentiometer doesn’t work (it sparks and smokes). Can anyone tell me what kind of rheostat/ potentiometer I need to replace it (a Mouser number would be great)? I don’t quite understand what the schematic is indicating.

Thanks a bunch
Ken


Schematics can be found here: http://www.nostalgiaair.org/PagesByModel/838/M0013838.pdf

I would contact Mark Oppat on that one.
I am sure he would have the switch for you.
murf



RE: Philco 77 volume control - Radioroslyn - 04-23-2012

That one to is going to be really hard to find as a new part. It's a dual pot one half controls the RF gain and the other half control amount of rf that is coupled from the antenna to the 1st RF amp.
Best bet is to try to fix the one you have or get one from a model 20,70,or an early 90 parts set.
GL
Terry

RE: Philco 77 volume control - Radioroslyn - 04-29-2012

There's a 77 chassis up for auction at you know where. Might make a good donor.
Terry

RE: Philco 77 volume control - kenlew0405 - 05-01-2012

Hi everybody,

Well, I feel like a real jerk! After all this research, getting the converter kit from Mark Oppat, and all the great help from the forum, I realized I was looking at the schematic for the wrong model! I am working on a Philco 65, not 77 (they look similar from the outside). So i guess my next question is, can I use the 10000 ohm pot that came with the converter kit to replace the (simple) 5000 ohm pot that the 65 has?

Thanks for all your help!

RE: Philco 77 volume control - morzh - 05-01-2012

Kenlew

Where on the 65 sch do you see 5K pot? I looked at the sch here in the Resources, and then on Nostalgiaair, and there is none.
if you look in their Rider manual, in the beginning there is the Mod. 87 that does have 10K pot, and then the 65 that does not.
And, the 77 does have 5K pot (dual, 5k/250 ohms) according to the sch.

Can you first straighten this all out?

RE: Philco 77 volume control - Ron Ramirez - 05-01-2012

http://www.philcoradio.com/tech/images/65.jpg

The volume control is part (25) on the Model 65 schematic; a 1,750 ohm rheostat. It controls the screen grid voltages of the two 24 RF amplifier tubes.

RE: Philco 77 volume control - morzh - 05-01-2012

Ron,

Yes, that one I saw, but it is in no way close to a dual pot, especially given the values of 5K/250 ohms or a 10K one.

I am not sure what Kenlew meant by 5K pot, and how he wants to fit a 10K i/o 1.7K one.

Now that he told he was looking at the 65 schematic, and not the 77, I have doubts that he really did.

RE: Philco 77 volume control - kenlew0405 - 05-01-2012

(05-01-2012, 04:41 PM)morzh Wrote:  Ron,

Yes, that one I saw, but it is in no way close to a dual pot, especially given the values of 5K/250 ohms or a 10K one.

I am not sure what Kenlew meant by 5K pot, and how he wants to fit a 10K i/o 1.7K one.

Now that he told he was looking at the 65 schematic, and not the 77, I have doubts that he really did.

Yes, I was wrong again. The pot is 1750, so I think the 10k is too much.


RE: Philco 77 volume control - morzh - 05-01-2012

Man, you've got to stop working late Icon_smile

RE: Philco 77 volume control - morzh - 05-01-2012

Other manual shows 2K, but that beside the point.

This pot needs to dissipate quite a bit, huh.....3W dissipation will make it 5 to 7 W rated pot at least.

I was about to suggest trying to trick that thing: you can connect a large, say, 6-8W resistor of 2.1K in parallel to the ends of the pot (the circuit will see an equivalent of 1.72K) and try to regulate it this way, but I am not sure what the 10K pot's rated dissipation is - needs to dissipate 1/2 Watt, so needs to be 1-2W rated at least.

You can try.....

RE: Philco 77 volume control - Radioroslyn - 05-01-2012

Another solution to your problem would be to change the manual gain control (the dual pot) to AVC (automatic volume control) This you would only need a single pot of 250K to a meg ohm along with a few resistors, couple of caps, and a diode like a 1N34.
Terry

RE: Philco 77 volume control - kenlew0405 - 05-02-2012

The wattage of the pot was my next question, so it should be at least 5w. Does it hurt to go bigger if I can find one @1750 ohms?

(05-01-2012, 05:33 PM)morzh Wrote:  Other manual shows 2K, but that beside the point.

This pot needs to dissipate quite a bit, huh.....3W dissipation will make it 5 to 7 W rated pot at least.

I was about to suggest trying to trick that thing: you can connect a large, say, 6-8W resistor of 2.1K in parallel to the ends of the pot (the circuit will see an equivalent of 1.72K) and try to regulate it this way, but I am not sure what the 10K pot's rated dissipation is - needs to dissipate 1/2 Watt, so needs to be 1-2W rated at least.

You can try.....



RE: Philco 77 volume control - kenlew0405 - 05-02-2012

You know it. Waiter, more coffee!

(05-01-2012, 05:07 PM)morzh Wrote:  Man, you've got to stop working late Icon_smile