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Model 77 wirewound resistor wattage question - Printable Version

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Model 77 wirewound resistor wattage question - VE Guy - 04-02-2021

I'm getting started on a Philco Model 77 lowboy.  As I expected it needs a lot of work, but in testing the 2 section wirewound resistor, one section tests OK (800 ohms), but the other section which should be 250 ohms, reads more than double.  I disconnected the middle from the circuit, but it still reads high.

My instinct is to replace it with 2 resistors, but what wattage should I use?  There's not enough data for me to figure it out.  2W, 5W, 10W?

Thank you.
Joe

RE: Model 77 wirewound resistor wattage question - Radioroslyn - 04-03-2021

Hi Joe and welcome,

I roughed it out the 250 ohm section and came up with 3w. You'll want the triple that so it won't get flaming hot. 7.5 w would fit the bill. The 800 section is the "cathode" resistor for the 45's. A couple of watts is fine.

GL

RE: Model 77 wirewound resistor wattage question - VE Guy - 04-03-2021

Thank you Terry,

So it appears Philco overbuilt the resistor for the 800 ohm end.  Since I have them in 10W, I'll go that route. There's plenty of room to put them in there.

Thanks again.
Joe

RE: Model 77 wirewound resistor wattage question - Radioroslyn - 04-04-2021

A couple of pointers on the old Philco tetrode trf sets.

1. When rebuilding the filter block (big black metal can) stick to the mfd value of the caps. If you don't it can significantly raise the B voltage.
2. Use film caps not electrolytic type. E caps can't take the ripple current and will short after a few months of operation. The 80 will turn a very angry red color! See pic below.
3. Check the resistance of the untuned side of the rf coils. Not uncommon for them to be open do to a chemical reaction.
4. Check resistance between any of the four pins on the spkr plug (unplugged). Should see nothing higher than 4k. Field coil is abt 3200 ohms and the the opt pri is abt 500 ohm. You can rewind the fc, spkr comes apart part pretty easily. If your a glutton for punishment you can rewind the transformer. A member just did one a few weeks ago.
5. Check the 250 ohm side of the volume control. It controls the cathode current on the rf amps. A lot of times they are intermittent or open @ the low end. The 5K side is always good which doesn't do much.

Don't know if you saw this diagram but it's got the parts values on it.
https://philcoradio.com/library/index.php/service-info/early-schematics/

I have It's older bro the 76. The only difference is you've got a tone control and I've got a local/distance switch.

GLOM
de N3GTE

RE: Model 77 wirewound resistor wattage question - VE Guy - 04-04-2021

Thank you for the help.  It's not my first time with an early Philco, but this is throwing me.  I'm not looking to taking that cap block apart, but I like to keep it as original looking as possible.

Again, thank you for the help and the schematic link.

Joe