http://www.nostalgiaair.org/PagesByModel...029646.pdf
I am working on a Philco 71 and I would like to replace the first portion of Part #52, a 245 - 185 ohm wire wound resistor. The 185 portion is fine but I get no readings for the 245 portion. Can I substitute a 250 ohm 1 watt resistor in place for the 245 portion of the resistor.
Also since this part appears to be not functioning properly, would it effect the DC readings I was getting? My AC readings were right on.
I would appreciate any thoughts or suggestions.
Is it the side of the resistor that connects from the ct of the hv winding to ground?
Hi, Phil27,
If the center tap of the HV Section of the transformer goes to the 245 Ohm section, then yes, there will be a big issue with DC voltages. If open and the HV center tap was still connected to the open leg, then there would be no "B-" and very little current.
If the HV Center tap is connected to the center tap of the resistor (chassis ground), then the grid bias of the '42 Audio Output would be 0V instead of the usual -16.5V on the grid. This would cause the tube to draw much current, causing a drop in B+ and likely causing damage to the tube or output transformer if the condition continues.
The standard tolerance for most resistors is 20%, therefore replacing a 245 Ohm Resistor with a 250 Ohm resistor is fine (this is an error of about 2%. The issue is that the 1 Watt resistor would probably smoke in time. you can do it temporarily, just to continue testing the radio, but measure the voltage drop and order a higher wattage resistor.
You might get away with a 2 Watt unit, but I would use a 5 Watt resistor at minimum. since this resistor is between the B Voltage Return and the rest of the radio (Chassis), the vast majority of High Voltage current passes through this resistor. Granted, if one applies Ohm's Law to determine the current through that resistor, and therefore the wattage needed, the 1 watt may pass the math, but there is a reason that they used wire wound resistors for that one.
Hope this helps.
Best Regards,
John "MrFixr55"
Yes it does help a lot. I’m new to radio restoration so I was at a loss as to how to fix the problem.
I did put in the 1 watt 250 ohm resistor and my DC voltages were there again. I will order a 5 watt 250 ohm to properly replace the 1 watt.
Thanks very much.
Hi Phil,
Glad that I was able to help! How does the radio sound? The '42 is a pretty solid Audio Output tube, so, if the speaker and the rest of the radio are good, the 71 should sound nice. The speaker is a big improvement over the earlier Model 20 "Pie Plate" Speakers. If you put in an Aux Input and feed an MP3 Player or an iPod / iPad into it, you will be amazed. (Also a good way to play period appropriate program material (Big Band / Swing Music WWII News bulletins, Roosevelt Fireside Chats, etc. through the radio). The only other major bugaboos on this radio are the "Autodyne" 1st Detector (Converter) and the dreaded cellulose insulators causing corrosion and an "open circuit" on the primaries of the RF Coils and the "Tickler" of the "Oscillator" Coil. Much about these in this Forum, most courtesy of Ron Ramirez, the Professor (Pro-Ph-essor?) of all things Philco!
Phil,
A 5 watt resistor will not be large enough. I know as I tried one plus I think I calculated it needed at least 7.5 watts. Instead ofmy retyping everything . Look at my post in Electronic restoration. WW resistor wattage 6452. Probably half way down the column.
The 12 w resistor worked nicely and ended up sinked to the chassis to help it run cooler. I replaced the 185 with a 2 watt which was over kill but I had it in stock.
If you have any other questions feel free to pm me.
Good Luck,
Henry