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Model 80 jr resistors getting hot - Printable Version

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Model 80 jr resistors getting hot - Xolo100 - 01-02-2021

Hi all I’m new to the forum and new to radio restoration. I restored 2 other radios in the past, but this is my first Philco restoration. I have two resistors that seem to be getting hot to the touch. It’s resistor nine and resistor 11. Resistor nine is a 9K 1 Watt and resistor 11 is a 16K 5 Watt. I replaced both resistors and the 9K resistor was replaced with a 9.1K 2Watt carbon resistor. The 16 K resistor was replaced with a 16K 5 watt resistor. The radio is running well and the performance is good and I’m receiving all stations across the band. All my voltages and resistance that I’ve measured seem to be close to the schematic measurements But I went without a miscalculation. The radio was previously worked on in the past because there were some modifications when I opened it up. The components basically match the schematic except there were a few differences. I’ll post some pictures of The underside of the chassis both before and after the work I’ve done. The radio has been recapped and all the resistors replaced because they were out of tolerance. I also replaced some frayed wiring. All the other resistors are cool to the touch it’s only these two resistors that are having the issue. Thank you for your suggestions.

Wayne

RE: Model 80 jr resistors getting hot - morzh - 01-02-2021

These resistors are supposed to be somewhat hot.
If you run without reducing AC voltage at 120V then the radio runs somewhat hotter, and 120V will make the power dissipation 20% higher compared to 110V.
Otherwise check your B+ voltage and if it is OK then you are seeing what it should be.

Your total dissipation should be around 2.5W if the B+ is about 250V. From this the 9K will dissipate a bit more than 1/3 of it and the 16K - less than 2/3 of it. It will make it noticeably hot. But kt should be OK. This of course is the first order approximation - I did not take into account the 36 tube grid shunting 9K resistor.

RE: Model 80 jr resistors getting hot - Xolo100 - 01-02-2021

Morzh

Thank you I'm going to go back and measure my voltages. I've been using a variac and isolation transformer with my set. I bring the voltage up to about 110 115. The radio has been performing well and the resistors do not discolor or have any other problems. I'm going to go back and check my voltages just for good measure. Thank you for the info. It looks like my b+ voltage is about 254
Wayne
Xolo100,

RE: Model 80 jr resistors getting hot - morzh - 01-02-2021

The only load of this divider is the Screen Grid of the 36 tube in the DetOsc.
It is shown as 165V at the dividing point. If it is what it should be then you are ok.

RE: Model 80 jr resistors getting hot - Xolo100 - 01-02-2021

I took a voltage measurement on the 16K resistor and I'm picking up 165 volts. I believe this should be okay?

RE: Model 80 jr resistors getting hot - morzh - 01-02-2021

Yes. It is Good.

RE: Model 80 jr resistors getting hot - Steve Davis - 01-02-2021

The new 16K will run much hotter than the original because of its smaller physical size. By the way, that great big original resistor was a 3 watt.

Steve

RE: Model 80 jr resistors getting hot - morzh - 01-02-2021

And those great big old one had a propensity to decrease in value, not to increase like the rest of them.

RE: Model 80 jr resistors getting hot - Xolo100 - 01-02-2021

Thank you I appreciate the knowledge you guys have. As a newbie I'm learning a lot. That's great information. I also just measured the voltage across the 9K resistor and it looks like it's coming in at 105 volts.

RE: Model 80 jr resistors getting hot - morzh - 01-02-2021

So....the Screen grid of the 36 DetOsc that says 165V is measured at 105V?

By 165V you meant then the voltage across the 16K resistor?
So then your B+ is then 270V?
What is the plate V of the tube, that says 245V?

Try to measure voltages to the Cathode of the tube.

RE: Model 80 jr resistors getting hot - n1ps - 01-02-2021

the 9K resistor has 3 watts of dissipation, so a 5 watt would be a little better.  You could use 3, 2W 27K resistors ganged together and get 6 watts for a little more safety and last longer...my .02 OM.  Radio looks in nice shape!
Peter

RE: Model 80 jr resistors getting hot - Xolo100 - 01-02-2021

I'm getting 165 volts across the 16K resistor. I'm getting 250 volts on the plate voltage for the detector oscillator 36 tube. I will make more measurements tomorrow evening when I get back from work. Sorry for the confusion.

RE: Model 80 jr resistors getting hot - Xolo100 - 01-02-2021

Thank you Peter I will take a look at that. I'm going to make new voltage measurements tomorrow just to double check my original readings.

RE: Model 80 jr resistors getting hot - Xolo100 - 01-03-2021

I found this page with the schematics. does anybody know if this note for the model 80 is relevant for the screen grid of the detector oscillator tube? I was under the impression that the screen grid voltage for the detector oscillator tube should be 165 volts but it's saying that it should be about 80 volts.

RE: Model 80 jr resistors getting hot - RodB - 01-04-2021

I don't see how the voltage can be anywhere near 80 volts unless the value of #9 & #11 are changed. The two resistors form a voltage divider to get the screen voltage. Maybe there's a substantial screen grid current.