Thermistor value for a BK 2040 SG crystal oven

[Pulsar_CB]

Hi everyone

Brand new here. To be honest I thought most online forums for cb radio are all but dead. But seems like this one is active. I should have joined years ago. I have a small cb repair and sales business and a youtube channel. I specialize in the older radios, mostly bringing them back to factory original. Undoing unnecessary mods and restorations. I'm an authorized President retailer as well. Hopefully I can help out others as well as get help.

Today I turned on my 2040 signal generator. I don't use it, but I like to keep it as a piece of cb history or backup. After about an hour of plugging it in, I smelled a slight burning. I immediately opened it up and found the crystal oven Styrofoam was melting. And the shields were hot hot ! I'm familiar with these units so I pulled off the cover and found the thermistor that works with the IC is all but a burnt up disc on the pcb. It's obviously getting way too hot. I tried to look up what type and value thermistor is supposed to be there and I can't get any info. The service manual just labels it as thermistor. I'm assuming it's a NTC but what value ? Anybody have this info or suggestion as to which one to use ? Its R302 in the attached schematic. I guess I could completely get rid of the resistors but I would.rather fix it right.

Thanks
Roland

 

[brandon7861]

I think you have more issues than just a thermistor. Consider the case where the thermistor is a dead short. What is supplying it enough current to get that hot? If all 15v were across the 2k resistor above it, that would only pass 7.5mA. Not cooking any styrofoam with that. So either R301 is also toast, or IC301 is.

As for the value. I emailed them, hopefully we will get a reply. In the meantime, we could make an educated guess.

The reference divider provides 6.38v to the inverting input. The divider with the thermistor therefore turns on the op amp output when the non-inverting input goes above that voltage. That would suggest picking a thermistor whose resistance falls with increased temperature and has a resistance of 1481 ohm at the temperature you want the OCXO to run at. Yes, NTC is correct.

And don't worry if you can't find the exact same one, all you need to do is change R301 so the voltage at the non-inverting input is right at the operating temp.

 

[nomadradio]

He's right, you know.

Seriously, the original circuit can't provide enough current to cook that part. Ohm/s Law won't allow it. This means that the surge current that caused the burn came from somewhere else. Like through a failed LM741 op-amp chip. Pretty sure that's the type number of the one in the schemo snapshot. Fortunately it's a generic item still available. I recommend installing a DIP socket when replacing this chip. If another one fails, the pc board's foil traces won't have to withstand another heat/cool cycle. If you heat a solder pad enough times the glue lets go and the foil lifts.

73