courier 1 cb base 6bq5 tube overheats

[klj2497]

Hello, I purchased a courier 1 changed the capacitors, and some resistors. luckily i found a mic that works. it transmittes,receives, hears great, everything works . but the Modulator tube 6bq5 is overheating. i traced to the rc-70 a 4 lead component between the 12bay7 and the 6bq5. the voltage is steady on lead 1 and 2 . 2 goes to pin 1 on 12bay7. lead 3 goes to the 2 pin on 6bq5. it should be 0v, but its leaking dc current into 6bq5 when i turn on the radio. then it creeps up from almost 0v to over 1vdc then i turn it off. the tube goes up over 250f. im thinking pc-70 is the problem i have not been able to find a pc-70. im thinking maybe i could make something that looks like the pc-70 out of 2, 500k resistors, 2, 250mmf capacitors , and a .01 capacitor. or maybe something else will work any help would be greatly appreciated Thanks Rob

 

[Dmans]

maybe i could make something that looks like the pc-70

Sounds like a good place to start. Easy enough to build and to try it out. Go for it!

73
David

 

[nomadradio]

I would suspect a shorted C304. Electrolytic caps fail more often than ceramic caps. Odds are the .01 grid-blocking cap in the module is a ceramic cap. Might also check resistance measure from pin 2 of the 6BQ5 to ground. If it's a lot higher than the 500k in the diagram, simply adding a resistor with that value from the pin 2 lug on the socket to ground might clear up the issue.

73

 

[klj2497]

Thanks for the info' I checked the resistance to pin 2 its .513m ohms. or 513k ohms. i have a peak atlas esr70 resistance/capacitance meter. so i checked the c304 10uf capacitor its 12.81uf. The esr is 4. The chart shows its low and thats good. I ordered some 510k 300v metal film resistors, and 240mmf 300v silver mica capacitors, i have some .01uf 2kv ceramic capacitors i think will work also.

 

[ExitThirteen]

I agree, I also suspect the PC-70, as it's likely letting DC "leak" onto the grid of that 6BQ5. TV's used to run something similar, and it was called a "couplet" back then, and they were a high failure component.

~Cheers~

 

[nomadradio]

"couplet"

Umm, "couplate"? As in a flat module?

And yeah, they go bad.

73

 

[klj2497]

Thank You my friends. the parts are on the way;

 

[klj2497]

its working. That courier 1 a75 sounds awesome with that plate modulation. and it has great ears. the couplate i made seems to work well The 6bq5 still goes up to around 300f idling but it might be okay. its less than before. so im going to keep an eye on it

 

[klj2497]

The courier a75 is working for the most part. except for the meter zero in the back being unstable. also I am only getting about 50% modulation. i purchased a service manual and did the transmitter alignment. i checked the mic, and wired another mic. the audio sounds good on a monitor radio its just missing the last 50% of its modulation.

 

[nomadradio]

Two basic possibilities: First, not enough mike audio feeding into the 6BQ5.

Second, not enough output from the 6BQ5 even with full drive feeding into it.

If trying another 6BQ5 makes a difference, you found the problem. A tube tester won't always tell you that the peak current a tube can pull has been reduced. The actual "quality" test is done at a far-lower current through the tube than what's needed for 100% modulation.

A decent-quality tester is good at identifying bad tubes. The accuracy level for predicting what is or isn't a "good" tube is lower.

73