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WaveyDipole · 26th Aug 2013, 12:48 pm

+Bill, thanks. The conversation has indeed gravitated to a specific radio fault so I guess this is correct. I see you changed the title of the thread to reflect that as well.

PJL, thanks for your responses. Regarding your questions:

1. What is the UL84 grid voltage?
Vg(p2)=2.4vdc/9.2vac; Vsg(p9)=160vdc/0.08vac

2. Does the buzz vary with volume?
No, it does not. It stays constant regardless of volume.
This is why I'm trying to figure ot the source of the AC voltage on the cathode and is comming from.

3. Do you have a spare UL84 or UABC80 you could try?
Unfortunately not. I do have a EL84 and an EABC80 in another radio but I believe these operate on different heater voltages.

4. Try a 0.1uF from UL84 grid to chassis, what happens to the buzz?
This kills the buzzing noise.

5. Try a 0.1uF from UABC80 grid to chassis, what happens to the buzz?
This has no real effect.

I also tried decoupling C43, but that made the buzz considerably louder.
So I guess this implicates the UL84?

26th Aug 2013, 12:48 pm	#13
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Possible Valve failure in a Bush VHF72 +Bill, thanks. The conversation has indeed gravitated to a specific radio fault so I guess this is correct. I see you changed the title of the thread to reflect that as well. PJL, thanks for your responses. Regarding your questions: 1. What is the UL84 grid voltage? Vg(p2)=2.4vdc/9.2vac; Vsg(p9)=160vdc/0.08vac 2. Does the buzz vary with volume? No, it does not. It stays constant regardless of volume. This is why I'm trying to figure ot the source of the AC voltage on the cathode and is comming from. 3. Do you have a spare UL84 or UABC80 you could try? Unfortunately not. I do have a EL84 and an EABC80 in another radio but I believe these operate on different heater voltages. 4. Try a 0.1uF from UL84 grid to chassis, what happens to the buzz? This kills the buzzing noise. 5. Try a 0.1uF from UABC80 grid to chassis, what happens to the buzz? This has no real effect. I also tried decoupling C43, but that made the buzz considerably louder. So I guess this implicates the UL84? Last edited by WaveyDipole; 26th Aug 2013 at 1:09 pm.