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Mitch-W · 7th Jun 2020, 2:21 am

I just repaired a radio with a similar arrangement not too long ago. It also had some hum at volume at 0. If I shorted the grid of the output valve (UCL82 in my case), the hum got almost away (but was still more than it should have been). The cause was the old 2x50uF mains reservoir that failed and also got leaky.

The story goes: the set had a bad 2x50uF mains reservoir (one section measured 0.1uF, the other 10uF), which I decided not to disconnect, but just parallel it with the two 47uF capacitors for the time of the repair. I noticed after a while that the 2x50uF capacitor got a bit warm after a while, but was hard to decide if it gets heated by the valve nearby or by pulling too much current.

For the test, I disconnected one of the bad sections (the one that measured less than 0.1uF) and the hum completely went away. My theory is that, either there was too much current being pulled through the output TX by the leaky cap, or there was some AC ripple that was coupled inside the 2x50uF can, since most likely one of the capacitors internally lost the ground connection.

I also did one more test: I pulled the rectifier valve out (to break the heater string), and used a solid state diode in its place. The set drew about 5W or 25mA at 220V and the mains filter got warm after 10-20 minutes. Anode voltage also only climbed to around 265V.

7th Jun 2020, 2:21 am	#14
Mitch-W Tetrode Join Date: Feb 2015 Location: Celje, Slovenia Posts: 70	Re: Ferguson 384U (Finesse) I just repaired a radio with a similar arrangement not too long ago. It also had some hum at volume at 0. If I shorted the grid of the output valve (UCL82 in my case), the hum got almost away (but was still more than it should have been). The cause was the old 2x50uF mains reservoir that failed and also got leaky. The story goes: the set had a bad 2x50uF mains reservoir (one section measured 0.1uF, the other 10uF), which I decided not to disconnect, but just parallel it with the two 47uF capacitors for the time of the repair. I noticed after a while that the 2x50uF capacitor got a bit warm after a while, but was hard to decide if it gets heated by the valve nearby or by pulling too much current. For the test, I disconnected one of the bad sections (the one that measured less than 0.1uF) and the hum completely went away. My theory is that, either there was too much current being pulled through the output TX by the leaky cap, or there was some AC ripple that was coupled inside the 2x50uF can, since most likely one of the capacitors internally lost the ground connection. I also did one more test: I pulled the rectifier valve out (to break the heater string), and used a solid state diode in its place. The set drew about 5W or 25mA at 220V and the mains filter got warm after 10-20 minutes. Anode voltage also only climbed to around 265V.