16-11-2015, 05:24 PM
Back to business:
The anode voltage on my KT61 checked out better that I'd thought. It seemed reasonably on spec. Having said that, I may attempt to change an AF coupling cap on the grid of the DH77 which it seems hasn't yet been replaced. Physically it looks a bit shoddy and tired. Where it gets difficult is the tube bases on this set are amazingly cramped. Just to get at the grid with a soldering iron from below may be a headache. I don't know why they cramped the bases to the extent it was done but I suppose there was no concept of the AF cap needing to be replaced within 20 years!
I did test the AVG diode load of the W77 and this was also very tricky as the leads of the resistors are too aged to make good electrical contact with the meter. I had to scrape a lot with a blade. I think it's at one Meg.
The AF coupling to the grid is shown on the schematic as a variable resistor with a variable resistance on the grid via the capacitor which I think is 0.02 mF. I'd be happy if I can snip this out and solder in a brand new capacitor.
I also replaced and repaired the aerial wire. The pin had been found to be bent and the aerial wire itself had lost most of its insulation. It was wise to deal with this in the event of a short while testing.
The radio seems to work O.K. in its limited way. It seems more "bedded in" but I now have the job of testing all the specs from the output valve downwards.
The anode voltage on my KT61 checked out better that I'd thought. It seemed reasonably on spec. Having said that, I may attempt to change an AF coupling cap on the grid of the DH77 which it seems hasn't yet been replaced. Physically it looks a bit shoddy and tired. Where it gets difficult is the tube bases on this set are amazingly cramped. Just to get at the grid with a soldering iron from below may be a headache. I don't know why they cramped the bases to the extent it was done but I suppose there was no concept of the AF cap needing to be replaced within 20 years!
I did test the AVG diode load of the W77 and this was also very tricky as the leads of the resistors are too aged to make good electrical contact with the meter. I had to scrape a lot with a blade. I think it's at one Meg.
The AF coupling to the grid is shown on the schematic as a variable resistor with a variable resistance on the grid via the capacitor which I think is 0.02 mF. I'd be happy if I can snip this out and solder in a brand new capacitor.
I also replaced and repaired the aerial wire. The pin had been found to be bent and the aerial wire itself had lost most of its insulation. It was wise to deal with this in the event of a short while testing.
The radio seems to work O.K. in its limited way. It seems more "bedded in" but I now have the job of testing all the specs from the output valve downwards.
