Quote:
Originally Posted by James Duncan
Quote:
Originally Posted by Phil G4SPZ
So far, he has successfully replaced the volume control and the receiver is working, but with constant hum. He has established that the HT at the anode of the output valve (pin 5) is only about 40 volts instead of the normal 200 volts. I have now asked Trevor to measure the valve’s cathode voltage to see whether the low HT is being caused by excessive current draw through the output valve or simply by a faulty reservoir capacitor.
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Remove the output valve, measure the anode volts, if then ok try another valve, lots of these available.
On the subject of hum (unrelated to low HT) please check for dc volts on the input grid of the valve, if any found replace the capacitor on the grid, does not need to be exact value, it allows the AF to get to the valve and isolates the grid from DC.
you will fix this radio as very simple circuit and also these preform very good
MM0HDW |
YES relocate the output Xmer, had to do that on three of these sets, this removed the hum but still check all other sources of hum, smoothing, leaky grid cap, bad valve.
The mods of 1971 radio mags greatly improved these sets. Fit the voltage stabilizer, improve the vfo stability. control of Rf stage etc.