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telstar · 14th Jun 2021, 2:32 pm

Hi, at long last I finally sorted the fault on the Strad 511 radio. The cause of the fault was the valve base for the 7C7 audio pre - amplifier. It had become tarnished inside the socket, so I gave it a good dose of WD40. This eventually cured the fault. It was causing poor connections including hum. When measuring voltages on the anode it made a sound like it was the grid of the valve. This was the only socket that had this problem, the others being O.K. So I gave these others the WD40 treatment
It was initially confusing to work on the radio as it varied from the layout on the later model on Trader 1014. One of the large resistors across the main electrolytic capacitors measured 12.6 K ohms instead of its designed 10 K ohms. This metal resistor had lost all of its colour code and yet the first resistor (after the 7Y4 rectifier) still measured 180 ohms (but lost its colour coding).
Most of the carbon resistors in the radio had exceeded their tolerance limits (30%), and this included grid stoppers.
The wax capacitors were replaced with modern ones, but the electrolytics were the only ones that did not require replacements.
I also decided to give the loudspeaker wires a longer replacement.
Still some work to do on flattening the back card, and the whistle effects on BBC Radio 4 L.W.

14th Jun 2021, 2:32 pm	#1
telstar Octode Join Date: Dec 2003 Location: Leeds, West Yorkshire, UK. Posts: 1,592	Strad 511 Hi, at long last I finally sorted the fault on the Strad 511 radio. The cause of the fault was the valve base for the 7C7 audio pre - amplifier. It had become tarnished inside the socket, so I gave it a good dose of WD40. This eventually cured the fault. It was causing poor connections including hum. When measuring voltages on the anode it made a sound like it was the grid of the valve. This was the only socket that had this problem, the others being O.K. So I gave these others the WD40 treatment It was initially confusing to work on the radio as it varied from the layout on the later model on Trader 1014. One of the large resistors across the main electrolytic capacitors measured 12.6 K ohms instead of its designed 10 K ohms. This metal resistor had lost all of its colour code and yet the first resistor (after the 7Y4 rectifier) still measured 180 ohms (but lost its colour coding). Most of the carbon resistors in the radio had exceeded their tolerance limits (30%), and this included grid stoppers. The wax capacitors were replaced with modern ones, but the electrolytics were the only ones that did not require replacements. I also decided to give the loudspeaker wires a longer replacement. Still some work to do on flattening the back card, and the whistle effects on BBC Radio 4 L.W.