After a good nights sleep I am now back on the job and ready to tinker with my radio set again
I've remeasured all the key voltages and things are looking good (all measurements taken with the disc set to 245V and spec data in brackets).
AZ31: Vh = 3.6 VAC (4); Va = 265 VDC (285); HT output ; 259 VDC (270)
EBL21: Vh = 5.9 VAC (6.3); Va = 230 VDC (245)
ECH21: Vh = 5.9 VAC (6.3); Va = 235 VDC (240)
My intention for this set is to use it as an amplifier and speaker with signal input through the 'PICK UP' connections rather than from the radio antenna. This is because I want to use it for FM stations rather than SW/MW/LW and I can do this by playing the FM stations picked up from my phone through the set. With this in mind I have directly wired in an audio cable across the 'Pick up' with a 3.5mm jack at one end so I can connect my phone or a bluetooth device. I have also removed/bypassed the switches S16 and S17 so the radio input is permanently muted. I have tested the new set-up and audio from my phone plays through the speakers nicely and at a satisfactory volume to listen to comfortably. So good so far
I have noticed an issue with the volume control (it occurred before I wired in the input cable so I have not introduced the problem). The volume pot has to be turned up to max before I get any audio rather than a gradual increase. Also, the tone control seems to affect volume. At its lowest setting (full left turn of the tone control) volume is audible but low volume. As I turn tone control to the right the volume increases.
I have taken some resistance readings for the two potentiometers. The volume control looks strange. The tone control appears normal and on spec. Any ideas what can be happening to the volume pot here? Is this characteristic of a faulty component? The variable output of R8 is connected to C18 (a black Hunts cap).
R8 (volume):
From ground tag to signal input:
240K ohms (this is very low as the spec is for 700K)
From ground tag to variable output
Zero @ fully anticlockwise; 260K @ fully clockwise
However at about three quarters clockwise it reaches 400K before reducing to 260K at the end. Interestingly the audio is only audible when the dial is past this 400K position.
R15 (tone control):
Across terminals 1 and 3 = 466K ohms (spec is 500K ohms)
Across terminals 1 and 2 (variable output) = 0 to 466K ohms
Across terminals 3 and 2 (variable output) = 466 to 0 K ohms