08-11-2019, 09:42 AM
I'm in the process of repairing one of the above early 1970s AM Car radios, which, at some time had suffered water ingress. This had caused the main smoothing capacitor to deform-it was buklging at the positive (Bottom) end. As a precaution I replaced two other electrolytics of the same make and, after having replaced the polarity c/o switch, the radio was working, but not up to scratch, with the centre rail at ca.+4.5v rather than + 7v or so. I traced this to a 560 ohm resistor in the emitter circuit of TR5 in the DC coupled amplifier, which had increased to around 750 ohms. Whether this was due to water ingress-no sign of water damage at the resistor's location, or simply due to the fact that the radio is 46 years old, I know not. The resistor in question, like most in these sets is a standard 0.5 watt carbon type. Other resistors measure within tolerance, though a 2k2 in the emitter circuit of TR4 reads about 2k5 on my DMM. The driver and o/p transistors (Motorola in-house types with numbers like A4k, P6D, etc., all check out OK. This is one of a series of Car Radios made by Motorola UK at Stotfold, Herts. in the 70s & early 80s. Prior to that UK Motorolas had been made by Radiomobile.