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Old 26th May 2011, 7:24 pm   #1
Malcolm G6ANZ
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Default Marconiphone 4124

I got this set mainly as it was one I grew up with. Its cosmetic condition is very good. Only a few light scratches and the vinyl is not torn or worn.
When it arrived the tuning indicator was at one extreme end of the scale and the tuner knob would not move it. My immediate thought was that the drive string had snapped. However on opening it I found that the string was intact and that the tuning capacitor was seized. Gentle cleaning with IPA (the cleaner not the beer) removed all the solid grease and allowed the drive mechanism to work normally. A small quantity of contact lubricant was applied and the action was nice and smooth.
9v was applied and I was greeted with silence. A finger on the slider of the volume control resulted in a hum from the speaker showing that the AF stages were working. The set uses the AF117 series of transistors in the RF and IF stages and knowing the high failure rate of these devices I checked them all before continuing. Each one was checked for shorts to the case/ screen and gain. Surprisingly they all worked with Hfe of around 150. When I refitted then I did not connect the screen lead. Power on again and again silence.
Measuring the collector voltage showed 0v This confused me for a short time until I remembered that the devices are PNP. Measure on the emitter gave various voltages around 4-5v. The bases were scoped and the LO was oscillating correctly and its frequency could be varied with the tuning. The signal could be followed until the last IF stage where it almost disappeared giving nothing on the detector diode. In this stage were two Hunts black caps. I replaced these with new ones ,reapplied power and signals were heard.
I left it on soak test for several hours with no faults.
The radio is sensitive and provides excellent volume with little distortion. An interesting repair as the transistors were fine and the real fault was the hunts capacitors.
Malcolm
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Old 26th May 2011, 9:51 pm   #2
mark pirate
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Default Re: Marconiphone 4124

Nice to see a set with af117's that still work!
Not suprised those hunts were duff, i always replace them on sight.
I have never seen this model before, but the chassis loks familiar....

Mark
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