Description
This page logs the repair job on a Grundig Type 2420 radio that would play a very aggressive and loud mains hum noise. This radio was manufactured by Grundig in Germany between 1963-1965. It’s a tabletop model that supports multiple processes of radio broadcast, from Long Wave and Short Wave to AM and FM. The device in question is rated to work at 220V and 50Hz.
Malfunction
Whenever the radio was turned on, a loud and aggressive mains hum could be heard, no matter which station or band was picked, even with volume knob was at 0. Bellow, I’ve added an audio file that records the noise while tuned to local FM station. Volume knob starts at 0 and I gradually increase it until you can hear the station.
Due to the age of these radios, it’s frequent to discover faulty or degraded components, and filtering capacitors are one of those components that are very prone to failing.
On the images to the right we can see the filtering capacitor that was causing this (it’s the silver cylindrical component on the middle-left). It's actually a triple filter capacitor (three capacitors in one shell). We can also see its capacitance ratings from the label.
After measuring the pins that should correspond to 8uF and 50uF, it was actually measuring 0uF. Easy conclusion: it just had to be replaced.
Solution
Unfortunately, these vintage capacitors can be rare and pricey, especially when their stock is very limited in more isolated countries.
After doing some research, my solutions was to use three normal contemporary electrolytic capacitors with very similar capacitance ratings and similar to higher voltage ratings, as shown on the picture to the right.
After soldering everything back together, the result was very satisfying. I attached an audio recording of it:
Playing around with the Long Wave band
Local FM radio station:
Photos Gallery (Click to Enlarge)
Written in
2016-09-24