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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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24th Mar 2019, 3:39 am | #1 |
Retired Dormant Member
Join Date: Mar 2019
Location: Ripley, Ontario, Canada.
Posts: 1
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Blaupunkt Franfurt Y series
I'm posting this as much to thank Leon Crampin for this help he provided here for another forum member : http://www.vintage-radio.net/forum/s...d.php?t=116715 I am not a radio guy, so having an expert that has actually worked on these old radios is a real help and I'm very appreciative.
Although the Blaupunkt Frankfurt radio I have owned for almost 40 years is the Y model USA spec Frankfurt (about 1967ish) and the radio that Leon was giving advice on is an earlier version, his advice was quite helpful. I'm nearly done the repairs and quite happy. This radio has always had super hot transistors and was a power hog in the years I used it from about 1980 until 1992. I pulled it out of the car as it was starting to act up and replaced it with an AM Blaupunkt of slightly older vintage. That old radio still works fine but its transistors also run hot and now I know how to fix it, thanks to Leon. Replacing the selenium diode that crept up to 1.4V drop in the audio amplifier section with a Schottky and silicon diode in series ( so about 0.75V drop as opposed to 1V drop originally) reduced the current draw from 1.4 amps to 1.2 amps. Still running too hot, so I looked at the transistor bias adjustments. First thing I found was the thermistor read 14 ohms cold when it should read 9 ohms cold and gets lower when hot. However the main culprit was the trim pot (R672 on the Y set I have) for balancing the biases. It is a 10 ohm pot (which can be read with a magnifier by looking at the back side) Once I replaced that pot with two 5 ohm resistors, the current draw dropped to just under 400 mA and the balance is 'good enough'. That, along with replacing the thermistor with a 4.2 ohm resistance to simulate a hot set, has the radio running at 330mA with more output power than before and the transistors and audio output transformer running stone cold. Also no more popping and hissing from thermal runaway or whatever from the AD148 transistors. Fred |
24th Mar 2019, 1:11 pm | #2 |
Octode
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,869
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Re: Blaupunkt Franfurt Y series
Thanks Fred, I'm really pleased you got this fine radio working - thanks for the acknowledgement.
Regards, Leon. |