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Old 14th Oct 2021, 10:01 am   #24
simpsons
Octode
 
Join Date: Feb 2007
Location: Harrow, London, UK.
Posts: 1,014
Default Re: Grundig 5399/u/s

Hi again Skip

So, to recap, with the stereo decoder board removed, there was no FM audio. This we can agree is no surprise as the 9 pin socket use to connect the board to the main chassis was now unplugged.

The fact that feint audio could be heard is likely due to the self capacitance in wiring but it says that the FM stages are working.

The 9 pin socket to the stereo decoder carries both heater and HT supplies to the decoder and presents raw audio on pin4. It also switches the decoder to mono, by earthing via the piano switch (stereo), pin 5. The decoder, when working, outputs stereo to pins 2&3.

Ship, just double check all this is happening in case there is a clue to what is going on.

So, as mentioned earlier, joining pins 2,3 & 4 should give you plenty of sound. My schematic doesn't show the decoder socket, which is really bad news, and for some markets it isn't fitted!!

To audio signal trace, we can either inject audio from a suitable source or using a an amplifier check the audio present on the Grundig from the ratio detector, onwards. A suitable starting point, as shown on the schematic, is R36 22k ohms.

There must be a reason why this fault is proving illusive, I wonder what are we are missing?

Chris
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