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Old 14th Oct 2021, 9:26 pm   #4
DMcMahon
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Join Date: Apr 2019
Location: Worthing, West Sussex, UK.
Posts: 6,587
Default Re: Grundig TK 146 Reel to Reel Check Out

Found there was a fourth fuse Si 4 (630mA) on the main board fusing the +17 volt to the output amplifier but this and all the DC supplies checked out OK.

Using scope proved that the audio signal was getting to the slider board but not leaving the slider board. While trying to see why, the audio started working again to the speaker
but kept coming and going intermittently. Could not conclusively see what was happening, but it looked like the volume control slider was intermittently going open circuit.

Trying to measure the slider in-situ gave inconsistent results so removed it to retest further. Had quite a struggle getting it out, it was soldered in by its 9 legs and my solder sucker really struggled and the legs were a tight fit in the holes.

4 of the legs are just the metal case fixings, 3 of the legs are the 100k log slider and the other 2 legs measure as 35k and their resistance varies wrt the wiper of the 100k slider with zero being around the middle position of the slider and either side of the middle the resistance increases up to the 35k when slider moved in that direction, these 2 extra slider legs are shown on the schematic but I do not really understand their function.

Measuring the resistance of the main 100k slider using AVO I could not fault its operation and no obvious sign of it going open circuit during operation, same for the 35k operation.

It does feel stiff and graunchy in operation so for that reason I am going to replace it, taking the part from my TK 141 (#2, # 1 sold) which is basically a spares machine, the TK 141 volume slider works well (in the TK 141) and feels very smooth in comparison.

David
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Last edited by DMcMahon; 14th Oct 2021 at 9:31 pm.
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