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Old 25th Dec 2019, 12:14 am   #147
G0HZU_JMR
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Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Marconi TF144H/4S

They might be referring to the connection '1' in the little square box at the top of the crystal calibrator circuit at FILT 1. This will have a 22k connection (R216) to 250V DC so some care would be needed here along with a high voltage coupling cap. This node in the circuit corresponds to the filtered audio output from the heterodyne mixer at the top of FILT 1. So it would seem logical that this would be the place to inject a test audio signal... but maybe not the safest!

If this is the right connection then I'd have expected a safety warning in the manual because there could be 100-250V DC via R216 here. It might give you a jolt and it might also zap any (modern low voltage) test gear you connect to it even with a DC blocking cap in the way.

They also ask for the range control to be set to a 'between 2 ranges' setting. Will this partial rotation of the range switch put the switch wafer in an open circuit position to isolate the mixer stage and prevent it loading the test point?
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Regards, Jeremy G0HZU

Last edited by G0HZU_JMR; 25th Dec 2019 at 12:26 am.
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