BUSH DAC90A - Low HT - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

PJL · 28th Jun 2017, 12:21 pm

The oscillator is self regulating through rectification on the oscillator grid. It will settle on the anode current necessary to give the gain required to maintain oscillation. Different wavebands will have different losses in the oscillator coils hence why the anode voltage changes. There will also be losses introduced by connecting the meter to the circuit!

Boater Sam · 28th Jun 2017, 1:08 pm

If you monitor the g1 pin voltage on the oscillator whilst changing wavebands you will find that it varies enormously due to the different coils and your DMM's response to the different frequencies, lower frequency i.e. long wave will raise the DMM readings, its response across the frequency range is not the same.

28th Jun 2017, 12:21 pm	#41
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: BUSH DAC90A - Low HT The oscillator is self regulating through rectification on the oscillator grid. It will settle on the anode current necessary to give the gain required to maintain oscillation. Different wavebands will have different losses in the oscillator coils hence why the anode voltage changes. There will also be losses introduced by connecting the meter to the circuit!

28th Jun 2017, 1:08 pm	#42
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: BUSH DAC90A - Low HT If you monitor the g1 pin voltage on the oscillator whilst changing wavebands you will find that it varies enormously due to the different coils and your DMM's response to the different frequencies, lower frequency i.e. long wave will raise the DMM readings, its response across the frequency range is not the same.