Gould OS255 Oscilloscope Repair Advice Needed... - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Chris55000 · 19th Jun 2020, 2:11 pm

Hi!

If the sections of the thick film R569 measure ok then there's no need to replace it unless physically damaged in some way.

I think you can get 3k6 1W metal film close tolerance resistors from DigiKey or Mouser but the postage on them will be fairly steep!

The exact voltage at TR707 collector when the e.h.t. is set up correctly will depend upon the rectified voltage from the e.h.t. rectifier doubler circuit across C711 & C712 and all the various resistance values in the chain going from the –1850V stabilised e.h.t. rail down to chassis, including the absolute values of the elements of the thick film network R720.

If you can set the set e.h.t. control to give –1850V with respect to earth at the power supply PCB point 715, then the e.h.t. should be correct and the exact figure at TR707 collector isn't too important!

Chris Williams

19th Jun 2020, 2:11 pm	#21
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,873	Re: Gould OS255 Oscilloscope Repair Advice Needed... Hi! If the sections of the thick film R569 measure ok then there's no need to replace it unless physically damaged in some way. I think you can get 3k6 1W metal film close tolerance resistors from DigiKey or Mouser but the postage on them will be fairly steep! The exact voltage at TR707 collector when the e.h.t. is set up correctly will depend upon the rectified voltage from the e.h.t. rectifier doubler circuit across C711 & C712 and all the various resistance values in the chain going from the –1850V stabilised e.h.t. rail down to chassis, including the absolute values of the elements of the thick film network R720. If you can set the set e.h.t. control to give –1850V with respect to earth at the power supply PCB point 715, then the e.h.t. should be correct and the exact figure at TR707 collector isn't too important! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed! Last edited by Chris55000; 19th Jun 2020 at 2:25 pm.