UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - HP 175A Oscilloscope

Radio Wrangler · 11th Jun 2018, 7:29 pm

If you build a HV probe out of a load of 10 meg resistors... Remember to check them first! If they're old carbon ones in your junk box, they may have gone high and turned into just that - junk!

It may be less trouble to buy some modern resistors.

If the cathode's independent supply is not good (leaky C, leaky insulation or a high resistor) then that leaves the grid a lot more -ve than the cathode and the CRT becomes cut-off.

You may be able to check the current inthe bias strings to get an indication if something is off.

David

11th Jun 2018, 7:29 pm	#9
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,870	Re: HP 175A Oscilloscope - Vanishing Trace If you build a HV probe out of a load of 10 meg resistors... Remember to check them first! If they're old carbon ones in your junk box, they may have gone high and turned into just that - junk! It may be less trouble to buy some modern resistors. If the cathode's independent supply is not good (leaky C, leaky insulation or a high resistor) then that leaves the grid a lot more -ve than the cathode and the CRT becomes cut-off. You may be able to check the current inthe bias strings to get an indication if something is off. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done