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

turretslug · 11th Jun 2018, 3:04 pm

ISTR it does indeed use separately generated CRT grid and cathode supplies, with the blanking circuit in the 0V return side of one of them- the sort of thing that makes you think, "hmmm, that'll be sketchy...." but Tek and H-P both made it work, and they're no fools.

You might be lucky and find that there's a leakage and loss of EHT through dust (and high-voltage circuitry attracts fine dust eagerly), you might be a bit less lucky and find that a capacitor is breaking down, less lucky still and have board insulation leakage or really damned and find that the HF generator transformer itself is breaking down. HF oscillator types don't have much oomf to them- in a way fortunate, as personal contact with EHT could be more survivable, but it does mean that it doesn't take much leakage to drag them down or quench oscillation. The initial fade-away, now nothing could mean that carbonised tracking has developed in something. I wonder if the old TV repairmans trick of holding a small neon bulb (the sort in a zillion mains appliances) near the EHT generator transformer would work?

If you come across a decent condition Simpson 269 meter with accessories, these have 100k/V sensitivity and scaling to 1,600V with a 240 Megohm screw-on probe that extends to 4,000V. Probably much more common in the US than here though- I got mine as NOS clearance at M + B Radio of Leeds many moons ago.

11th Jun 2018, 3:04 pm	#6
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,398	Re: HP 175A Oscilloscope - Vanishing Trace ISTR it does indeed use separately generated CRT grid and cathode supplies, with the blanking circuit in the 0V return side of one of them- the sort of thing that makes you think, "hmmm, that'll be sketchy...." but Tek and H-P both made it work, and they're no fools. You might be lucky and find that there's a leakage and loss of EHT through dust (and high-voltage circuitry attracts fine dust eagerly), you might be a bit less lucky and find that a capacitor is breaking down, less lucky still and have board insulation leakage or really damned and find that the HF generator transformer itself is breaking down. HF oscillator types don't have much oomf to them- in a way fortunate, as personal contact with EHT could be more survivable, but it does mean that it doesn't take much leakage to drag them down or quench oscillation. The initial fade-away, now nothing could mean that carbonised tracking has developed in something. I wonder if the old TV repairmans trick of holding a small neon bulb (the sort in a zillion mains appliances) near the EHT generator transformer would work? If you come across a decent condition Simpson 269 meter with accessories, these have 100k/V sensitivity and scaling to 1,600V with a 240 Megohm screw-on probe that extends to 4,000V. Probably much more common in the US than here though- I got mine as NOS clearance at M + B Radio of Leeds many moons ago.