HP 1725a - CRT ghosting - Page 1
Forum section: electronics / repair

author: g4tuz
author_rank: Newbie
timestamp: March 26, 2022, 06:06:16 pm
content: Help please. HP 1725a, ebay purchase.The CRT imaged appear brighter than real life due to camera response, the ghost is there at normal brightness.Received with broken knobs and some badly remounted y amplifier PCB. Dismantled and cleaned the timebase switches, reassembled with all working OK. Left to soak test, just noticed a sudden brief brightening from the CRT in my peripheral vision. Since then I have a ghost around the CRT beam. This effect is most easily seen in x y mode.All LV voltages are within spec with no ripple. 20V unregulated is at 23 VDC with some ripple as to be expected. Additional capacitance added to this line has no effect on CRT display. -3kV supply is present and correct.Focus, astigmatism and pattern work as expected.I am struggling with identifying some edge connector pin numbers and wire colour codes due to the poor quality of available downloaded manual. Does anyone have a good quality pdf copy of the manual - especially the circuit diagrams.Also any clues as to possible cause? My current thinking is related to CRT voltages.TIA

author: factory
author_rank: Super Contributor
timestamp: March 26, 2022, 08:14:52 pm
content: Artekmanuals have a pdf, should be better than the crap scanned by Agilent.http://artekmanuals.com/manuals/hp-manuals/Also try looking for any free pdf 1715A manuals, it's not the same in the front end, but does share some parts.No idea what the problem might be, did you clean the push switches, dirty contacts on the beam finder switch are a common problem with HP scopes.Note: The CRT heater in the 1715A & 1725A is fed from a winding on the high voltage flyback transformer, if the HT circuit runs away it can ruin the CRT.David

author: g4tuz
author_rank: Newbie
timestamp: March 26, 2022, 08:29:29 pm
content: Thanks, that's useful info re 1715A. Yes all switches cleaned including a strip and clean of the timebase board rotaries. The -2915 is almost spot on so that would imply that EHT and heater should be OK, but never say never. Will check cct to see which supply is referenced for regulation purposes.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: March 27, 2022, 12:52:25 am
content: HiAlthough it is difficult to be certain from your photographs it looks like the CRT face and implosion guard/gratitude need cleaning to remove the ghosting.As to the sudden increase in intensity, can you dim/turn off the beam with the intensity control?The exact voltage of the 2950 supply is not critical, the difference in voltage between the Cathode and the Grid of the CRT is critical and difficult to measure accurately. This voltage is provided by a number of high value resistor chains, a little known in today’s low voltage world is that high value resistors can change value with applied voltage. This change can sometimes be transitory with the resistor changing back to the correct value when the voltage is removed (instrument switched off) or it can be permanent. I suspect that you have a resistor that has changed in value in CRT supply.Please note that most of the resistors in the CRT supply circuit need to have a high voltage ratting.G Edmond