HP-3580A Spectrum Analyzer, display faint, how to test/replace A11's CR-5 ? - Page 1
Forum section: electronics / repair

author: slbender
author_rank: Regular Contributor
timestamp: July 12, 2018, 07:33:09 am
content: Hi, allYes I am busy working on many sets (Amber 3501/3501A's and my HP3561A's) but my HP Audio SA is quite dim, can barely see the 'CAL' in a dark room, and its squished off at the right side of the CRT. I suspect CR5 has become a high Z over the years and needs replacement. This is on the H V board A11 and that is hidden under a metal can, which I can not figure out how to get out without disassembling the front, rear and side - basically the whole case.Anyone done this task and/or have any shortcuts to getting to this part, mechanical disassembly does not seem to be present in my Service Manual...Thanks!-Steven

author: slbender
author_rank: Regular Contributor
timestamp: July 12, 2018, 08:05:24 am
content: The part, if I can get to it appears to be rated at 7,500 Volts and 200 mA, so the modern parts for a typical microwave oven should work, they usually are rated at 12,000 Volts and I think 500 mA. Didn't anyone ever have this problem before???-Steven

author: rf+tech
author_rank: Frequent Contributor
timestamp: July 13, 2018, 02:30:22 am
content: Hi slbender,The place to start is always to verify the power supplies, paragraph 5-53. Post the results of your measurements.Is the +158 Vdc correct? You say the zero marker is squished off to the right, this sounds like a horizontal deflection problem, in addition to the dim display.What are the results of the Display Adjustments, paragraph 5-58? Is there any noticeable change in the display?It may be necessary to verify that the CRT gun is properly biased, that the grid is not holding the electron gun near cutoff. Both elements sit around -2.9 kVdc so extreme caution is required to measure the differential between the two. Varying the intensity control should produce something like -50 to -15 Vdc between the grid and cathode, corresponding to minimum and maximum intensity.If you have a suitable HV probe, carefully measure the CRT cathode (pin 2). If the -HV is low, then proceed to check the HV oscillator and that the control amplifiers Q23 and Q22 are not throttling back Q21 base bias.Rule out the easy to test items first before jumping to the conclusion that CR5 may be defective. It is entirely possible the CRT emission has fallen off with many hours of operation. If all supply voltages are correct, the -HV correct and grid to cathode bias is reasonable, then low emission is pretty much confirmed.I do have a 3580A and can make some measurements, if needed. I use my 3580A as an auxiliary digital storage display, connected to an HP141T.RF+ Tech

author: slbender
author_rank: Regular Contributor
timestamp: July 14, 2018, 02:29:26 pm
content: Thanks RF+tech, my maybe suitable HV probe is hiding somewhere and trying to check those high voltages always gives me the Willie's. So I thought the obvious would be the old style series diodes that are a known failure mode, like in the KLH Nine electrostats circa 1970. Guess I wish I had those, almost did at one point. Anyway, yes I should check the obvious, including the lower voltages, but since I once had an older 3580a with the mechanical crank, I guess it was foolhardy to sell that working unit in order to buy this later version, with the red led readout, which of course has multiple problems. Oh well...-Steven