HP54502A Oscilloscope fail A/D test and cal  - Page 1
Forum section: electronics / repair

author: Mr Lurpak
author_rank: Contributor
timestamp: February 25, 2025, 09:29:42 pm
content: I have a HP54502A in fairly good condition I want to fix for my workbench. It fails on A/D test (pass D/A and rest) on CHANNEL 2 only. I have no schematics, just the SM, and if anyone knows of link to any it would be appreciated.Anyway, have anyone else successfully fixed this error before and what was it? I did see a blog online with at short in PS but ALL voltages seems OK and 5.2 adjusted according to SM. I have swapped the 2 ICs with heatsink around just to eliminate that. In lack of a schematics does anyone knows which IC is the DAC (A/D converter) so I at least can have a look around it and compare channels (CH 1 works perfect).Any hints are welcomed.........Error code/Fail code 0000 0000 0101 1110PS. the Dallas chips has been replaced before I got the scope with a DS1230Y so guess that is not the reason?I got the schematics for a 54601a is that too far off or similar in chip sets used?Cheers

author: colorburst
author_rank: Regular Contributor
timestamp: February 26, 2025, 03:37:34 am
content: Does channel 2 work, are there any issues aside from not passing the test? What happened when you did the self cal after changing the Dallas block, was there any error?U106 is the channel 2 ADC. If you'd like to check, the power rails for that chip are listed below. -2.4V come from different regulators for each of the ADCs, the three other rails are shared.+5.2V: pins 9, 30+3.5V: pin 7-2.4V: pins 6, 25-27, 38-5.2V: pin 29GND: pins 2, 8, 24, 28, 34

author: Mr Lurpak
author_rank: Contributor
timestamp: February 28, 2025, 10:51:29 pm
content: CH1 is working normal. Just CH2 that fails. Have not yet tried to replace the Dallas chis as it seems to have been replace at some point. Issues is that Ch2 seems to be random noise, compared to Ch1 (working)I checked the power at Power supply and all good but will have a look at the the U106. So different ADC for each channel?, so I could swap them to test if it's the chip or just a component around it?Much appreciated.Cheers.

author: colorburst
author_rank: Regular Contributor
timestamp: March 01, 2025, 01:56:11 am
content: Re: random noise on channel 2, does it respond to changes in V/div or is it always the same level?Check input impedance with a DMM, make sure 1) it's not stuck in 50 ohm and 2) 50 ohm termination is in fact close to 50 ohm and not blown open.Does the channel respond in any way to a known signal, e.g. AC cal? Feed the signal to both channels and set trig from ch1, see if you can get any sort of output.Yes, each channel uses a separate ADC. You can swap them, although not permanently as it may affect the factory cal. Though I think a bad ADC is unlikely.It's possible you have a faulty attenuator instead. Could be dirty relays or a gross overload in its previous life. You can swap these, too.

author: Mr Lurpak
author_rank: Contributor
timestamp: March 04, 2025, 08:32:44 am
content: Thanks, very good hints.I will test when I get the time (just a hobby) but I did notice a big difference, in voltage measured on the 2 LM337, as in one having 20mv and other 2V on output, near the ADCs so will investigate that as well. Hinted by your first post on the voltage. 100% not the ADC as I have swapped them and fault stays on Ch2 - so that is always a good thing as probably hard to find.BTW,after replacement of DALLAS IC - I loaded default values from service menu, on this scope and another HP54503A - now in calibration the mode it seems to not recognise the BNC connection from DC CAL (something like check BNC connector error) on all CH (old DALLAS chip CH OK but fail A/D) is there something I missed? does the BNC needs to be 50 Ohm terminated or just direct BNC-BNC? (tried 4 different cables that normally works fine)Cheers.

author: colorburst
author_rank: Regular Contributor
timestamp: March 04, 2025, 02:34:54 pm
content: If you're checking the LM337s, U71 is the one that provides the 2V rail for ch2, U68 feeds ch1 and can be used for comparison. It's possible that the regulator is fine but there is a shorted cap across the rail.The cable just needs to be BNC-BNC, the scope can turn on 50 ohm termination by itself when needed. For some timing calibrations, you'll need two identical cables.Hard to say why it doesn't recognize DC cal, would be good to look at it with another scope while it does detection. If there is no output, you might have a problem with the DAC (U55) or the DC cal buffer (U69). The DAC is powered from a local regulator that maintains the full scale output at 5V.