HP 1630A goes BANG - Page 1
Forum section: electronics / repair

author: Tim T
author_rank: Contributor
timestamp: December 22, 2022, 04:08:58 pm
content: Several years ago, I obtained a HP 1630A logic analyzer thinking it might be useful one day, and other than an initial powerup check it has sat unused. Well, that day has arrived, and all appeared to be fine over several short sessions of maybe 30 minutes to an hour or so just to learn how to use the thing. Then, when about 3 hours into a session for pulling out some bus data – BANG. I killed the power within a few seconds and if memory is correct the fan and CRT continued to work over that interval.The interesting part is no smoke or smell, and in tearing down the 1630a I have yet to find any obvious damage. I replaced the RIFA safety cap that is exposed on the power supply board before starting to use the analyzer.Is there a non-destructive way to check the two X2 and two Y caps within the sealed EMI filter?Thanks,

author: TheMG
author_rank: Frequent Contributor
timestamp: December 22, 2022, 07:32:27 pm
content: Take it out of circuit and measure capacitance between L-N, L-G, and N-G.Capacitance values may be labelled on the EMI filter, or should be able to find them on a datasheet. If not, L-G and N-G readings should be about equal, and less than the N-L reading.

author: Tim T
author_rank: Contributor
timestamp: December 23, 2022, 12:40:11 am
content: Right, a photo would have helped.I read N->GND at 6.12 nFSame for GND->LN->L is 290 nFN->L resistance is 10.31MInductance across L is 2.88 mHSame for NThanks for help. For some reason I was thinking it would be more complex.So it looks like I need to dig deeper to find the fault.

author: coppercone2
author_rank: Super Contributor
timestamp: December 23, 2022, 12:52:54 am
content: well thats a pleasant story for the holidaysI heard some kind of high pitched like almost whistling noise from something like a fluid was leaving from something last week but I can't find where it was lol.. this is why I put extreme effort into grounding robustness for all equipment I have.

author: srb1954
author_rank: Super Contributor
timestamp: December 23, 2022, 06:09:51 am
content: Quote from: Tim T on December 23, 2022, 12:40:11 amMeasure the L-G and N-G resistance, preferably with a proper insulation resistance tester that applies 500VDC to the capacitor under test. Any resistance under 10Mindicates a failing Y capacitor.Also test the L-N insulation resistance test with the insulation resistance tester to see if the insulation resistance drops below the 10.31Mpreviously measured. Any significant drop in insulation resistance indicates a failing X capacitor.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: December 23, 2022, 10:46:09 am
content: if something said BANG, there must be some kind of visual damage to find,I would also expect some part of the circuit is not working uptimal now,you need to keep looking

author: DC1MC
author_rank: Super Contributor
timestamp: December 23, 2022, 10:51:38 am
content: I had once a 300pF cap fully disintegrating, with a loud bang, but no smoke or traces, only the (short) legs remained, was difficult to found, especially since the basic functionality of the device was not visibly affected.

author: Tim T
author_rank: Contributor
timestamp: December 23, 2022, 06:19:22 pm
content: Yea, the lack of smoke or obvious damage is why I was thinking it may have been the sealed EMI filter.One nice thing about the 1630 series is the CPU and acquisition boards can be easily removed from the motherboard, as well as ability to disconnect individual ribbon cables for CRT and keyboard from the CPU board, making narrowing down the problem area a bit easier in principle. So far, the power supply alone looks to work fine, though unloaded, the -2.4 rail is running at -3.4. Did learn that this SMPS does not like to start when using a variac to bring up voltage, so it was a bit of a "grit your teeth and duck" moment when first powering it up at full mains.Found something odd with the -5.2v path on the motherboard showing just 17 ohms to ground. The only things on the MB are connectors and branches off the -5.2 to four resistor networks (each with a 10 nF ceramic cap to ground) that appear to be voltage dividers which if my thinking/math is correct will supply -3.0 volts to the acquisition boards, but this is a guess. The otherwise very comprehensive service manual doesn't say much about the MB. I lifted each cap just to check and still had the 17 ohms. Anyhow, I am getting -5.2 v at the ps test point, with no over-current warning LED.Next step is to add back the cpu board.