Re: Help me get started on a middle-aged HP 'scope power supply?
That's definitely on my list of things to check, but I while I can see it trigger in free-running mode on another scope, I haven't managed to catch it in one-shot memory yet.
The weird thing is that when I see it trigger, the startup "current" (really voltage on U4), looks completely differ than what I see when it starts cleanly. This might just be because the spike is so brief and what catches the eye when it starts is the post-spike running characteristic signal, while the...
willray
chronic dabbler
Last Activity: 04-04-2013, 05:54 PM
Joined: 11-15-2012
Location: Columbus Ohio
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Re: Help me get started on a middle-aged HP 'scope power supply?
No, no, those are out because I had them out for testing. They're all quite happy and back where they belong now.
Will...Leave a comment:
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Re: Help me get started on a middle-aged HP 'scope power supply?
Right - except the primary overcurrent (U4->CR9) protection is only tripping sometimes, and when it does, it's dues to a brief excursion on the rectified T4 output exceeding 2.6V. The rest of the time (even though the diode was still shorted), it's only seeing a fraction of a volt coming from T4.
I certainly can't rule out that it's right on the border of tripping every time, and it's just my dumb luck that it comes up sometimes, and fails others though. That'd be about the average...Leave a comment:
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Re: Help me get started on a middle-aged HP 'scope power supply?
I agree that that should be the case, but this isn't obvious to me from the design of the circuit. Of course, I'm nowhere near close to a professional, and I'm long on theory, and what practice I once had is decades decayed, so something not being obvious to me says more about what I know, than about the thing being discussed.
But... A dead short in CR19 just turns -2.4VRet and -2.4V into an AC circuit, which is going to be largely filtered towards 0V by LC T6 and C48. A significant circulating...Leave a comment:
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Re: Help me get started on a middle-aged HP 'scope power supply?
Yeah - it's out now. I only realized that it was shorted right at the end of my last discussion with the power supply. Silly me assumed for some reason, that either the 2.4V regulator had gone, or that the rectifier failed open, and so it took a long time for me to get over to that corner of the board.
Replacement is winging its way this way from Digikey....Leave a comment:
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Re: Help me get started on a middle-aged HP 'scope power supply?
Ah, I meant as in, the crowbar chip is a voltage sensor, so they're going through some effort to get it to trigger on current. With the (to me) strange non-voltage-divider input they're using to present the voltage to the crowbar, it's not obvious what current/duration profile actually produces the 2.6V input that's the actual sensed trigger.
I originally thought the fault might just be false triggering, but although I can't get a clean picture of the signal when it triggers, I'm now...Leave a comment:
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Re: Help me get started on a middle-aged HP 'scope power supply?
Just an update for anyone watching the idiot bumble around...
The scope is still not running. The overcurrent sensing circuitry is definitely seeing a brief voltage transient that's getting clipped off by the crowbar. Since the crowbar isn't directly sensing the current, and since it's shutting down the PWM rather than _causing_ the over current, I'm still not sure where the current surge is coming from.
With the bare power supply, it doesn't happen all the time, and it's so brief I haven't...Leave a comment:
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Re: Help me get started on a middle-aged HP 'scope power supply?
Ain't she a beauty? Who has the real estate to lay out a board like that anymore?
Simple circuits for simple minds like mine - my antiques and I go well together.
That region of the board is where things are leading me at the moment. I don't know if the one that's tripping is tripping because there's really an overcurrent in the primary, or if it's just screwed up sensing - either way it's easy enough to test.
As an aside, they aren't using...Leave a comment:
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Re: Help me get started on a middle-aged HP 'scope power supply?
I know - my apologies - I'm only near the 'scope occasionally (it's in my shop at the farm), and was hoping that the "stands up good voltages with no load, but can't make current" behavior was a completely obvious clue to those better educated than I.
So - I went and had a chat with the 'scope - it's a HP 54200A (a detail I'd managed to forget previously), and snapped a couple photos of the power supply board. I'm not sure how high a resolution that badcaps will let me post,...Leave a comment:
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Re: Help me get started on a middle-aged HP 'scope power supply?
So far I'm not impressed with what I've found, though I'm doing better than the last time I looked for service docs for this one. My previous experience with HP service documentation was that they were sufficiently detailed for even me to be able to follow and fix most anything. In this case the diagnostics make the assumption that if the self-test of a single voltage is good, then you're done - less helpful than I expect from HP...
I've managed to find a scan of a service manual for...Leave a comment:
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Help me get started on a middle-aged HP 'scope power supply?
Greetings BadCaps gurus,
I come seeking wisdom. It seems I've managed to go through life so far, getting lucky simply making guesses at bad components in power supplies based on appearance, smell, heat, discoloration etc, but I'm now trying to resurrect an end-of-the-CRT-era HP 'scope, and its power supply is proving that I'm but a dilettante.
I've done the obvious - pulled and tested, or tested in place the electrolytics, and even replaced common suspects that didn't test bad, and gone over the board for obvious signs of overheating and such. So far, none of the...
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