Author Topic: HP 54602B oscilloscope problems repair (Read 12334 times)

bd139 · « **Reply #25 on:** February 11, 2018, 04:49:16 pm »

Before I forget, I broke the board down into rough logical functions before I attacked it. This is not exactly the same as mine. I didn't have photos at the time so I was reverse engineering it from schematics.

Disclaimer: this isn't necessarily exactly the layout - this was "where to look for shit" to start with. Lots of pissing in the dark here

bd139 · « **Reply #26 on:** February 12, 2018, 04:07:40 pm »

Further debugging. Very interesting so far. I did some checks on the circuit design and there's a 100k/0.1uF between the DAC and the LM324 which controls the summing amplifier. If I inject a voltage into the input of the LM324 then I can bring the trace back into the range of the ADC and get a trace back on all channels. This confirms this is entirely a problem with the DAC controlling the vertical position. However it is pulling 25mA when I do this. I think this might just be a ceramic cap or two that have fractured short when it was dropped. If I had an IR camera I could probably see it. The DAC is well protected from output short however which is good.

Dac is an HP 1SJ2-0102. Worst case not totally unobtainable if I have to replace it.

Found an interesting technical article on the architecture and design of the scope series as well which helps with the understanding: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1992-02.pdf

The DAC and filters are in the top half of the image above labelled "clock gen". They aren't clock gen. That's the DAC. Remember what I said about pissing in the dark - I was completely wrong

bd139 · « **Reply #27 on:** February 12, 2018, 04:27:55 pm »

I'm an idiot. Been staring at the wrong LM324. There was another one hiding under one of the DL marked packages (have no idea what the hell they are).

This one does NOT look healthy - note the burn marks. I will trace it out and order a replacement.

bd139 · « **Reply #28 on:** February 12, 2018, 04:33:24 pm »

Ordered 2 from RS. £2.88 including delivery. 2 because if I don't work out why it blew up correct the first time it saves me another day of waiting

Good excuse to fire up the T12 here and use it

I reckon it might be voltage rail collapsing causing the input / output to be driven past them.

Specmaster · « **Reply #29 on:** February 12, 2018, 04:37:15 pm »

No you're not an idiot, that much we all can vouch for. Hmm pins 1 and 14 are outputs, thats give away then, 1 for each of the affected channels.

Specmaster · « **Reply #30 on:** February 12, 2018, 04:39:03 pm »

Would that T12 be the mains switcher or are we still on the external supply one?

bd139 · « **Reply #31 on:** February 12, 2018, 04:43:19 pm »

External supply one. Mains one hasn't arrived yet

Found a TH LM324 lying around In the junk box. Resisting wiring up a bodge impatiently with kynar

Specmaster · « **Reply #32 on:** February 12, 2018, 04:46:29 pm »

go ahead and do it, whats the difference between that and the M version I wonder?

It would give you more debugging time if there was another reason for the old one failing, so that if this one failed, you might find the reason before the new arrives from RS?

bd139 · « **Reply #33 on:** February 12, 2018, 04:58:18 pm »

Can't find a reason. No shorts, no bad components. Am wondering if it just overheated. it's wedged underneath DL1 which looks like a large thin film array which is kicking out some serious heat. The other LM324's on the board are quite warm.

Specmaster · « **Reply #34 on:** February 12, 2018, 05:01:18 pm »

In that case, I'd be inclined to pull that sucker out and install the other one and try it, if it works OK, when the RS delivery comes, you'll have 2 spares in stock.

bd139 · « **Reply #35 on:** February 12, 2018, 05:04:39 pm »

Yeah looking like a plan.

Going to persist a little longer and see if I can trace out a bad cap as well as there are a ton of ceramics across the output of this amp in various places which could cause this. If they are low impedance suddenly, then the opamp will still try and correct its output and eventually blow itself up again by trying too hard. They're only rated for max dissipation for a set amount of time.

Specmaster · « **Reply #36 on:** February 12, 2018, 05:08:31 pm »

Well pin 14 certainly has been getting hot, but there looks like there might be a burnt hole on pin 1, at the bottom of the pin 1 ident marker, is it a hole or burn mark? If so than you are going to be looking for more than 1 cap?

bd139 · « **Reply #37 on:** February 12, 2018, 05:16:54 pm »

That's the orientation mark. It's pitted and burned.

For future reference, here are the output pins for the DAC:

DAC pin 23 - channel 4 vertical position
DAC pin 1 - channe 2 vertical position
DAC pin 4 - channel 3 vertical position
DAC pin 11 - channel 1 vertical position

Specmaster · « **Reply #38 on:** February 12, 2018, 05:20:56 pm »

What are pins 1, 7, 8 and 14 doing on the op amp?

bd139 · « **Reply #39 on:** February 12, 2018, 05:38:34 pm »

Hang on a min. I have traced this further out.

It goes from U14, the DAC at 0-3v approx to U1 which is an OP-400 instead of the expected LM324. applies a -2.5v offset to it giving it an output range of +1.6 to -1.6v on pin 14.

Tracing that output now with continuity tester. Low tech but it works. This is a 6 layer board apparently so YMMV with that method

The schematics I have are for an HP 54600A which are vastly different apparently. There are no schematics for the 54602B

Specmaster · « **Reply #40 on:** February 12, 2018, 05:45:12 pm »

PM me your email address, I have a copy of the HP 54600B which also covers the HP 54602B service manual.

bd139 · « **Reply #41 on:** February 12, 2018, 05:52:21 pm »

Ooh interesting. PM on the way

bd139 · « **Reply #42 on:** February 12, 2018, 07:22:06 pm »

Found it at last.

The LM324 is responsible for buffering the voltage reference from the main AD588 reference for the input voltage on the hold ICs. The hold ICs are expecting 0.5v as the reference voltage but the LM324 has blown up and the output is being pulled down. This is saturating the transistor which is holding it at -1.2v.

The only reason channel 1 and 3 works are because they are exceptionally high gain amplifiers in the control loop on that side of things which are saturating providing offset. No sign of anything nasty around the LM324 so will just swap it out and see what happens. There's a tant across the output I will test however as if that thing goes short it will sink 54mA constantly through the opamp which will eventually blow its arse out.

This is working off an HP54600A schematic but it traces out the same:

There are a couple of 100nF ceramic decoupling caps as well on the 0.5v rail so will check that too.

Specmaster · « **Reply #43 on:** February 12, 2018, 07:35:51 pm »

Ok, what are you waiting for, whack that lm324 in and lets see....

bd139 · « **Reply #44 on:** February 12, 2018, 08:09:10 pm »

Just tested the one I have and two of the opamps in it are dead. It was a pull out of something else. Grr. Will have to wait until tomorrow. Might yank the old one out tonight though.

Specmaster · « **Reply #45 on:** February 12, 2018, 08:21:26 pm »

I would, the more prep you can do today, the sooner you will know tomorrow when the new ones arrive, if it works. The suspense is killing me....

bd139 · « **Reply #46 on:** February 12, 2018, 08:31:05 pm »

Killing me too. Knowing ParcelFarts it will turn up at 6pm

Ran the circuit up in LTspice and actually looked at it properly this time. It's not applying any offset. the output should be close to 5v. What it's doing is removing any digital noise picked up from the reference before it hands it over to the hold ICs. I think that either the hold ICs are holding it at -1.2v (two transistor BE drops) or the extra mmbt3904 depicted above has died.

To quote the LM324 datasheet: "Short circuits from the output to V+ can cause excessive heating and eventual destruction. When considering short circuits to ground, the maximum output current is approximately 40 mA independent of the magnitude of V+. At values of supply voltage in excess of 15 V, continuous short-circuits can exceed the power dissipation ratings and cause eventual destruction. Destructive dissipation can result from simultaneous shorts on all amplifiers."

Specmaster · « **Reply #47 on:** February 12, 2018, 08:38:00 pm »

With it possibly not turning til late, then more prep done tonight the better, then all you have to do is drop it in and run up the T12 again, a few dabs of solder applied and then BANG it will either go again or hopefully fire right on all channels

bd139 · « **Reply #48 on:** February 12, 2018, 08:41:55 pm »

Will do that shortly. Curry-o-clock

Specmaster · « **Reply #49 on:** February 12, 2018, 08:43:13 pm »

Haha beat yeh to it, just finished mine.


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Author Topic: HP 54602B oscilloscope problems repair (Read 12334 times)

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