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Old 25th Aug 2020, 11:06 pm   #61
Bufo Bill
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Default Re: HP 180A: chances of repair?

A quick update: I'm waiting for an IEC socket and lead, and have given the internal areas a vacuum/ dust. We are currently in self isolation prior to my dad going into hospital for knee replacement surgery, so we are trying to get everything sorted in readiness. Progress with this project will be slow I fear.
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Old 27th Nov 2020, 9:54 pm   #62
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Default Re: HP 180A: chances of repair?

Hi guys, a hoover and IEC socket (and my father's knee surgery and broken hip) later and we have a seemingly working unit. I suppose it will need recalibration?
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Old 27th Nov 2020, 11:00 pm   #63
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Default Re: HP 180A: chances of repair?

There's calibration and then there's calibration.

These things are pretty stable, so do some checking and then some cross-checking and thinking before ever touching an adjuster.

There should be gain and offset screwdriver pots on the front panel so you can use the calibrator waveform source (in the mainframe) along with your probe and set the right size on the CRT. Nobody bothers, but as a plug-in scope with ALL of the Y amp in the plug-in, a twiddle may be needed to suit the CRT.

Going deeper there are Bal adjustments, then bandwidth-flattening twiddlers and then tweaks on some attenuator ranges.

Gain and offset are part of the mainframe for X. Then there are trigger gen bias (tunnel diode - very touchy) and some time/div cal twiddlers for the faster speeds.



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Old 28th Nov 2020, 6:30 pm   #64
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Default Re: HP 180A: chances of repair?

Thought things were too easy! We have an intermittent/low power signal. The inputs are filthy so will give them a good clean and see where that leaves us.
I looked at that De-Oxit, and the cheapskate in me says there must be a cheaper option. Any recommendations? Would switch cleaner applied with a cotton Q-tip be any good?
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Old 29th Nov 2020, 10:14 am   #65
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Default Re: HP 180A: chances of repair?

All scopes are sadists, just then you think you've fixed it, pow! Here's another fault sucker! They're like grumpy cantankerous old men ....or women. I feel your pain, know how it feels. Remember what the fish said, " just keep swimming"

Wouldn't use cotton buds, they leave behind tiny fibres that get caught in the contacts, a thin bit of card or paper soaked in cleaner works ok if you can get access that is. SW cleaner is complicated, some is for cleaning and leaves no residue, others like DeOxit coat the SW contacts in some whizz bang chemical. you have to watch the cleaner doesn't knacker the wafer too. Recently I ran out of Servisol and bought some spark gap cleaner from a local car shop as a stop gap, it's a mix of ether and iso alcohol, it seems to work ok, but I tested it on a small area first.

When buying SW cleaner you really have to read the label and do some research, it's like having a kid with a nut & wheat allergy : )

Andy.
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Old 29th Nov 2020, 7:23 pm   #66
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Default Re: HP 180A: chances of repair?

Thanks for the encouragement and advice Andy.
I did as you said with a test patch, it worked well. Unfortunately I think the problem may lie in the probe.
It's an old HP 10004b, so not surprising it's a bit knackered really.
Rather than an adjustable screw, it has a little pot with a wheel adjuster, like on a Walkman or little Trannie radio. This is not adjusting the signal very much at all.
I will get some pics up of the 10V Square Wave, and the Probe pot.
Hat in the ring right now, I would love to rebuild the probe if the thing is faulty, but I suppose that rather depends on whether the faulty part(s) are available or not!
Cheers from Bill.
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Old 29th Nov 2020, 7:56 pm   #67
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Default Re: HP 180A: chances of repair?

Here's the screen. V/CM = 0.5 (signal takes up 2cm squares on graticule).TRIGGER= 2milliSeconds. Middle image shows pot in probe, (marked "Erie"), right hand image shows two other screw adjusters.
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Old 29th Nov 2020, 9:21 pm   #68
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Default Re: HP 180A: chances of repair?

The component marked "ERIE" is a variable capacitor not a pot, the manual with parts list and diagrams can be found here (assuming you haven't already have it); https://bama.edebris.com/manuals/hp/10004b/

You need to select DC on the switch above the input BNC to get the square wave to look right.

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Old 29th Nov 2020, 9:47 pm   #69
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Default Re: HP 180A: chances of repair?

Cheers David, I thought it had to go on AC! That may be it. Will let you know. Thanks for the link, very useful.
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Old 29th Nov 2020, 9:58 pm   #70
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Default Re: HP 180A: chances of repair?

For setting up the probe compensation the DC setting is needed as the calibrator output is a square wave.
The AC setting adds a capacitor in series with the input so you can block the DC from the signal, this is used for observing waveforms that are part of a larger DC signal (for example the ripple on a PSU rail).
If you were looking something like TTL signals (0V to 5V) then the DC setting would be used.

It all depends on what you are working on, for some things you would need to be aware of where you connect the ground lead as well, for example you don't want to short out a supply rail or signal ground that isn't connected to case ground/earth through the probe ground.

David

Last edited by factory; 29th Nov 2020 at 10:04 pm.
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Old 29th Nov 2020, 11:04 pm   #71
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Default Re: HP 180A: chances of repair?

Hi again, still having no luck. Do you think I am right about that cap, or is it more likely in the scope itself? 25pF seems small to go bad like this. Should I start testing the probe first or the scope? I suppose the cap adjuster mechanism could be broken?
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Old 29th Nov 2020, 11:26 pm   #72
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Default Re: HP 180A: chances of repair?

You should be able to check the scope without the 10x probe, connect either of the calibrator outputs directly to the input, if the wave still isn't square then it's either a bad connection in the AC/DC switch or a fault somewhere else.

You could also try the probe on the other scope input if you haven't already done so.

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Old 29th Nov 2020, 11:42 pm   #73
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Default Re: HP 180A: chances of repair?

Thanks David, will let you know how I go on. I did try both inputs, the same problem occurred.
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Old 30th Nov 2020, 7:38 am   #74
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Default Re: HP 180A: chances of repair?

That sq wave looks terrible, usually if your probe is out the tops of the waveform are flat but show either LF or HF attenuation, IE the tops remain flat, but are sloped either left of right. That waveform is curved which I think would point more to the vertical IP maboggins than the probe.

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Old 30th Nov 2020, 11:28 am   #75
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Default Re: HP 180A: chances of repair?

As you adjust the top of the waveform should change from sloping down to sloping up, a flat top being the right adjustment. Maybe when the probe 'wheel' is turned the capacitor vanes are not moving.

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Old 30th Nov 2020, 12:12 pm   #76
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Default Re: HP 180A: chances of repair?

That's possible, but we need to find out if the probe is at fault or the oscilloscope, hence why I said to try connecting the calibrator directly to the scope input (without using the 10x probe).

David
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Old 30th Nov 2020, 12:34 pm   #77
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Thumbs up Re: HP 180A: chances of repair?

HP180 inputs.
The display indicates that the input capacity is over compensated. I suggest you ascertain whether it is the probe or the input circuitry of the scope.

So connect the calibrator directly to each channel of the input in turn, by a direct wire link. and no probe. If that shows a reasonable square wave, then you know to check the probe. If not, then the input compensation of the Y Amp need attention. Each range has separate compensation, so try each range in turn. It is helpful to keep to DC input on the Y Amplifiers.

I would be very surprised if the output of the calibrator itself was defective. But you can try a different square wave if you have generator of some sort. Wave about 1khz is needed. Most AF generators produce something quite good enough.

The manual of the HP10004 probe is very informative, and makes clear that you only need to adjust the capacitor (Erie) by the external wheel for low frequency compansation, and can normally ignore the two little adjustable resistors which are for high frequency correction. I attach the circuit.

I assume that the input capacitor C1 across the main 9M resistor is sound. That takes the main voltage stess, and could be failed. 8.5pfd. Try substitution ( 8 to 10 pf) and see.

One remote possibility is a faulty probe cable. As you can unscrew each end from the probe bits, it is easy to check with your ohm-meter. Be aware that the cable is made of resistance wire. It is about 100ohm for a 3ft cable. This is a usual feature of all quality probes as it suppresses spurious reflections. A fact not widely known.
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Old 30th Nov 2020, 3:50 pm   #78
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Default Re: HP 180A: chances of repair?

Hi, thanks for your replies. I tried linking up both channels (one at a time) to the calibrator, similar problems resulted. I connected up a signal from a Farnell LF1 unit, which worked fine with the probe and could be calibrated using the wheel control. However there seems to be a problem with the V/CM control on both channels. Sometimes the signal cuts out between switching channels. Dirty switches maybe? Or something else?
Cheers from Bill.

Last edited by Bufo Bill; 30th Nov 2020 at 3:55 pm.
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Old 30th Nov 2020, 5:29 pm   #79
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Default Re: HP 180A: chances of repair?

Sounds like the calibrator output on the 180A is the problem then, do you have another scope if needed?
The signal cutting out sounds like dirty switch contacts, a common problem with stuff that hasn't been used for many years.

David
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Old 30th Nov 2020, 6:52 pm   #80
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Default Re: HP 180A: chances of repair?

David, thanks for replying so quickly. I'll clean the contacts then. I may have mentioned i had to hoover the scope, so that explains that I hope! No other scope available right now, would there be any danger in putting that part of the restoration on the back burner, or can it do more damage if left untended to?
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