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Old 11th Aug 2020, 10:29 pm   #41
turretslug
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Default Re: HP 180A: chances of repair?

Oh- both 180As here (1 populated with the "usual" 1801/1821 complement, the other an enbalmed "parts mule" frame) have just the slide-clip quick-release covers. Admiittedly, undoing them is quite a deliberate act, but as someone who was an enquiring and sometimes meddlesome child at around the time these would have been shiny and new, I can see that it would be easy for, well, anyone to have access to EHT panel (generator itself guarded, but plenty fizziness on the rest of the PCB), PSU and X-amp HT, unsleeved mains switch contacts and so on. In the day, it's likely that this sort of instrument would have been restricted to professional environments and personell simply by virtue of cost but years down the line, there are probably quite a few in a more "domestic" setting. Certainly, it struck me that there's something of a caveat with things like this as regards current legal wording,

Colin
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Old 12th Aug 2020, 10:30 pm   #42
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Default Re: HP 180A: chances of repair?

Can anyone advise me on probes? Will I have to make them, buy old stock, or can I buy something new but generic online?
Cheers from Bill.
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Old 13th Aug 2020, 7:36 am   #43
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Default Re: HP 180A: chances of repair?

All of those methods are possible.

You just missed Woodchips selling off a collection of nice probes. Keep your eyes open and pounce when something turns up. Your probes don't have to be the same brand as the scope. At different times Tek and then HP have made probes with ancillary contacts the scope could read and take into account the attenuation in what volts/div was displayed.

Some people will find this contentious, but I strongly recommend you look for x10 ratio probes. The occasions when you need a x1 probe are very rare and x1 probes load a lot more capacitance onto whatever you are probing.

There are probes with a little switch to select between x1 and x10 but they have two problems. Firstly the load they impose on the x10 setting is not as good as a purely x10 probe can give, secondly the switch is round about where you hold the probe and it's too easy to knock so you ger a scaling on the scope that's tenfold out. Amazing how far you can get in trying to fix something that was actually OK. Personal experience.

Have x10 probes, and maybe a separate x1 and then you'll have no doubt on what you're using.

Look at teh BNC inputs on the scope amplifier plug in, and you'll see the input impedance printed as a resistance and the parallel capacitance. Note these.

Good scope probes have printed on them something like 'To compensate 8-20pF' And this is a good clue to whether they will work to full bandwidth with your scope.

Attach probe. Put probe ground pin on the ground post near the calibrator outputs, and then probe the calibrator output (I think you have two different levels) Look at the squarewave you get and start homing in on the rising edge. With a small non-metallic screwdriver (or a plastic one with a tiny bit of metal blade at the end) adjust the trimmer in the connector end body of the probe to get the best shape. Quick risetime, squarest corner and flattest top. You have to compromise a bit. If you r probe won't do the capacitance of your scope, you run out of adjustment before you get the best response.

Lower bandwidth probes have more adjustment range, wider bandwidth probes can't have so much. For a 50MHz scope you won't want any of the newest whizziest multi-hundred MHz probes (You wouldn't like their prices, either).

David
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Old 13th Aug 2020, 9:39 am   #44
Craig Sawyers
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Default Re: HP 180A: chances of repair?

This table of current Tektronix probes illustrates David's points perfectly.

https://www.tek.com/passive-probe

The only 1x probe (P6101B) is a mere 15MHz bandwidth. Tek have been making that since Adam was a lad.
And the wider the bandwidth, the lower the input capacitance. From 12pF at 50MHz down to 3.9pF at 1GHz.

Back to the 180A

What probe you need depends on the vertical plugin. The only one what is suitable for conventional probes is the 1801A, and that is 50MHz. The 100MHz 1802A has 50 ohm inputs, and opens up a whole new discussion about 50-ohm probes.

So assuming (the OP might already have said) a 1801A, any 10x probe with a 50+MHz will do the trick. BUT it has to compensate a 25pF input capacitance for the plugin.

That is quite a high value by the way. The current crop of Tek probes will just squeak 25pF at the extreme end of their adjustment for the 50MHz and 100MHz models.

And if you are buying from a "cost effective" source, be very careful to check the compensation range.

Craig
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Old 13th Aug 2020, 1:11 pm   #45
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Default Re: HP 180A: chances of repair?

David and Craig, thanks for your excellent and clear explanation,
Cheers from Bill.
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Old 13th Aug 2020, 3:23 pm   #46
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Default Re: HP 180A: chances of repair?

To give an idea of the input capacitance of vertical amplifiers, these are the Tektronix 7000 series plugin amplifiers with a 1M input resistance:

7A13 - 20pF
7A18 - 20pF
7A15N - 20pF
7A26 - 22pF
7A16P - 20pF
7A12 - 24pF (a very old 1968 design - one of the first Tek amplifiers)
7A22 - 47pF

However the 7A22 is a rather special plugin, differential, with selectable low and high frequency filters, a 10uV/div sensitivity, and a maximum bandwidth of 1MHz. The Tek-recommended voltage probe was the P6062A, X1/X10 switchable, and could compensate up to 47pF. With a 1MHz bandwidth plugin X1 bandwidth of a mere 4.5MHz was not an issue.

There are an equal number of wider bandwidth units with 50 ohm inputs.

This is just to point out that 25pF on the HP amp is a somewhat high input capacitance, needing some care with probe choice.
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Old 13th Aug 2020, 7:47 pm   #47
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Default Re: HP 180A: chances of repair?

The original probes for the 1801A (50MHz) vertical plug-in were the following, taken from the 1968 catalog;

10004A, length 3 1/2 ft
10006A, length 6 ft
10005A, length 10 ft

These were eventually replaced by 10004/5/6 'B' versions, lengths are the same for each, the compensation range is 20 to 30pF, from 1972 catalog.

They were still listing the 1801A in the 1984 catalog (the last year), probes are now 10004/5/6 'D' versions.

The higher bandwidth vertical plug-ins use different probes as the input capacitance is lower, some have 50 inputs instead.

I've also had problems with modern probes not having the correct compensation range for older oscilloscopes.

David
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Old 13th Aug 2020, 10:46 pm   #48
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Default Re: HP 180A: chances of repair?

Cheers for the info guys. I'll suspend things until I have a probe, and as I've just been hit with a vet's bill and I'm not a big earner that maybe a wee while. Hopefully not too long though.
Cheers for all the help from Bill.
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Old 14th Aug 2020, 8:02 pm   #49
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Default Re: HP 180A: chances of repair?

If anyone's still reading, the cat is very pleased with himself, having made such a fuss they sent him home early. This is a good thing in more ways than one as I don't have to pay so much money as I thought as they revised the bill for me. Have therefore been able to secure a 10004b probe from an American on ebay. Hopefully won't be too long before I'm ready to get going.
Cheers from Bill.
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Old 14th Aug 2020, 9:03 pm   #50
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Default Re: HP 180A: chances of repair?

I would have thought it would have been better to see if the scope is actually going to be repairable before splashing out on probes.
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Old 17th Aug 2020, 9:57 pm   #51
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Default Re: HP 180A: chances of repair?

You should be able to verify that it powers up & displays a horizontal trace without the plug-ins fitted, see section 5-9 of the manual and try step e (connect 10V calibrator output to horizontal ext input).
Some basic checks of the plug-ins should also be possible without the probes.

David
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Old 17th Aug 2020, 10:34 pm   #52
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Default Re: HP 180A: chances of repair?

Without the plug-ins the 180 has no connections at all to the Y plates and they could acquire charge in any direction, the spot may be off the screen in the Y direction, but often will be on.

The mainframe will contain the X amplifier which is somewhat offset to match the timebase. Operating X shift may bring a spot on. Selecting ext input for X will centre things up. Applying a sig gen to X input will give a horizontal trace. Note there is a trace rotation trimmer to align horizontal with the graticule.

Tek on their 7000s used the abbreviation 'Trace Rot' on a little bezel around the access hole for their trimmer. HP shouldn't tease Tek about this, On the first iteration of the 3724A, they abbreviated "Noise analysis mode" to "Noise Anal" and made a 2-shot injection moulded keycap with that legend. You couldn't make it up!

David
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Old 17th Aug 2020, 10:57 pm   #53
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Default Re: HP 180A: chances of repair?

Or the 465/475 that sported the legend "A lock knobs" on the horizontal mode switch.

Craig
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Old 18th Aug 2020, 6:03 pm   #54
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Default Re: HP 180A: chances of repair?

I had a bit of a closer look at mine and found another transistor had been removed. A new replacement was sat with its wires just poked into the holes without solder, it's amazing it hadn't dropped out and got lost! This transistor is another FET and is Q103 on the diagram - it's a 2N4860, so whether that's a good replacement for the original, I would only guess. The other transistor that's been removed is Q301, which is another FET, and luckily I've got it and it is the original. I did test it a long while ago and it seemed to be good and is numbered on its case as 5-020 754, there is another mark after the 4 which looks like just a line and not a number. The two transistors are shown in the second picture below, the first picture shows where the components have been removed from on the board.

My scope has the 1801A D/C vert. amp. (50MHz). 1821A TB etc. plug in. Unfortunately, the manual that's with the scope dates from 1968 for this particular unit, but the unit itself is dated April 1983.

The first picture below shows the board (I see what Woodchips meant about the wiring harness etc., running along the bottom and the general access). You can see where Q301 has been removed at the top left and Q103 near the bottom left, also tunnel diode CR304, which again I've got and seemed to test OK a while ago, so hopefully can go back in. I didn't remove any of these components, this is just how it came to me. The second picture shows the two transistors - the first one an unused/unfitted replacement obviously obtained by the previous owner and the second one is an original removed by the previous owner. The next picture shows part of the parts list with the three components listed and markings by a previous owner. The last two pictures show parts of the circuit diagram showing where the removed components are in circuit.

Maybe this will spur Bill into action with his scope - or maybe it'll put him off...it might even put me off mine yet
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Old 18th Aug 2020, 6:09 pm   #55
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Default Re: HP 180A: chances of repair?

1855-0020 is the full part number. It's a high Idss FET we used mostly as an analogue switch.

2N4391 if my memory is right.

There should be an HP part number/commercial numbers listing on the interweb, somewhere.

David
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Old 18th Aug 2020, 6:38 pm   #56
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Default Re: HP 180A: chances of repair?

1855-0020 is listed as an SFE793 .

SFE793
N JFET
TO-18
500 mW max
Gm=18mMho at Vds=20V
BVgds=40Vmin
Idss=50mA min 100mA max
Vp/Vgs(th)=10V max
Igss=500pA max

Craig
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Old 18th Aug 2020, 6:47 pm   #57
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Default Re: HP 180A: chances of repair?

So not dissimilar to the 2N4391 spec. You've got the same level of knowledge about HP stuff as I (hope...) I have about Tek 7000 geaer ;-)

Craig
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Old 18th Aug 2020, 8:36 pm   #58
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Default Re: HP 180A: chances of repair?

That is a properly good PTH board. You can use a solder sucker from the TOP side, just add a bit more solder before trying sucking. Pull the component from the top side, suck holes out, fit new component with carefully trimmed leads, solder from top.

Easy to work on, once you know how.

David
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Old 19th Aug 2020, 8:30 pm   #59
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Default Re: HP 180A: chances of repair?

I'm finally getting somewhere with the low voltage power supply board from my late 1960's 180A, I got fed up a while ago with not being able to measure voltages without two of the fuses instantly blowing.

To test the +100V & -100V regulator circuits required three power supplies, two higher voltage supplies to supply the inputs & a third low voltage supply for the +105V zener based supply (only used on this board), two 2N3055 pass transistors were also needed as these are mounted externally to the board at the rear of the oscilloscope.

The +100V supply was working fine, but the -100V was stuck at the input voltage. This was eventually traced to an open circuit 1854-0071* transistor (Q416), once replaced the output voltage could now be adjusted. The voltage reference neon (V402) also reduced in brightness to match the other one.

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*I've had these fail in other HP equipment before, note that they used several suppliers for this part and they can have different pinouts.

David
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Old 19th Aug 2020, 11:17 pm   #60
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Default Re: HP 180A: chances of repair?

1854-0071 was HP's default small sig NPN it was a Texas Instruments TIXSsomething or another. We stopped using them and went onto 1854-0215 (2N3904)

The GP PNP used to complement it was 1853-0020 (2N3906, so you can see why we changed the preferred jellybean NPN to 2N3904)

1854-0071 are now known to have a corrosion problem. replace on sight if you want things reliable.

David
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