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Old 23rd Jul 2021, 3:08 pm   #61
Bufo Bill
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Default Re: HP 180A Oscilloscope.

The 1820c was a case of putting in a bid so low I thought I had no chance of winning, but when I did win not only did I feel guilty for chipping the guy so badly, I then had the nasty surprise of the USPS cost!
I don't know whether it's just because it's a simpler unit or if its because of the trigger not being controlled (supplied?) by tunnel diodes as WME_bill says but the 1820c is so much easier for me to operate correctly, so a real win for me there. I guess its a combination of both, as I am still quitea greenhorn with scopes.
Plus the board of USPS should have no worries about ordering large quantities of fizz and caviar for the shareholders meeting this year either, it appears I paid for both.
Bill.
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Old 25th Jul 2021, 11:26 am   #62
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Default Re: HP 180A Oscilloscope.

THe 1820 was the very basic entry-level timebase. It's far out-numbered by the 1821 dual timebase, so model specific spares will be a lot harder to find. The C suffix means it's one of the last updates.

HP did some dual timebases that were rather covert to not intimidate people, they had controls putting it over as an 'expansion' function.

The need for a simpler timebase was limited because any firm prepared to stump up the prices of a 180 system was going to want to go all the way.

The way round is to simply learn which controls to park and not use, then the 1821 drives just like the 1820. Eventually you'll come across something the delayed timebase will make easier and then you find that the second timebase drives just like the first one and you only have to worry about the delay setting on the multi turn pot. You set the scope to run normally, with just a bright-up on the traces at the time the delayed timebase will run. Select the delayed timebase and there you are! The delay pot allows you to scroll through the whole waveform the first timebase is covering.

It's do-able in small, gentle steps.

David
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Old 26th Jul 2021, 4:46 pm   #63
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Default Re: HP 180A Oscilloscope.

Thanks David, that's a very useful description. Will have to get my 'scope books down from the attic and have another delve into the subject.
Many thanks from Bill.
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Old 26th Jul 2021, 6:42 pm   #64
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Default Re: HP 180A Oscilloscope.

Scope books might not be much of a help. Most of the ones on the UK market were very out of date and covered scopes with old fashioned 'synchronised' timebases not the modern triggered jobs.

The triggered ones seem more complicated from outside, but once you start using them you'll say they are an awful lot easier to use. Controls do specified and useful things. You don't need to twiddle and hope!

You can best learn by doing. A scope, a scope probe, and a variety of things to provide interesting signals.... with the odd hint from on here, they'll get you going. If you start feeling too comfortable, it's time to dig a little deeper

David
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Old 26th Jul 2021, 7:08 pm   #65
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Default Re: HP 180A Oscilloscope.

Soooo . . . Got the 1821 unit back in, things went well at first, then suddenly the display cut out. I can't get a sausage using the timebase in "Normal" mode, but using the "Auto" setting it comes up fine. I have also started getting that previous problem with the EHT again. I will build an EHT Probe at some point this week and I will measure the rails tomorrow.
There's thunder and high humidity here just now, so will wait until tomorrow to get going. Hopefully the weather will be a bit fresher. And hopefully I will be too!
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Old 26th Jul 2021, 8:49 pm   #66
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Default Re: HP 180A Oscilloscope.

The normal mode won't show anything until the trigger level is adjusted, that is how it's supposed to work, the auto mode will show a trace even with no input signal.

Don't know if this would help, I picked up a few things I didn't learn at work from it.
https://www.youtube.com/watch?v=8ZKMrzTGxLQ

David
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Old 26th Jul 2021, 9:16 pm   #67
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Default Re: HP 180A Oscilloscope.

Quote:
Originally Posted by Radio Wrangler View Post
THe 1820 was the very basic entry-level timebase. It's far out-numbered by the 1821 dual timebase, so model specific spares will be a lot harder to find. The C suffix means it's one of the last updates.

HP did some dual timebases that were rather covert to not intimidate people, they had controls putting it over as an 'expansion' function.

The need for a simpler timebase was limited because any firm prepared to stump up the prices of a 180 system was going to want to go all the way.

David
I can't remember buying this, but I found I have a 1824A timebase with the expansion function.
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It triggers fine, but the 1801A that came with it only works on channel B.
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Last night I had a look through the manuals for the 1820C & 1824A on the Keysight website and was surprised to see they share a number of boards (apart from the main board & the extra board for the expander switch). The IC's used are obsolete, but still plenty of old stock available.
They are 1826-0086 = UA776HC ? (Op-Amp), 1820-0806 MC10109P (strange ECL OR/NOR gate) and 1858-0004 = CA3049 (transistor array).

As for the 1820C, I can't understand why they didn't give it a new model number, as it is quite different in design to the 1820A, which used all discrete components including the tunnel diodes and the front panel layout is different.
Can't comment on the 1820B, as I can't find any pictures showing the circuit boards.

David

P.S. A big thank you to Radio Wrangler for sending me the CRT bezel.
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Old 26th Jul 2021, 10:07 pm   #68
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Default Re: HP 180A Oscilloscope.

Hi David (Factory), thanks for the link. Also, many thanks for the information on the IC's for the 1820c.
Re 1821A, I had tried using the trigger level control, but still wasn't able to get a response. I tried this on several positions on the timebase dial that had previously been happily showing a nice square wave. No luck.
Cheers from Bill.
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Old 26th Jul 2021, 10:22 pm   #69
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Default Re: HP 180A Oscilloscope.

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I can't get a sausage using the timebase in "Normal" mode, but using the "Auto" setting it comes up fine.
That is correct operation for a triggered timebase... both your 1821 and your 1820.

In Normal mode it will wait for a trigger before starting a sweep. It will wait until the cows come home.

It will only sweep once you have both a valid signal and your trigger controls set up to trigger from that signal.

This is a catch-22. How the hell are you supposed to be able to set up the trigger controls if it won't let you see the signal?

'Auto' is the answer. If it hasn't had a trigger for a while, it fires of a sweep anyway. Not having been triggered, the sweep won't be synchronised properly to the signal but it does show you tha tthere is a signal there and what voltages it goes through. If the signal is repetitive frequently enough that once triggering, triggers come often enough, the 'Auto' timer never gets to trigger any extra sweeps. So the scope is normally used in the auto setting, not in the 'Norm' setting! Of course, this confuses the pants off most people.

So use 'Auto' as your default mode. If you have problems with extra sweeps out of synchronism with the other runs, flip it into 'norm' to see if it cleans things up.

If you want the scope to sit there only ever running from a valid signal as you wait to trap some rare event (very useful with a storage scope) you need 'Norm' to prevent auto sweeps spoiling things.

If you get extra sweeps due to multiple valid trigger points in a waveform, there is a 'Holdoff' control on some timebases. This is a timer that runs at the end of a sweep, disabling triggering for a while. Fiddling with this can clean things up. When not in use, park it at the minimum end of its adjustment range, some timebases even have a pot with a click at that end.

David
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Old 27th Jul 2021, 7:10 pm   #70
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Default Re: HP 180A Oscilloscope.

Hi thanks for the clarification guys.
Before I noticed David's reply I checked the rails. While the DC measures fine on all test points the +100V rail keeps fluttering with AC when measured (no AC on the other rails), i get momentary readings from 2V up to 20V AC.
I also noticed on the A1 board of the 1821A resistors r310, r312, r126 and r120 are getting very hot, there is slight discolouration of the board surrounding these resistors consistent with excess heat.
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Old 27th Jul 2021, 7:15 pm   #71
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Default Re: HP 180A Oscilloscope.

With my first problem AC on the HT+ rail, I think I will check c408 first as it seems to be the nearest cap to the test point, we'll see where that takes us, then you can probably tell me where I should be looking!
Cheers from Bill.
Edit: maybe it's the massive electrolytic on the back of the board!

Last edited by Bufo Bill; 27th Jul 2021 at 7:28 pm.
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Old 27th Jul 2021, 8:26 pm   #72
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Default Re: HP 180A Oscilloscope.

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Originally Posted by Bufo Bill View Post
Hi thanks for the clarification guys.
Before I noticed David's reply I checked the rails. While the DC measures fine on all test points the +100V rail keeps fluttering with AC when measured (no AC on the other rails), i get momentary readings from 2V up to 20V AC.
I also noticed on the A1 board of the 1821A resistors r310, r312, r126 and r120 are getting very hot, there is slight discolouration of the board surrounding these resistors consistent with excess heat.
I'm not surprised those resistors get hot, they have to drop the best part of 100V. The board in mine is also discoloured & has a cutout to help with airflow, if the solder joints look dry it would be worth resoldering them.
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The same goes for the big resistors in the 1801A on the A4 output board, those are definitely dry jointed on the second 1801A I have tried.
Be aware the transistor insulators are BeO beryllium oxide and yes I did remove that polystyrene chunk before testing.
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The ripple on the +100V rail could be the big capacitor (C405) if it isn't the smaller one (C408), I remember you've already had one go bad on another rail.

David
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Old 27th Jul 2021, 8:36 pm   #73
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Default Re: HP 180A Oscilloscope.

Cheers David, well remembered too. Will let you know how I get on.
Many thanks from Bill.
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Old 30th Jul 2021, 3:37 pm   #74
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Default Re: HP 180A Oscilloscope.

HP180A
Flutter on the +100v rail. Have a good look at the little neon stabilisers in the power supply. I have found these often give much trouble as they get older, and your varying AC rail sounds a possible fault condition. Replace with the Philips PM3262 solution using a couple of transistors and a zener diode. I have posted on this in the past. Try a search under Voltage Reg Tube in 2020.
wme_bill

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Old 31st Jul 2021, 1:52 pm   #75
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Default Re: HP 180A Oscilloscope.

The Philips circuit for the solid state replacement of the neon voltage reference tubes is here.
https://www.vintage-radio.net/forum/...5&postcount=24

You will need to alter the resistor ratio for 82V (instead of the 103V for the Philips scope) as mentioned in the link.

David
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Old 31st Jul 2021, 4:36 pm   #76
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Default Re: HP 180A Oscilloscope.

Just be a little careful with the choice of zener diode. Some types have their voltage specified at currents like 5mA, 10mA and 20mA. One of these running at 2mA will be well below the knee in its curve, so the accuracy and temperature stability will be a lot worse than spec.

Discharge tubes like neons and VR150/30s exhibit hysteresis between striking and extinguishing potentials. Zeners don't and circuits like the 'amplified zener' pointed to above don't, so they don't need as much care as the neon circuits to not make a relaxation oscillator of the application circuit.

That said, the zener amplifier circuit can create some negative resistance and RF oscillation. Be ready with ferrite beads just in case, though the high-ish resistance drive to the base of the darlington transistor ought to help keep it tame. Just make sure there isn't capacitance on the node in the middle of the potential divider/base.

David
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Old 1st Aug 2021, 10:05 am   #77
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Default Re: HP 180A Oscilloscope.

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Aye, that's the glass.... now why have I got Roger Bannister in my head... Phase two? Phase three was Mr Tebbit and during the miner's strike at that, so there were all sorts of security concerns for that opening.

David
Hi David,

I wonder if you were mixing Roger Bannister with another runner, Chris Chataway who opened phase two?

Peter
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Old 1st Aug 2021, 11:22 am   #78
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Default Re: HP 180A Oscilloscope.

Ah-ha!

That's it. Trying to think where the plaque for that was. Phase 2 reception stairs?

Thanks, Peter.

Hugh's books are being distributed over the next couple of days.

David
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Old 1st Aug 2021, 12:41 pm   #79
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Default Re: HP 180A Oscilloscope.

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Ah-ha!
Hugh's books are being distributed over the next couple of days.
David
No guesses where I got the Chataway name from.

Peter
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Old 1st Aug 2021, 9:21 pm   #80
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Default Re: HP 180A Oscilloscope.

Today I had a little time to carry on with diagnosing the internal trigger problem with the 1801A/1821A set, that the rack width 180A came with.

I went through part of the 1821A adjustment procedure for the main & delayed sweep using the external inputs, TP204 & TP404 were both found to be out of spec at 1.6V & -0.4V, they were adjusted to within 15mV of 0V using R238/R422. There was nothing wrong with trigger symmetry and I stopped at this point, as the other adjustments are not trigger related. Strange how the tests & adjustment procedure in the manual only use the external trigger inputs.

OK that leaves two more possibilities for the internal trigger problem, the internal trigger amplifier in the 1821A, or a lack of a sync signal from the 1801A vertical plug-in.
At this point I decided to swap the 1821A timebase for the known working 1824A timebase, that now also displayed the same lack of internal trigger.
Next weekend I will have to trace the signal through the sync amplifier board (A6) in the 1801A with another scope.
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