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Old 11th May 2021, 6:49 pm   #1
Bufo Bill
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Default HP 180A Oscilloscope.

Hi, having some trouble with my HP180A. I am using an HP1806A and a HP1821A. I am getting a very variable signal like the voltage is unstable. I have been in contact with WME_bill who suggested either the power rails (supplied by neons) or a problem with the EHT supply. I am suspecting a problem with both areas, as the TB seems unusually erratic, but rather terrifyingly i just heard something that sounds like capacitors spewing round the back of the unit. Not had it apart yet, waiting a bit for the caps to discharge. I took off the side panels and couldn't see anything oozing.
Cheers from Bill.
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Old 12th May 2021, 6:03 am   #2
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Default Re: Hp180a

Does it do this on all sweeps or just a few? I had a very similar problem on a Tek 585A but noticed the oddness was only on certain sweep speeds, EG 05mS, 5mS etc and tracked it down to a resistor that form formed part of the RC timing.

I also had the same loud pop and squeesh sound, aha, a caps gone I thought, it wasn't, I checked them all, must have been the resistor.

Andy.
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Old 12th May 2021, 3:37 pm   #3
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Default Re: HP 180A Oscilloscope.

Had a look inside, and the good news is no goo.
I think from looking at the display with its varying intensity and size, I think we have an EHT problem, if you guys agree.
I would normally give the machine ten minutes to get going, but I am worried to do so at present, I worry about damaging the tube.
I could really do with your advice on what to do guys.
Cheers from Bill.
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Old 12th May 2021, 3:42 pm   #4
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Default Re: HP 180A Oscilloscope.

If the variation in the display size effects both X and Y then yes, PSU or EHT.
As long as you keep the brightness turned down, and the display is not blooming, it should be OK.
Too much voltage on the heater could damage a tube, but its usually a separate winding.

dc
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Old 12th May 2021, 3:59 pm   #5
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Default Re: HP 180A Oscilloscope.

Cheers for the prompt reply Dave, I reckon I might let it warm up properly this evening and see if it calms down any.
Many thanks from Bill.
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Old 12th May 2021, 7:16 pm   #6
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Default Re: HP 180A Oscilloscope.

Right, once the unit is properly warmed up we still have a lot of unwelcome EHT/PSU variation.
I have a schematic, I'm willing to give it a go (carefully), but wonder exactly where to start. Measuring voltages maybe? Is there a particular part known to fail? Transformer output voltages perhaps? In your hands here.
Cheers from Bill.
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Old 13th May 2021, 4:56 am   #7
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Default Re: HP 180A Oscilloscope.

Yep, measure voltages, all the LT rails then the HT & EHT. Do you have a high voltage probe?

Andy.
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Old 13th May 2021, 12:04 pm   #8
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Default Re: HP 180A Oscilloscope.

I have an AVO 8 MK3, so not sure it will do the tube.
Many thanks for your help.
Bill.
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Old 13th May 2021, 12:41 pm   #9
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Default Re: HP 180A Oscilloscope.

Is it still making odd noises ?

In addition to Andy's suggestions I would measure the PSU ripple (crude way is to measure ac volts on the the dc rails). Also check for any PSU capacitors that are getting significantly warmer than their surroundings.

A venting electrolytic may only make a visible mess if its venting from the bottom. If its the right way up there might be nothing to see, other than steam! Once it boils dry it may not be dissipating much power so it cools down - unfortunately it won't be much of a capacitor either

dc
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Old 13th May 2021, 1:45 pm   #10
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Default Re: HP 180A Oscilloscope.

Cheers Dave.
Just having a good look at the schematic, trying to work things out. I am progressing slowly but surely with reading them.
Re the odd noises, they stopped when I removed the side panels, which seems either very odd or a total coincidence! When I get going I will check temperature of the caps, thanks for the tip. Not seen any steam at all.
Bill.
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Old 13th May 2021, 3:40 pm   #11
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Default Re: HP 180A Oscilloscope.

Okay I found out what that noise was. V301 & V302 are lighting intermittently, and the noise I heard seems to be related to them. Arcing maybe? Test point 301 is close by but nothing I own will measure that much voltage (-2950 V). Do you guys need a schematic to read? I can take a pic of the relevant portion if required. I think that is is allowed, I did it once before if I remember correctly. If not I can email anyone willing to help.
Also I don't have anything healthy enough to reliably read capacitance.
Many thanks from Bill.
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Old 13th May 2021, 3:57 pm   #12
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Default Re: HP 180A Oscilloscope.

Tell a lie, my "best" multimeter does capacitance okay!
Bill
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Old 13th May 2021, 4:18 pm   #13
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Default Re: HP 180A Oscilloscope.

The grid bias (and the anode?) is generated from an HF transformer. There looks to be feedback around that so I guess the 2 breakdown tubes are there for safety - they shouldn't be conducting!

Interestingly, that is all powered from the 15V rails 'raw' supply. If C412 was in poor nick it could cause problems there. Might be worth testing for Cs and ESR.

dc
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Old 13th May 2021, 4:27 pm   #14
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Default Re: HP 180A Oscilloscope.

Will get onto it after Tea Dave, many thanks for your help with this.
Bill.
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Old 13th May 2021, 5:48 pm   #15
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Default Re: HP 180A Oscilloscope.

Have we confirmed all the main power supply rails are OK and have no excessive ripple?

Quote:
Originally Posted by dave cox View Post
The grid bias (and the anode?) is generated from an HF transformer. There looks to be feedback around that so I guess the 2 breakdown tubes are there for safety - they shouldn't be conducting!

Interestingly, that is all powered from the 15V rails 'raw' supply. If C412 was in poor nick it could cause problems there. Might be worth testing for Cs and ESR.

dc
Actually the regulator circuit for the EHT also requires the +15V and +/-100V rails too. The unregulated (approx 23V according to manual) supply only feeds the oscillator circuit via fuse F301.

If the main PSU is OK, then it could be something open circuit or breaking down in the EHT circuits and/or feedback. The two neons are there for power up protection (may light once & then go out in normal operation) and to prevent the EHT supply going too high in the event of a fault.

The CRT heaters are fed directly from the mains transformer, I seem to remember this 180A is from the 1960's from the previous thread.

Do you have either a good scan or an original manual of the correct age for your scope? I have an original from 1969 to refer to.

David
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Old 13th May 2021, 5:52 pm   #16
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Default Re: HP 180A Oscilloscope.

Hi. Removed one wire from C412, measured 1.150nF. Measured with both leads attached and very little difference. Don't have a dedicated ESR meter, so voltage measurement 5mV and couldn't get a current reading. What did I do wrong?
Bill.
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Old 13th May 2021, 5:56 pm   #17
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Default Re: HP 180A Oscilloscope.

Hi Dave crossed post.
Checked DC voltage only on rails, will get back to you soon.
Bill.
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Old 13th May 2021, 5:59 pm   #18
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Default Re: HP 180A Oscilloscope.

It's a 2100uF at 40V according to the manual, I would disconnect it from circuit & check again, but make sure it's discharged before using a meter to check the capacitance.

I've had a few low voltage can capacitors fail open/low capacitance in this way in HP TE.

David
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Old 13th May 2021, 10:39 pm   #19
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Default Re: HP 180A Oscilloscope.

Double check your measurement and typing
A Quick test would be to patch another 2200uF 40V in, if it reads less than ~1000uF.

dc
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Old 14th May 2021, 5:53 am   #20
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Default Re: HP 180A Oscilloscope.

The quickest and easiest test for a cap is to flick you meter onto AC volts after taking the DC reading. Any rail that shows lower than spec DC or high AC needs further looking at. If you've no ESR meter etc next is to test for leakage. If no leakage tester rig up an adjustable hi voltage power supply - simple to do if you have a few bits - then disconnect the positive wires to the cap after taking loads of pics of wiring etc. Next put an ammeter in series with your hi voltage PSU, connect to cap on test,then power up PSU noting the current reading.

You need to be systematic and a bit organised when fault finding on scopes and "big stuff", get a book, write down the rail voltages out of the manual, where the TP's (test points) are located then the results both AC & DC. SW off, sit down and look at the results and re-read the manual to re-check the results are right.

If you get stuck you can borrow a few bits if you want like my high voltage probe, ESR meter and I could knock you up a cap leakage tester; do you have a variac?

Andy.
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