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Old 14th May 2021, 1:26 pm   #21
Bufo Bill
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Default Re: HP 180A Oscilloscope.

Okay, not very well today, but will fill you in on last night . . .
Removed suspected failed cap C412, it is nowhere near 2100uF.
While I was removing it I smelled burning which seemed to be coming from beneath the metal EHT cover. As the unit had been switched off for several hours, I pulled off the cover and found C309 was visibly burned on one corner.

Andy's advice is most welcome as I have never worked on something this complex and even the way to go about this diagnosis and repair is new and daunting to me.

On that point, I was wondering whether or not to continue the project until these two obviously defective Capacitors have been replaced? It seems daft to my untrained mind to continue until the obviously bad bits are replaced, but maybe I am wrong?

Andy, I have a Variac, I may get back to you on the EHT probe.
Cheers from Bill.
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Old 14th May 2021, 2:20 pm   #22
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Default Re: HP 180A Oscilloscope.

I guess C309 is a ceramic or film HV?
( I don't have a BOM )

Maybe its just burnt from the original soldering ? If it is open circuit I would guess the scope would still work as there are some caps a bit further along to iron things out. If shorted / breaking down it could be a problem. Is it easy to remove? (you can only hope!)

dc
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Old 14th May 2021, 3:05 pm   #23
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Default Re: HP 180A Oscilloscope.

Looks ceramic to me Dave, great big thing, flat like a "nice" biscuit. The whole top corner is black and the outer layer is burned away there. It is mounted on what looks like a printed circuit board so I can't see what it connects to underneath.
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Old 14th May 2021, 3:10 pm   #24
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Default Re: HP 180A Oscilloscope.

Looked at the schematic and I don't think it will be difficult to remove and replace it.
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Old 14th May 2021, 3:27 pm   #25
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Default Re: HP 180A Oscilloscope.

Sounds like its had a hard life, so add to the replace list
Do you have the spec for C309 ?

A replacement for C412 should be easy, unless its long and thin, C309 maybe less easy to sub ...
Photos would be good!

dc
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Old 14th May 2021, 4:12 pm   #26
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Default Re: HP 180A Oscilloscope.

The whole machine has had a hard life! Former skip material I'm afraid.
C309 is 0.01uF rated 5KV. I see one on RS for 6KV.
C412 has plenty of options.
Will get you some pictures up too.
Cheers from Bill.
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Old 14th May 2021, 5:30 pm   #27
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Default Re: HP 180A Oscilloscope.

Seems the HT transformer board is the only clean part in mine, C309 is above the two rectifiers.
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Lots of space to fit replacements for the large can capacitors, the fourth is hidden under the LV power supply regulator board.
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Old 16th May 2021, 4:24 pm   #28
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Default Re: HP 180A Oscilloscope.

David, thanks for posting, here's my C309. And what I think used to be the foam insert that made it overheat.
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Old 16th May 2021, 9:56 pm   #29
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Default Re: HP 180A Oscilloscope.

I just realised that I never posted the rail voltages. TP401: 100.1VDC TP402: 14.93VDC TP403: -12.96VDC TP404: -100.1VDC.
Next time the scope is running will try measuring VAC.
Cheers from Bill.
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Old 17th May 2021, 9:14 am   #30
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Default Re: HP 180A Oscilloscope.

The foam is there to improve electrical isolation. This is probably more of an electrical breakdown than an overheat, maybe a crack in the capacitor casing and has 'arced' through the foam. Maybe when it was assembled there was too much pressure in that region - maybe

Fingers crossed, replacing those two parts should bring you closer to a working scope.

When you replace the HV cap, be sure to carefully clean the surroundings with alcohol - especially anything that appears to have blackened. Carbon / soot is very conductive! Remove the existing foam, it should be replaced but I'm at a loss for a replacement at the moment, maybe Andy (DA) or David (factory) knows of something that could be used ...

BTW, it was very good that you followed your nose!

dc

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Old 17th May 2021, 1:08 pm   #31
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Default Re: HP 180A Oscilloscope.

Hi, put the replacement parts in and the problem seems to have cleared up. Do I just enjoy the scope or are there further checks or adjustments needed?
Cheers from Bill.
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Old 17th May 2021, 3:36 pm   #32
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Default Re: HP 180A Oscilloscope.

HP180.
If it is working, then leave it well alone. For something made from 1966 it is lasting very well. So do not disturb it. They are very good oscilloscopes.
But do need to be used relatively frequently. A rest seems to represent the opportunity for other minor faults to appear.
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Old 17th May 2021, 6:09 pm   #33
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Default Re: HP 180A Oscilloscope.

Quote:
Originally Posted by dave cox View Post
The foam is there to improve electrical isolation. This is probably more of an electrical breakdown than an overheat, maybe a crack in the capacitor casing and has 'arced' through the foam. Maybe when it was assembled there was too much pressure in that region - maybe

Fingers crossed, replacing those two parts should bring you closer to a working scope.

When you replace the HV cap, be sure to carefully clean the surroundings with alcohol - especially anything that appears to have blackened. Carbon / soot is very conductive! Remove the existing foam, it should be replaced but I'm at a loss for a replacement at the moment, maybe Andy (DA) or David (factory) knows of something that could be used ...

BTW, it was very good that you followed your nose!

dc
Looks to be a sheet of clear plastic under the foam, hp used that nasty foam where parts/boards are liable to move with vibration or for isolation from vibration. It also tends to absorb moisture and corrode PCB traces & parts it's in contact with as a result, being 50+ years old it's often found crumbled everywhere inside some test gear, having been distributed by the cooling fan, note: this scope doesn't have a fan.

David
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Old 17th May 2021, 6:43 pm   #34
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Default Re: HP 180A Oscilloscope.

Thanks to all who have helped. I would be interested to hear of any substitute for the foam, it was indeed a crumbling horrible mess, but had not yet crumbled to the point of total disintegration, very lucky. I doubt a substitute exists, but I would love my pessimism to be proved wrong.
Cheers to you all, Bill.
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Old 17th May 2021, 7:46 pm   #35
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Default Re: HP 180A Oscilloscope.

The foam is probably polyurethane, I used some offcuts of new lawnmower pre-filter foam in some recent repairs for an isolated board, but for the large strips under covers I just clean it off with a brush straight into the outside bin and forget about it.

David
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Old 17th May 2021, 8:50 pm   #36
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Default Re: HP 180A Oscilloscope.

Many thanks David, good to know you don't think it's essential, it seems to me that it was just there to make sure C309 didn't chafe against the perspex plate.
Have a good evening, Bill.
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Old 17th May 2021, 9:41 pm   #37
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Default Re: HP 180A Oscilloscope.

The foam is to stop vibration shaking the capacitors and causing hinge-like flexing leading the the leads fatiguing off. Some have to endure lots of transportation in their working lives. So during development HP products spent a lot of time on shaking tables doing accelerated life testing to find weaknesses.

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Old 17th May 2021, 11:23 pm   #38
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Default Re: HP 180A Oscilloscope.

Ah, someone did tell me the RAF used them in their aircraft, now you mention it. That would cause a fair bit of vibration, I expect!
Cheers from Bill.
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Old 18th May 2021, 9:38 am   #39
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Default Re: HP 180A Oscilloscope.

There's a new foam appearing in packaging now, most likely polypropylene, in thicknesses from 1mm (plastic packing pouches) to a few inches. I'll wrap a strip round a hot water pipe for an accelerated life test
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Old 18th May 2021, 9:51 am   #40
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Default Re: HP 180A Oscilloscope.

Anyway, I came back here to mention that local friend G6OXW came round last week to borrow a plugin and extender board to use with his 182C mainframe which is a later version of the 180A because his baseline has been jumping up + down by a few dB

His original fault had been breakdown of the CRT heater winding on the mains tfmr which he resolved by hanging a modified toroidal tfmr on the back of the instrument. I haven't heard yet how he's progressing with the jumping (hint hint)
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