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Old 12th Nov 2022, 10:11 pm   #1
Bufo Bill
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Default HP 1805A Restoration

Hi all, my HP 'Scope addiction has reached a critical stage; My 1805A is the final plug-in from my hoard needing restoration.
A good clean and judicious application of "Bill's Mixture" to the contacts shows that there are not many faults on this unit.
The .1Volt position on channel A chops and changes between a correct reading and a reading about double the correct Voltage.
The selector lever on the channel A switch assembly has broken off leaving a very short stem requiring a Screwdriver blade to change position. I wonder if there is some way of reconditioning the lever? Possibly creating a mould and casting in resin? Spares are probably unavailable unless a scrap unit appears on ebay by some miracle.
I Hope you will follow along with me on another adventure!
Cheers from Bill.
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Old 14th Nov 2022, 5:54 pm   #2
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Default Re: HP 1805A Restoration

Okay, so I took apart the broken rotary switch assembly, and I think that I can make a quick and nasty replacement from a piece of brass sheet. The lever on the other rotary switch seems very fragile also, so long term I will plan on machining a pair of lever mechanisms on my lathe.
Good news with the other problem of the 0.1V setting on Channel A chopping and changing; more use of the plug-in seems to be ironing out the fault. At least the problem seems to be occurring much less frequently, so I'm not going into surgery on it just yet.
Cheers from Bill.
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Old 14th Nov 2022, 9:02 pm   #3
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Default Re: HP 1805A Restoration

Haven't got a 1805A here, they seem less common than the 1801A, only seen them offered in the US.
The levers look similar to those found on the 17xx series scopes, well apart from the colour of the plastic, might have a spare from one of those. Are the 1805A ones made of plastic or plastic & metal?

Well the pdf manual is actually good quality for a change, the attenuator uses cam switches, I'm guessing a dirty contact for the X2 (S11/S15).

I'm sure there are plenty more to find, to add to your HP scope collection.

David
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Old 14th Nov 2022, 10:15 pm   #4
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Default Re: HP 1805A Restoration

Hi David, good to hear from you, hope you are well.
The switch levers are all plastic. Easily broken I imagine.
If you do have a spare I would be very grateful, I will post a photograph or two tomorrow to confirm, then PM you.
The .1V seems to be behaving itself tonight, so I think you have it right again David.
I keep looking at the 1809A, 1804A units, but it's hard to justify right now.
I really like the 1805A. It's really nice to use. Mine must be as near to indestructible as you get. It was really grubby when I got it, but very little wrong bar that Switch lever.
Cheers from Bill.
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Old 15th Nov 2022, 2:34 pm   #5
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Default Re: HP 1805A Restoration

Hi, I looked at images of a 17xx machine (1706 I think). Yes the switch levers look identical bar the colour. I have posted pictures of my intact assembly and the remains of the broken lever for those following along with me.
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Old 15th Nov 2022, 9:58 pm   #6
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Default Re: HP 1805A Restoration

Looking at the scale on the cutting mat, I would say mine are the same size, but grey colour instead.

David
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Old 6th Dec 2022, 6:16 am   #7
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Default Re: HP 1805A Restoration

David very kindly donated a pair of levers, and all is well with the .1V reading now. I was going to take a picture earlier, but the scope trace is being a bit choosy about working just now. C'est la vie. Enjoyable problem finding in my immediate future!
The 1805A has been a nice easy repair, I was due some luck after the 1825a struggle!
Thanks if you've been following along.
Cheers from Bill.
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Old 6th Dec 2022, 7:15 am   #8
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Default Re: HP 1805A Restoration

Wouldn't be surprised if the whole attenuator arrangement was the same as the later 1740 100MHz scope. The 1740A tube is the same as the 180A.

Used an 1805 amp in the 180 on my bench at work for many years. The vert output was handy for piping into counters etc. It worked flawlessly. Then the lab budgeted for a few new scopes and I got a shiny new 1740.

David
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Old 6th Dec 2022, 11:46 am   #9
John Caswell
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Default Re: HP 1805A Restoration

The knob looks to be identical to the ac/dc/50R flip knob on the 1840 range , as David says, although it is a different colour - 1840 series is grey. Not sure about the attenuator switch, I think in the 1805 it is a rotary type whereas I know in the 1740 it is a cam type.
John
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Old 6th Dec 2022, 2:01 pm   #10
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Default Re: HP 1805A Restoration

David (Mr Wrangler); in my extensive survey of 1805A owners (you and me) it seems the unit stands the test of time very well. It's certainly got a great range of functions, a lovely workhorse for you and a great toy for me.
John, if I am reading the manual correctly it does use cam switches. The unit is encased in an aluminium shell with holes for trimmer cap access so you can't really see the construction.
Cheers from Bill.
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Old 6th Dec 2022, 3:56 pm   #11
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Default Re: HP 1805A Restoration

The promised picture. We'll put this morning down to "Operator Error " (the primary Error being trying to engage the brain at 5AM)!
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Old 7th Dec 2022, 11:00 am   #12
John Caswell
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Default Re: HP 1805A Restoration

Bufo Bill wrote
John, if I am reading the manual correctly it does use cam switches. The unit is encased in an aluminium shell with holes for trimmer cap access so you can't really see the construction.
Thankyou useful info good to know.

John
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