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Old 8th May 2021, 12:54 pm   #1
RogerWalker
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Default HP 1742A scope

Hello all - been away a while so good to be back. I am trying to fix an HP1742A.

First, I can only find indistinct copies of the user/service manual on-line. The pages showing parts lists and component types are particularly bad - I can for example, just about discern the word "transistor" but the type or any other info is blurred and illegible. I can understand why the copying of this manual proved difficult - in typical HP style the pages vary from single A4 to three shrunken A4s side by side. A truly nightmarish prospect at the scanner/photocopier! WME Bill referred to this phenomena in one of the posts here from yesteryear relating to this model.



Secondly, this scope features circuits that automatically shut down the high voltage oscillator circuit to protect the expensive CRT, in the event of over- or under-volts on the low voltage PSU lines - or, it seems, for any reason, at the drop of a hat!



My unit has 'normal' low voltage supplies, but something has shut down the HV oscillator [as yet, I know not what] and it is proving difficult, short of removing all the components, one-by-one, to see whether they are duff, to fix it. There is no HV PSU activity, so no sign of a trace.



A scope like this is at the very top end of my skills bandwidth, so if anyone has come across this fault before, I would love any advice you can offer.



In the meantime, if anyone has a legible copy of the manual [or wonder of wonders, an original HP jobby], I would love to hear from you!
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Old 8th May 2021, 2:45 pm   #2
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Default Re: HP 1742A scope

Hello,
maybe you can take this very readable scan of the HP1740A manual and use it to fill in the bits. From what I can see, many images and parts list sections are identical. And the HP1742A manual is not so unreadable that you have no visual information to match. In many cases, numbers and words are recognizable, especially if you have the HP1740A manual for comparison. Hope this helps in some way.

Denis
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Old 8th May 2021, 6:29 pm   #3
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Default Re: HP 1742A scope

Have you got any part numbers from the boards? Some parts are shared in the 174x scopes but an old thread suggests the power supply board in the 1742A has differences to the 1740A.

David
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Old 8th May 2021, 9:12 pm   #4
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Default Re: HP 1742A scope

Hi

Just a note from experience!

I had a 1725a, which developed a fault in the HT regulator circuit (feedback bent open circuit) which resulted in the HT going very high, with flashover on the EHT.

After diagnosing and fixing the HT feedback through the gate circuits, and restoring the HT to correct voltage, found that the heater had blown, as it uses the HT feedback to regulate the heater (same transformer, regulation done through primary windings!)!

I would consider disconnecting the Heater supply to the tube if faultfinding on the HT PSU. There is NO protection from a runaway HT regulator.

Your 1742 may be different, but just be careful.

Peter
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Old 8th May 2021, 9:21 pm   #5
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Default Re: HP 1742A scope

THe 1725 is a completely different scope to the 1740.

The 1725 runs hot and has consequent reliability issues. 1740 series are fairly reliable, but there is virtually nothing in common. I may even hace a paper copy of the 1742 manual, but it isn't where I last saw it.

David
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Old 8th May 2021, 9:26 pm   #6
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Default Re: HP 1742A scope

Many thanks to all who responded. Peterg0rsq in particular who might've saved the CRT from extinction. I have downloaded the 1740A manual and will take a while to compare and contrast. Wish me luck! I have attached a copy of the HV PSU circuit in case anyone can suggest something . . .
Attached Files
File Type: docx hvpsu 1742a.docx (96.5 KB, 14 views)
File Type: docx hvpsu diag.docx (176.3 KB, 17 views)
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Old 8th May 2021, 9:32 pm   #7
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Default Re: HP 1742A scope

Quote:
Originally Posted by PETERg0rsq View Post
Hi

Just a note from experience!

I had a 1725a, which developed a fault in the HT regulator circuit (feedback bent open circuit) which resulted in the HT going very high, with flashover on the EHT.

Peter
Do you still have the 1725A? I have a spare CRT from a 1715A if it's any use.

David
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Old 9th May 2021, 8:31 pm   #8
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Default Re: HP 1742A scope

Quote:
Originally Posted by factory View Post

Do you still have the 1725A? I have a spare CRT from a 1715A if it's any use.

David
David

I believe the 1715A is a 200MHz scope, and the 1725A is 275MHz, so presume if the tube fits the bandwidth may be limited.

However it would breath life back into what is otherwise a good scope.

I will PM you for more details.

Thanks
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Old 9th May 2021, 8:59 pm   #9
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Default Re: HP 1742A scope

Quote:
Originally Posted by Radio Wrangler View Post
THe 1725 is a completely different scope to the 1740.


David
David

You are right that they are completely different scopes,. but a quick look at the HT PSU shows an almost identical circuit. The 1740 uses an amplifier in the feedback loop, but the 1725A does not.

If this feedback loop goes wrong (as in my scope one of the capacitors was breaking down giving a few K to ground in the feedback) the chopper goes out of control sending the HT very high, trying to close the loop. Unfortunatly the heater winding is on the same transformer, so it too will increase in voltage, and in my case blows the heater in the tube.

https://www.vintage-radio.net/forum/...1&postcount=26

Maybe I was just unlucky (or a little careless) but definatly worth considering when working on the HT PSU
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Old 9th May 2021, 9:08 pm   #10
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Default Re: HP 1742A scope

Quote:
Originally Posted by PETERg0rsq View Post

David

I believe the 1715A is a 200MHz scope, and the 1725A is 275MHz, so presume if the tube fits the bandwidth may be limited.

However it would breath life back into what is otherwise a good scope.

I will PM you for more details.

Thanks
Just checked and both scopes use the same tube 5083-5552
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Old 10th May 2021, 6:01 pm   #11
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Default Re: HP 1742A scope

Not having much luck troubleshooting this fault - there are a few resistors a tad high, but no leaky capacitors so far. The oscillator transformer shows continuity in all the right places. Resistance checks on the oscillator transistor don't look right - I can't read the type number on either the device or the partslist. It is a Motorola npn power device. If anyone knows which type it is, I may just buy another and substitute it - there don't appear to be any drastically dodgy components that might blow it up, so worth a gamble?
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Old 10th May 2021, 6:37 pm   #12
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Default Re: HP 1742A scope

I found the HV osc transistor for the 1740A and have ordered one - HP part 1854 0433 - eight quid on a well known auction site. Fingers crossed it will be what's causing the lack of EHT.
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Old 10th May 2021, 9:40 pm   #13
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Thumbs up Re: HP 1742A scope

Quote:
Originally Posted by PETERg0rsq View Post
I believe the 1715A is a 200MHz scope.
I have a 1715A here: yes, it is a 200 MHz 'scope.

Al. / May 10th.
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