Author Topic: HP 1727A oscilloscope with multiple issues.  (Read 3464 times)

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Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #25 on: May 02, 2021, 06:00:36 pm »
I know it has been a few days but I have been working on the scope bit by bit. I am focusing on the the storage board (A17) right now, using the waveforms given in the manual. Everything is OK except for the storage mesh output from the output amp, this is the circuit that I mentioned had issues and shorts from the beginning.

I did find one thing that was causing some erratic behavior on the screen, and that was a switch on the back panel of the scope that had failed so that it was open all the time except for when you were putting pressure on the lever. Interestingly this switch is not in the schematic, and the manual is a little different layout wise as it pertains to A17. The switch is labeled Brightness; maximum/normal, and as far as I can tell it just joins or opens a connection between pins 6 and 8 on U4, which is just a trace in the schematic. Sadly this did not fix all the random behavior, the trace still flashes randomly sometimes.

I do have a question about something on the schematic, on page 8-59, what is Q10 doing? I can see that its a PNP, so with the base grounded it would be on if the collector was more positive than ground or vice versa? that looks odd to me, what's going on there?
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #26 on: May 02, 2021, 07:26:18 pm »
I have identified another potential issue on A17, there are two analog switches, U8 and U7, and U8 plays a big role in the storage mesh control. The issue I see, and I could be overlooking something, is that the  pinout of U8 does not correspond with the schematic or the PCB circuit. The manual gives part number 1826-0502 for the IC, which crosses to an ON-semi MC14066BCP, here is the datasheet I found; https://pdf1.alldatasheet.com/datasheet-pdf/view/174197/ONSEMI/MC14066BCP.html

I find it hard to believe that there is a mistake in the HP manual, so is it possible that this is not the right datasheet? U8 and U7 are the only ICs on this board that do not have a HP part number on them, and although they have the same P/N, one is a plastic package and one is a ceramic, leading me to believe that they have been replaced? I will keep looking for a different datasheet but until then U8 is confusing.

 
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #27 on: May 05, 2021, 07:17:36 am »
Quote
I did find one thing that was causing some erratic behavior on the screen, and that was a switch on the back panel of the scope that had failed so that it was open all the time except for when you were putting pressure on the lever.
I think you're on something.

Quote
The switch is labeled Brightness; maximum/normal, and as far as I can tell it just joins or opens a connection between pins 6 and 8 on U4
All of this seems quite strange. It seems this is controlling the flood gun grid (TP3) and it seems that it would inhibit signals which turn off and on the flood guns which I believe may result in the weird things you've been seeing because they are a very important part of the behaviour of this CRT. Can you tell if such a switch is "original" or was added later? its function seems more like a hack or a debug tool rather than something you would really need to use...

Quote
Sadly this did not fix all the random behavior, the trace still flashes randomly sometimes.
So you're saying that the ghost image is now flashing rather than being all the time there? I think this may be an improvement.... then you have to trace the timing of the signals which may cause the flickering. Or perhaps it's still the switch which is not connecting properly... honestly I would bypass the switch for the time being.

Quote
what is Q10 doing?

https://en.wikipedia.org/wiki/Common_base

EDITED : U4 is an Emitter Coupled Logic (ECL) gate. In ECL you don't couple gates by voltage but rather by current.
TTL is a current-sinking logic since a current must be drawn from inputs to bring them to a logic 0 voltage level. I think by connecting the two gates output together, they are realizing a AND logic gate here.
That transistor I think is (EDIT: besides providing the logic function "AND") a current buffer which interfaces the result to a voltage-coupled device (the CRT flood gun grid) by means of a load resistor (R46 and R47) which would be too hard for the gate to drive directly. Note how the emittercollector is referenced to -100V supply.

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I find it hard to believe that there is a mistake in the HP manual
Mistakes may be much more common than what you may think. I would just really trust the PCB because at some point it must have worked. Looking at the datasheet I see only pin 3 and 4 might have been swapped in the schematic... I don't have the PCB to confirm and trace where the pins go.

Quote
U8 and U7 are the only ICs on this board that do not have a HP part number on them, and although they have the same P/N, one is a plastic package and one is a ceramic, leading me to believe that they have been replaced?
Now if you think somebody has had a go, keep you eyes more open than usual around those signals and check them all bu t I wouldn't be surprise if they used different packaging or slightly different ICs at the factory.
« Last Edit: May 05, 2021, 12:14:54 pm by cosenmarco »
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #28 on: May 06, 2021, 01:56:17 pm »
I want ahead and bypassed the switch for now, even though it was working properly. As far as I can tell this switch is original, there is labeling printed on the back panel that looks the same as everything else.
I am not sure if I mentioned it before but I have noticed a few other differences between the storage board I have and the HP manual. The differences are mostly in the board layout , but I suppose it is possible that U7 and U8 are not the right ones for this particular board. I did verify that the circuit board matches the schematic, so pins 3 and 4 are swapped on the actual IC.

As for the flickering its not just the ghost image, its everything, and not always everything at the same time, I will look into it more later but for now there is some work I need to do on the storage board before anything else.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #29 on: May 07, 2021, 09:20:13 pm »
Ok, I have an update and a major discovery, hopefully :)

As of right now everything on the storage board A17 works fine, I compared what I have to the table on page 8-53 of the manual and everything looks good as far as the storage stuff goes. However In tracing and measuring the floodgun stuff in that same table I found that the floodgun accelerator was nearly zero volts, when as you can see it should be about +16. So I did what I always do first and started measuring power rails, and the one involved with the particular section I am looking at is the +53V rail. Low and behold there is not +53V, but 15V.

This is where it gets interesting, I attached two pictures of A13, gate assembly/amplifier, one of the top and one of the bottom. As you can see on the big plastic connector at the top, somebody has disconnected the +53V rail and ran a jumper wire to the +15V rail. This it where I stopped for now because I am not sure how to proceed, should leave it how it is? or should I put it back to factory configuration? The puzzling thing here is that the wire used looks like factory wire, it is the same hard plastic with a colored stripe, in this case its green.

This is interesting for sure but it raises more questions as to the history of this scope.  ???
 

Offline 0culus

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #30 on: May 08, 2021, 04:50:14 am »
I have a 1727A that works perfectly fine, and has this ghost trace as well. I'm interested to see what you find out about it. This isn't a very common model at all. It is also huge...I'd say it's stretching the definition of "portable oscilloscope".  >:D
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #31 on: May 08, 2021, 02:11:03 pm »
I am slowly working through this thing bit by bit, and hopefully I will eventually find the source of the problem  :-BROKE

It is also huge...I'd say it's stretching the definition of "portable oscilloscope".  >:D

I attached a picture of how much space this thing takes up on my bench, it's massive and heavy!


EDIT: I don't know why my pictures are always upside down on forums I post to.
 

Offline 0culus

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #32 on: May 09, 2021, 06:47:06 pm »
I am slowly working through this thing bit by bit, and hopefully I will eventually find the source of the problem  :-BROKE

It is also huge...I'd say it's stretching the definition of "portable oscilloscope".  >:D

I attached a picture of how much space this thing takes up on my bench, it's massive and heavy!


EDIT: I don't know why my pictures are always upside down on forums I post to.

Yeah, mainly due to that storage tube. Like I can't seriously see it being portable due to the length and weight. A tek 454 is more what I'd call portable.

I'm definitely interested to see the result of your troubleshooting, since mine has that ghost trace too. Other than that it works perfectly, so I had written it off as a characteristic of the storage tube.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #33 on: May 10, 2021, 01:12:49 am »
I had written it off as a characteristic of the storage tube.

I suppose it is possible, I never really thought about that. I really should do some more research into storage scopes since I have no prior experience working with a scope like this. I doubt something like a second trace would be a normal characteristic, but you never know!

 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #34 on: May 12, 2021, 06:36:21 am »
Quote
I really should do some more research into storage scopes since I have no prior experience working with a scope like this.

I'd be interested in what you find out!! Please keep me posted.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #35 on: May 12, 2021, 04:00:44 pm »
I did a little research and it seems that there are not too many of these things out there, I did find two other examples of scoped with the same second smaller trace, or at least it appears to have it. The first link is an ebay listing for a 1742A not a 1727A, however it looks like it has a smaller trace in the middle(the bright part), however it doesn't look like the 1742A is a storage scope, so perhaps the second trace is not caused by the storage bits? The second link is actually an old thread from this forum that was about a 1727a with the same problem, unfortunately that thread did not get anywhere and it went dead over 7 years ago. with that being said the only picture I found of a 1727a without a second trace was in the third link, which seems to be a history of HP scopes, its actually kind of interesting.

a good point mentioned in the other forum thread is that with storage scopes you could possibly get reflections at high intensities, so it is not out of the question that a reflected trace is normal to a degree. Because of that fact it may be a better idea for me to look at a couple of other issues that this scope has, and then once they are fixed I can go back to the ghost trace problem.

https://www.ebay.com/itm/254966332232

https://www.eevblog.com/forum/testgear/hp-1727a-beam-light-when-retrace/msg419172/#msg419172

https://www.hpmemoryproject.org/wb_pages/wall_b_page_02.htm

 earlier I made post about a weird modification on the gate board, and I decided to rewire it back to the way it is on the schematic with a jumper wire. low and behold it did not change a thing, so either there is still a problem with that circuit or it has no visible effect on the screen. I also managed to blow the little 18.2V zener while testing it  |O so I had to replace it with one that I had that is close enough to 18V.

 

Offline dzseki

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #36 on: May 12, 2021, 07:21:22 pm »
HP 1720A (non storage) owner here. I can't add much to troubleshoot the storage part, but once done I would be curious to see how well it fares with bandwidth? My unit strugles to meet it for sure, in fact mine barely goes beyond 250MHz...
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Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #37 on: May 12, 2021, 08:28:19 pm »
I don't have any kind of test equipment that can go anywhere near 275MHz. I have a PC scope that can only do 10MHz bandwidth and an old function generator that can barely do 1MHz, so unless I have an opportunity to test this scope with a piece of equipment at my college I probably will never be able to stretch its legs. would be neat to see though.  :-+
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #38 on: May 12, 2021, 09:30:46 pm »
Small update on the scope, I found the source of the random flickering of the screen. R42 (a potentiometer) had gone bad on the LVPS, so the HV collimator voltage was bouncing all over the place. I have to order the right pot but I threw in a random 50k pot I had and it works fine.
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #39 on: May 13, 2021, 07:28:30 am »
I think some ghost trace at high brightness is to be expected. The HP manual even reports it as I pointed out on 21st of April
In your case though I think it's not normal that the ghost trace has higher brightness than the legit trace.

@dzseki
Quote
My unit strugles to meet it for sure, in fact mine barely goes beyond 250MHz...
Consider those units are 40+ years old. Think of what has become of all capacitors and resistors along the very complicated signal path to the CRT... So I wouldn't complain. Bandwidth is tricky to debug.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #40 on: May 13, 2021, 02:47:04 pm »
The more I work on this scope and dig through every corner of it for problems, I am noticing more small differences between my scope and the manual, including modifications that seem like they were done at the factory so to speak. I have no way of knowing if the hardcopy of the manual I have is original to the scope, but it has this page; (see picture), which seems to suggest that this manual and scope was shipped out to the customer before the manual was complete. This would explain the lack of the chapter on service in my manual, since the service section was probably not done yet.

It seems to me that it is a real possibility that my particular scope was produced and sold very early on in the 1727A production, and thus is a little bit more prototype than most. further lending to this theory is the fact that the serial number is as follows; 1947A 00075, does that mean that my scope is the 75th unit?

If Oculus is still watching this thread, I would be interested to know what the serial number of his scope is  ???
 

Offline 0culus

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #41 on: May 14, 2021, 03:37:12 am »
The more I work on this scope and dig through every corner of it for problems, I am noticing more small differences between my scope and the manual, including modifications that seem like they were done at the factory so to speak. I have no way of knowing if the hardcopy of the manual I have is original to the scope, but it has this page; (see picture), which seems to suggest that this manual and scope was shipped out to the customer before the manual was complete. This would explain the lack of the chapter on service in my manual, since the service section was probably not done yet.

It seems to me that it is a real possibility that my particular scope was produced and sold very early on in the 1727A production, and thus is a little bit more prototype than most. further lending to this theory is the fact that the serial number is as follows; 1947A 00075, does that mean that my scope is the 75th unit?

If Oculus is still watching this thread, I would be interested to know what the serial number of his scope is  ???

OK, this is real interesting. Mine is 2452A1208. the first two digits generally indicate years since 1960 for instruments newer than 1960 or so. The date, however, indicates the engineering revision date. So yours is a 1979 revision and mine is another revision that was done in 1984. Here's where it gets fun. I consulted my -hp- catalog collection and the 1727A does _not_ appear in the 1979 or 1980 catalog. I don't have a 1981 catalog, but it's definitely in the 1982 catalog. You, sir, may have a prototype on your hands. This might explain the weird bodges and stuff.
 

Offline dzseki

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #42 on: May 14, 2021, 08:26:19 am »
The more I work on this scope and dig through every corner of it for problems, I am noticing more small differences between my scope and the manual, including modifications that seem like they were done at the factory so to speak. I have no way of knowing if the hardcopy of the manual I have is original to the scope, but it has this page; (see picture), which seems to suggest that this manual and scope was shipped out to the customer before the manual was complete. This would explain the lack of the chapter on service in my manual, since the service section was probably not done yet.

It seems to me that it is a real possibility that my particular scope was produced and sold very early on in the 1727A production, and thus is a little bit more prototype than most. further lending to this theory is the fact that the serial number is as follows; 1947A 00075, does that mean that my scope is the 75th unit?

If Oculus is still watching this thread, I would be interested to know what the serial number of his scope is  ???

OK, this is real interesting. Mine is 2452A1208. the first two digits generally indicate years since 1960 for instruments newer than 1960 or so. The date, however, indicates the engineering revision date. So yours is a 1979 revision and mine is another revision that was done in 1984. Here's where it gets fun. I consulted my -hp- catalog collection and the 1727A does _not_ appear in the 1979 or 1980 catalog. I don't have a 1981 catalog, but it's definitely in the 1982 catalog. You, sir, may have a prototype on your hands. This might explain the weird bodges and stuff.

This is indeed strange, as the 1727A was introduced in the April 1982 issue of HP Journal:
https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1982-04.pdf

Though HP journal not neccesarily articled the newest instruments, but there was no point to advertise a years old tool either...

BTW in that article they went into highlight the strengths of the new tube (and less the drawbacks obviously).
HP 1720A scope with HP 1120A probe, EMG 12563 pulse generator, EMG 1257 function generator, MEV TR-1660C bench multimeter
 

Offline 0culus

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #43 on: May 14, 2021, 03:18:48 pm »
The more I work on this scope and dig through every corner of it for problems, I am noticing more small differences between my scope and the manual, including modifications that seem like they were done at the factory so to speak. I have no way of knowing if the hardcopy of the manual I have is original to the scope, but it has this page; (see picture), which seems to suggest that this manual and scope was shipped out to the customer before the manual was complete. This would explain the lack of the chapter on service in my manual, since the service section was probably not done yet.

It seems to me that it is a real possibility that my particular scope was produced and sold very early on in the 1727A production, and thus is a little bit more prototype than most. further lending to this theory is the fact that the serial number is as follows; 1947A 00075, does that mean that my scope is the 75th unit?

If Oculus is still watching this thread, I would be interested to know what the serial number of his scope is  ???

OK, this is real interesting. Mine is 2452A1208. the first two digits generally indicate years since 1960 for instruments newer than 1960 or so. The date, however, indicates the engineering revision date. So yours is a 1979 revision and mine is another revision that was done in 1984. Here's where it gets fun. I consulted my -hp- catalog collection and the 1727A does _not_ appear in the 1979 or 1980 catalog. I don't have a 1981 catalog, but it's definitely in the 1982 catalog. You, sir, may have a prototype on your hands. This might explain the weird bodges and stuff.

This is indeed strange, as the 1727A was introduced in the April 1982 issue of HP Journal:
https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1982-04.pdf

Though HP journal not neccesarily articled the newest instruments, but there was no point to advertise a years old tool either...

BTW in that article they went into highlight the strengths of the new tube (and less the drawbacks obviously).

Thanks, I knew there was an article about it in the journal, but I couldn't find it last night. I'd say it was formally released for 1981 at the earliest.

It is a very interesting and rare scope today...cue my excitement about seeing this thread. I use mine occasionally but it's size makes keeping it out impossible because of the amount of bench space it uses.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #44 on: May 14, 2021, 06:00:31 pm »
Thanks for the interesting information, I was looking at board A13 and it is very different than the layout in the manual, I know it was slightly different but now that I looked at it harder Its layout is completely different. This of course makes it difficult to trace signals If I don't know which component is which!  ??? I am focusing on A13 right now, just to make sure everything there is working since the beam intensity  does not respond to the knob on the front panel.

Just my luck to get a prototype scope without knowing it,  |O  :-DD
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #45 on: May 15, 2021, 07:35:07 pm »
I think you're lucky, you may now try to sell it to collectors for 3X what you paid for it  >:D
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #46 on: May 15, 2021, 08:24:44 pm »
Ha Ha,  :-DD that would be funny.

In all seriousness my ultimate goal is to fully repair and refurbish this scope, hopefully learning as I go  :) Thanks to you people on the forum for being here for the ride!

The point I am at right now is troubleshooting A13, There is nothing on TP1 that resembles what is shown in the service section, so I need to figure out how to trace that signal back without an accurate board layout. I am not sure how accurate the schematic is but I presume it is pretty close if not the same.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #47 on: May 17, 2021, 09:04:13 pm »
Ok, I found a dead transistor on A13 in the output amp section, Q14. Although it is in a metal can It does not seem to be anything special except that in the parts list it lists a frequency FT=600MHz. Unfortunately the manufacturer part number it gives in the manual is not helpful, as it's just the HP number. I should stop here and note that this particular transistor is not the original one, since according to the manual Q14 is the same as Q1,3,4,5, and 6, which are all plastic. I was not able to find a part that matched the P/N given in a cross reference table for HP numbers either.  :palm:

I found this part on mouser.com just by filtering BJTs by 600MHz, I don't know if that is the best way to do it but it at least got me a result: https://www.mouser.com/ProductDetail/Central-Semiconductor/PN918-PBFREE?qs=l7cgNqFNU1iR8p7FRKw7wg%3D%3D . The collector emitter voltage is only 15V but it is all 5V stuff anyway, and with my limited knowledge I would assume that this circuit would not be too picky about the transistor specs, provided that it met the required transition frequency, but if anybody has any other input or an alternative transistor choice let me know!  :-+
 

Offline dzseki

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #48 on: May 17, 2021, 09:45:50 pm »
NPN or PNP? A popular NPN TO can transistor was the 2N2369 at that time. Can you insert the schematic?
HP 1720A scope with HP 1120A probe, EMG 12563 pulse generator, EMG 1257 function generator, MEV TR-1660C bench multimeter
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #49 on: May 17, 2021, 09:59:51 pm »
It's NPN, I didn't think about the schematic.

 


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