Author Topic: HP 1740A Channel B not working and flickering on A  (Read 3342 times)

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Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #25 on: May 13, 2021, 04:09:19 am »
I didn't know about the blue screen filter. I found some pretty cheap ones online, so if this flickering ends up getting fixed, I'll order one.
Turning the scale illumination fully CCW makes the background "grey" exactly like what I was describing happened sometimes for no apparent reason. Interestingly, turning it fully CW makes the pot rotate as well, and the terminal attached to the yellow wire touches the CRT housing and makes it really bright.

Anyway... I removed A8 and inspected it all and it looks all fine to me, although I've attached a whole lot of pictures of it both removed and back in, as well as some pictures of the back of A7, and close-ups of the interface connector pin solder joints. I sprayed Deoxit in the interface connector twice before reconnecting and putting it all back together, but that hasn't changed anything unfortunately. I am wondering though how it's possible for the connector to be the issue, given there is no flickering for timebase selections smaller than .5 mSec/Div. Would that not instead indicate a problem with the switch? Although as I said, pressing on the switch plates changes nothing, only flexing the PCB eliminates the flickering.

Do you see anything suspicious in the pictures? Do you think I should remove A7 to inspect those solder joints (and post some pictures)?

EDIT: Here's a link to the pictures: https://ibb.co/album/c6PnLh
« Last Edit: May 13, 2021, 04:17:45 am by PurpleAmaranth »
 

Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #26 on: May 13, 2021, 10:20:03 am »
Those PCB connectors were never a good choice , not even gold plated, so I always make 100 % sure they are clean.
As my 1740 showed with A8 causing trace collapse at switch on until the unit warmed up and the initial indication was the time base switch with with operation seemed to fix it!.
 Any way your thoughts are heading in the right direction I think. Looking at the switch contacts on the photos where you can see some of the PCB gold plated contact it does look rather dirty , not sure if that's an artifact of the photo taking or its real!. I would take the switch housing of the PCB for another photo of the gold switch traces before giving it a good clean with a cotton tip .
 There is one other possible failure that I have encountered and that is component legs can impart some stress into the component giving internal intermittent connection which you overcome with the flexing. Although I would start at the switch first. Given you can stop the flickering with mechanical movement there is a bad connection somewhere.
 
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Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #27 on: May 13, 2021, 10:29:52 am »
One last thought , don't discount a dry joint particularly around components that get warm to hot as that thermal cycling over the age of this unit could easily introduce those and the joints still might look good but are not. Real close inspection with magnification needed. Unfortunately this is going take some effort top in down the culprit.
It will be worth it though as these are nice vintage CRO's with a crisp bright display.
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Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #28 on: May 14, 2021, 02:54:12 am »
So I took apart the switch to look at the gold plating (which I had already sprayed with Deoxit before). Here are some pictures: https://ibb.co/album/5MXsG9 . The gold is very hard to take a picture of, but it all looks really clean. The one thing is that there are two tones of gold, one a shade darker than the other, and this darker shade is not totally uniform, and seems to fade a bit in some areas in the counterclockwise direction, but the lighter shade which is present still looks very good.

I looked at each of the components' solder joints with a magnifying glass, and they all look perfect as far as I can tell. I did attach a picture to this post, and as you will see, part of one of the light green PCB lines (I don't know what they're called) appears to have what looks like solder or something of that colour on it, is that normal?

I haven't put it back together yet in case there's anything else to take a look at in particular. Is there any other way of testing apart from visual inspection, I don't seem to see anything visually wrong, although I'm hoping the pictures can tell you something!

Thanks!
 

Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #29 on: May 14, 2021, 06:34:52 am »
So I took apart the switch to look at the gold plating (which I had already sprayed with Deoxit before). Here are some pictures: https://ibb.co/album/5MXsG9 . The gold is very hard to take a picture of, but it all looks really clean. The one thing is that there are two tones of gold, one a shade darker than the other, and this darker shade is not totally uniform, and seems to fade a bit in some areas in the counterclockwise direction, but the lighter shade which is present still looks very good.
Thanks!
From the photo's at least the gold contacts appear dirty or worn looking at the darker 'tone' which is where the wiper contact slides. In the end its too hard to tell actual condition from the pic's. However assuming the photo's are representative of condition they certainly need a good cleaning. I have used a soft white pencil rubber to gently remove crud from gold contact surfaces on PCB's  with no issues, make sure its not one of those combo PEN/PENCIL types but a PENCIL only. After that clean with IPA and re-lube the contact surfaces again. Don't forget the contact springs on the black rotors (be careful not to flick one out of its position, as it will likely disappear on the floor forever  :P.

I looked at each of the components' solder joints with a magnifying glass, and they all look perfect as far as I can tell. I did attach a picture to this post, and as you will see, part of one of the light green PCB lines (I don't know what they're called) appears to have what looks like solder or something of that colour on it, is that normal?
That trace on the left half way looks odd , check for continuity, also give it a swipe with the iron and a small amount of fresh solder to be sure proper bridging of any corrosion induced breaks.
Also be mindful that although the joints look okay that there still could be a component leg that is 'loose' in the middle of an otherwise acceptable solder fillet. I had one such type of joint in an old fluke voltmeter that had an intermittent fault caused by such a bad joint !. Could only see it by visual inspection and moving the component up and down through the solder joint and observing the lead moving !.
I will say though that this type of failure in not common, usually a dry joint is fairly obvious.

I haven't put it back together yet in case there's anything else to take a look at in particular. Is there any other way of testing apart from visual inspection.
Well after cleaning the switch contacts and cleaning up that suspect trace as well as  being happy that all components on that PCB are properly soldered its re assemble time then back to trying to track down the poor/intermittent connection, (if still present!), by carefully prodding and flexing sections of the PCB/components and surrounds with a plastic 'stick' etc. trying to isolate the issue area .

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Offline Alex_Baker

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Re: HP 1740A Channel B not working and flickering on A
« Reply #30 on: May 14, 2021, 06:35:56 pm »
Hello, I'm just lurking on this topic because I am working on a HP 1727A that has a couple of problems, you might have seen the topic for it on this forum.

Although the 1740 is a bit different than mine I am still interested in how it turns out!  :popcorn:
 

Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #31 on: May 15, 2021, 04:13:59 am »
So I took the switch plates off again, and rubbed the gold plating and the little metal springs on the black rotors with IPA until they looked perfectly clean, and then a few more times for good measure. That does appear to have gotten rid of the vast majority of the "two-toneness" and made it a much more uniform gold colour. (It does looks like there were some very light scratches now that it's clean, but nothing major). The q-tips did get black, so I guess it needed it! I lubed it back up, put it back together, tried it again... and it's still flickering. So I assume it's not the switch that's the problem?

I checked the suspicious-looking trace, and there is continuity. Do you still think I should add some solder to it anyway?

I looked carefully at all the components again, and C11 (on board A8) looks a bit odd. I've attached a picture of one of its legs, it looks like a "drip" of metal on it. And although this isn't really visible in the picture, the leg has turned a hue of purple very near the connection to the green side of the capacitor (same side as the drip). And a last thing is that it wiggles when I try to roll it move it back and forth, significantly more than C18 which looks very similar. Does any of this sound suspicious? Should I take A8 back out and test C11's capacitance using the DMM? Will that even work if it's connected to the rest of the circuit?

Otherwise, what else is left to do? Prodding with a plastic rod? Taking out A7 to inspect the solder joints?

And Alex, I've actually been lurking on your thread as well! I hope you manage to fix the problems you have, you sound like you know lots more than I do! The 1727A looks very similar to mine, and I believe it's just the storage version. Even the inside looks very familiar.
 

Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #32 on: May 15, 2021, 06:18:31 pm »
I realized I can access C11 without taking out A8, so I tested the capacitance and I get 102μF, so looks like it's not C11. I guess the next step is carefully prodding with a plastic rod to try to isolate the issue  |O
 

Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #33 on: May 16, 2021, 02:16:43 am »
While that may seem tedious, its one of the tried and tested methods for isolating mechanicaly introduced failures. And if temperature related then add heating and freezing to the mix.
As it is affected by flexing the PCB assembly that's currently the best avenue of isolating the area.
Besides a plastic rod such as the outside barrel of an old ball point pen etc is better than poking fingers around the live parts  :P.
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Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #34 on: May 16, 2021, 07:29:18 am »
Well... I spent some more time on it and although I've narrowed the problem down, I'm now very confused. I set the Time/Div to 2mSEC, which is where the flickering starts to happen. By flexing the PCB in many different directions, I figured out that the problem came from the lower right quarter of the PCB (A8) when looking from the front of the scope.

I also found that the flickering stopped when that area was pulled towards to the front, not pushed towards the back. So I took an old pen and a thicker wooden rod and first tried to press on each component in that lower right area, but that never stopped the flickering. I then tried pushing each component side to side, but that also did nothing. So I turned my attention to the switch, even though I was fairly confident it wasn't the switch given the time spent on it already.

I tried pressing the front plate of the switch in the lower right quarter area into the board, but that changed nothing. So then, although not expecting anything interesting to happen, I tried lifting the lower right quarter of the rear switch plate up a bit (between the two pin-like metal structures that act as stoppers) with a fingernail, and somehow that eliminated the flickering.   :o I tried it again, many times and every time, the trace stabilized.

I made sure the PCB wasn't bending at all, and then tried to figure out how this was possible. My first thought was that maybe lifting the bottom right of the rear plate caused the top left of the front plate to press down harder, but pressing on that had no effect. So I took off the rear plate and checked that all the springs were making contact, and they all seem to be nice and springy so I don't think that's a problem. I did notice however that the area I'd been lifting was the area with the springs. I put it back together, but left the C-clip off, as well as the round stoppers that go on the switch axle.

From there, I tried varying the pressure, and I found that nothing I do on the front plate has any effect. But the rear plate can have its area opposite the contacts completely off the board, and its area with contacts just slightly on, and it works, but if I press down a bit more, it stops working. I found that there's a very specific amount of pressure that works optimally. Not enough and I get a dot on the screen, too much and I get the flickering.

I'll spend some more time on it tomorrow in the hopes of finding something notable, but do you have any idea how it's possible that lessening the pressure on the contacts would eliminate the flickering? That sounds very counterintuitive to me. Anything you'd recommend trying in particular to further isolate the problem? It definitely appears to be in the switch.
 

Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #35 on: May 16, 2021, 09:53:04 am »
Random thought, what about a trace fracture inside the switch area or at one of the via's. As to the contact pressure  :-//.
Did you get a sense that the PCB gold contacts were heavily worn, can't really tell from the picture.
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Offline Brakeboard

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Re: HP 1740A Channel B not working and flickering on A
« Reply #36 on: May 17, 2021, 07:57:35 am »
Hi guys, Been loitering on this thread. I have a 1741A, the same as the 1740A but with storage.

I have no Vertical or position function on my scope. It's so clean inside it seems worth salvaging. Thought I will be lucky and find a simple power supply issue. just like David's repair vlog: EEVblog #803

But alas I've narrowed it down to the custom HP A3A1 amplifier IC. An unobtainium part it seems.  Is there any way to fix this? Or do I need a parts machine to resolve the issue?

Any thought would be appreciated.
 

Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #37 on: May 17, 2021, 09:47:52 am »
Are the DC voltages at its outputs balanced?. Note that these voltages are set further up the chain by resistors, (don't have the manual in front of me currently, so can't give part no.s ATM.).
I think I mentioned about this in the other thread noted in my second post here.
Upshot is check the resistors for drift ,usually high and can easily upset DC bias conditions.
 This could make the hybrid A3A1 seem dead is no signal displayed with an input applied.
I had a 1741 with this exact fault, fixed the bias and bingo both channel's working again.
Of course if the hybrid amp is toast then a parts unit is the only solution. Hopefully its not toast !.

EDIT : check R1-R5 on the vertical O/P board A5 for incorrect values to start.
« Last Edit: May 17, 2021, 11:25:15 pm by lowimpedance »
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Offline Alex_Baker

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Re: HP 1740A Channel B not working and flickering on A
« Reply #38 on: May 19, 2021, 12:43:10 am »
Not sure if this is relevant or not to the flickering problem, but I found a solution to an intermittent issue on my scope by cleaning the gold contacts on the PCB rotary switches. They were covered in a layer of grease that over time hardened into a nasty tacky paste :P. I am not sure if this is true of your scope but it might be worth a look, a light rub with acetone and a cloth cleaned mine right up.  :-+
 

Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #39 on: May 21, 2021, 12:32:31 am »
It's been a while since my last reply... things got busy! I havent come up with much more since then... If I remove the C-clip that hold the two switch plates together, and then move the rear switch plate totally outwards such that no contact is made, there is very rapid flickering that is at the limit of what my eyes can perceive, so looks more like rapid dimming and brightening. As I move the rear plate inwards, I get a slightly thinner line, and a stable display, and then if I press it up against the board (where it should be), the usual flicker is back.

So I move the rear plate slightly outwards again, and by varying the pressure in different areas (in particular pulling the area opposite the contacts outwards), I get a stable display. If I then hold the switch in this position relative to the board and move the A8 PCB and the switch assembly jointly in one direction or another, the display remains stable. It therefore seems that the problem is not with a PCB component or with a trace fracture within the PCB, but rather with the switch itself.

Given that nudging the front plate in any way has no effect on the flickering, I suppose the issue is with the rear plate or the rear gold contacts. So I tried bending the little springs outwards just in case they weren't making good enough contact from having flattened over time. But that changed nothing. The assembly looks perfectly clean. But I am in the process of scrubbing it all with IPA and then I will lube it up and reinstall it. However, given I've already done this, I don't expect much.

It's hard for me to tell the condition of the gold contacts. They look nice and shiny and golden, but when holding the PCB at a certain angle to the light, I do get reflections akin to a grey/black 'mosaic'. And it does look like there are some circular scratches. Having not seen good condition contacts, I really do not know. But they're definitely not in particularly bad condition. I'll try reassembling after rubbing with some more IPA. But if it still doesn't solve the issue, I really don't know what to try next. Deoxit makes a product called Deoxit Gold which they claim chemically removes any oxidation on gold, so I may try getting some of that as a last resort.

Any other ideas?

And Alex, I'm a little wary of using acetone on a PCB, are you sure it's alright to use?
 

Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #40 on: May 21, 2021, 12:47:58 am »
Unfortunately its getting to the point where you will need access to another oscilloscope to monitor the waveforms as shown in the manual etc to see what happens to them when the issue occurs.


Its good to be wary of acetone, If you really had to then small amount on a cotton tip and apply carefully at the problem spot only. I have never needed to use it on any electronic (ie PCB ) assemblies myself, of course metal  mechanical stuff OK as required.
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Offline Alex_Baker

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Re: HP 1740A Channel B not working and flickering on A
« Reply #41 on: May 21, 2021, 12:58:53 am »
I used acetone because it's what I had, rubbing or isopropyl alcohol would probably be better, but for the 5-10 seconds that any acetone would be in contact with the PCB I doubt it would really be much of a problem. There is no silkscreen you need to worry about wiping off either (on the 1727 at least).
 

Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #42 on: May 22, 2021, 01:22:13 am »
Put it all back together, and it's still flickering. I'm out of ideas. I don't know anyone with an oscilloscope that I could borrow. If I manage to get one somehow, I'll post again. Thanks for the help so far!
 

Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #43 on: May 23, 2021, 04:08:20 am »
Well, looks like I'll need more help anyway. I was playing around some more with the switch assembly on A8, gently prying with a screwdriver, and with the 1V cal connected to channel B, in the hopes of finding, well, literally anything. That probably wasn't smart given a screwdriver is metal. But it was going fine, and then suddenly the trace disappeared. And now no matter what I do, I can't get a trace on the screen. There's nothing that shows except the green background. I don't know if it was my faut or just a badly-timed coincidence...

What should I try first?

Edit: I see no visible damage, and there were no sparks when it happened, so I'm leaning the coincidence route, but I don't actually know of course

Edit #2: I should've mentioned. I get no trace on either channel.
« Last Edit: May 23, 2021, 04:18:15 am by PurpleAmaranth »
 

Offline Alex_Baker

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Re: HP 1740A Channel B not working and flickering on A
« Reply #44 on: May 23, 2021, 07:53:46 pm »
check the +120V power rail, if the rail is above or below a certain threshold the HV is disabled, causing no image. If +120 is ok, then I don't know, just an easy thing to check.
 

Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #45 on: May 23, 2021, 11:29:58 pm »
 The 120V is not on A8 PCB, but easy to test anyway and while checking voltage rails also measure the +/-15V on A8 (C19 and 21 pos and neg respectively wrt TP3).
If all present then its quite possible one of the transistors has popped, use your DMM diode mode to check for junction shorts etc.
Symptom sounds like the main sweep trigger is not happening.
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Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #46 on: May 24, 2021, 01:20:43 am »
I tested the voltage rails. All are fine. I just checked the +/- 15V on A8 and those are fine too. So I started checking the transistors by following the instructions here: https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07 .
On Q12, I get: BE 0.75V, BC 0.75V, EB 2.657V, CB OL, CE OL
Q11: BE 0.755V, BC 0.753V, EB 2.776V, CB 1.258V, CE 1.04V

Q10: BE 0.701V, BC 0.678V, EB OL, CB 1.377V, CE 0.805V
Q9: BE 0.752V, BC 0.750V, EB 2.353V, CB 1.917V, CE 0.707V

Q3: BE 0.332V, BC 2.407V, EB 0.333V, CB 0.719V, CE 0.809V

I didn't test the others because they aren't labelled EBC, is the order of leads still EBC for all of them? Do any of those values look suspicious to you? They don't seem to correspond to my link but I suspect that's because they are still connected on the PCB.
 

Offline lowimpedance

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Re: HP 1740A Channel B not working and flickering on A
« Reply #47 on: May 24, 2021, 04:51:12 am »
 The values seem to be reasonable given there may be interconnections and other components causing odd readings, like other semiconductor junctions , low value resistors and largish capacitors which charge up when your probes are connected (usually noted as slowly drifting value change on DMM).
The other transistors may follow but Q7 does not !. Note the small 'dot' in the component side copper next to each transistor, that is the Emitter. Use that as a guide to work out what the others are by cross referencing to the schematic.
 Next steps would be to measure the voltages on the transistor leads and compare with the values noted on the schematic, investigate any major differences.
Double check the front panel button selections for auto trigger / main sweep etc. Does beam find button result in any bright dot/trace on screen ?, Is the RESET lamp on or off?.
In NORM mode the RESET lamp is on when waiting for a trigger event and during the trace sweep (when triggered) and will go off during the retrace and re arming of the trigger cct. Randomly rotating the TRIGGER level pot below the reset lamp will cause a trigger to happen and a sweep (RESET lamp should blink).
If you select NORM mode sweep and EXT. trigger in then hold a piece of bare wire to the EXT trigger input BNC centre pin you should be able to trigger a sweep.

BTW as you can still get the scale illumination (green background) the HV is OK. as this CRO uses a floodgun in the CRT for the even back ground glow as opposed to using a row of lamps above and/or below the front of the tube. Although in most normal situations that is generally turned off, so no green background.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #48 on: May 24, 2021, 08:20:07 pm »
Interesting about the Emitter corresponding to that dot, I didn't know that.
So I measured the rest, and here's what I'm getting:

Q8: BE 2.186V, BC 1.447V, EB 0.717V, CB 0.707V, CE 2.075V
Q7: BE 0.567V, BC 1.079V, EB 0.567V, CB 0.722V, CE 0.818V

Q6: BE 0.601V, BC 0.625V, EB 0.997V, CB 0.961V, CE 1.396V
Q5:

Q4: BE 2.031V, BC 1.446V, EB 0.718V, CB 0.710V, CE 1.899V
Q2: BE 1.922V, BC 0.534V, EB 0.674V, CB 0.532V, CE 1.977V

Q1: BE OL, BC OL, EB 0.724V, CB 0.717V, CE 1.901V

Q5 is apparently a JFET, which I’ve never heard of before, but looks like it’s similar to a MOSFET, I’ll have to read more into it. I tested that using the resistance setting, although I’m not sure it tells us much… But at least nothing seems shorted out.
Q5: GD & GS 6.86MΩ, DG & DS rapidly increasing in the MΩ range, DS & SD about 190Ω

Do any of the above values look odd?

Pressing BEAM FIND results in a bright dot on the screen.
RESET lamp is off.

Selecting NORM mode, the RESET lamp is still off, and rotating the TRIGGER level below it does not change anything, the RESET lamp stays off.

Selecting NORM and EXT, and putting a piece of bare wire into the EXT trigger input BNC centre pin changes nothing, RESET lamp stays off, and screen stays blank (only green background).

Good to know for the HV!

I’ll measure the transistor voltages and post again once I’ve got them.
 

Offline PurpleAmaranth

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Re: HP 1740A Channel B not working and flickering on A
« Reply #49 on: May 24, 2021, 09:41:53 pm »
Alright, so this took a little while, but here are the transistor voltages, with the voltages that they're supposed to be according to the schematics in brackets, I put ... when no voltage was indicated.

Q1: E 1.699V (1.7V), B 1.32V (1.4V), C 0V (0V)
Q2: E 1.699V (1.7V), B 0.98V (1V), C 0.89V (0.7V)

Q3: E 0.89V (0.7V), B 0V (0V), C 0.862V (1V)
Q4: E 7.58V (…),  B 14.15V (1V), C 0V (0V)

Q5: G 0.863V (1V), S 14.66V (…), D 2.256V (0.8V)
Q6: E 0V (0V), B 2.256V (0.8V), C 14.14V (1V)

Q7: E 14.17V (13.3V), B 13.35V (12.7V), C 14.14V (1V)
Q8: E 13.64V (1.5V), B 14.15V (1V), C 0V (0V)

Q9: E 12.92V (1V), B 13.63V (1.5V), C 12.90V (14.6V)
Q10: E 5.193V (3.5V), B 5.88V (0.5V), C 12.93V (14.6V)

Q11: E 11.64V (0.2V), B 12.31V (0V), C 14.82V (15V)
Q12: E 1.072V (0.8V), B1.477V (1.5V), C 5.193V (3.5V)

Q13: E -9.66V (-9.7V), B -9.13V (-9.2V), C 0.86V (1V)

As you can see, many of the transistors have very wrong values. Any ideas what could be wrong?
 


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