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HP 180C Oscilloscope repair attempt - no traces and hissing noise.

KanoTech

New Member
Hi all,

I noticed a wealth of knowledge and advice on these forums regarding oscilloscope repair, so I had to sign up!

I while back I purchased a HP 180C.
I have had it in storage a while and have finally pulled it out as a project to work on.

Unfortunately it seems that I have no beam at all, I can not get any trace to appear on the CRT.

I have already gone through some other forum threads on the topic of fault finding and repairing CRT based oscilloscopes.
The other threads have been a wealth of knowledge and I have proceeded to try and narrow down the issue following some other similar symptoms.

I already have the service manual downloaded from Keysight website.
I have gone through the low voltage checks, all rails; -100V, -12.6V, 15V and 100V are all fine.
I have a HV probe, so I have checked the HV test point and get the expected 3150V so the HV is functioning.
I have checked the 6.3VAC heater voltage and that also seems ok.
I have cleaned all the HV section and the relevant part on the A3 board.
I have removed the post accel suction cap cleaned underneath and put back in place.
I have removed and put back in place the CRT end connector and connections on the neck - in case there was a bad contact. (I have not cleaned them though).

My fear is that the issue is the CRT, however I want to make sure I have covered everything else off first.

I do hear a 'hissing' sound from the CRT when the HV is turned on, it sounds like a tyre inflating, but the sound is constantly there.
Searching for this symptom I read that it may be the HV finding another path to ground, could this be the case?

I have tried intensity and beam find, however nothing on the CRT at all.
Also I noticed that the CRT Scale knob has no effect, I can not get the scale to illuminate, could this be another hint to what the issue in the CRT may be?

I have also gone through and tested the gate amplifier circuit, Test point 1 which should show the blanking/unblanking waveform. I do get this waveform, at approx 30Vpp with intensity fully ccw.
Note that I am performing these tests without plugins, I have applied a 100kHz signal to the Z-axis input on the back for testing.

I seemed to have hit a wall in terms of my knowledge and I am not quite sure what to try next.

Any help, hints or guidance would be appreciated.

Thank you.
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
but the sound is constantly there.
Searching for this symptom I read that it may be the HV finding another path to ground, could this be the case?
That would be my guess. Could the pcb have absorbed moisture? Any carbonised path due to arcing?
 

KanoTech

New Member
Hi alec_t, Thank you!

Although the unit was in storage in a garage for a while (over a year), it has been in an room with reverse cycle air conditioning for at least 6 months.
So I'm not sure if moisture would still be present.
I'm uncertain about it's history before I purchased it.

I can't see any evidence of arching, the HV power supply assembly and gate amplifier board both look fairly clean for their age, just a thin layer of accumulated dust which I have already cleaned off.

I note that the low voltage power supply board A1A1 looks like it has had contact with moisture in the past, as some of the transistor cans look oxidised.

I will have a further look for evidence of arcing.
 

KanoTech

New Member
Apologies for the back to back posting, however I thought I better share my findings before anyone invests more time into replying.

Finding no further evidence of arching or component issues, I proceeded to remove the CRT from the unit for inspection.

At first all looked fine, and as I started wiping down the tube I could hear something rattling inside, looking closer I could see a few tiny pieces of broken glass sitting inside the tube :(

Looking closer at the end of the neck, I could see inside the rear connector the glass was cracked, exactly where the hose clamp tightens around the back end.
This is a disappointing find, however at least I know what the problem is.

I have attached a picture for reference.

I can only assume that the unit was mishandled at some stage, or someone had over-tightened the clamp.

I will be on the lookout for a replacement tube, my part number is 5083-3552.
Looking at the manual other options included all of the 5083-35xx series.

I found a mint condition 5083-1952 which I believe was for the HP 180A.
I would need to see if this would be compatible, otherwise I'll keep an eye out for the correct series.

If I ever find a replacement, I'll post my findings here - success or not.

Thanks again.
 

Attachments

gophert

Well-Known Member
Most Helpful Member
I thought I better share my findings before anyone invests more time into replying.
very polite and honorable of you to share your disappointing findings for the stated reason. Better luck On your next purchase.
 

unclejed613

Well-Known Member
Most Helpful Member
I do hear a 'hissing' sound from the CRT when the HV is turned on, it sounds like a tyre inflating, but the sound is constantly there.
near the electron gun there should be a patch of metallic coating on the inside of the glass. this coating is usually about the size of a large coin and is there to remove any residual oxygen after the tube is evacuated. if instead of shiny metal it looks white, flaky, or powdery, you have an air leak in the tube. if you run the scope in the dark, you may see blue, purple, or pink light inside the glass because the tube is arcing internally. the metallic coating is called the "getter". \EDIT: i just saw your post about the broken neck.
 

KanoTech

New Member
Thanks unclejed613,

Indeed the patch has turned white and looks powdery, so this confirms the air leak.
Thank you very much for this information, at least now I have an easy visual way to identify if the CRT vacuum is intact.
If I come across potential replacements I will check this first.

I wonder if it is possible to seal up the crack by doing a bit of glass blowing, then evacuate the tube again.
Could the 'getter' be used again to remove the residual oxygen after evacuation using a vacuum pump? Is an induction heater used for this purpose?

Again, this repair is more about learning new (old) skills.

Thanks again.
 

Boxnut

Member
I have seen a lot of this equipment fail being stored and not used. My Tek 465 is about the only piece that hasn't gone the same way.
Your arcing hiss could be an arc path on the circuit board you can't see, that or the CRT has gone gassy and needs replacing.
Easier finding a good pull out CRT than repairing one. Vendors recycling old parts are trying to get new prices so not cheap. Perhaps a DOA scope you can get cheaper and rob that for the CRT + parts prone to fail.
Consider this stuff is 30 years old or more, now obsolete and no longer supported. first class equipment if you can properly refurbish it.
My Wavetek 1061 is so bad from sitting, I am butchering and modifying it. Worked good 12 Years ago then stored.
Finally getting a replacement spectrum analyzer gone haywire just sitting. Between the two, should have a good piece and some good parts left over.
 

unclejed613

Well-Known Member
Most Helpful Member
Could the 'getter' be used again to remove the residual oxygen after evacuation using a vacuum pump? Is an induction heater used for this purpose?
no the same getter won't work. the metal used is barium or other reactive metal that oxidizes rapidly. they use an induction coil to flash the getter after the tube has been evacuated and sealed. there used to be TV shops that had equipment for rebuilding CRTs, but parts for an oscope tube are not something you will find. oscopes are precision instruments and the dimensioning tolerances are far more critical than in a TV tube. i'll link a video i found for a CRT rebuilder here in the USA that's still doing a lot of work. it shows the whole process and is fascinating to watch...
 

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