Author Topic: HP 141T CRT restoration  (Read 561 times)

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Offline cosenmarco

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HP 141T CRT restoration
« on: April 12, 2021, 08:33:38 am »
Hi folks,
I'm attempting to restore a HP 141T display unit to its glorious beauty and I took the CRT out.
Initially I thought it was broken because the socket in the back was rattling (why on earth they used such sockets... :( ?), some chips of material came out of the collet and it looked to me the getter inside the tube was white.
Then I cleaned it up and the milky looks of the getter was because of dust and looking down the collet I see that the glue they used is crumbling. So it seems to me the CRT could still work, so I'm proceeding with electrical checks and fixes of the unit so that I'm able to test the CRT itself.
There is also a ribbon cable for the storage meshes and the HT which started delaminating. The lines test ok for continuity but I feel the urge to do something to put it in a better shape.

Any suggestion for how to improve the situation?
Would you recommend to try to remove the socket and cleanup the glue remains or would you just recommend (if the CRT works) to leave it like that?
I could use silicone rubber for example to secure the socket...
What would you suggest to do in order to fix the ribbon cable?

EDIT Bonus question  ^-^ : Also all the foam in the receptacle of the CRT has all crumbled. With what could I attemt to replace? Rubber gasket?

« Last Edit: April 12, 2021, 08:38:46 am by cosenmarco »

Offline BrokenYugo

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Re: HP 141T CRT restoration
« Reply #1 on: April 14, 2021, 08:26:55 pm »
The loose tube base should be fixed, I've never done one but I'm sure the vintage TV restoration crowd has some preferred procedures.

Offline Smoky

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Re: HP 141T CRT restoration
« Reply #2 on: April 14, 2021, 11:15:11 pm »
I have a few vacuum tubes that I would like to fix too. They are 845's with loose bases.

I recall someone recommending Permatex Ultra Copper. It doesn't have metal in it and it doesn't conduct electricity:

You may want to experiment with it first and put it through some tests.

To release the socket pins from the tube wires, have one soldering iron applying heat on the sides of each pin and use a vacuum de-soldering tool to suck the solder out from the tip.

I never tried it but I will soon  :-+


Offline cosenmarco

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Re: HP 141T CRT restoration
« Reply #3 on: April 16, 2021, 12:52:15 pm »
Thanks Smoky for your reply.

I recall someone recommending Permatex Ultra Copper

Unfortunately I'm in Germany and this product is not available. I have two-component liquid silicone which hardens after mixing the two components in about 20 minutes. I was thinking it may act as a good shock absorber between the socket and the glass. Also Silicone in general can withstand high temperatures (this one up to 200C and it's non-conductive. This is it for all Germans following:

To release the socket pins from the tube wires, have one soldering iron applying heat on the sides of each pin and use a vacuum de-soldering tool to suck the solder out from the tip.
Ok thanks. I may take the socket out and clean it better... I'm just scared of screwing up something.

I never tried it but I will soon
Well let me know please how it turns out.

Offline cosenmarco

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Re: HP 141T CRT restoration
« Reply #4 on: April 29, 2021, 07:44:31 pm »
I was able to pull the socket by de-soldering the wires, sucking with the de-soldering station and additionally with my 3rd hand (joking... I have no 3rd hand) wiggling them so they don't bind to the side of the pins finally (not so) carefully wiggling the socket and pulling it out

I cleaned the crusty glue and then checked that everything was ok then I was... puzzled about how I could re-assembe it: the chances that every wire goes right into the little hole in every pin to slide the socket back in are qual basically to winning the national lottery so I soldered extension wires which I inserted first through every pin of the socket....

I thought this was a good idea and then this is where I started swearing because the two wire bonded together would often bind when the blob was about to enter the pin from the back side....
I also wanted to put something between the socket and the glass so I cut a couple of stripes of silicone (which I mixed and poured myself to a thin flat layer ca. 1/2mm thick). The re-fitting the socket was an odissey of wires disconnecting, re-soldering, wiggling, wires which would bend rather than enter the pins  :palm: ... in the end I was able (after intensive swearing and ca. 1 hour) to fit the socket back in place with the silicone between the socket and the glass, re-solder the wires and that side of the CRT was done.

Now to the flat cable. I removed it by de-soldering it from the lateral pins of the CRT and tried to understand what to do with the de-lamination.
I tried to clean it up with alcohol but the alcohol itself was causing further de-lamination so before completely de-laminating it,  I started re-gluing from the center using 2-component epoxy glue in the hope that it would provide enough isolation. After this little dance of glueing / cleaning / glueing I had a 80% re-glued flat cable so I decided I wanted to test it.
Before installing it back tested the isolation between the tracks using my UNI-T UT505B and at 1000V it was showing an isolation of ca. 500MOhm between the HV track and the next.... I decided it was time to test this CRT in the unit.
I reinstalled the flat cable and re-connected everything and started the unit with current-limited supply (bulb limiter with 2X40W lamps in parallel).

To my bad surprise I was hearing arcing and sure enough the problem was the flat cable, so I decided for another approach.
I had a piece of high voltage cable which belonged to a Hameg HM604 so I decided to replace the flat entirely. The two meshes are at low voltage so they got a couple of small wires and the HV cable I soldered directly onto the pin and stripped out some isolation to make it as flat as possible for the part which goes to the side of the CRT so that the CRT could fit in place. Picture is below.

I did put it back in place for a test and, to my surprise, the CRT started doing something!!
In "conv" mode nothing showed up but I could see it glowing green internally (is there a phosphor layer on one of the internal meshes???) while in "std" mode I could see an uniform glow (from the flood guns) which was intensity modulated by the "persistence" control. No trace was visible on the screen.
Anyway convinced that there was nothing else I could do to the CRT itself I put back the gum grommet, I wrapped the sides in 50mm Isolation tape and I installed it back in place. With the additional thickness of the tape and grommet the CRT was not fitting any longer for something like 1/2mm so I took the dremel and enalrged a bit the entrance  >:D It's anyway not visible once the counter frame is screwed in....
Ah, I also added on the top and bottom (internally) of the CRT metal shield a layer of 20mm large x 3mm thick isolation sponge-like rubber to replace the previous one that had crumbled.

Looots of work anyway.... being this a hobby I'd say no harm done but definitely not viable for a commercial restoration of the unit.... well now that I know how to do it, I would probably need 1/4 of the time but sill we're talking about 3 to 4 hours and then the hazards of handling the CRT itself in so many non-conventional ways and then still not knowing if the CRT works because I get no trace and the plug-is are also broken and I have to fix them... wish me luck.
« Last Edit: April 29, 2021, 07:46:34 pm by cosenmarco »
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Offline mag_therm

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Re: HP 141T CRT restoration
« Reply #5 on: Today at 06:18:56 pm »
I would like to add this older thread, in case details are of use to anybody else, as I've just finished a similar repair on the HP141T spectrum analyser.

Referring to the  CRT Safety Summary at front of "Operating and Service manual, HP141T Display Section",
the CRT and other parts I substituted are no longer available for HP141T  as far as I could see on internet.

I think substituting parts in vintage hobby equipment is OK, but I would not recommend to anyone to do the following repair -of soldering new cables onto a cr tube. Alternatives are finding a donor HP141T . Also the4552B IF section has bnc outputs for vertical and scan that could be adapted for an external oscilloscope etc.
Another option might be to convert the 141T back to non-storage using either its existing tube without the 6600 V accelerator grid (see notes below) or a another available non standard tube.

With cosen's advice in #4 above  I removed the tube (I think the original one from 1972) without pulling on the glass-to-plug adhesive.
After loosening the hose clamp, a small flat blade was used to pry the harness socket, going around the circumference unsticking the pins one by one "clicks"  until they were all sliding, then the socket was pulled off by prying with a bigger screwdriver to avoid tension on the plug.
The black bezel on the front panel has to be removed by pinching vertically and rotating downwards.
Then after a few other details...., the tube can be pulled out from the front.

The failure here was also flashover of the 6600 V post accelerator -wire(2) to  4.2 volt storage mesh -wire(6) and 158 V to collector mesh -wire(602) inside the original ribbon cable running to the front electrodes.
The ribbon had delaminated and the copper tracks were corroded and green.
I replaced this cable with cores of AWG 22 in separate heatshrink sleeves  running from the terminal, and soldered  onto the electrode pins and insulated with hot glue. -  covered the exposed tube with a heavy towel, wearing safety goggles and a mask.
Dielectric grease was added in the chassis connector so it squeezed out on re-clamping.

There were two other components failed on the boards:
a) 82 Volt regulator V1 on low voltage board ( Fig 8.10)
About a year ago the original neon regulator had failed and I converted V1 to a string of (5) 18 V 0.5 Watt zeners.
Now I am not sure if this failed ( one zener went open) and caused the 6600V to flashover in the delaminated cable or vice versa.
Anyway not liking the idea of using zeners string  (essentially they are avalanche diodes) I have converted the regulator again, this time to a 3 terminal high voltage regulator TL783C using R1 = 68 Ohm and R2 = 2 series 2200 Ohm RN70, with a 0.68 Uf 630 V input capacitor.
b) diode CR26 on the Pulse Circuit Fig 8.16.
The diode failed to "leaky when reverse biased" with the 158 Volt on collector mesh -wire 602, resulting in the storage mesh being pulled up to about 120V and no trace.
The diode tested OK on a meter. This "leaky" failure of CR26 has also been reported on another forum.
I replaced it with a 1N4007 rather than a fast recovery diode.

Some notes that may assist in fault finding associated with the tube:
The HP141T here can be run in non storage mode as follows:
unplug and tie  the yellow wire  feeding -2350V  to the tripler. That disables the tripler and the 6600 V supply to the crt
unlug and tie  the yellow wire feeding -2450V to the intensity pot. Now the grid will float to cathode gun , -2444 V via CR28 etc
Press "Conv, non storage" on the front panel.

The HP141T will start up with a trace intensity  not adjustable , but of sufficient brightness to be usable.
That gave me the incentive to continue poking around to pulse board to find the leaky CR26.

Here are some photos of tube removal etc

Presently, The HP141T is all reassembled and fully working on non storage, slow and fast storage modes with a sharp spot and good brightness range.
Wishing good luck and patience to those keeping these old things working!
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