Iwatsu SS-6122 Oscilloscope CRT

[wzk]

Hi all...this is my first post, but have been sporadically reading through the forums here and really appreciate the breadth of knowledge available here to those who thirst for it.

I have an Iwatsu Oscilloscope model # SS-6122 given to me by a kind guy not too far away from me in a "to-be-repaired" state last year and finally got around to it just a few weeks ago. There was no display, nothing at all on screen, while the fan seems to be running perfectly fine. I started troubleshooting and tearing it down, and eventually found that the CRT filament or heater (it's indirectly heated cathode) is open. So the CRT has to be replaced. I hate to just let it go, seems like a perfectly good scope for troubleshooting purposes, in the hope of finding CRT replacement one day. Nothing turn up on my searches other than used ones on Ebay which I did not really think would suit with what I had in mind - it's just does not make sense to buy a broken or semi functioning scope just to take the CRT and then I will be left with another set that would be up for repair again.

I ventured around and found a site that sells used and new CRT's mostly for Tek's in BC, Canada called Sphere Research and chanced upon their posting of a NOS, Kikusui COS 6100 Oscilloscope CRT. This Kikusui model is about the same specs as my Iwatsu. Since both scopes are produced from the same country, Japan, and maybe about the same era, I asked for pictures of it for comparison. Unfortunately, partial inspection of the external looks did not seem to point out that they are the same. My Iwatsu CRT had four pins sticking out on the side all connected somewhere on the boards (I am guessing these are for the horizontal and vertical deflections but not 100% sure) while the brand new Kikusui CRT only had two. Both CRT's uses Electrostatic deflection. The number of pins in the schematic are the same:14. My knowledge is limited and I could not really know for sure if these two CRT's are interchangeable, even if I have to slightly modify the interconnections, and hence I need the expertise of this forum. Here are more details and pictures:

Iwatsu SS-6122    : CRT p/n "SS 8212 B31" (this CRT is the same as that of Iwatsu model SS-5711 based on schematic) 
Kikusui COS 6100 : CRT p/n "150BE B31"

I've included schematics of the CRT section here and pictures of the brand new CRT for the Kikusui COS 6100. Hope anybody can help me. Thanks in advance for whatever information you can throw at me.
Let me know if you need more information.

Cheers...

-Wzk

[tautech]

Welcome to the forum.

Shame and you're best to keep a watch for a parts donor rather than trying to shoehorn another CRT into it.
It's very unlikely anything other than the correct one will produce reliable results.

Luckily you haven't had to shell out anything for it other than time thus far.
Hope you stumble on a cheap donor.

[wzk]

Thanks for your quick reply tautech, that is the other thing I am hoping for here, if there's anybody willing to donate me a CRT to resurrect my old analog scope.

[tautech]

Thanks for your quick reply tautech, that is the other thing I am hoping for here, if there's anybody willing to donate me a CRT to resurrect my old analog scope.

All I can suggest is that you be patient as you never know what some member or viewer has tucked away that they're unable to repair. Give it a week or two.
Good luck.

[scopeman]

Have you tried using a CRT tester that has the ability to weld the filament? Years ago I took a 20uF 450VDC electrolytic cap and charged it with a DC supply and then connected it while charged (disconnect the charging supply first!) across the filament pins and then rapped on the neck of the tube with a wooden pencil to vibrate the filament until the filament connected across the cap. The resulting arc welded the filament in place and I got a few years use out of a CRT from a HP1200A scope.

If you succeed you may want to consider putting a 100 milliohm resistor in series to limit the inrush current as the inrush current stresses the filament the most.

Sam
W3OHM

[wzk]

Hi scopeman,

I've never heard of such, but I've always wondered if there was a way to reconnect the filament inside that vacuum. Wow, that gives me another option to pursue! Would CRT rejuvenators have that feature also? I'll start looking into those devices, who knows one will pop up along the way that is affordable.

wzk

[Mr Nutts]

How about getting the old CRT repaired?

https://www.thomaselectronics.com/repair-overhaul

[wzk]

Mr Nutts, thanks for the suggestion, do you know how much would they charge typically for this type of repair?

[Mr Nutts]

Sorry I've no idea how much they charge.

[scopeman]

Before trying to weld the filament make sure that the problem is not in the contacts of the socket. Verify the filament connections at the pins of the CRT with an ohmmeter first.

I looked and my Sencore CR70 (one of the most advanced instruments for CRT work out there) and it can not weld a filament so I doubt that any others can.

However don't despair as I believe you can make a simple circuit that may work (see attached). I would use a heavy duty switch for S1 so that you don't weld the switch contacts!

Verify all connections and operation of the switch before connecting the actual CRT.

Please make sure that you have read all of the notes on the circuit before use and remember SAFETY FIRST!.

Watch the DMM voltage as the input resistance of the DMM used will slowly discharge the capacitor when the switch is in the weld position. So hopefully the part will weld quickly. If needed just flip the switch to charge to restore the charge on the capacitor.

If you do see the filament weld then use a low voltage power supply to supply the correct filament voltage to the pins per the data on the manual schematic and see if the filament lights and holds.

Good luck and let us know how you make out!

Sam
W3OHM

[wzk]

Hi there scopeman,

I went down to the pinouts on the tube glass itself, so yes, I'm sure the filaments are open.
You are right, there's nothing to lose as the filament is already dead anyway.
Don't worry, I have experience working with high voltages and will take the necessary precautions.
One thing I do not have is the face shield, so will have to get that beforehand.
It takes time to set-up properly, so I will not be able to do it within this week, but will get to it eventually.
Also the highest power supply I have is 200V at 200mA, so I have to assembly a rectifier / voltage doubler supply from the 120Vac mains to get to 400Vdc.
Any idea how many Amps I am aiming for?

Also I noticed, you used 82uF in the schematic, I remembered you said 20uF in your first post, can you clarify?
Thanks again!

-wzk

[scopeman]

Probably either cap would work. Might try 20uF first then 82uF. I would at least have safety glasses with side shields when handling the CRT. I don't believe that there is any risk of implosion when welding as even with 82uF and 400V that is only 32mJ so not a lot of energy there.

You just need enough current to charge the cap so a voltage doubler will get you close. I would use an isolation xfmr and a variac (remember variacs don't isolate) if you have one. with a 120V line you will have 2 X SQRT2 X 120V =339V probably close enough. 1A 600 or 1000PIV diodes like 1N4007's will be just fine.

Maybe you can find a ham close by that has what you need to hack this all together. If I was close by I would be more than willing to help as I know what it is like to have a great but otherwise non working instrument (been there too many times and done that!).

Best regards,

Sam
W3OHM

[wzk]

Hi scopeman,

Agree with you, 339V peak will likely get close to 400V across the caps. I plan to put everything in a plastic box, caps and switches inside.
The other wrinkle is I do have a variac but do not have an isolation transformer, so I still have to look for one.

The 20uF will get us to 8 mC (Q=CV) or 1.6 Joules (W=1/2QV or 1/2CV²)
The 80uF will get us to 32.8 mC or 6.56 Joules.

Thanks again for the reply; I will start gathering things together.

-wzk

[wzk]

Hi there scopeman and all,

I managed to assemble the contraption to charge the caps using 20uF and 12Ohm 5W resistor (only one I had on the bin).
Still trying to get the rest of the system ready. I think I'm going to put together a voltage tripler and just adjust the variac to get as close as possible to 400Vdc.
My biggest hurdle is finding an isolation transformer at the moment. I'll just keep plugging in...

See pic 

-wzk

[scopeman]

Good Morning,

I think that 339V will be close enough.

Sam