Holy smoke from TEK 465B graticule illumination driver

[SergeDr]

Some day ago I managed to smoke the driver transistor for the graticule illumination by messing around with the tray containing the bulbs underneath the CRT.
What I found out:
- The transistor Q4190 on the motherboard (A4) is quite exotic, according to the parts list it is an SPS3414 NPN.
- The bulbs are 6.3V 200mA T-3/4 types, 2 each in parallel. (assembly A10)
- The power rail that drives this circuit is at 9.25Vdc, quite odd. Or this a symptom of an underlying issue?
- The service manual does not show the full schematic.
- Scope seems to work normally after its mistreatment.

Can anyone help with:
1) the selection of a suitable NPN type? My gut feeling makes me believe that any power type with an Ic max of 1A and Vce max of 25V or above will do.
2) share some insights regarding this trivial part of the scope.

Info based on the download on the Tek site: 465B_SVC.pdf

Thanks,

Serge

[SergeDr]

It helps when one reads the manual somewhat earlier on the day.
Or posting invokes positive vibrations.
Found all info in the pdf.

Thanks to all who lent this post some attention.

Serge

[bdunham7]

Central Semiconductor to the rescue!  Here's your transistor:

https://www.mouser.com/ProductDetail/Central-Semiconductor/CEN-U45-PBFREE/?qs=%2Fha2pyFaduiF4sdxlcD%252BVLARIstVQ3wDtsgQDjBZZvtxV0SONArcRw%3D%3D

[SergeDr]

Great!

Thank you for this post, much appreciated.

How did you came accross? The type number is nowhere near SPS3414 from the parts list.

Best regards,

Serge

[bdunham7]

How did you came accross? The type number is nowhere near SPS3414 from the parts list.

A lot of test equipment was sold the the US and other governments and one of the conditions of those sales was that parts and service information would be available.  There is a system for parts where they are organized and given a number, called a NATO Stock Number or National Stock Number, abbreviated to NSN.  There are surplus part companies that buy up this stuff and if you just do a google search for a part under the HPAK/Tektronix/Fluke/etc proprietary part number, you'll get hits that include the NSN from NSN resellers.  Some of those will list a cross-reference of part numbers that have the same NSN--many of those 'proprietary' parts are just relabeled and not so special.  In this case SPS3414 gets you an NSN of 5961-01-005-9745.   If you dig a little, you will find this page:

https://www.nsncenter.com/NSN/5961-01-005-9745

And you can see that one of the cross-references is a more generic part number, MPS-U45.

MPS-U45 can then be cross-referenced in the usual manner.  It is available as an NOS on eBay and it crosses to NTE272.  However, I like to check Central Semiconductor as well because they are an excellent, reliable source for a lot of 'trailing edge devices in legacy packages'.   Their parts are available through regular, reputable suppliers so that you don't have to deal with 30-year-old or counterfeit parts and they don't gouge your eyes out like NSN-suppliers will.  NTE Parts Direct has the NTE272 for $36.34 USD.  Mouser has the CEN-U45 for $ 2.73.

As you go through this process, you do have to check the datasheets and make sure that there hasn't been an error somewhere in the chain, otherwise you may end up with a triac instead of a J-Fet or some other ridiculous result.      

[SergeDr]

Thank you again for this elaborate and well documented answer.

An eye opener to me.

I managed to salvage the lamp with the broken leg and ordered the transistor right away today.
The darlington I choose had a much smaller Hfe and had a TO-220 package.

The drop in replacement is a much more elegant solution to hide the damage caused by my - and  let's call it right away - clumsiness.

Best regards,

Serge

[james_s]

Looks like it's just an emitter follower with a Darlington transistor buffering the voltage out of a potentiometer divider. I would expect just about any NPN Darlington of sufficient current rating would work in that circuit.

[SergeDr]

Thank you for confirming my gut feeling.

Yes, indeed you're right, but I'm happy to have a drop in replacement.

And should one not threat this old dog with some respect in repairing him?

Best regards,

Serge