Tektronix 475 Troubleshooting 151 0367 00 [MPSH05] (Guaranteed Bad Parts)

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In troubleshooting the 475A I have run across a part that should be considered suspect until proven good. The 151-0367-00 These transistors have developed a weird failure mode This video goes a little deeper in to the failure
** Repair exposes the Tech to mains electric and should only be done by trained personal. Errors can be fatal. **

[@Sloxx701]

I've restored/rebuilt a few Tek scopes (TDS 540, TDS 420A, 2225) but I will say that the basket case 475 that I rebuilt was an exercise in pain tolerance. I still have the 475 and it 100% works now and I love it, but I swear these scopes are absolute maintenance whores. I am relatively new to the hobby. I've watched a few of your videos and your channel is great and you are way more knowledgeable than I am about this stuff. Thank you for sharing that knowledge with everyone here.

[@apollorobb]

i do a bunch of vintage audio repair and the black leg syndrome is a big deal with a bunch of the small signal transistors in them and it wreaks havoc in them .

[@southerner66]

Very interesting. I was given a 475 scope in apparently pristine condition, but it will not trigger. I also have an old Heathkit Transistor Tester that does leakage testing. Transistor leakage tests have saved me a lot of trouble on several occasions, including on an old combo organ I was given because everyone else gave up on trying to fix it.

[@ZenwizardStudios]

After looking in to this further these parts are not at fault. The diode is there due to the speed of the part. if the scope is not triggering I would try tuning up the trigger section before anything else. Tunnel diodes are none for a small amount of drift it might be as simple as needed a calibration touch up.. The transisters in the trigger section of a 475 are at an fT of 1.2Ghz.

[@southerner66]

@@ZenwizardStudios Thanks for the update. I had put this scope on the back burner because I had heard that there were parts in it -- and I think they may have been the tunnel diodes -- that were unobtainable as repair parts. But perhaps I'll go through the functionality checks you outline, along with the calibration/alignment and see if that will get it working.

[@ZenwizardStudios]

There ARE tunnel diodes in the scope. The are the main active switching element in the trigger. The good news is in the 4xx series scope they are not known for being problematic. In most of the scopes that I have looked at that appeared to have serious issues 90% of the time it was power supply related. Usually a missing or low voltage rail. One of the 4xx scopes I have fixed was not sweeping at all. It turned out that the -8 Volt rail was missing on the sweep / timing board. There are some extremely exotic capacitors on the timing and sweep board that should NOT be changed these were made in how by Tek for Tek and replacement would be multiple $100’s the good news here is these really never fail and are always good (I have never had to replace any yet) They are the main timing capacitors for the sweep speed and are of any extremely high quality. And they were all a matched set to get the sweep speed to work correctly.

[@potang008]

What is the replacement transistor you ended up using?

[@ZenwizardStudios]

KSP10BU. Pin out is different so lean bending needs to happen. These were also not just used in the 475. And should be checked and replaced no matter where they are found.

[@potang008]

@@ZenwizardStudios Is there a specification for the original transistor SKA6516? I can't find it anywhere on the internet.

[@davesdigitaldomain]

I just checked these transistors in my 475 and they're all coming up with the same symbol in my pro atlas transistor tester are you sure these are leaky? Seems strange that every single one is leaky I would expect to find at least one that's ok. what was the problem with your 475A before changing the transistors?

[@ZenwizardStudios]

I am about 98% sure unfortunately. The trade name of the transistor is the MPSH05. From all the information I have been able to gather the reverse protection diode should NOT be there. It would appear this is a process issue that has cropped up over time. The newest batch of the transistors do not show the revise protection diode at least for what I have been told. depending on where the transistor is in circuit it might effect the circuit it might not as this is a emitter -> collector leak.

[@swp466]

I just came across this myself and will be swapping mine out just on principal (cheap insurance). According to the 475 service manual, this part number is not dependent on serial number, except for Q688, Q698 which start at S/N B010162. Incidentally, I believe the list on the video had a bit of dyslexia in one of the transistor numbers. On the video list is Q689, but on the parts list in the service manual, there is no Q689 (but there is a Q698, which is listed as 151-0367-00). I am suspecting that the list in the video is wrong since all the transistors in the service manual have strictly even numbered parts. Also, in the service manual, there are more of this transistor than you show on your list. Q390, Q420 (for a small range of S/N), Q1124, Q1134, and Q1184 (for a small range of S/N) are also the suspect part.

[@daikonsparks118]

same here, all 151-0367 were damage. No Q689 -> Yes Q698. On the other hand, some transistors 151-0434-00 have the same issue, just a few, in my case Q562 and Q188. Great video, very helpfull

[@csx5945]

are these leaky transistors across all 475 serial numbers?

[@ZenwizardStudios]

I am not sure if it is across all 475 serial numbers however this Tek part number 151-0637-00 was used in more than just the 475's and should be check in all applications. The Early ones are guaranteed faulty. The later production transistors MIGHT be faulty but should be checked.