Tektronix 475A Trigger Transistor Testing 151 0367 00

A huge thank you again for all that continue to Join us in the lab for repairs and alignments. The channel would not exist with out all of you so thank you again for dropping by.
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This Video was in response to some discussion on the TekScopes mailing list. It centers around the 151-0367-00 Part number and questions weather these parts should be replace on site components. In the end mysteries are unraveled and some questions were answered but as with all good challenges some questions still remain.
** Repair exposes the Tech to mains electric and should only be done by trained personnel. Errors can be fatal. **

[@oturgator]

This is pure gold! Thanks

[@Ozymandiuus]

I've got a couple of 485s that will reluctantly but nevertheless dutifully trigger and display up to ~1gHz. Of course I need to feed it a very clean and strong signal, and the vertical is at its edge as well. I don't know off hand what the transistors are in the trigger or vertical, but I'd guess they are a higher frequency than those in the 475s, or at least of a variety that was more stable up in the thin air of these higher frequencies. Thanks for the very interesting video. Just what this vintage Tek enthusiast needed to finish a rough day.

[@ZenwizardStudios]

In the 485 Tek switched from discrete parts to custom Tektronix IC's that were made in house . These chips were a trade secret for a LONG time and are actually the same in the vertical amplifier, and were used in LOTS of places especially in the 7000 series frames and plugins.

[@southerner66]

Do the candidates as replacement transistors for this part exhibit the diode behavior you're finding in the original parts? That might be a good test to show or discuss.

[@ZenwizardStudios]

At the moment I do not think that these are bad parts as they are 1.4 Ghz components the reverse breakdown voltage of the high fT parts is quite low.

[@hughgilbert390]

The diode indication could be emitter-base leakage or breakdown. A 2N3904 has an emitter-base breakdown around 7V but many rf transistor have a much lower breakdown.

[@ZenwizardStudios]

This would be emitter-collector breakdown around 0.7 Vdc.

[@jspencerg]

The dca75 will output curve traces when used with its pc software. The currents and voltages are limited, but one can specify different parameters to compare. You might also run it as a diode to examine that behavior. The current changes when you touch transistor because the electron Fermi level increases with increasing temperature. You show the faster 475A triggered faster than the 475, no surprise. What frequency response do you expect the 200MHz 475 to have?

[@ZenwizardStudios]

At least on this one 475 (200 Mhz) the vertical amp and the tube brightness ran out of gas before the trigger did, even with the replaced trigger transistors. I could get the scope to trigger even after my eyes did not see anything on the screen. The camera is more sensitive to the beam so I got a little further looking through the lens. But the intensity on the 475 was cranked all the way to max brightness.

[@simonlawson2236]

Surely if the scope calibrates OK with the original transistors then let them be. I think you have demonstrated very well they seem upto the task.

[@ZenwizardStudios]

That is where I am at. Some of the units I work on are not mine and I need to do as instructed by the customer. My current recommendation is let them be but if the trigger is acting weird something to check on the short list.

[@andymouse]

Maybe talk to the guys at Peak they might know if such a device exists or this is an un usual failure mode? I haven't seen that symbol with a diode from emitter to collector before and I would have thought the TEK forums or perhaps EEVblog would be a good place to start. Interesting subject...cheers.

[@ZenwizardStudios]

There has been a LOT of discussion on the telescopes group about this topic. This video was made because of the findings during that discussion. There ARE parts where the reverse diode is present and also expected. It can be sometimes called a suppression diode H sweep circuits in televisions are known for having the suppression diode in the sweeping element.

[@andymouse]

Fascinating ! do these parts have a name or perhaps a part number or a supplier where I could read about or even buy one please ? are they only small signal stuff in the TO92 cases please ?....Thanks!!@@ZenwizardStudios

[@ZenwizardStudios]

I will see if I can dig some up. One use for them other than H sweep circuits is in relay coil drivers. They short cut the free wheel diode and just made it internal. I have seen some of them before.

[@andymouse]

Awesome thanks !@@ZenwizardStudios

[@ZenwizardStudios]

A modern part that does have the E->C diode in it that should be there is a Darlington transistor SOT223 package part number BSP52 as an example. This is still a multi node part though.

[@BerislavBradarić]

Hello Zen Maybe my impression is wrong, but the left oscilloscope seems to handle the trigger better, and you have less trouble with it. Done thoroughly and precisely as always, there is always something to learn from your videos. greeting

[@ZenwizardStudios]

This is not wrong but there is a few things going on there. The scope on the left is a much newer serial number 475A, 250Mhz of bandwidth. The 475 NON A is only a 200Mhz scope and it is also MUCH older production. Given the way the tubes are focusing I would also say much higher hour. The vertical amp on the scope on the right will be inferior to the one on the left based solely on V bandwidth. This will cause the trigger at a given frequency to have to work harder on the scope on the left. The actual trigger detector is the tunnel diodes and in both units there are unchanged and original equipment. Zen