Help needed with Tektronix 475 component identification - Page 1
Forum section: electronics / repair

author: Unknown
author_rank: Unknown
timestamp: November 28, 2016, 12:30:49 am
content: Hi all,acquired an old Tektronix 475 today which smoked when turned on.The source was a piece of toast, which was formerly a resistor... alongside a tantalum cap on the interface board, under the unit, (IMG_1).One thing that immediately stood out is how many electrolytics had been swapped for tantalums... ??The service manual is here..http://www.ko4bb.com/manuals/179.210.75.44/Tektronix_475_Oscilloscope_Service_Manual.pdfAnd similar (475A) schematics are here...http://www.tronola.com/Tektronix_475A_Oscilloscope_Schematics.pdfThe resistor in question connects to Q980 (img2) which is supposed to be an TO92 N channel J-Fet, but which actually had a TI 945 NPN in its place..The only problem is that I cannot find this resistor referenced in any part of the manual.It is in the Service Manual images (only without ref number - see img Resistor_1 -right under the 'J4') and it is not in the schematics (unless it is connected to DCPL3 - whatever that is?)Any help will be greatly appreciated.

author: Unknown
author_rank: Unknown
timestamp: November 28, 2016, 01:50:10 am
content: Perhaps someone who has a 475 would be so kind as to slide the case back far enough to read the resistor value?Perhaps?The board in question is below.Follow the orange spot just above the white connector strip in the upper-central area.

author: Unknown
author_rank: Unknown
timestamp: November 28, 2016, 02:19:28 am
content: Why would they leave out certain components?the 2.2µF 20v cap which was also cooked is also nowhere to be seen in the manual and without looking beyond than 3 inches further, there is another 2.2µF cap totally ignored... even though all the surrounding components get a mention!?

author: helius
author_rank: Super Contributor
timestamp: November 28, 2016, 02:22:14 am
content: Quote from: Gazucha on November 28, 2016, 01:50:10 amOK. It's100 Ohm, 5%.Sorry, it is 10 Ohm!The schematic situation is not always perfectly clear as there was more than one revision of 475 boards.It's also worth noting that Tek considers tantalums (correctly) to be a subtype of electrolytic capacitor.

author: Unknown
author_rank: Unknown
timestamp: November 28, 2016, 02:27:27 am
content: Fantastic!I did wonder if that was the value, as I knew the first band was brown and one of the images showed two further 'dark' bands before a gold, but I just couldn't get that tongue angle right....Thanks mate... Seriously appreciated!Now to get this babe back on her feet.p.s. I see the tantalum/electrolytic relation, just was under the impression that tantalums were more reliable but considered a fire risk under extreme conditions?Anyways... thanks again. Now just gotta find that JFet in Rio!

author: Unknown
author_rank: Unknown
timestamp: November 28, 2016, 02:33:49 am
content: No, hang on... it's a 1 ohm....Only joking.Thank you for helping. It'll come back to you...

author: tggzzz
author_rank: Super Contributor
timestamp: November 28, 2016, 10:50:46 am
content: Since you provided such a clear explanation of what you do and don't know, it is a pleasure to helpIt is R965 and C965.R965 is 10ohm 5% 0.25WC965 2.2uF 20V tant bead on a 15V rail; I would prefer that to be 25V, and the capacitance is not critical. If that's faulty then I would think of replacing similar caps, provided you can do that without damaging the board.That's in one of my 475 manuals that is stored on my machine with the name "tektronix-475A-dec84-SCOPE-INS-14678.pdf"; it may have been a different filename when I found it.There are several different types of tantalum, some extremely reliable, some less so. Epoxy tant beads are less reliable, especially when operated near their rated voltage (Tek 485s have 15V caps on a 13V rail, doh!)(And that's my post 212)

author: Unknown
author_rank: Unknown
timestamp: November 29, 2016, 12:55:26 am
content: Thanks Tggzzz,that helps.Not quite sure why it was so difficult to find the component details, but your input is well appreciated from this end regardless...

author: Unknown
author_rank: Unknown
timestamp: December 01, 2016, 01:25:44 am
content: Just to edit..

author: tautech
author_rank: Super Contributor
timestamp: December 01, 2016, 02:24:57 am
content: Quote from: Gazucha on December 01, 2016, 01:25:44 amUse the Modify button.