Electronics > Repair

Tektronix 475A power issue

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705jas:
HI

Thanks for the suggestions.  I have lifted and tested just about everything I can think of and the only thing I seem to have confirmed, by lifting the leg off the Collector of Q1456 is that the problem seems to be only on the 5v Unregulated line.  What I can't see is where that line goes?

cheers..Allan

Chris56000:
HiI

The wassock that uploaded that grotty 475A manual to the TekWiki couldn't be bothered to scan the layout diagrams for the PCBs as well as the schematics either, and repairing Tektronix gear without proper and complete documents is very much a hit–and–miss affair!

I would pay particular attention to where Q1456 and Q1482 are mounted to make sure these two transistors  are not missing any insulating bushes or mica washers underneath them, otherwise the "+5V UNREG" line, (marked "+8.4V" on the diagrams I can find) will be directly shorted out, which would soon cook up and damage the bridge rectifier!

Another possibility is the wiring between C1452 and the collector of Q1482 – is it shorting to the metalwork anywhere?

It is unlikely that the scale illumination control has a direct short to earth in it as there's a limiting resistor feeding it, and in any event, this comes off the +15 V line, so a short to earth in the scale illumination control wouldn't cook up the bridge rectifier!

Chris Williams

Gyro:

--- Quote from: Chris56000 on June 08, 2023, 01:46:42 am ---The wassock that uploaded that grotty 475A manual to the TekWiki couldn't be bothered to scan the layout diagrams for the PCBs as well as the schematics either, and repairing Tektronix gear without proper and complete documents is very much a hit–and–miss affair!
...

--- End quote ---

The schematics are on the TekWiki page (as Reply #6)...  https://w140.com/tekwiki/images/f/f1/Tek_475a_schematics.pdf  Granted, no layouts.

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