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Old 30th August 2008, 02:39 AM   #1
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Default Tektronix 475 PSU Troubleshooting..........

Hello All,

I am the proud ?????????????? owner of a 475 that just started to blow
line fuses on power-up. I have got the best schematics I can locate
on the internet but they leave everything to be desired so I am here
to ask for some assistance. Maybe I have been looking at the diagrams
too long but I am having a hell of a time getting access to the psu
board so I can test for resistances. Any suggestions as to where to
start? I already have the cover off and need to know where to go from
there.

Cheers,

David
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Old 30th August 2008, 12:31 PM   #2
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bump...................
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Old 30th August 2008, 02:22 PM   #3
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If it blows fuses on power up, look for the dead short after the transformer primary.

Fire it up with a variac, so the AC voltage starts at zero and stays low, measure the line current, and see how much of a dead short you have...

Chances are you are looking for a shorted electrolytic filter cap.
You can look for that with an ohmeter, no power applied, no schematic needed.

Beyond that, you'll need to look for the voltages being present on each supply line going out from the PSU.

There are some regulators in the circuit probably, you might have a shorted regulator, but that'd not likely make the fuses blow.

Sure you have the correct size (amperage) fuse installed?

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Old 30th August 2008, 06:11 PM   #4
ilimzn is offline ilimzn  Croatia
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The regulators in the 475 have foldback short circuit protection so a short downstream will just shut down the particular power rail. Look for a shorted cap, blown diode in one of the bridge rectifiers, etc. It might be possible there is a non-regulated power line (don't have the schematic handy unfortunately), look for any tantalum caps used as decouplers on it. The 475 is (unfortunately) full of them, and for the most part they are trouble free, but mine had a cap that would intermittently go short which drove me nuts - everything would check out cold, but when power was applied, a rail would be missing. If there is a tantalum in parallel with the unregulated power or where there is no short circuit protection, it may well blow the mains fuse.
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Old 31st August 2008, 12:45 AM   #5
eyoung is offline eyoung  United States
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I know you know this but, while you are poking around inside your scope there are some very high voltages some excess of 3000 volts, that being said I bought a scope on ebay that had similar ailment. It had a bad cap in the HV supply for the CRT, a $12 capacitor fixed it. good luck ...

Regards, Elwood
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Old 31st August 2008, 01:27 AM   #6
GK is offline GK  Australia
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Old 31st August 2008, 12:38 PM   #7
d3imlay is offline d3imlay  United States
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I had a 4 channel analog tek scope that sat for a few years. When I turned it on one day i just slammed it on. I knew better than that. The screen started to come up and then I heard a pop that sounded like an electrolytic venting. The power supplies were buried and I couldn't get to them without major disassembly. I ended up selling it for about $50.
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Old 31st August 2008, 12:44 PM   #8
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I did the resistance checks at all the PS test points and found the following with no power going to the 475:

+110 - Starts low and gradually climbs beyond the spec of 11k
+50 - 2.665k (Spec 2.7k)
+15 - 63.0 (Spec 63.0)
+5 - 45.2 (Spec 46.0)
-15 - 67.5k (Spec 480.0)
-8 - 34.2 (Spec 32.0)
+UNREG 50 - Starts low and climbs beyond the spec of 14k
+105/160 - Starts low and goes beyond the spec of 12k

Could someone please help me understand where to go from here.

Cheers
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Old 31st August 2008, 12:59 PM   #9
d3imlay is offline d3imlay  United States
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Look for shorted diodes and transistors. Shorted means less than 1 ohm between pins.
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Old 31st August 2008, 02:15 PM   #10
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Should I be looking for those shorts in the circuits that are not measuring within spec or should I be checking the supply circuits in a certain order? This is truely my first troubleshooting expedition so I am kind of lost but.............. I am willing to go the distance for success. Thanks for all the help guys.

Cheers
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