Tektronix 475 PSU Troubleshooting..........

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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
 
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? ;)

_-_-bear
 
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.
 
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
 
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.
 
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
 
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
 
Again, did you check that you have the correct size (amperage) fuse in the AC mains??

Do you not have a way to reduce the AC voltage going into the scope for the purpose of troubleshooting it?

How about a fixed transformer that produces say 5-12vac at the requisite current needed ( a few amps)??
:rolleyes:

Put that in series with your DVM measuring current and see what the draw on start up looks like. Unless it is a dead solid short, the fuse shouldn't blow... One with a few different taps, like 5-10-15 might be more useful than a single secondary, but any is better than none... you can measure voltage across a resistor to measure current too, and have some degree of current limiting depending on the value of resistor on hand (use a biggish power resistor).

If ur going to work on solid state gear with it, you'll probably want a "variac" (variable voltage AC auto transformer) anyhow, maybe now is a good time to get one from ebay or some surplus dealer, or maybe from someone here...

It would be much much easier to try to bring up the AC voltage from zero and look to see if the DC voltages rise equally... you'd be able to identify the area of trouble quickly that way. (at least it is very likely). But the fixed transformer will suffice.

So, now you can fire up your scope on very low voltage and look at the resulting unregulated voltages to see if they stay in proportion with each other. The low one is where the problem is... :D

_-_-bear
 
I have gone through (2) 1.5amp 250v fuses so far. This is the appropriate fuse listed in the manuals as well as the fuse holder. I will locate my variac (20amp) and get it up and running. Thank you for the help. You folks have been great!!! Standby for further elementary questions...................

Cheers
 
make sure those are slow blow fuses. charging caps create a current surge that will blow fast fuses of the same current rating.

actually, you are going to need to check the actual resistances within the power supply BEFORE the regulators. don't worry about those readings that are higher resistances than specified. you are looking for a short, not an open (at least not yet, anyway). there are a few components in a power supply that will cause a line fuse to pop instantly. 1> electrolytic caps shorted, 2> rectifier diodes shorted, 3> shorted regulator components, and 4> a shorted winding in the power transformer. if the primary side has MOVs for spike protection, one of these shorting can cause the fuse to go too. i would say that the most common failure would be a shorted diode, and caps next. caps with high ESR can cause the diode to overheat, or cause regulator components to overheat.
 
OK, just to verify,.....................

Can I SAFELY use my DMM in the ohms setting to check from post to post on the capacitors for a shorted one?

If I get infinity then it is good and if I get ANY continuity then it is shorted right?

PS....... I have my variac up and running now!!!!

I am just very concerned with the shock hazard involved cause this is my first troubleshooting exercise and I want to ensure a safe learning experience. But, I know I can do this, I just need a little guidance.

Cheers
 
It appears that C1412 is the first bad cap. I must remove the Trigger Generator and Z-Axis Logic Board to gain better access to the caps so while I am in there I will just replace all 6 of the large filter caps.................

The only bad part about this discovery is I am not able to confirm the visual inspection of the C1412 with a DMM because I lack the understanding of checking the caps and I am waiting for a better copy of the PSU schematic to get here. So...................it makes it difficult for me to understand which solder joints on the back of the main board to check for continuity. I see where the six filter caps are located on the front and the back of the board but the Trigger Generator and Z-Axis Logic Board is in the way of me actually seeing the pin outs for the caps and where they meet on the board. There seems to be 3 solder joints near every one of the labels on the back of the board for the caps and I don't know which to check. This combined with my inexperience causes me to wonder..................I am getting some pics so I can help tell the story. Standby...............

Any suggestions?

Cheers
 
C1412 was actually opened up on the top like it blew up. Kind of

yea, that'll be the one :clown:

its not so un-common, they overheat and boil the electrolyte, on a good day the cap vents (through a safety device) on a bad day it bursts ...

it a good tip to wear safety googles when working close up on live PSU's - anyone whos had a big cap explode near their face will agree. watch out for the little ones as well, especially tants :smash:
 
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