Tektronix 7613 blowing fuse immediately

[Stu]

My Tektronix 7613 is my favorite oscilloscope, because I feel most comfortable usinng it. I feel I know what it is doing, and nothing is hidden in some setup menu anywhere. If it has the option, the knob or switch is right in front of you!

I put my scope in storage (indoor, fully air conditioned) a few years ago. It was working perfectly fine when I put it away. What's a few years for a roughly fourty year old scope?? Apparently too much!

I dragged my old friend out of storage, plugged it in, and as soon as I press the power button it blows the fuse. It's blown two fuses so far.

I wonder, logically what would short out just sitting there? Only thing I can think of is a cracked winding on the transformer, which is scary because it has a gazillion different coils and is totally custom. (P/N 120-0708-00)

I measure only 2 Ohms resistance between the hot and neutral on the AC receptacle.

Not really sure where to begin, it's a bit overwhelming inside the scope. I see the power supply is kind of modular, so I was going to take it out, but there are wires going everywhere, and it looks like I'll have to take other parts of the scope apart to disconnect and remove the power supply.

I has hoping for some suggestions before my bench turns into hundreds of screws and parts everywhere:)

Thanks,

-Stu

[Toasty]

Welcome to the Forum!

Pull all the modules and try powering it.

If it still does it, it's time for surgery.  It might be something simple.  Have hope! 

Please update your profile with your country.

T

[edavid]

Pull all the modules and try powering it.

That is not going to make any difference, it's shorted on the primary side.  Got to be either the line filter or the transformer.

[voltz]

Hi,

Its a case of isolating the fault. So here are some steps you can follow.

If you are ok with working with mains voltages (caution here), place a mains lamp in series with the mains inlet. This will stop further fuse blowing and give you a visual if the current draw goes down to normal (dim lamp).

Try to disconnect all secondary voltages from the transformer. If this is on a connector, great, if not it may mean de-soldering them.
Does the high current draw persist? If so then it looks like the transformer primary is short or there may be something else shorting like a mains filter cap / line filter.

More likely to be a short at the power supply circuit itself after the secondary. Filtering capacitors are highly suspect at this age and can go short. Or it could be the rectifier diodes / bridge rectifier.

If you still have not located the fault from after those points, then it means going deeper, after the power supply.

[Toasty]

... More likely to be a short at the power supply circuit itself after the secondary. Filtering capacitors are highly suspect at this age and can go short. Or it could be the rectifier diodes / bridge rectifier. ...

My reasoning exactly. 

T

[edavid]

More likely to be a short at the power supply circuit itself after the secondary.

No, this does not fit the facts.  OP said:

I measure only 2 Ohms resistance between the hot and neutral on the AC receptacle.

The problem is on the primary side.

[Stu]

I took the power supply out. Cell phone cameras make things so mich easier! I'll definitely be depending on those photos later..  so many wires going so many places. Apparently I have some sort of optional BNC signal ports on the back and the wires run through the power supply frame to various places.

I measure half an ohm resistance across capacitor C813 as well as C814. Haven't desoldered them yet, so this is an in-circuit reading.

http://bama.edebris.com/download/tek/7613/Tektronix_7613_Service_Oct1974.pdf

These are two of the large filter caps.

[Toasty]

@edavid -

I thought he was saying he "normally" had 2 ohms at the power plug.  He did not say he had the switch on.

Granted, 2 ohms is waaay too low....

Looking at the schematic, I'll agree either (hopefully) there's a short in the wiring to the transformer, or (sadly) the transformer is shorted.

T

[Stu]

Ive gone ahead and pulled it all apart. I've desoldered C814 (9600MFD 30VDC) (32mm x 75mm) Mallory p/n 290-0506-00 or 235-8223Y
I have 1.6-2.0 ohms across it pulled and out of circuit. The service manual says the impediance should rise as the capacitor fills. Is this normal at empty, or is this cap toast?

I've also pulled C809 (1800MFD 75WVDC) and across the terminals the resistance starts off low in the single ohms and rises steadily to about 35M Ohm. This would indicate the cap is charging off the batteries in my multimeter (as expected, right?)

The resistance does NOT rise on C814 if I hold the probes to it for a while.

I'm thinking I need some capacitors...

Thoughts?

Thanks

-Stu

[Toasty]

Problem is on the line power input side of transformer, not the output .  Check the replies ^ ^ ^

Check from power connector to power switch to transformer.

T

[voltz]

If those caps are short, then yes they certainly need replacing. And it follows that if repeated tries to start the scope were carried out, its possible that the over current has shorted the transformer internally. So its likely to be a knock on effect, shorted caps leading to a  broken transformer. The diodes/rectifiers would have been driven hard too..

If you follow the procedure i mentioned, you will isolate the all the problems one by one. Its not just the transformer! 2 Ohms by the way is only the DC resistance of the primary. This is not the same as its impedance at 50/60 Hz which will be much higher in operation. So you can forget that value at AC, although yes it sounds too low just from practical experience.

If you work through it logically, isolating problems one at a time rather than just diving in and changing caps, you will get there without causing more damage and saving time in the long run.

Hope this helps.

[bd139]

Extensive 7603 power supply repair in here: http://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_7000_series_mainframe/rep-und-kal-7603/Reparaturbericht%20TEKTRONIX%207603.htm - worth having a read through for pitfalls.

Run it through Google translate.

[Cyberdragon]

I have a dead (no short though) 7603 I can measure the transformer from.

*measuring*

confirmed: a little over 2

[The Guy]

If the scope was working when you put it away AND if it was kept in a dry place it is really not likely that the transformer went bad.

I´m thinking probably a cap.... but what do I know... lol

 

[Toasty]

Can you disconnect the transformer from the cap board?  The try powering it and see before going forward with the caps replacement.

T

[David Hess]

I measured an input resistance at the IEC connector of my working 7603 of 1.6 ohms so I doubt there is a problem with the transformer.

If the capacitors are shorted which I think is the most likely explanation, then the associated rectifiers may be damaged as well and should be tested or just replaced.

[Armadillo]

I wonder why it takes so long and few episodes to troubleshoot this?

Hi Stu;

It is always a pleasure to know  online friends from afar in this lonely planet, so could you update which part of the earth you are from? I am sure your countrymen would be pleased to come to your aid.

You should unplug connector P890 and then turn on with a 100watts incandescent light bulb in series. If the light bulb extinguishes quickly, then the trouble is not the transformer. Then the troubleshooting of the other parts can be more systematic. 

[David Hess]

I wonder why it takes so long and few episodes to troubleshoot this?

It has only been 2 days and they are off on a wild goose chase making things slower.

[voltz]

I wonder why it takes so long and few episodes to troubleshoot this?

It has only been 2 days and they are off on a wild goose chase making things slower.

exactly. See my first post - i did try