Scope repair (again)

[zachtheterrible]

Hi everybody, I'm taking a second crack at repairing my tektronix 475A scope, especially after I burnt the front of my other scope to bits while welding next to it one day. Yeah i'm an idiot, covered it up with a towel so there wouldn't be any tiny little burn marks, but instead the towel caught on fire and the rest is history

Anyway, I replaced all the filtering caps because the scope was originally having problems with a display that was obviously being affected by the mains hum. You could even see it in ground mode. But after I replaced them, I couldn't see anything on the display.

So I turn it on today and hear a loud "SNAP!!" I immediately unplug it and look for (and smell for) any damaged parts, but couldn't find anything. So I plug it back in and this time there is no snap or anything. Then I turn it off and busy myself with something else for 2 hours before coming back to it. I turn it on and am greeted once again by the loud "SNAP!!" I'm hoping this might give someone a clue as to what might be wrong with it. There are also three lights on the front which indicate UNCAL, they are all lit up.

I also bought the repair manual off ebay for a mere 2 dollars.

 

[kchriste]

The obvious thing to check is that the new caps are in with the correct polarity and voltage ratings. Most likely the problem is in the highvoltage circuitry because the backlighting/indicators work.

 

[zachtheterrible]

All the caps are fine. That was the first thing that I checked.

When I put in the new caps a couple months ago, I didn't put them in right. They are regular through-hole caps, and the board is double sided. The original caps were connected to both sides of the board by a big bead of solder going through the same hole as the original caps. When I soldered in the new caps, I didn't connect that bead of solder. Today though, I realized what had happened and I soldered both sides. Is there any chance that the caps not being connected could have damaged anything? I wouldn't think so but i'm not sure.

I would imagine that if they had, I would've heard that loud snap sound when they were soldered in originally, but I didn't.

 

[HiTech]

I would suspect the high voltage supply or it's associated wires. Make sure they haven't rubbed off some insulation near the chassis, etc. Look for signs of arcing. Remove the housing, dim or turn off room lights and watch for any corona action near the HV supply and wires. BTW, what is a Tek o'scope doing near a welder? Shame on you! That's like storing a FN Browning rifle in a corner of a damp, mildew saturated basement! Now if you had an EICO or B&K o'scope, I'd place it in series with the welder's electrode clamp and set it to 250 amps!!!!

 

[zachtheterrible]

The Tektronix scope is the one that I'm trying to fix. My old scope (the one that got burnt) was actually a B&K, so it sounds like that would be fine with you!

 

[zevon8]

The three "uncal" lights just mean that the timebase and the two voltage per division little red knobs are not at their "zero" or calibrated positon. There is a centre red knob on all three dials that lets you vary either the timebase or the voltage per division ( depending on the knob in question) from the marked value for fine adjustment. If you turn the small red centre knob it will click to a home ( calibrated ) position, this is where it normally should be, so that the dial markings are what you can make youre readings based on.

The load snap is likely a high voltage discharge, something might have gotten dislodged when you worked on the capacitors. Maybe the high voltage regulation is no longer working.

I assume from your mention of a welder that you have been doing some metal fab work. I hope you didn't let metal dust get all over the inside of the Tek scope. Any grinding or sanding and there would be very fine metal dust EVERYWHERE in the room/garage. Obviously this would be conductive, and generally a disaster inside any piece of electronic gear.