Beginner scope, old analog vs. digital handheld

[DennisCA]

Started opening it  up, it's not as easily accessible as I had hoped. The board which has the connectors is inside the silver casing to the bottom behind the front:


I am afraid that to get the silver casing out I willalso have to remove the PCB for the front control panel, and desolder these two wires, kind of look like they might have been desoldered before:

[DennisCA]

Here's the front control boar and the board with the BNC connectors.


The board with the BNC connectors is enclosed in a welded shut box. I have absolutely no idea on how to get it out. I'm stumped. Break the welds and force the box open perhaps. I can see nuts on the inside that I think are holding the BNC connectors and board to the front of the inside, so I think opening up the box is the ony way to go.

EDIT:

Check the AC/DC switch contacts - that signal could be capacitatively coupled through a layer of dirt

They looked clean as far as I could tell, the inside of the scope looks clean overall.

[DennisCA]

No more time last night for anything more so I went to sleep. This morning I started wondering.... Put the soldering iron to the welds and they melted. So looking a little bit better. Going to work now so we'll have to see later. Think I need to go into the city and get some deoxit and electronics cleaner as well. Might as well give all the pots a clean.

[james_s]

It's pretty common for shielding to be soldered over sensitive stuff like that, it would be highly unusual for it to be welded. Take your time and be careful not to damage anything. Remember that you have two identical channels so you should be able to make comparisons pretty easily. You should have a 1M input impedance, so follow that until it you find where it becomes open circuit.

[Wallace Gasiewicz]

One channel not working is quite common in my experience. The problem can be in input attenuators or in the first JFET, usually a dual FET
Usually if it is the attenuator (if your unit has one) it is usually just one of the attenuators and does not affect all ranges.
I do not have the schematics in front of me but these are the common problems usually because someone did not use a 10X probe and fed in too much voltage into DC.
If you can find the first JFET test it with as ohmeter and compare it to the other channel. If it is this problem, it is easy to fix.

Wally

[DennisCA]

Aaaahhhhhhh last night left me feeling so horrible...

I opened up my oscilloscope and I opened up the AT board enclosure. I found the error almost at once. An obviously broken solder to the BNC connector well that was a simple fix I thought! I resoldered it and reassembled and... whole scope was acting erractic and weird, I even lost the traces sometimes. Lost the ability to modulate the amplitude (v/div). Was feeling really down, thinking I broke the scope with ESD, I had it on a mat though and I kept touching an earthed metal piece now and then. I was pretty sure I'd gone and ruined something. Went to bed feeling pretty lousy.

Fortunately I got a tip from someone else and I started looking at the ribbon cables. Turns out one of them had broken but looked whole. I soldered in another cable for test and hey it works again! Now I am feeling really satisfied

Both channels work identically now, feeding a 3khz sinewave from my computers headphone socket:



I still need to properly replace that 3-ribbon cable however.

[Wallace Gasiewicz]

Great news...
And just think....you did not pay anything extra for the learning experience   HA!
Your tenacity is to be lauded..good repair.
Perhaps leaving the soldered in ribbon might be preferable. if it fits nicely. If this unit uses plug in ribbon connectors, they are not always easy to replace.

Now you can use the scope to repair other scopes, since you had so much fun with this one.

[james_s]

That's great news. This is also a great illustration of the warnings often given about old analog scopes, finding one that is truly fully working is easier said than done these days. It is also a good example of the sort of problems that often come up and how they can usually be repaired with a bit of perseverance. Now if it ever has any sort of problem again you will feel a lot more comfortable opening it up and troubleshooting.

[nigelwright7557]

I have 2 scopes.
An analogue scope I got off ebay second hand for £45.
A USB PC scope I got off ebay for less than £20.

For storage stuff I use the USB scope. I did a lot of DCC model railway work with that and it was great.

For general analogue work I use the other scope.
Had it 10 years now so its paid for itself many times over.
My max working speed is less than 20MHZ usually so this is fine.