Tek 2430 Works Fine for 15mins Then Freezes


John Morris
 

I've got a TEK 2430 that seems to work just fine for about 15 mins. Sometimes more. All tests PASS.
I've input a function generator and all different inputs look fine on both channels. Then after 15 mins and sometimes more, I'll look over at it and the screen is still showing a nice looking sine wave, for instance that I've input, but when I go to make a change, the scope is frozen with that image and no other controls seem to make a change.
Relays click when I make some control changes but the screen remains frozen with that sine wave image.
When I turn the scope off and wait 20-30 secs and then turn it back on, it will show the standard horizontal line but will not detect an input.
I can turn the scope completely off and after a few hours it will again come up and work just fine but eventually the same thing happens.
Scopes are not my expertise so if anyone has an idea where to start looking, I'd sure appreciate some guidance.
Thank you. John / K7RLD


 

Is the fan running? Is there a lot of dust on the heat sink fins?

On 3/20/2021 5:40 PM, John Morris wrote:
I've got a TEK 2430 that seems to work just fine for about 15 mins. Sometimes more. All tests PASS.
I've input a function generator and all different inputs look fine on both channels. Then after 15 mins and sometimes more, I'll look over at it and the screen is still showing a nice looking sine wave, for instance that I've input, but when I go to make a change, the scope is frozen with that image and no other controls seem to make a change.
Relays click when I make some control changes but the screen remains frozen with that sine wave image.
When I turn the scope off and wait 20-30 secs and then turn it back on, it will show the standard horizontal line but will not detect an input.
I can turn the scope completely off and after a few hours it will again come up and work just fine but eventually the same thing happens.
Scopes are not my expertise so if anyone has an idea where to start looking, I'd sure appreciate some guidance.
Thank you. John / K7RLD



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Siggi
 

On Tue, Mar 30, 2021 at 10:02 AM John Morris <k7rld@comcast.net> wrote:

Scopes are not my expertise so if anyone has an idea where to start
looking, I'd sure appreciate some guidance.
You'd generally start by looking at the power supplies which may simply
need recapping by now.


 

Welcome John!

You should begin at the beginning: open her up and do a visual inspection, you'd be surprised what will jump out at you, even if you're a novice (I assume that you are not actually a novice to electronics, based on the call sign, but even a novice to scopes can still spot some signs of trouble). Next, clean the insides as needed using a static safe brush or compressed air. Finally power it up and have a look at the power rails (as Siggi suggested).

Take pictures. Take notes. People here will be more than willing to assist you with further diagnosis and repair.

You might want to have a look at this document: https://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

Also, download this service manual from TekWiki: https://w140.com/tekwiki/wiki/2430

The TekWiki is a source for a great deal of information, both general and specific, about Tek scopes and other instruments. Here is the "reference material" page: https://w140.com/tekwiki/wiki/Reference_material
and here is the main page: https://w140.com/tekwiki/wiki/

Finally, some of the 2400-series scopes can be damaged if they are run for more than a few minutes with the case off. Some of the hybrids really need the forced air cooling, so if you are running it with the case off (to check the power supply rails, for instance) you should direct a fan to be blowing air over the main board with all the heat sinks. I'm not sure that the 2430 is one of the ones that can be damaged that way, but better safe than sorry.

-- Jeff Dutky


Steven Bender
 

Hi John,

I have a 7603 that is a mainframe scope which did something similar, 15 to 30 minutes then frozen. I also have some 2445’s that do need caps and/or possibly diodes.

The 7603 had 2N3055 or similar TO-3 pass transistors, due to decades of heat, solder on their B and E pins became faulty, needed solder removal and resoldering.

On a 24xx first look for silver cased 100 uF. / 25 vdc low volt caps, they are a known problem in the 2445, check the associated 3A shottkey diodes on "diode test". Look for any darkened “Burn Marks” on PS circuit board.

I have some non-tek units from the 1980’s which gave lots of yellow/orange drop tantalum bypass caps, across Op Amp voltage lines that are usually +/-15 volt or +/-18 volt lines and elsewhere - tantalums, that had impurities in their manufacture, which 2, 3, or 4 decades later will either glow, burn black, or smoke, damaging nearby trimmers, or other parts, typical and definite signs of a failure, partial short, or dead short. As such, for 16 volt, 20 volt, or 25 volt tantalum parts, replace with similar value 35 volt parts, usually cures it. While electrolytic have also improved over the decades, to change to an electrolytic, in general triple the value, and expect to replace again in twenty years :-).

Steven


John Morris
 

Thanks for the suggestions, Steven. I'll be diving back into it after this Easter weekend.

John


John Morris
 

Thanks, Jeff. All good suggestions which will be be implemented after the Easter weekend.
I will report back.

John


John Morris
 

Fan runs and no dust, Tom.


John Morris
 

Thanks for the primer on the TEK Scopes and how to make the best use of this fine group, Jeff.
I took the scope apart per the service manual and drilled down to the low voltage supply board. I saw where C244 had been changed and the replacement was down on its side to get it into the area supposedly. The service person had cut out the old 180uf 25V electrolytic in the control circuit, left the leads though, and replaced it with a rather hefty 50v at 220uf soldered to what was left of the old leads. I could see some distinct overheating on the board had occurred where the original electrolytic had been. (See attached pic) . . . well I don't see how to attach a jpg to the response. Sorry.
As I moved the replacement aside, one of its leads popped off one of the remaining old leads at its solder point. The remaining lead attach point had obviously had some heat stress as well as the attach point that came undone. The solder in the lead junctions was not shiny but rather mottled.
(Its obvious why the repair was done that way, to avoid having to remove the MAIN board to get to the underside of the low voltage board to repair it the right way.)
Now, one could imagine that one or both of those attachment points lost some conductivity when they heated up causing the freezing issue I'm having. But, why is it heating up that much? That is my next question.


Mark Vincent
 

John,

By what you said about the heating, without a photo, the jackleg tech had a cold solder joint. That sure looks like the main source of your freezing up when warmed. The lead(s) could have been arcing from a cold solder joint causing the heating. By soldering on top of the board that close to the underside pads, the heat made the solder joint poor on the underside where the traces are. It is best to change all the caps with long life high temperature low ESR ones. Something like Nichicon UHE would be a good type for the 100V and less and ULD or UCY for over 100V electrolytics. Ones like 1 to 4,7mfd electrolytics can be replaced with film, if you want to use this type. The cap values can be raised, e.g. 180mfd to 330mfd. Mr. Yachad might have a parts kit for the 2430. If your scope has the RIFA caps on the mains input, change them now! They will be a clear amber case. Most likely with cracks. Get at least 300VAC types. They will be the line filters, usually ,068mfd.

Mark


John Morris
 

Thanks, Mark.
Yes, the RIFA caps are there. Two of them. They'll get replaced.
Your heating theory of the cold solder joint is very plausible. I'll start replacing the electrolytics and see what happens.
Now its a matter of getting the low voltage board out or its bottom at least exposed.
BTW, how do I attach a jpg to a message?

John