Electronics > Repair

Tek 2445A repair help

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okney1lz:
Hi all,

I'm a lurker, but finally decided to get in the proverbial game.

I recently acquired a 2445A for a song.

I had to just about power wash the grime off of it, but it does power up and show a trace. I replaced the seized fan with new unit from Mouser that I had laying around. Part # AD0812HB-C73-LF
However, after about 2 minutes the trace and readout starts jittering a lot.

Here's a video of what I'm talking about:



I also noticed a buzzing or high speed clicking sound. Maybe an arcing sound?
I did the requisite voltage check at J119, and obviously there's a problem with the +15v going on.

Here are the results:







I used a couple references for ground in order to double check myself.


I have the 2445 and the 2465B manual. The board layouts are a bit different, but close. I'll keep looking for a 2445A specific manual.

Here is the 15v regulator circuit from the 2465B manual, a close approximation of what I'm dealing with.



Where to go from here?

I'll try to keep this on topic and post pictures to help others.

Thanks,

Lawrence

mij59:
Hi,

Looks the same as the in the service manual of the 2445a.
Check the voltages of U1260-317.

okney1lz:
So, I tested the voltage regulator, and it looks good to me.



So, still open to suggestions. :)

Here is another video, better showing the symptoms it's experiencing:



I don't think turning channel 2 on had anything to do with it, as it happens after a bit of time with no probes attached. Just coincidence. Also, it will sometimes clear up for a bit, then go crazy again.

Also, you can here the crt buzzing and carrying on a little better in this video.

Thanks for any input,

Lawrence

tautech:
Get the Tek scope troubleshooting pdf from this post:
https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/msg477909/#msg477909
There are some tips in it for troubleshooting the SMPS

--- Quote ---I also noticed a buzzing or high speed clicking sound. Maybe an arcing sound?
--- End quote ---
That symptom is described.  :)

Failing E-Caps?

The 317 readings, can you monitor them to see if they are stable?

Do you have another scope to monitor PSU rails for ripple and when it misbehaves?

okney1lz:
So,

Will read recommended repair manual tonight, so forgive anything that may overlap. Also, it's really annoying that a lot of these manuals are password protected, allowing print only. I have a great OCR program that would be awesome to make these searchable. Oh well

Have another scope on the way, but it's probably wasted too. lol I'm a glutton for punishment. I have 10 8640B's waiting in line, that need functional checks and gone through.

With the Vreg ruled out for the most part, it's output seems to be stable, I focused on where the +15 was coming from before it reaches the J119 header.
It looks like the DLY REF 1 headed from U2521 through U2620 (High Speed OpAmp).

This is the schematic I'm working with:



A quick check of supporting components in the path turned up a bad resistor, R2720. Bottom of the red circle. I replaced that, and things have improved. It still freaks out, just not as bad and it takes a bit longer. Even a blind squirrel gets a nut once in a while.

However, the sound is definitely coming from the back of the crt, so maybe all of this is moot, if that's garbage. Maybe a problem with HV side of things?

Again, any suggestions are welcome.

Lawrence

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