Tek 2445A repair help

[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

I also noticed a buzzing or high speed clicking sound. Maybe an arcing sound?

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

[tautech]

Maybe a problem with HV side of things?

Quite likely.
The Tek pdf has plenty in it to guide you through faults in the CRT HV area.
Some study required. 

[christopher iles]

Hello,

When I was a TV engineer years ago, it was not unknown for arcing to occur around a dirty anode cap on a CRT. Put the 'scope on in a darkened room and look for arcing/sparking around the cap; it will be clear to see if that is the problem.

Should that be the case, removal of the cap and cleaning the cap and CRT normally effects a complete cure.

Just a thought, hope it's helpful.

Regards,

Chris.

[mij59]

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

Please check the +15V on J119 at start up and when the problem occurs.
It seems that the character generator and the triggering are affected,m must be some kind of power supply problem.

[okney1lz]

Alright, I've rechecked everything, with the right schematic(Almost 2465B) this time! And a better bench top meter.

Oh, and I almost forgot, a bunch of reading material.

It seems the -15 volt leg is dropping (or is it adding) to -14.5 Volts.

As the HV circuit turns this -15 to the proper A/C levels for the crt, I have to believe this is the root of the display issue and noise.

I'm scanning the schematics now to localize the source of the -15v, and trace back to it.

See, I just needed some prodding in the right direction, and not to get ahead of myself.

Will update over the next few days.

[okney1lz]

So, here goes. I need someone to check my math, so to speak.

From this:


When I touched pin 2 of the regulator with my meter, I got the buzzing to stop. I tried re-flowing the solder, but it didn't help. I know this regulator needs some load to work, so I figure my meter changed something enough for it to stabilize for a moment?

-5 volts is measuring -7.23, with a spec of -7.1V from low voltage board. It is also stable through the described malfunction.
-19.2 is dead on and stable.

C1331 is 0.1 uf 20% 50V ceramic
CR1330/32 are 400V 1A Rect. Diodes


I'm leaning towards the regulator or a diode. Your best guess?

Bad regulator(LM7912CT), supporting components, leaky diode, or U1207(LM324N) OpAmp?



Thanks,

Lawrence

[tautech]

When I touched pin 2 of the regulator with my meter,

Pin 2 is the -19.2 V supply, that doesn't make sense?

Have you checked the supporting resistors 1331-34 out of circuit? Lift one leg.

[okney1lz]

I know it doesn't make sense, but as they say on the internet, true story. lol

Resistors checked fine. Added in blue:


Suggestions for next area/component to test?

Thanks again for all the help!

Lawrence

[tautech]

The 7912 is configured as adjustable with the 324 controlling it.
Does the 7912 get warm/hot? this may indicate it is overloaded and the reason the -15 rail is low.
I suspect all is well here and the problem is elsewhere. But I have been wrong before. If you have the parts available, replace them to eliminate this part of the circuitry as a problem.

Study the schematic for any tantalums on the -15 rail.

As the HV circuit turns this -15 to the proper A/C levels for the crt, I have to believe this is the root of the display issue and noise.

Don't be too sure of that. HV stresses componentry much more and has more frequent failures.

Can you post the HV and CRT schematic so we don't need to download it.

[okney1lz]

So, I decided to test the diodes too. I got a iffy reading on cr1330, so I replaced it with a 1N4004 I had.

This made the scope stable for about 25 minutes. I ran all of the tests and exercises. Thought I was home free, however, I noticed something new, the trace seems to bend down left of the center graticule marker. It definitely wasn't doing that before. So either a new problem that I caused or something that was masked.

Then it started again with the buzzing and carrying on. So, I'm thinking there's still a problem somewhere. Argh!!! lol

I'll have to disassemble again to check the voltages and if heat is an issue. This will probably have to wait until next week though, I have a ton going on right now!

Here's a try at posting the schematics:






And a link to, hopefully, a larger/clearer pic: http://thefinezzagroup.com/2445A%20Info/CRT_HV_Schematic_002.JPG

Thanks for the input so far everyone!

Lawrence

[tautech]

C1951 looks suspiciously like a Tantalum, please confirm? At location B7 on the CRT HV schematic.
Also check & test C1971 100 uF. After the inductor on the -15 V rail.
Confirm voltages listed around T1970.

EDIT

the trace seems to bend down left of the center graticule marker

Is the trace a straight line? If so, Trace rotation adjustment required.