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577 Bad CRT?


DW
 

Thanks for useful and interesting information, this will give me some repair options if needed, for now I am looking into a high voltage probe before I proceed any further.


Tom Lee
 

There could be several causes to the flashing. Sorry if I am repeating some advice you've gotten, as I haven't the patience to read the entire thread.

Grid-cathode shorts are not an uncommon problem, and intermittent ones can cause intermittent brightening. These shorts can be external as well as internal. Check the area around the crt pins on the little board that holds the socket (if there is one), and along the entire path that G1 takes, from CRT back to the intensity control circuitry. Clean up any carbon tracks you might find, and touch up any solder joints that look suspicious.

If, after extensive troubleshooting, you become absolutely certain that the crt itself is to blame, you have nothing to lose, so you can attempt a couple of fixes. The first is purely mechanical. Orient the unit so that the crt faces downward. Then gently, with a well insulated tool, power up the unit and tap on the rear of the crt. With luck you may be able to dislodge the fleck of cathode material that may be the cause of the problem. The reason for the face-down position is to make sure that the fleck, if there is one, falls out of the way. The grid-to-cathode spacing is extremely small, perhaps a mm or two, I would guess.

If tapping has no effect, then you go to Danger, Will Robinson mode. Details are here: https://www.repairfaq.org/samnew/tvfaq/tvrescuesc.htm

In my TV service days, I was the designated DWR tech (kids were expendable; boss was not). I rescued many CRTs this way, both back then, and afterward (most recently an ancient Mac Plus). There's a risk of killing the CRT, so you don't do this until you've ruled out everything else.

Good luck. That curve tracer is a beauty, and well worth repairing!

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 1/17/2021 20:50, DW wrote:
Tam

I am crossing my fingers that hopefully the CRT is okay but in the case that it isn't I have another broken 577 which I think I can salvage a CRT from but in any case your offer is appreciated.




DW
 

Tam

I am crossing my fingers that hopefully the CRT is okay but in the case that it isn't I have another broken 577 which I think I can salvage a CRT from but in any case your offer is appreciated.


Tam Hanna
 

If the CRT has croaked, do NOT scrap the unit.


If my health allows, I will have something for you shortly.


Tam

--
With best regards
Tam HANNA

Enjoy electronics? Join 15k7 other followers by visiting the Crazy Electronics Lab at https://www.instagram.com/tam.hanna/


DW
 

I actually included a short compressed GIF demonstrating the problem, notice the pattern of the trace with a transistor under test how each segment briefly flashes with a higher intensity than other segments of the trace. I turned intensity fully counterclockwise where the display went completely black, oddly the flashing segments of the trace were briefly visible before immediately disappearing to darkness.

Also I probed R1218 as shown in the schematic image as you suggested in hopes I would see any occurring spikes, I only noticed a constant DC voltage (no spikes) which would vary depending on the intensity knob setting so I guess the intensity circuit is good all the way up to that point.

The final path to check is before C1242, a 0.01uF capacitor which is holding back 3000V, I will definitely need a high voltage probe here

https://groups.io/g/TekScopes/album?id=259525&p=Name,,,20,0,0,0

Thanks for the reply


Harvey White
 

Just suspicions, but.....

If the flashes are aperiodic, with no relationship to anything, then I'd suspect something is breaking down.  Sadly, you'd need an HV probe and a scope to see what voltages are peaking.  I'd suspect that whatever it is reduces the grid-cathode bias to zero (or even positive, so no idea).

If they are periodic, then you can try to find out what is causing the problem (first half of curve, for instance, but no idea how that could happen), then track it down from there.

Harvey

On 1/17/2021 7:37 PM, DW wrote:
I have a interesting issue with a 577 D1 storage CRT where random portions of the trace will flash brightly at random times on the screen. I tried turning the brightness to minimum where the trace disappears but the flashing still occurs where the trace is located. Is this a symptom of a CRT going bad? Thanks





DW
 

I have a interesting issue with a 577 D1 storage CRT where random portions of the trace will flash brightly at random times on the screen. I tried turning the brightness to minimum where the trace disappears but the flashing still occurs where the trace is located. Is this a symptom of a CRT going bad? Thanks