19K4900 HOT and Fuse Blowing

cwstrong

Member
Joined
May 21, 2008
Messages
110
Reaction score
0
Location
LONGWOOD, Florida
Prior to doing a cap kit, the chassis had a burn hole straight through the board at jumper, J19. I replaced the jumper and fired up the the monitor, to get only a small blue dot in the middle of the screen.

I went ahead and completed the cap kit, replaced the HOT and VR as well as the diodes at D501 - D504. Fire it up and fuse F501 is blowing, shorts the HOT and causing the ceramic resistor mounted above the VR (STR381) to get hot as hell.

I've checked for cold solder, bridges, everything I can think of, but no luck at determining what could be the problem. It's possible the flyback could be failing, but even that seems coincidental, since the fuse at F501 was not blown before the cap kit, etc.

I've followed Randy's flowchart but this problem seems to reside outside of it unless it is the flyback.

Any suggestions would be appreciated!

Thanks in advance!
 
did you put the insulator back between the H.O.T. , VR and the heat sink.

i would not suspect the fly just yet.

take some close up pics of the underside of the chassis and of the jumper work you redid.

there is a short in there somewhere.

Peace
Buffett
 
did you put the insulator back between the H.O.T. , VR and the heat sink.

i would not suspect the fly just yet.

take some close up pics of the underside of the chassis and of the jumper work you redid.

there is a short in there somewhere.

Peace
Buffett

Yes, the insulators were replaced for both the VR and HOT and tested for continuity before power up. All was good. Jumper fix and chassis images are attached.
 

Attachments

  • IMAG0115.jpg
    IMAG0115.jpg
    100.5 KB · Views: 21
  • IMAG0118.jpg
    IMAG0118.jpg
    109.3 KB · Views: 19
Last edited:
take a razor blade and clean the carbon off the board in the burned hole.
it can arc to the adjustment trace you have cleaned off beside it.

check the big brown/red/green poly caps in the power supply and in the horizontal circuit.

the poly in the degauss circuit will show a dead short in circuit so you will need to pull one leg to test it.

make sure they don't have a dead short between the legs.

and check the two diodes in the degauss circuit also.

Peace
Buffett
 
Checked the poly caps across the entire chassis as well as D505 and D506 and everything checks out. I also cleaned up the carbon at J19 for good measure. Same results.

FYI - I swapped out the "bad chassis" with a known good one to rule out anything with the tube, etc. The monitor came right up, so the culprit is definitely in the chassis.
 
Last edited:
ok there is a short or shorted part on there somewhere that's for certain.

check the 4 rectifier diodes again to be sure.

then check all the big resisters on the chassis to see if they are way out of spec or cracked.

then for a more drastic test you could swap the fly's on each chassis and see what the results are.

Peace
Buffett
 
Checked all ceramic resistors and all were within spec as well Q351. Double-checked again for cold solder cracks, etc. Everything looked good.

B+ is still reading 160v across the 130V line that leads to J33 and to the Q352 (HOT) C emitter. What could be causing the high B+ voltage? I'm definitely running out of ideas. :(
 
check R504 and see if its in spec or not.

please take a clear pic of your 4 diodes that you replaced in the rectifier circuit i want to see if they are installed correctly.

do you still have your old VR if so measure it and compare it against your new one to see if the match.

a bad VR will keep it in shutdown as well.

also do a continuity check from the center leg of R313 to pin 2 of the VR.

if that trace is not complete it will be in HV shutdown.

Peace
Buffett
 
Back
Top Bottom