Author Topic: Learning CRT repair.. black screen, no raster, but sound?  (Read 1075 times)

0 Members and 1 Guest are viewing this topic.

flgliderpilot

  • Jr. Member
  • **
  • Posts: 11
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #15 on: February 10, 2018,11:17:52 »
hi have known the 12v supply to be bad for this problem, try hot air on this transistor to see if voltage fuctuates
regards
astracat

Ok.  Thanks for the help everyone, I'll keep digging.  Sounds like the flyback is probably not the main problem then.

But the high pitched squeal, while not very loud, is a sign that the flyback is on it's last legs right?

« Last Edit: February 10, 2018,17:29:11 by flgliderpilot »

Mulja tv repair portal

Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #15 on: February 10, 2018,11:17:52 »

flgliderpilot

  • Jr. Member
  • **
  • Posts: 11
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #16 on: February 11, 2018,15:09:28 »
Back to the TDA3505 again.

I followed the composite signal on my scope all the way to the TDA3505.

I also checked sand castle going into the TDA3505.

Both look good.

No modulation on RGB lines from the TDA3505, just a fixed DC voltage of around 2V (going from memory).

TDA3505 also seems to be grounding the 1-5V from the brightness and contrast circuits.  I see a fixed .8V on those lines when I should see an adjustable 1V-5V. VCC also shows .8V which is really suspicious since it matches the contrast and brightness pins. 

There is a 4.8K resistor between 12V and VCC pin 6, and I do see 12V going into the resistor, but .8V on VCC.  Resistor tests good. 

Everything connected to the TD3505 tests good.  Chip shorted again?  I'm not very familiar with how this chip works so I'm not sure if it's in a cutoff state or if it's just dead.

I ordered another chip from a different seller just in case, it's possible I got a bad chip from ebay.. wouldn't be the first time.

I did check EVERYTHING else in the schematic, section by section, and found everything working correctly, and all expected voltages to be correct, except for TDA3505. 

Also, shorted the first set of RGB output transistors fed by TD3505,  and the guns came on, so TD3505 is definitely offline or dead.
« Last Edit: February 11, 2018,15:21:39 by flgliderpilot »

astracat

  • Hero Member
  • *****
  • Posts: 222
    • Email
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #17 on: February 13, 2018,15:00:05 »
hi if r574 is high resistance which sounds like your problem it should read 4.7 ohms
regards
astracat

Mulja tv repair portal

Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #17 on: February 13, 2018,15:00:05 »

astracat

  • Hero Member
  • *****
  • Posts: 222
    • Email
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #18 on: February 13, 2018,15:05:10 »
sorry forgot to say its on main board

flgliderpilot

  • Jr. Member
  • **
  • Posts: 11
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #19 on: February 14, 2018,11:35:01 »
Astracat good call!  Funny thing is I discovered that myself this morning. I thought 4E7 on the schematic meant 4.7 (K), but I was thinking about it and 4.7K would be a huge resistor to put before a chip that draws 100ma.  So I went out and checked it again... 12V before the resistor, and 5V after the resistor.. hrm.. then I checked the resistance of the resistor, OPEN!?

I checked another resistor of the same color bands (yellow purple gold ???), and sure enough it was a 4.7 ohm resistor!

D U H

I replaced the resistor with two 10 ohm resistors in parallel, re-installed the original TDA3505, and it fired right up.

I suspect the cap tied to that resistor shorted and killed the resistor (slowly).. so it wasn't brown, but it overheated and died over time.

Thanks for all the help fellas.  Having my first monitor repair under my belt feels good.




« Last Edit: March 23, 2018,11:39:21 by flgliderpilot »

astracat

  • Hero Member
  • *****
  • Posts: 222
    • Email
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #20 on: February 14, 2018,14:19:07 »
hi and well done but 2,10k in parallel is still hi suggest to replace with correct value
regards
astracat

Mulja tv repair portal

Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #20 on: February 14, 2018,14:19:07 »

flgliderpilot

  • Jr. Member
  • **
  • Posts: 11
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #21 on: February 15, 2018,20:14:33 »
Yes a little high, but the VCC range for the TD3505 is 6V-24V, and now it's getting 12.03V with a 5 ohm.  I'm not too worried about a .3 ohm difference in this scenario. FWIW all of the 4.7 ohm resistors actually measured 6 ohms on my meter (which probably isn't all that accurate anyway).





« Last Edit: February 15, 2018,21:11:12 by flgliderpilot »

astracat

  • Hero Member
  • *****
  • Posts: 222
    • Email
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #22 on: February 17, 2018,07:33:01 »
hi its ok with 5ohms but 2x10k is 5000ohm so presume you must have done a typo error
good luck with future repairs
astracat

flgliderpilot

  • Jr. Member
  • **
  • Posts: 11
Re: Learning CRT repair.. black screen, no raster, but sound?
« Reply #23 on: March 23, 2018,11:39:57 »
Yeah sorry typo, two 10 ohm.

 

Powered by EzPortal