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Learning CRT repair.. black screen, no raster, but sound?
downunder:
Yep, seems like you're on the right track. Keep at it. Bruce
astracat:
hi,notice this is a monitor vid signal from computer ,if 120 voltage correct it may be poor earth connections on flyback tx suggest you resolder these first or make sure you dont have a frame collapse as that will blank the screen
good luck
astracats
ps final anode cap is between 18/20 kv so keep hands away from that
Keith:
Hi,
Working from my fading memory! Some of these monitors had 2 inputs TTL and analogue and I think there was a switch to select the input, make sure you are using the correct input.
Keith
And some only had TTL inputs!!
flgliderpilot:
Hey guys thanks for the advice. Ok, so I got ripped off on my isolation transformer, seller was a scammer and it never arrived : |
I decided I'd work without it and omit the oscilloscope. Grounded one lead with a clip and only held one probe when powered up and stayed clear of power supply and HV section.
So I first checked all of the transistors and they seemed good in diode test mode. I checked the power supply outputs and they are good. I applied video to both inputs on the monitor (composite and digital RGB) and neither worked so this let me bypass a huge section of the schematic until those signals meet at the TDA3505 IC.
VCC was supposed to be 12V at the resistor before pin 6, which I saw. After the resistor I saw 0V. I pulled and replaced the cap in parallel with this resistor, and now I had .9V so I am at least going in the right direction.
I checked every cap surrounding this chip and connected in any way to 12V and all seemed ok.
I checked the resistance from pin 6 VCC to pin 24 GND and it did not seem to be shorted. I decided an output pin must be shorted to ground further along, so I desoldered all output pins with no improvement.
I reconnected outputs, and desoldered the ground pin of TDA3505.
With ground pin disconnected I have 5V at VCC. So it was actually VCC shorting to ground.
After this I was able to press the green/monochrome button on the front of the monitor and the display would change from grey to green, but still no display (obviously since TDA7305 is off), and I can see retrace lines because screen control is turned up. So the section AFTER this chip appears to be working.
My conclusion is that it must be a bad TDA3505, the chip is shorted out.
I'll order one today and see what happens next!
Not as scary as I expected as long as you discharge everything before hand.
In the meantime I think I'll desolder all pins of TDA3505 except VCC and GND just to be sure since I have to desolder them when the new chip arrives anyway.
Keith:
An isolation transformer will only protect you from the live AC mains. You still have to work with caution as you will still get a shock from anything after the rectifier.
Keith
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