Sony KV 1212

[DaveWM]

Got it

it was a 4.7uf on the boost supply that powers the video output transistors.

I found it while scoping the CRT back, it was on the collector but not on the base (the decrease in signal on either side of the blanking pulse.

what was throwing me off was the sams did not have a boost supply HV, rather just the reg mains B+ which I had changed out. This set had a diode where a choke was supposed to be, the diode rectified pin 1 (again not on the sams) and the filter was across the boost voltage and the B+ (which seems odd since if the horz does not kick off the boost will be zero and the B+ mains will be 160, the cap is rated at 50v....hmmm maybe when that startup cap went out it took this one with it.

[DaveWM]

The fix explained

http://www.youtube.com/watch?v=hMCH6spmaXE

The results

http://www.youtube.com/edit?o=U&video_id=Ow7x2sRWaEU

and some stills for Carmine

[dieseljeep]

Originally Posted by DaveWM

Got it

it was a 4.7uf on the boost supply that powers the video output transistors.

I found it while scoping the CRT back, it was on the collector but not on the base (the decrease in signal on either side of the blanking pulse.

what was throwing me off was the sams did not have a boost supply HV, rather just the reg mains B+ which I had changed out. This set had a diode where a choke was supposed to be, the diode rectified pin 1 (again not on the sams) and the filter was across the boost voltage and the B+ (which seems odd since if the horz does not kick off the boost will be zero and the B+ mains will be 160, the cap is rated at 50v....hmmm maybe when that startup cap went out it took this one with it.

Dave! All kidding aside, that was the one I was thinking about! I found that cap bad in several sets, when there was strange shading in the raster.
Compared to the newer sets, even Sonys, the early Sony models were really complicated.

[DaveWM]

it was pretty easy once I gave up guessing and got the scope. You could clearly see the issue present on all 3 cathodes of the CRT. tracing back to the collectors of the drive transistors and it was there as well, but the bases looked perfect. I figured it had to be in the power supply to the collectors.

What got me was I thought they were supplied by the mains B+ (per the schematic) but as I trace the pcb the orange supply wire I saw it going somewhere else. then I see a diode where a choke was supposed to be (the pcb even had both symbols choke and diode printed on top), and the only thing else was a wire to the flyback to a pin that was not ref in the schematic.

hmmm, that was when I realized it had a boost supply and of course the 1st thing to check was the filter cap. I had check the other caps in the LV flyback supply but missed this one completely since I did not know it existed.

oh and this is a bit odd the BOOST for the screen comes from the horz out transistor, while the voltage supply I refer to comes from the flyback, Whats odd is the fly does not power the yoke there is a another transistor and transformer that does that. So not even sure what to call this supply from the fly that is used for the video out, its only about 15v higher that the mains B+, maybe I should be calling it flyback B+ 3 while the flyback LV supply is B+ 2 and the mains would be B+1 its a bit confusing. buddy of mine is getting me the correct schematic, that should help.

[DaveWM]

ok some corrections

this is the SCC 17 B/C model of the KV1212 chassis.

there is a 129v supply from a 1/2 wave main supply.

there is a 131v supply from the flyback primary side

there is a 18.2v supply from the flyback secondary side

there is a a 530v boost supply from the primary of the yoke trans former (there is a separate output transistor and transformer to drive the yoke).

thats a lot of power supplies to keep up