RCA (Thomson) F20TF10 in a coma

[kc8adu]

Re: RCA (Thomson) F20TF10 in a coma

digikey for both and choose first class shipping .

I can get the regulator from Parts Express (and too much in shipping) but haven't found a source for the fuse yet.

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

No need to at this point but thank you.

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

In the interest of time I did not repair this 2nd MP50 fuse but used a .5A glass fuse with leads for troubleshooting. After replacing fuse there is NO high voltage on the anode and I can hear HV hiss from somewhere.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

Turn off the light and in the dark it will be seen place leak

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

I can't see it in the dark but it sounds from the area of the 6v regulator, the hiss sound changes over a period of 1/2 second from quiet to louder quickly then down to quiet again.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

You do not forget to connect the ground wire from the CRT to the U02 on the CRT board ?

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

I did not forget, it is connected and confirmed good to mainbaord ground as well.

I checked with KV probe on connections in the area of 9 and 6 volt regulators (area where the hiss sound originates) with no results.

I pulled one leg for the fuse and got HV back on the CRT anode.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

Probably some sort of a bug in the installation. Clarify in there circle you're talking about the fuse

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

This pic is before replacement.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

Need to check the electrical circuit of the fuse and measure the resistance of the stabilizer 7806 pin 1-ground, and pin3 - ground. And show the actual photo.

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

The fuse ckt is ok. 7086 pin 1 is 1.1M to ground and pin 3 is 4K to ground. Fuse is circled, 7806 has the arrow.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

What news? I want to show fuse repair

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

The MP 50 below the fuse circled in the 2nd pic is the repaired fuse, the fuse wire I used from a glass fuse goes around the outside of the body on the right side. That checks ok.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

I see it. Now what is missing, all the voltages are?

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

UB on mainboard has changed from 117.9v to 136.5v
CRT pins 11, 8 & 6 has changed from 177-180v to 102-105v
CRT anode has 0v
VG2 has 75 to 534vdc to 0.0v to 0.1v rotating screen control
BU004 pin 5 changed from 188.8v to 134.9v
BU003 pins changed:
2 was 1.2v now 0.3v
3 was 1.2v now 0.3v
4 was 1.2v now 0.3v
6 was 8.7v now 0v
Mainboard 12v, 9v & 6v test points are good.

I am thinking again it is some part in the flyback assembly.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

Check the voltage IV01 (TA1268) 1: - 9 volts on pin 9,26,46,48. 2: - 3.3 volts on pin 18

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

26 has .7v
48 has 0v
The rest are ok.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

Pin 26 (IV01) is power a master oscillator flyback , it is necessary to check the resistance pin 26 on the ground. Trace the entire chain of the pin 26, there should be a zener diode and probably low-resistance breaking resistor from pin flyback

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

There is no conductivity from IV01 pin 26 to ground.
The zener checks ok out of circuit.
The resistor is 2.2 ohms out of circuit.
Traced to emitter of TP390, an NPN that tests good out of circuit. The base ties to 9VS2, which has tested good when powered.

That is as far as I've gotten so far.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

Collector TP390 what is connected, you can see how the chain? I'm on a photo I can not check, fuzzy printing, may be even take a picture of this area from two sides. Principle of operation is that when the power is turned on the first 26 pin comes from the power supply, and when it begins to run flyback line scanning, the power to pin 26 comes with flyback