RCA (Thomson) F20TF10 in a coma

[SteveNielsen]

Can't find schematic for this, just partial svc info. Most every Google result is in cyrillic font. Right now want to know heater pins on the lg philips a51qdj279x CRT. One site says 4 & 5 or 9 & 10 is common.

Mains fuse ok.
27KV on CRT anode.
I can hear a pulse in the speakers when 1st plugged in.
Power button has no effect.
No heater light in the CRT neck.

"WHAT THE HELL IS THIS RUSSIA?" - Larry The Cable Guy

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

crt board

[kc8adu]

Re: RCA (Thomson) F20TF10 in a coma

last time i had a set with hv but not much else like no heaters and some supplies missing it had a crack under the fly.
heaters run from winding on the fly.

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

Thanks kasfamily. I just realized that I have that in a diagram in the partial manual I dl'd. I didn't look closely enough at it.

[Andrew F. Ali]

Re: RCA (Thomson) F20TF10 in a coma

It could also be an open load resistor connected between the FB pins and CRT Heater pins. It is usually about 6Vdc.
If as you say 27kv is on the CRT Anode connect a signal source to the AV input or Antenna and see if the rest of the system is working.

[Clydeuk]

Re: RCA (Thomson) F20TF10 in a coma

The 2 heater pins on a CRT are usually next to each other. You may be able to see which pins are connected to the heater by looking through the neck. If not using a meter on a low ohms or continuity setting try metering between adjacent CRT pins, you will be looking for a low reading or a beep if on continuity. Those will be the connections for the heater, then trace continuity back from here to the source, usually as stated the LOPTX (or FBT USA).

CRT heaters can go open circuit but this is very rare, the usual causes are as stated either a series resistor or choke, a crack in the PCB or a bad solder joint.

It is rare for a transformer to fail in this way, but if it has you can usually codge up a heater supply with a couple of turns of wire around the limb of the TX and disconnecting the original supply. This was also a useful dodge if the CRT had a heater-cathode short as it isolated the heater from ground.

The supply is around 6.3v AC but will be at a high frequency (>15KHz) so don't expect any meaningful reading on a normal DMM.

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

It could also be an open load resistor connected between the FB pins and CRT Heater pins. It is usually about 6Vdc.
If as you say 27kv is on the CRT Anode connect a signal source to the AV input or Antenna and see if the rest of the system is working.

Is that to say the heater voltage should be 6vdc? Is that load resistor likely to be on the mainboard near the FB or on the CRT board ont he back of the neck?

It was connected to an AV source when it went dead and wasn't getting a picture or sound at that time.

There is most definitely 27kv on the anode, measured with my trusty Polaris 651 HV probe.

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

This spot by the flyback shows signs of heat. I checked the resitors and diodes in this area and they are all fine.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

show photo crt board

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

crt board

BTW, on this CRT board connectors BU003 & BU004 are switched, i.e. BU003 s the 6 pin and BU004 is the 5 pin. Tracing visually the heater pins on the tube are 9 & 10. Just for clarity, is the heater votage AC or DC?

[kc8adu]

Re: RCA (Thomson) F20TF10 in a coma

ac right off the fly with maybe a low value resistor.

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

Thank you kc8adu. As I thought it might be.

I measured 5.4 ohms across pin 9 & 8 on the crt so I think the heater is ok. I've got 14.5 vac with power applied. I can hear the audio amp, degaussing coil and crt hv when plugging in ac cord.

Here are the crt board pics kasfamily requested.

[Andrew F. Ali]

Re: RCA (Thomson) F20TF10 in a coma

How can you have heater voltage and heater resistance OK and no light up...I can't understand that...something is amiss hear...

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

I don't think that the heater in this crt is visible through the neck if that's what you mean. The crt seems to be ok but I don't get anything displayed. Power and menu buttons don't do anything. Even without any input the menu should come up.

BTW, all the e-caps are Rubycon or Lelon.

[Andrew F. Ali]

Re: RCA (Thomson) F20TF10 in a coma

OK so you have HV, Heater filaments are lighting up but you have no Raster and X-Ray protect cct not shutting down the set in addition audio is OK, too. From the FB Trans you would be producing ABL voltage 200Vdc that go to the CRT and also the ABL cct.
There are two resistors that are in parallel typically 180K-ohm and 560K-ohm that send about 5.2V via a Zener Diodeto the ABL cct. If these resitors go open then you would get no screen light up (Raster)

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

pin 9 & 10 on the crt board the heater

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

Thanks kasfamily, I got that. Andrew F. Ali, I measure 187 vdc on pin 5 of BU004 (marked BU003 on block daigram) on the crt board. The block diag says should be 160v. I've got 179v on pin 11 of the crt, which I assume is grid1. (I can't find a reliable pinout of the crt.)

What do you folks think, should I pull the main board out and start testing parts? Seems like the logical thing to do next.

I also have 8.7vdc on the 9VE pin 6 of BU003 (marked BU004 on block daigram).

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

Still say 1.Heater voltage is at 9 and 10 pins of the CRT ? 2.The heater glows CRT (better to look in the dark) ?

[SteveNielsen]

Re: RCA (Thomson) F20TF10 in a coma

Yes, heater voltage is at pins 9 & 10. I turned off the lights and do in fact now see the heater glowing. It is not visible with room lighting on. Thanks.

[kasfamily]

Re: RCA (Thomson) F20TF10 in a coma

Turn on your TV and rotate clockwise to the right on the control "screen" TDKS