Help needed repairing a Ferguson set with TX80 chassis

[G Squad]

Hello all

I desperately need help repairing my wife's TV set, it was given to her by her late grandfather so has sentimental value

Some background...

The original fault was mechanical, the spring broke on the earth wire across the back of the tube, and fell in the power supply! BANG! I managed to trace the fault to the main flyback switching transistor (TP10 short cct), replaced this (and the spring!) and all was well

Then a few months later, smoke and death. Discovered that one of the PCB pads on the main PSU cap (CP31) had lifted, so the cap was not in circuit. Fixed the track and replaced the cap for good measure. Now the set fires up (alomst literally ) after a few seconds I get smoke and arcing from the scan coils.

They measure OK cold (3.4R for H and 12.2R for V) but there's obviously a problem. I don't dare to run the set for long enough to get a 'scope on it and take a look at the H and V voltage waveforms. I've been through the entire PSU with the component tester on my 'scope (with it switched off) and can't find anything suspicious.

Anyone got any ideas? Is this simply insulation breakdown in the coils? If so how do I go about repairing it?

Any help would be greatly appreciated!

Thanks
Gordon.

[Guest]

Maybe some other teck lifted the cap on purpose so he didn't have to go through the tedious process of repair, and found the easy way of fixing the problem,not expecting customer call back. Check these out you may find you fault here.

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Anonymous
RCA TX chassis
Wed Dec 24, 2003 1:41PM
24.235.162.109


RCA
E13320SE
TX825TC
Auto-programming stops at channel#95 &then locks up.
Replace LM07=AFT coil
Comments:When unit locks up,you cannot power on/off,no front panel or remote functions.The screen locks on ch#95.Adjusting the LM07=AFT did not help.Replacement of LM07 corrected problem...

RCA
E13320SE
TX8251C
no or intermittent startup
check/replace TP12/TP15
Comments: TP 12 or TP15 are transistor part numbers. One or the other will be in the set, NOT both. Upgraded RCA replacement part is # 198752

RCA
E13361GT
TX82R
Intermittent operation
Solder pin #10 of flyback

rca
f19204
tx86tb
high voltage ok / screen blacked out
replace rt40 68k/.5watt resistor
Comments:located on crt board

RCA
F19426
TX809
Turns itself on, slow reaction to front panel controls,no video, no sound.
Replace TL04 blanking transistor near Hor Driver #198743/BC548/ECG85
Comments:TX809 TX-TVD809 Turns itself on, slow reaction to functions,no video, no sound. Replace TL04 blanking transistor near Hor Driver #198743/BC548/ECG85

rca
f19204
tx86tb
high voltage ok / screen blacked out
replace rt40 68k/.5watt resistor
Comments:located on crt board

[Frank]

[G Squad]

[Frank]

Did you replace that cap?

[minnie]

I believe he did. Did you make sure you observed correct polarity?

[G Squad]

Yeah, I did, and I put it in the right way round

It's all over now... whilst trying to 'scope around for the fault the horizontal deflection coil burnt out.

Unless anyone has a Samsung DSG-0992JL yoke for sale?!

Thanks anyway guys.

[torbjorn]

If i remember correctly, the TX80 chassis is in fact a Thomson chassis, and it is used (with other type designations) in a lot of televisions carrying various Thomson brands (Saba, Telefunken, Nordmende, Thomson etc).

I have no schematic available for the moment, but a major problem is that the power supply is a switched-mode series-passing design (buck converter). If the switching transistor gets shorted or if there is a fault in the control circuitry causing it to be constantly "on", the supply voltage to the line output will be way too high. Although there is a zener diode that is supposed to fail short in such cases and cause the mains fuse to blow, this protection does not always work and there are often much consequential damage.

It is very important to NOT connect a such set directly to the mains after repairs in the power supply. Instead, use a variac and increase the voltage slowly and observe what is going on. Have a meter connected to the B+ line and be prepared to switch off immediately if it increases above the nominal voltage. Remember that there is no galvanic separation in the power supply, so the entire chassis is "hot". It is a good idea to use a isolation transformer too.

The deflection coil is relatively standardized, you could probably try other coils from similar 14" or 20" CRTs