GDR made portable TV (Robotron Combi Vision)

[dan_man]

Ordered one!

I had a look at the board again and noticed a very corroded T 1.6A fuse right behind that EHT cage (looking from the upper edge of the board).
It was fried and also came apart.

When I replaced it and switched on the set, the screen went on immediately!
Unfortunately though, this didn't go on for long. The fuse went and so did a second one. I am out of 1.6A fuses now... Also, there was no sound while the screen was on. Only wenn the fuse burned through, the sound went on immediately.

I think (but am by no means sure) that the fuse went because I fiddled with that screw you can only access via screwdriver from the outside. It is not that vertical frequeny adjuster. I messed with that ominous screw a few days ago without knowing what I was doing. It is three screws, but only the one in the middle is accessible from the outside (see picture that shows whole PCB, bottom right)
Anyways - for the short time it did work, the screen was flickering as I was expecting, but the centre was darker and there was a vertical stripe.


Cheers,
dan

[dan_man]

Update:

bought some 1.6A time-lag/delayed fuses. Takes them longer to melt.

New fuse, TV comes on, picture fades until black, no sound, fuse melts, sound works again. When I put in a new fuse, the EHT hisses but no picture.
I doubt that my observations will be revealing, but I thought I'd give it a shot.

Thanks anyways for the friendly and patient input here.

Have a great evening,

-Dan

[ben]

The line stage is clearly drawing too much current. That would explain:
a) the repeated blown fuses
b) the lack of sound as the PSU is put under load

check for starters
-capacitors and diodes in the stage.
-line output transistor, is mounting (mica insulating washer?) and any associated caps or diodes
-scan coils resistance.

The fact you got a picture suggests a leaky component rather than a dead short.

[julie_m]

Originally Posted by dan_man

New fuse, TV comes on, picture fades until black, no sound, fuse melts, sound works again. When I put in a new fuse, the EHT hisses but no picture.
I doubt that my observations will be revealing, but I thought I'd give it a shot.

Something in the line output stage (the circuit within the tin box, downstream of the fuse) must be drawing so much current that (a) it is melting the fuse and (b) there is not enough power left for the audio stages. However, it is not a dead short circuit; there is enough power to generate the high voltage.

The next step really is going to need for you to get hold of the wiring diagram, in order to work out what might be the prime suspect components.

EDIT: Crossed with Ben. GMTA.

[dan_man]

is that the line output transistor? It is a three-pronged component mounted to a large aluminum plate near the PSU. Note the brown stain that extends from it. It doesn't smear.

http://**********/a/g4ZZ1

-What is a mica insulating washer and where is it supposed to be?

[ben]

Glad someone else agrees with my ramblings Julie!

As for the picture, it's not possible to see the markings on the component so it could be a voltage regulator for all we know. We'd need to see exactly where it goes in the circuit.

The mica washer refers to a thin insulating sheet (that may or may not be present) between the component and the metal it is bolted on to. A quick test of its integrity on a set like this might be to unscrew the transistor and briefly try the set and see if the fuse holds. running the set like that for more than a couple of minutes would, however destroy the transistor due to overheating!

[Lloyd 1985]

Hi,
My guess is the line output transistor is the metal cased thing bolted to the outside of the cage, where the line output transformer is.

Regards,
Lloyd.

[ben]

Quite right Lloyd, I'd not seen that picture. I'd bet on that being the LOPT as it is right next to the line OP transformer. The other 'transistor' in post 25 is likely a regulator and should be left alone at this point.

[julie_m]

Originally Posted by ben

Glad someone else agrees with my ramblings Julie!

Right back atcha Seriously, though, I do think it's the most obvious explanation consistent with the symptoms.

Originally Posted by Lloyd 1985

Hi,
My guess is the line output transistor is the metal cased thing bolted to the outside of the cage, where the line output transformer is.

Mine too.

I still kick myself over all the 1980s-era CRT TVs I've seen where a poor connection on the line output transformer caused an arc which ruined the line output transistor, but passed up the chance of some easy money as they mostly were live-chassis sets and that was the really scary high voltage bit (and those large colour sets use even higher voltages than mono portables).

If the line output transistor has failed so as it is partially-conductive all the time (which would be unusual, but not inconceivable), then the stage could just about run but will draw too much current. The picture would be a narrow strip up the middle of the screen (because the oscillator output is reduced; some of the energy that should be going into the beam scanning coils is ending up heating up the transformer and the transistor), dim and ill-focussed (because this is also causing low EHT).

Anyway, here is the first thing you can do when you get your multimeter .....

CARE: DURING THE FOLLOWING TESTS, DO NOT TOUCH ANY METAL PART OF THE SET! HANDLE YOUR METER PROBES ONLY BY THE INSULATED PARTS. IF POSSIBLE, HAVE AN ASSISTANT STANDING BY THE POWER SWITCH, READY TO TURN IT OFF IF YOU GET INTO TROUBLE.
With the selector switch of your meter set to 200V DC (or 200V ⎓), the black probe on the 0V rail (say, the negative lead of a convenient two-ended electrolytic capacitor; but check the copper side of the PCB to be sure, not every capacitor is returned to chassis) and no fuse in place, switch on the set and apply the red probe of the meter -- which must be plugged into the appropriate socket for measuring DC Volts -- to each end of the fuseholder in turn. One end (probably the end nearest the line output stage) will be at 0V or at least very low, the other end will have some voltage on it. Note the two readings. Turn off the set and allow several minutes for the power supply smoothing capacitors to discharge. Fit a new 1.6A fuse. Switch on the set and quickly measure the voltage at one end of the fuseholder (it does not matter which, as the intact fuse is a dead short circuit so the reading will be the same at each end). Switch the set off as soon as you have a reading, and in any case before the fuse blows.

Post your readings, and stand by .....

[dan_man]

Thanks for your replies! The multimeter should arrive soon. As I have an exam coming up nextweek, hence I shall do the measurements and anything distracting/jolting afterwards (procrastination, you know ).

I took a photo of what you thought was the LOPT. I wonder if there was a bridge between the two contacts (probably not, but if you look close, there's a wire stump sticking out from the lower one).

The seems to be a brown insulation mat underneath. As the thing is riveted, I can't remove it just now to check if the insulation is brittle.

best,
Daniel

[hans]

Rivited transistor. That shows that this is a German quality item. It would never break...

There should be no bridge where you have drawn the line. The "brown mat" is there just to prevent the transistor housing to touch the chassis.