Orbitel TR-005 - Contrast Potentiometer

[Thibault]

My Orbitel always had a finicky contrast knob. The picture would go from very flat to extremely contrasted by barely touching the know. And sometime it would just stay very flat, no matter where the know was set to.

Now, as I was I trying to tweak the contrast, the picture just disappeared, I have no more snow/noise on screen, just a solid grey picture, just like if the contrast was set to zero.

The brightness control still works fine though.

First question: could the contrast potentiometer be the culprit?

Second question, do you know what the rating is? I removed the case and isolated the potentiometer and I think it may be 1000 Ohm, but if anyone could confirm I’d greatly appreciate.

[Alex KL-1]

Without a schematic, is difficult to determine circuit behaviour, but some contrast biasing circuits sometimes include the potentiometer (the fixed ends), so a completely open potentiometer in this case will can vanish the image (some chance in models using all 3 wires of potentiometer). Or cable shorting (in other cases).
In any case, is good to substitute anyway to restore proper contrast adjustment, and test to see if all problems are solved, before any futher analysis.

[damen]

According to the Sams Photofact diagram the contrast control is 1000 ohms. It is connected to a electrolytic capacitor that bypasses the emitter resistor on the video output transistor. If the control is open, or disconnected from the circuit, there should still be a picture, but faded.

[Thibault]

Thanks, I’ll start by replacing that potentiometer with a 1000 Ohm one and circle back if that did not solve anything.

I am extremely new to this, but I guess we all have to start somewhere!

[zeno]

Keep in mind the Orbitel is one of the few modern sets worth $$$.
So save the old part ! You can take them apart & swap out the
element, mounting, shaft etc.

good luck

73 Zeno
LFOD !

[Thibault]

Originally Posted by zeno

Keep in mind the Orbitel is one of the few modern sets worth $$$.
So save the old part ! You can take them apart & swap out the
element, mounting, shaft etc.

good luck

73 Zeno
LFOD !

Oh yeah, I paid a pretty penny for it! Good call on saving the original part though!

[zeno]

Originally Posted by Thibault

Oh yeah, I paid a pretty penny for it! Good call on saving the original part though!

You can mouse it for now but we would put it in a plastic parts bag & put it inside the set. The pots in most JA sets back then were pretty bad. They
got dirty & froze up. A little WD40 worked great.
BTW I worked at Sony factory service @1978. Bad pots & switches had what
was called "crystaline growth" Its from salt air when shipped hence
those sillica packets..........

73 Zeno

[Thibault]

Another great idea to leave the old part inside the set!

So quick update, I replaced the potentiometer and I can now control the contrast properly, it’s behaving perfectly!

But there is something is definitely odd with the TV.

I get reception on some channels and a solid pictures on most.

For example, I can receive channel 20 just fine, but on channel 70, I can’t get anything. The other sets will pick up either signal just fine (I have a couple of agile modulators).

Anyone has any idea why a set would only be receptive to some channels only?

A week ago I could still use any channel on the orbitel, and channel 70 was super clean….

[zeno]

Take cover off UHF tuner. You will see the shaft. Along it are U shaped
grounds. A few drops of WD40 on each & run it back & forth.,
DO NOT bend anything in there, its how they align it.
That fixes most.

Zeno