Super Video XE Problem

[+Larry]

Against my better instincts, I performed the Super Video XE upgrade on my recently upgraded 65->130XE. (OK, it is a *little* clearer/sharper than before I did the mod, but not by much.) However after about 8 hours of use, something strange started happening. Very slowly, the screen display started becoming darker, until the foreground was almost, but not quite as dark as the background. "Oh Shoot!" -- maybe a little stronger than that... I thought that this was likely related to the (luminance) video output transistor, Q3. In the mod, a 10 uF tantalum capacitor is attached to the collector of Q3 to on-board cap C47 or C48. So leaving everything else the same, I clipped the new 10uF capacitor, and the video was restored almost to normal. The display is *slightly* darker than before the mods, but it is quite stable (no darkening). I first thought that this must be a "weakness" in the transistor Q3, but after clipping the capacitor, I now think this must be a bad tantalum capacitor. (?).

 

What is the theory of adding capacitor to the collector -- it is designed to boost the output from the collector? When it failed, did it start acting as a resistor instead of a capacitor (hence reducing the output)?

 

-Larry

[guus.assmann]

What is the theory of adding capacitor to the collector -- it is designed to boost the output from the collector? When it failed, did it start acting as a resistor instead of a capacitor (hence reducing the output)?

 

-Larry

Hi Larry,

As far as I can tell, this capacitor has to "clean up" the voltage supply to the transistor.

This way, variations in the video signal, causing current fluctuations, will be prevented. (May show as "ghosting")

A tantalium-capacitor is very bad if connected in reverse or for over-voltage.

You may also want to check the resistor in the supply-line.

The cap may have acted as a resistor and drew too much current. This has lowered the voltage for the transistor.

BR/

Guus

[+Larry]

What is the theory of adding capacitor to the collector -- it is designed to boost the output from the collector? When it failed, did it start acting as a resistor instead of a capacitor (hence reducing the output)?

 

-Larry

Hi Larry,

As far as I can tell, this capacitor has to "clean up" the voltage supply to the transistor.

This way, variations in the video signal, causing current fluctuations, will be prevented. (May show as "ghosting")

A tantalium-capacitor is very bad if connected in reverse or for over-voltage.

You may also want to check the resistor in the supply-line.

The cap may have acted as a resistor and drew too much current. This has lowered the voltage for the transistor.

BR/

Guus

 

Hi Guus-

 

Thanks for the info. I don't *think* I connected it backwards, but since I clipped it out, there is no way to tell, now. This was a 16V cap. I'd think that was enough, but I'm going to order some more caps, and I'll get some 35V.

 

For anyone interested, I found a nice site with a lot of basic info about components: http://www.interq.or.jp/japan/se-inoue/e_capa.htm

 

Thanks again, and good to hear from you!

 

-Larry

[puppetmark]

Against my better instincts, I performed the Super Video XE upgrade on my recently upgraded 65->130XE. (OK, it is a *little* clearer/sharper than before I did the mod, but not by much.) However after about 8 hours of use, something strange started happening. Very slowly, the screen display started becoming darker, until the foreground was almost, but not quite as dark as the background. "Oh Shoot!" -- maybe a little stronger than that... I thought that this was likely related to the (luminance) video output transistor, Q3. In the mod, a 10 uF tantalum capacitor is attached to the collector of Q3 to on-board cap C47 or C48. So leaving everything else the same, I clipped the new 10uF capacitor, and the video was restored almost to normal. The display is *slightly* darker than before the mods, but it is quite stable (no darkening). I first thought that this must be a "weakness" in the transistor Q3, but after clipping the capacitor, I now think this must be a bad tantalum capacitor. (?).

 

What is the theory of adding capacitor to the collector -- it is designed to boost the output from the collector? When it failed, did it start acting as a resistor instead of a capacitor (hence reducing the output)?

 

-Larry

 

 

I had that exact problem once. I hooked up the 10uf Cap backwards. It can be tough to read the polarity small caps.

[+Larry]

I had that exact problem once. I hooked up the 10uf Cap backwards. It can be tough to read the polarity small caps.

 

Hi Puppetmark-

Well, that makes it pretty likely that is what I did! And you're right, the "+" marking on those particular caps is certainly *toward* one side, but I can see how I could have turned it around. Thanks for the info/confirmation!

-Larry