KV9200 Issues

[old_tv_nut]

I have a gut feeling that the variation in spot size with G1 to cathode voltage (that is, with beam current) outweighs the variation due to G1-G2 voltage change, but it would be interesting to know.

[vol.2]

So I went ahead and did this. It looks a little sharper to be sure, but I found a couple more issues while I was messing around with bringing the brightness and color back to normal.

First weird thing is the AGC. That's the only internal video level control I get, so I went to set it up and the voltage is way off what the Sams says it should be. Sams wants 2.5V-2.7V at the emitter of Q207, the 1st Video Amp. I see between 600mV-1.6V maximum as I twiddle it.

As the voltage at the emitter goes down, the video signal gets stronger, which I didn't expect, but I'm just learning here I guess.

I wish I had the actual SM, but this is the best I can do otherwise. Is there any reason the voltage there would be off by what seems about double?

I checked and set the B+ when I did a recap of the set.


EDIT: OK I found the SM. It looks like Sams is wrong. It's supposed to be 2.6V at the BASE not the EMITTER, and readings are with a colors bars pattern displayed or nothing. Sams didn't mention any of those details.

[vol.2]

Having some time to actually view material on it and not just stare at test patterns, I can definitely say that it's a bit sharper than it was. Putting the G1 to ground seems to have helped, and there's plenty of headroom for brightness past where it's currently set. Also checking the AGC and afterwards redoing the white balance and screen settings has made an enormous improvement in the over color reproduction.

I am still seeing a slightly diminished bright red and blue bar, but it's nowhere near as bad as it was before.

I wonder if it isn't also necessary to redo the IF sections because I've never touched them. I haven't done TV IF adjustment before and I'm not sure exactly what to do.

I want to also note that the image jumps around a bit while it's running. It doesn't do it constantly, but every so often it will bounce in and out of vertical hold seemingly.

I wonder if there's any way to make the image more stable?

[old_tv_nut]

Tap on things and wiggle connections to see if some particular thing is sensitive to vibration.

[vol.2]

Over time with using it with the G1 at 0V the wiggliness has almost disappeared, so I'm not terribly worried about it anymore.

I have noticed with this set that the image seems to get a little brighter on the right side of the screen, so I decided to investigate the power supply for the RGB output transistors. I noticed that the 162VDC supply was coming in a little bit low, and with a little bit more of an AC component than I expected to see. I also scoped the derived 130VDC supply and found that it was around 125V

The OG diode was a HF-1Z with an epoxy blob on the back, so I swapped it for a 1N4937 and now the voltages are closer to what they should be. I see the 130V at around 128-129, and the 162 is about 160. The AC component is halved.

This got me thinking if maybe there was something I could do to improve the performance of the output transistors on the neck board.

I wonder if swapping the 2SC1127 transistors out for something more modern, or with higher gain would improve things at all, especially since pegging the G1 to zero volts required that I raise the G2 to compensate a bit?

I know it's got to at least be able to handle the conditions of the original component, but I'm wondering which characteristics I need to pay most attention to in order to tune the circuit in a beneficial way?

The 2SC1127 has an hFE of 1000, so would I be looking for something that was just a bit higher or something?



Also was looking again at the focus voltage now that I've done the G1 mod and I realized that it's actually having an effect to use a potentiometer on the focus. Only issue is that it's loading down the G2 voltage and making me turn up the G2 close to it's limit. I think I can prevent this with another resistor or something, but I'm not sure how big it would need to be.

[vol.2]

As far as the Focus Voltage project goes, I am still trying to work out how best to set it up in order to prevent loading the G2.

I have the 1 megaohm potentiometer that purchased for this project, along with some various power resistors, the largest of which is 27k 2W. I can always buy another couple of resistors, but I'd like to do it with what I have on hand.

The original Focus circuit has a 470k 1/2W resistor in series with the Focus Grid. I'm not entirely sure if it's capable of handling a modified circuit that includes a resistive divider to pull down the focus voltage, but this is what I'm currently thinking about:



I also have some 1.6kV 1.5A diodes I could maybe use to isolate the G2 from the Focus voltage circuit, maybe preventing the load down?

Any opinions or advice on this?

TIA

[zeno]

Without looking back......
SAMS & most other SM's show wave forms & voltages with a clean signal & given pattern, it is noted in the docs. AGC can vary WIDELY from no sig / full signal.

L-R shading for me has ALMOST ALWAYS been the filter cap for the @200 V
supply. Its across all brands ! Thats hundreds of sets. Just blanking & video coupling caps were found a few times. IIRC there is one in you Sony.

As far as TNR / IF alignment go DONT DO IT ! When 17 yrs old my first TV
repair job I was told NEVER F with it or I will get fired ! He meant it. FAFO.
Yes there are a few traps you can do by eye but thats rarely needed.

time for a brew
Zeno
LFOD !

[vol.2]

Originally Posted by zeno

Without looking back......
SAMS & most other SM's show wave forms & voltages with a clean signal & given pattern, it is noted in the docs. AGC can vary WIDELY from no sig / full signal.

L-R shading for me has ALMOST ALWAYS been the filter cap for the @200 V
supply. Its across all brands ! Thats hundreds of sets. Just blanking & video coupling caps were found a few times. IIRC there is one in you Sony.

As far as TNR / IF alignment go DONT DO IT ! When 17 yrs old my first TV
repair job I was told NEVER F with it or I will get fired ! He meant it. FAFO.
Yes there are a few traps you can do by eye but thats rarely needed.

time for a brew
Zeno
LFOD !

Thanks Zeno.

I did observe the color bar requirement in the Sams when setting AGC. I didn't mess with the IF! I would not as I don't have the right equipment to do up right anyways and I'm not so bold.

L-R shading for me has ALMOST ALWAYS been the filter cap for the @200 V
supply.

Yes, there's a lytic cap there it's been replaced with the same value. C524, 4.7uf, 50V. I could swap it out for something higher I guess? Like maybe a 10uf.

Any ideas about controlling the focus voltage without loading down the G2? I thought maybe if I put a diode between the 530V and the 1M pot it could prevent that, but not sure if it's the right approach.

[damen]

C524 voltage rating should be much higher than 50 volts. Try a 5 or 10 meg focus control so as to not load down the supply voltage.

[vol.2]

Originally Posted by damen

C524 voltage rating should be much higher than 50 volts.

I thought it did seem kind of weird while I was looking at the schematic today, so I used a 250V 10uf cap to see if I got any improvement in the evenness of the brightness, but I did not.

However, Sony used a 50V part there. That's what I pulled out of it (it was dead, but it said 4.7uf 50V), and that's what the schematic says it should be.

Try a 5 or 10 meg focus control so as to not load down the supply voltage.

I don't currently have one that big. I have 1.6kV diodes though! Would it make sense to throw a diode between the 530V and the 1M potentiometer to prevent load? Like this:

[Alex KL-1]

The diode only will serve to make a voltage offset; will not isolate anything (unless the focus grid supply current...). Is better to invest in the resistor values.

[zeno]

C524 is probably stacked on the B+ instead of going to GND like
most sets so 50 V rating used. Fisher & GE also did this. Most
others used a 4.7 or 10mfd 315 V to GND.
Zeno

[vol.2]

Originally Posted by Alex KL-1

The diode only will serve to make a voltage offset; will not isolate anything (unless the focus grid supply current...). Is better to invest in the resistor values.

Okay, I thought the current loading was the issue, but I guess you're saying that it's actually a voltage drop on the screen?

The issue I'm having with this is that when I put the 1M pot in the circuit with the other side to ground, it creates a load on the screen supply and the screen dims. I thought it was a current issue, but I guess it would make sense that it's creating an additional resistive divider for the screen potentiometer. There is a lot going on in this circuit, and I'm having trouble visualizing exactly what happens when I put the pot in. It's like this, and the arrow is where I would put the proposed diode in place:



Otherwise, are you saying that I can prevent the voltage drop by using a higher value resistor to supply the Focus Grid, with another one to ground on the other side? I think I mainly need help understanding exactly why the G2 is becoming dim in order to fix it because I don't know how to do the calculations for the resistors if that is so.

[vol.2]

Originally Posted by zeno

C524 is probably stacked on the B+ instead of going to GND like
most sets so 50 V rating used. Fisher & GE also did this. Most
others used a 4.7 or 10mfd 315 V to GND.
Zeno

Yes, it's stacked on the B+. There's a .01uf film or ceramic (I'm not inside right now) going to ground between B+ and 162V Source. I guess I could try replacing that instead, or increasing it a bit if it looks okay. I've seen some film caps go bad I guess, but it's very rare.

[Alex KL-1]

Originally Posted by vol.2

Okay, I thought the current loading was the issue, but I guess you're saying that it's actually a voltage drop on the screen?

The issue I'm having with this is that when I put the 1M pot in the circuit with the other side to ground, it creates a load on the screen supply and the screen dims. I thought it was a current issue, but I guess it would make sense that it's creating an additional resistive divider for the screen potentiometer. There is a lot going on in this circuit, and I'm having trouble visualizing exactly what happens when I put the pot in. It's like this, and the arrow is where I would put the proposed diode in place:



Otherwise, are you saying that I can prevent the voltage drop by using a higher value resistor to supply the Focus Grid, with another one to ground on the other side? I think I mainly need help understanding exactly why the G2 is becoming dim in order to fix it because I don't know how to do the calculations for the resistors if that is so.

If focus adjustment are great at near top, you can add a some Mohms resistor from "cold" wiper from potentiometer to ground (in series with potentiometer), reducing the loading, and making the focus adjusment less coarse.