Motorola ts18 contrast control

[timmy]

Ok I have a restored ts18 Motorola chassis and if I use a green scope tube on my other sets which is another ts18 and ts4d it is very bright and excellent contrast however this recent ts18 I have just finished restoring. If I put the same scope tube on this chassis the contrast seems to work fine but to get it not to kind of wash out at hi brightness I have to lower the brightness but the other sets I don't have to. The scope tube I would say is new, that's how good it is. So I am wondering if the 3 caps on the contrast control could be bad in such a way that it is preventing the bright from being raised more. It seems like there is a balance between the contrast control and the brightness.where if the bright was up and contrast was at half it then looks cloudy and then advance the contrast up and it darkens the pic. The ts18 only has the caps on the contrast pot. This chassis don't seem to have enough of range in the darker areas so I was thinking those caps on the pot. What do you think mr bandersen as you seem to know these sets very well ?

[bandersen]

Very unlikely those caps are bad. They are ceramic and very reliable. I think it's most likely the voltages on the CRT are different in your TS-18. Check G, A1 and A2 (if you have a HV probe).

[timmy]

What should the voltages be on those connections, 5 kv or so? Well G is ground so there is nothing on that one just A1 pin 5 and A2 pin 9 . Not exactly sure but pin 5 is focus voltage 1600-2400 Vdc and pin 9 5-6 kv if I am correct. Pin 3 I did the mod for the retrace.

[bandersen]

Compare the voltages to your other sets that perform better. Check the cathode voltage too.

[timmy]

Originally Posted by bandersen

Compare the voltages to your other sets that perform better. Check the cathode voltage too.

What pin is the cathode and what should the cathode voltage be ?

[bandersen]

Pin 2. Here's the 7JP4 data: http://www.nj7p.org/Tubes/PDFs/Frank...vania/7JP4.pdf

Between 0 and 220 according the the Sams service info. That's what the brightness control goes to.

You said the CRT works great in another chassis. That's why I suggested you compare voltages between the two of them.

[timmy]

Ok the voltage on the pin 5 green wire A1 is focus voltage and I have good focus but the voltage is not making it to 1kv the others A2 5kv cathode 228 Vdc turn pot opposite is around 49 Vdc. I don't know exactly what the focus voltage should be but all 3 connections on the focus pot is under 1kv. The 2.2meg resistors are not that far out as well as the 6.8meg resistors at the 6sL7 . I don't know if the focus voltage is low will it have anything to do with the contrast and brightness. The SAMs I have for this ts18 chassis don't state what the focus voltage should be except the crt datasheet shows recommended 1600-2400vdc.

[timmy]

Ok so I'm going with the voltages are good. Now if the mod is not done for retrace lines, 220k resistor with the cap , then you have lines left to right on the top of the screen going half way down and fading once done they are gone with hi brightness. I have 2 other sets I done this mod on and both are good however this one I got done has the 220k resistor done already looks factory and checked the resistor and its 220.7k. What I don't get is the lines you get if this mod is not done, on the top I have the same lines on the bottom left to right up half way up the screen the complete opposite . I do notice at hi brightness the lines are there but certain scene changes they almost disappear. I can lower the brightness and the lines are gone but the brightness suffers and I'm left with a somewhat dark picture when the tube is capable of being bright. So this has me baffled why the lines are on the bottom as if the retrace mod was not done and the lines just went to the bottom rather then the top where they would normally be. Any ideas would be great...

[tom.j.fla]

OK, so you are telling us that the voltages are fine but you have retrace lines, after the retrace mod was done. Resistor good did you check the cap.? There is one or two things it could be bad cap. or wiring error, so check again the part and work. All the best,Tom

[timmy]

Originally Posted by tom.j.fla

OK, so you are telling us that the voltages are fine but you have retrace lines, after the retrace mod was done. Resistor good did you check the cap.? There is one or two things it could be bad cap. or wiring error, so check again the part and work. All the best,Tom

It's just the opposite where the lines are, the bottom rather then the top. The wire ,number 3 goes to ground unless the retrace mod is done and it was by the factory and the resistor is fine the cap I don't know even if it's bad I don't think it's going to reverse where the lines go.

[earlyfilm]

Originally Posted by timmy

. . . . . If I put the same scope tube on this chassis the contrast seems to work fine but to get it not to kind of wash out at hi brightness I have to lower the brightness but the other sets I don't have to. . . . . .

First, I agree with B. Anderson, to check for voltage differences between the problem set and the correctly working TS18.

Tim, from what you describe, I'd be more likely to suspect low high voltage, which on an electrostaticly deflected CRT often looks more like a weak and gassy CRT, but on a magnetically deflected set, it usually shows up as image bloom.

First verify that the feedback coil on your 1B3 is in the correct place. If it is a round coil spring, it centers with the bottom folded lip of the 1B3 plate. If it is a flat band, the bottom edge matches the bottom edge of the 1B3 plate. Older 1B3s have a much longer plate than the newer ones, so the sweet spot won't be in the same place on all 1B3s.

If no improvement, borrow the 1B3 and 25L6 HV Osc tubes from the better performing set, and retest.

Since the B++ only feeds the sweep, HV Osc and Video Amp, confirm that is is working correctly by measuring the voltage from the high end of the Brightness control to Ground on both sets.

Also check the voltages on the video output tube on both sets and try substituting that tube.

A final check would be to check the value of four resistors in the HV cage. Ozone can cause resistors to change value.

(The above trouble-shooting tests are listed in the order of probability.)

James

[timmy]

Originally Posted by earlyfilm

First, I agree with B. Anderson, to check for voltage differences between the problem set and the correctly working TS18.

Tim, from what you describe, I'd be more likely to suspect low high voltage, which on an electrostaticly deflected CRT often looks more like a weak and gassy CRT, but on a magnetically deflected set, it usually shows up as image bloom.

First verify that the feedback coil on your 1B3 is in the correct place. If it is a round coil spring, it centers with the bottom folded lip of the 1B3 plate. If it is a flat band, the bottom edge matches the bottom edge of the 1B3 plate. Older 1B3s have a much longer plate than the newer ones, so the sweet spot won't be in the same place on all 1B3s.

If no improvement, borrow the 1B3 and 25L6 HV Osc tubes from the better performing set, and retest.

Since the B++ only feeds the sweep, HV Osc and Video Amp, confirm that is is working correctly by measuring the voltage from the high end of the Brightness control to Ground on both sets.

Also check the voltages on the video output tube on both sets and try substituting that tube.

A final check would be to check the value of four resistors in the HV cage. Ozone can cause resistors to change value.

(The above trouble-shooting tests are listed in the order of probability.)

James

the hv is a solid 5 kv weather the bright is up or down my other sets are at 4.5 kv and they are fine. But it's possible that these tubes are not as good as the ones in my other sets. I tried the crt in my other set but the lines were Bearly there at full brightness but at full bright it was Alittle washed out suggesting possible gas in the tube as the other tubes are at full bright and they are bright with no lines. But if I can find something or a reason why the lines are stronger on this set rather then the other ones it would be nice to get rid of even 90percent of them.

[earlyfilm]

Originally Posted by timmy

the hv is a solid 5 kv weather the bright is up or down my other sets are at 4.5 kv and they are fine. .

That is good to hear.

Then I'd check the B++ at the high side of the brightness control to ground against the other set, and substitute the video amplifier tube and compare voltages in the video amplifier circuit.

If you had a scope, you could measure the video signal on both sets starting with detection, through the video amplifier and to the CRT step-by-step and isolate the problem.

For the record: I misspoke, in my previous post. I should have said the B++ mostly supplies the sweep, HV Osc and video output. It supplies other circuits as well, but the cathodes of those tubes connect to and supply the regular B+ . These types of circuits use less parts and saves money in manufacture, but can cause nightmares in servicing, since a voltage or current problem in a circuit that is performing fine, can cause an issue in an unrelated circuit.

These "stacked B+ circuits are easier to see in the Factory schematic than in the Sams. Both are on ETF website Technical section.

James

[timmy]

The high end of the brightness circuit is 228vdc so that's right on.

[Electronic M]

It is possible the factory retrace circuit is a bad design....If you know which resistors and caps form the retrace blanking circuit, then you can try different values of capacitors and resistors to try and improve it's performance (as I've seen some do on some sets)...