Are these things I should consider bad
 

I've noticed a few things that I'm not sure about. I've posted pics.

The first is the damper on the HOT. The leads look like they've encountered a decent amount of heat. The leads are no longer silver. The same goes for the zener diode in the ABL circuit. All other diodes in the set have silver leads with no signs of discoloration.

The last one is a composite resistor in the ABL circuit. I don't know if this is a defect in manufacturing or if it's heat damage. Could be someone touched it with an iron.

Thanks.

Don't know aobut the resistor. If you know the correct value, you could measure it, or just replace it as a matter of course.

If those diodes have actual silvered leads, it is normal for them to tarnish over time. Any actual heat damage would likely cause a failure, so if working, leave well enough alone.

That's the thing. I don't know if they are good or not. With my limited knowledge, I've been able to find the ringing on the ABL. I've learned quite a bit lately from many here, but I can learn only so much via threads in a forum. That leaves me with the only thing I know, replacing components. Since I have no real training in electronics, I don't know if a zener diode in the circuit only works or doesn't work. Or if that diode could be semi-working and allowing the overshoot that is causing the ringing. I also am unsure how ringing and resistors relate or if they do at all.

I have the NOS zener and the resistors will be here on Saturday. I'm just going to replace them all and see if it helps.

You can rest assured that a damper diode either works or fails - there is no in-between. If it fails, the horizontal sweep doesn't work and you probably will blow a fuse. So, if your set is basically working and your complaint is those jail bars, it's not the damper diode. The zener, on the other hand, could fail open and the effect on operation might not be so obvious.


There are a couple of ways to check the Zener in operation. One is to measure the DC voltage across it and see if it is correct. This presumes that the Zener is conducting during normal circuit operation. But sometimes, a Zener is used in a circuit for over-voltage protection, and then it only conducts if there is a fault; in this case the voltage could be anything less than the specified value.

You can also check with a scope, but this is more complicated unless one end of the zener is grounded, because you have to check both ends and subtract one from the other to see if the voltage difference is constant and equal to the correct value.

This is where my lack of knowledge comes in. Take a look at the pics in this post:

http://videokarma.org/showpost.php?p...22&postcount=3

The schematic of the tripler with the brightness limiter and zener makes my head spin. If I'm reading it correctly, the zener is connected to the ground of the tripler and chassis ground. There's also a spark gap going across those two points. I can't seem to figure out how the zener gets its 7.5V for operation connected to two grounds. :no:

...

The Zener is not connected to two grounds, because the spark gap is an open circuit until the voltage gets very high and causes a spark. It is there probably to protect the Zener from being killed in case of a CRT arc.

I don't know if the Zener is supposed to be always regulating at 7.5 volts, or just limiting the voltage to 7.5 volts if it tries to go higher. You can find out by measuring the voltage from the top of the Zener to ground (or scoping the top) to see if it is 7.5 volts, or if it is less.

The problem was there before the recap. I didn't mention it because I figured it might have been a bad cap. That and whoever worked on it before messed up just about everything (pincushion, convergence, greyscale, etc.). It looked like crap, yet the person selling said it worked and looked great. It really wasn't until I started cleaning it up that I realized how bad the bar actually were.

I will put on the safety cover on the probe and let the plastic rest on the HOT to see if I can get a signal. I'll do the same for yoke and yoke wires.

Now the base of the HOT get the horizontal pulse from the horizontal board. I checked all those points on the horizontal board and they matched the waveforms given. The HOT's signal on the base has to go through the horizontal transformer. I haven't scoped the base of the HOT, so maybe that should be next. Maybe there's a problem with the transformer. The SM does show a waveform for the base of the HOT. It says it should be 7V.

Hang in here with me. Now isn't the 500ohm 7w resistor connected to the ground of the tripler? If so, then isn't the zener connect to that (ground) AND to the chassis point (ground).

I've attached a pic of the underside. I've circled the connections in question. In the left circle is the black wire which connects to the 500ohm resistor on the tripler ground, the white/green wire to the brightness limiter pot's T1, one leg of the zener and one leg of the spark gap. The right circle is a ground point (soldered to the chassis' top side) that contains the other leg of the zener and spark gap. I'm hoping you can understand why I see nothing but grounds and no voltage for the zener.

I can't tell much from the fuzzy picture, but I suspect that the "ground" terminal of the tripler is connected to ground only through the 500 ohm resistor and series zener diode as well as the pot and 10 ohm resistor.
Even if the 500 ohm resistor were connected *directly* to ground that would not short out the diode, as there can be a voltage drop across the resistor if current is passed through it. Do an ohms law calculation and you can determine how much current is needed to raise the voltage to the zener voltage.
jr

Here's a better picture. The red outline is the ground post for the diode and spark gap. The yellow outline is from the 500ohm resistor on tripler ground connection, other lead of the diode, other lead of the spark gap and terminal one of the brightness limiter pot. The wiper of the pot goes to base of ABL transistor and terminal two of the pot goes to ground. The green outlines are the spark gap and the diode.

The reason I'm going though all this that I am not an expert in electronics and am still learning. Until I can understand how all these connections can work when they all seem to go to ground and not one of them seems to have any connection to a power source. If I can understand where the voltage for the diode and transistor bias is coming from, it will help me greatly. Is there voltage on the ground terminal of the tripler?

Must be... the designer chose a 7 watt rating for the 500 ohm resistor... that would imply that something like 0.1 amp or more (DC or average pulse) is flowing through the resistor or else the designer would have chosen a lower rated (cheaper) part. The "ground" lead of the tripler must be at least 50 volts (DC or average of higher pulses) above ground for the 7 watt rating of the resistor to make sense. Without knowing the digram of the tripler or what it is connected to, the source of the voltage is not known... but for sure the "ground" terminal of the tripler is *not* at ground potential.

jr

So I can use my DMM to test for voltage at the resistor. I am doing my best to understand how all this works. Looking at the schematics I think I might understand how the zener is working. If the voltage coming off of the tripler (though the 500ohm 7w resistor) goes above 7.5V, it opens and sends the excess to ground? Does that sound right?

Yes, I think that you should be able to measure the voltage... it is likely a combination of DC and AC spikes, perhaps measure with both DC & AC scales. My Tek DMM 870 has a combination "AC+DC" range which is ideal, but I am not sure that many other meters have that function.

"Opens" is not the word that I would choose to describe the action of the Zener diode, it will *conduct* at its zener point and not allow the votage to rise above that point, unless its current ratings are exceeded, in which case it *will open* (burn out) and the voltage will likely go "sky high".
jr

What you called the "ground" of the tripler is not ground, it is just the low point and has some positive voltage on it due to being connected to ground through the other parts of the circuit, not directly to ground. If the Zener was connected on both ends to ground, it would indeed be doing nothing.

Okay, I scoped the HOT by placing the plastic safety tip against the shell. That's the fist pic and I see no ringing. The second pic is the focus wire and I'm assuming that's ringing on it, but I don't know if that can create the bars. Also, with the probe touching the yoke wires, I get roughly the same as the focus, but it shows less ringing the closer I get to the wires. So I'm not sure if it's really ringing or just that the probe is picking up interference.

So what would you suggest next?

If I put the probe anywhere around the flyback, I get the same as in the focus picture in the previous post. Lots of ringing. Of course I don't know if that's just because I'm not making electrical contact and just picking up on the radiating signal.

I scoped the leg of the diode that connects to the 500ohm resistor that feeds the wiper of the ABL pot, blah, blah, blah. This is what I got. It's an ugly waveform, that's for sure, but what does it mean? It was with a volts/div of .2 and a time/div of 20us.

It is really difficult to follow what is going on with the schematic incomplete and in a different thread.

Can you post a bigger chunk, so we see where the ABL signal from the pot wiper goes? Somewhere into the video amps, I assume.

Scope the B+ voltage that feeds the video amps, and look for ripple/ringing there.

Check the zener diode using the diode check on a DMM as a first go. That will conclusively check for opens/shorts. I'm betting it is still good though. Im thinking you might have a bad bypass cap in the video amp somewhere.

The wiper goes directly to the base of the ABL transistor. From there it leaves the collector, through the contrast control and then on to the base of the base of the third video amp.

I'll do the B+ tomorrow. The DMM will tell me if the diode is working as far as passing in one direction. Wouldn't I need to supply 7.5V to open for testing in the other direction? Would a 9V battery do the trick?

Okay, here's the B+. The DMM has it at 129.8V and the waveform is attached. It doesn't look clean at all. Not sure why there are two on the scope. Scope had the 10x probe, volts/div of 1 and the time/div of 10us.

Okay, I just scoped the B+ at the test point which gets it directly from the source and something odd is happening to the waveform. Hopefully someone here has the answer.

Take a look at the two attached pics. The first one is the rippled wave form with a thin trace line. Then take a look at pic two. The line is starting thin and spreading out, then back, then out, etc. It takes about 4 to 5 seconds for the process and it keeps repeating.

So what would cause this?

I think TCA is taking tripler ground to mean GROUND and of course its not.

OK TCA the "ground" on the tripler is grounded in SOME sets.
On those the ABL will usually be taken at R356, the "low" end of the
FBT HV winding.
Think of yours as the low side of the tripler.
R362 & R359 will have the HV current through them, the brighter
the pix the more current so more voltage across the resistors.
The Zener diode will keep the voltage at its cathode at
7.5 V or less.
Under 7.5V the zener does nothing.
Result is the ABL control will adj. between apx 0 V & 7.5 V
If a dark pix it will adj. between 0 & < 7.5 V & send that to the
ABL transistor.

Hope that helps

73 Zeno:smoke:

Makes perfect sense. I will file it away in my memory and hope what I've learned will help with the next lesson. :)

Is that the main B+ or one of the video outputs ???
The video out B+ is 240 VDC & filtered by C270. This cap
is notorious for causing jailbars in almost any brand.
Check C270 before going further.
In any case there is 5V of garbage there that shouldnt be.........

73 Zeno:smoke:

It is the B+ directly off of the transformer and the pin is noted on the topside by a tag that says "B+".

So I should be locating the 240V B+? I'll see if I can locate it on the schematics.

Oh, and C270 was changed during the initial recap when I got the set.

Here's the B+ 240V that feeds the video outputs. Again, I don't know why I sometimes get two traces. I don't have a reference waveform to compare it to, but it does seem that there is ringing in it.

There is no way to tell if this is significant if you don't tell us the volts/division setting of your scope so we know what the peak-to-peak value of this waveform is.

Edit: I see you did tell the settings on a previous waveform. I suggest you check the filter cap as suggested by Zeno.

What is the amplitude of that ringing on the 240V line? What is the DC voltage?

Can you try temporarily bridging another cap between 240V line and ground to see if it improves the bars or cuts the amplitude of that ringing?

Post the section of the schematic that shows C270 and this supply for those of without a Sams handy...

It had the 10x probe on volts/div of 5 and time/div of 20.

I'll double check the cap. I is new and tested good w/in spec when I tested it before install.

You'll have to forgive my lack of knowledge. I'm learning how to use the scope as I go along. All I can tell you is it was 5 volts/div with 10x probe and 20us time/div. How can I get the amplitude of the ringing? Do I increase decrease the volts/div and use the vertical scale just on the ringing portion?

As for the cap, I know I need one over 240V, but is there a particular value I should use? Also, should I parallel it with the current one or can I just find a 240V source and put the cap on it to ground?

I've attached the section of the schematics with the C270 a 10uf 300V. The T207 is the Flyback.

The waveform looks to be about 1.5 divisions peak to peak, so that could be either 7.5V or 75V, depending on whether or not the "5V/div" is taking the 10:1 probe into consideration.

75 vpp ripple on a 240V supply is a problem. 7.5V, not so much.

The doubled waveforms might be caused by improper triggering. Make sure you are triggering on whatever channel the probe is connected to, and if your scope has a TVH trigger mode, use it. Try rocking the trigger level back and forth around zero, and see if that makes the trace clearer.

I would start with the same value cap as you already have installed, and putting it right in parallel should be fine if it actually goes to ground. The attachment isn't showing up for me...

The probe was in the 10x mode. Also, the triggering is from a Leader LCG-396 Pattern Generator.

The cap is connected the following way. There's a lead from the flyback to a leg of a diode, the other leg of the diode connects to a leg of a 150 ohm 2W, the other leg of the resistor goes to the positive side of the cap and a lead that connects the 240V around the chassis and finally the cap goes to ground.

I'll have to find a local source for a 10uf 300V cap. The local Radio Shack doesn't have them in-store. Must be a place around here somewhere. Not looking forward to paying 7 bucks for shipping of a single cap.

Got any dud CFL bulbs? They usually have a 10uF 250-450V cap in the base...Though not exactly as likely to be good as a new cap.

But some scopes can automatically detect the probe setting, and adjust the vertical scale range to reflect the probe attenuation and prevent confusion. This requires both a scope and a probe that support this feature. Do yours?

If your scope is doing this, then the scale really is 5 volts/division. If your scope or your probe doesn't do this for you, then the scale is actually 50 volts/division. This makes all the difference in the world when trying to interpret the waveforms as ripple on a power supply rail.

I have a Tektronix 465M. No auto settings for the probe.

I've been thinking about the waveform. It's a 240V source, so the 5V/Div would be too low to show it and the 10x probe would make it 50V/Div and it would cover almost 5 divisions. What I got was way under that. I'm going to use the DMM to go all the way back to the boost diode and when I find the point it's 240V, I'm going to scope it. With a setting of 5V/Div and the probe in 10x, I should get a form that covers over 4 divisions, right?

Thanks for the heads up. I have a crap load of new 26 and 13 watt CFLs from China and I don't mind gutting one or two. I'll let you know what I find.

Where is your baseline set? Are you operating the scope AC or DC coupled?

Typically, for checking ripple on a power supply, you would operate AC coupled, so you only see the AC component of the signal, and ignore the DC. If you are running DC coupled, the baseline should rise a bit less than 5 divisions (250V) from zero, with ripple superimposed on it. You can do a quick sanity check with a DMM, by measuring the 240V supply twice, both in DCV range (should show ~240V), and in ACV (should show close to zero if no significant ripple is present).

From what you are telling us (5 V/div with a 10:1 probe and no auto priobe ranging), the scope is actually showing you 50V per division, or a waveform of ~75 Vpp. This is WAY too much noise on a supply rail, and would easily explain the jailbars.

You really need to learn how to use a scope effectively in order to get useful information from it. Take a couple hours and watch this GREAT YouTube training session:

http://www.youtube.com/watch?v=8ZKMrzTGxLQ

Gonna get some popcorn and watch that video.

Okay, just did a scope of the 240V line just after it comes off of the resistor into the main feed (where the capacitor is and the where the 240V gets distributed). I had the scope set to AC coupling, V/D set at .5, the T/D set to 20us and probe at 1x. The rippled waveform covered 1.75 divisions peak to peak. So that would make the AC less than a volt, correct?

.5 V/div and a 10:1 probe is 5 V/div. A signal 1.75 divisions high would be 1.75 x 5, or 8.75 Vpp.