25K7191 Heater Voltage Low / CRT Rejuvination

[Kevin Mullins]

Been doing a slew of monitors lately, but have one now that I need some opinions on.
(and schematic interpretation)

P538 main chassis -

Capped and usual solder joint stuff done to it....works fine..... except that it's quite dim, hard to adjust, come to find out the heater voltage is low. Should be around 6.3v AC, but only getting between .5 - 1.5v. Looking at the schematics obviously this voltage is derived straight off the flyback. (R213 on the neckboard is good, voltage is low on either side of it)
What I'm trying to figure out is where on the opposite side of the flyback winding does the voltage come from. That particular winding according to the schematic is kinda split between two others.

Basically I want to check that the voltage feeding the flyback is good that leads up to the heater pin on the neckboard...... but where?

Make sense ?

At first I was thinking it was the tube and started to gear up to try and rejuvinate it, but am in the process of building an adapter for the tube for this old Sencore CR161 I have with no manuals.
But then when I measured the heater voltage to have an idea what to set it at I went "Hmmmmmm", because I knew what I saw just wasn't right.
(and would cause the dimness which is the problem)

Schematic was too large to upload, but here's a link:
http://arcarc.xmission.com/PDF_Monitors/Wells%20Gardner%2025K7191.pdf

[qrz]

a scan derived supply will track B+.
if that is low, all the windings will be off as well.....

also bear in mind the voltage applied to the fils is pulsating dc,  rather than filtered dc.
usually, it is a negative going signal.  meaning , the downward peak will be at groung potential.


qrz

[Kevin Mullins]

a scan derived supply will track B+.
if that is low, all the windings will be off as well.....

I had actually considered that and double checked it..... it is spot on.
And I mean when it says it should be 130v it is exactly 130v.

also bear in mind the voltage applied to the fils is pulsating dc,  rather than filtered dc.
usually, it is a negative going signal.  meaning , the downward peak will be at ground potential.

"pulsating DC" ? - I had only found documentation saying it should be 6.3v AC, maybe they were referring to the "pulsating" aspect.
I would assume this to be difficult to measure with a basic multimeter then. (I don't have a scope)
The ground potential you described might explain why when I reversed my leads the value would vary slightly. (which I thought was odd for AC)


Question though - I was just going through and checking what voltages I could going into and out of the transformer and came across an oddity to me.
(refer to pic)
When measuring points 4 & 5 on the transformer, the 24v line measured fine, but the 12v line had nothing.

The general circuits look identical as best I can tell leading to the transformer.

On the 24v circuit I can read the voltage on either side of D13 and either side of R91.

But on the 12v circuit I can only read off the cathode side of D14 and no reading on either side of R92.
Tested the diode out of circuit and then even swapped with a different one, no change.
It's like the 12v just dead ends there which puzzles me because I would have assumed it would read just like the 24v circuit did.
Granted it doesn't ground right off like the other circuit does, so maybe I need to follow it further on. Just looked at it again and it does tie to ground, just on the other side of the transformer along with the heater.

Electrolytic caps are new...... should I maybe look at the .001 mylar cap?

Or am I just missing something here?

Like I said, I'm just trying to verify all voltages at the transformer to see if there is something missing causing the low heater voltage.

[Rickn]

First off , when the voltage comes off the flyback like this it can only be accurately measured using a "True RMS Volt meter"

Even saying that it is a little misleading.

Waht happens is that the flyback is running at what ever the scan rate is ( standard res is 15.75). The diode and filter network attempts to turn it into DC, but the filter is not 100 percent efficient... therefore a misleading measurement.

So for now, is there an orange glow in the neck showing that the heater is on... if so I would not get hung up on it.

Rick@niemandisplays.com

[Kevin Mullins]

So for now, is there an orange glow in the neck showing that the heater is on... if so I would not get hung up on it.

Yes, the heater is definitely on.
The monitor is working fine except for the dimness and general adjustments are either not enough or make it worse. (almost from either a totally dim to an overexposure effect extreme)

But I wanted to investigate the heater voltage before I jumped into trying to shoot the tube.
(which I've never used this unit before)

[qrz]

another option would be to make ur own filament winding.

just loop 3-4 turns of insulated wire ( 22ga is ok) around the core and connect to the fil  .
wires must be parallel and not overlap.   approx 2 volts per loop is produced.
be sure disconnect orig filament connection !!!
usually a good idea to run a 2.2 ohm 2w res in series with the fil.

qrz

[MonMotha]

If the filaments in the tube are glowing, it's probably at least pretty close.  Unfortunatley, a True RMS meter is about the only way to measure this.  It's a "choppy" signal, not a nice 50/60Hz sine-wave AC like cheap multimeters are designed to handle on their AC setting.

The "make your own filament winding" technique qrz describes is a fairly common technique I've seen in repair guides, though I've never done it.  It's often used to isolate HK shorts.  I've also seen people advocate boosting the filament voltage some (often using this "wind your own" technique) to bring nearly dead tubes back for a little while.  Running heaters at too high voltage (and it doesn't take much - 10% over is a lot) has a rather adverse effect on the lifespan of the tube, so it's best to save that for a last resort.

If you happen to have a beefy bench supply or a suitable transformer, I've heard of people isolating the heaters and apply exactly 6.3V (DC or RMS AC) for testing purposes.  If that suddenly brings it to life, you know that you do have a problem with your heater voltage.  If nothing changes, then it's probably a weak tube (assuming other causes have been eliminated).  A bench supply or variac is great for this because you can attempt the "boost" technique in a controlled manner.

(As a note, most of my knowledge of this stuff isn't from CRTs but rather RF and audio tube stuff, so the "real" monitor/TV guys feel free to correct me)

[Kevin Mullins]

After getting a couple mentions from ya'll in respect to the heater "glow" I decided to step back from that and take a different look at things. The more I looked at it the more it looked "about right". Even if I couldn't measure it properly, it just looked right to me. I didn't want to go adding windings or anything like that as I knew it didn't have any direct H-K shorts or anything like that. So I decided to go back to my initial assumption of it being a weak tube, as it was also mentioned as a possibility.
Well, that meant breaking out the Sencore unit I had never played with and of course it didn't have the right adapter with it I needed. So once again I'm at the BYO point.
One of the adapter sets that was with it already had one that had been kinda hacked up to be modified for some other tube, so that made it an easy choice. (as I usually wouldn't just hack something like this up) After grabbing a few pinouts and digging around through my parts boxes I found everything I needed. Started with a K4600 neckboard that had been busted up anyways, pulled the crt socket and went after it.
And of course I don't have the manuals either for this unit, but most of it seemed pretty self explanatory by looking at the control panel.
First thing to do was set the gun balance, checked the emissions on each gun and they actually looked really good. Checked for shorts, none. (which I already kinda expected) But I go ahead and I go to "Remove Shorts" on each gun, needless to say the first one caught me off guard and scared the crap outta me as I was watching the lights on the panel and not the tube itself. Thought I was going to have to clean my shorts. So I do that to each gun and rechecked emissions and whatnot and all still looks good, meaning I hadn't broken anything yet. And each one gave off a pretty good crack/spark.
There are three settings for Rejuv, but I only tried the first two as the second one made the heaters come up really hot, hot enough I wanted to stop right there. And hitting the button after the heaters warmed up didn't seem to do anything.
So I slap the neckboard back on and ...... WOW !!  What a difference. Much brighter, had to go back and readjust everything back down. And now for the most part everything seems to adjust as they normally would. The colors were back and vivid again as before they were there, but just crappy dull.
Definitely going to use this Sencore CR161 unit more often now. I have an older CG141 signal generator too I need to figure out as well.

But all in all the monitor looks really good now and I was just happy to learn some new stuff along the way.
(even though I did start to over think things and complicate it a bit)