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 Post subject: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Wed 04, 2021 6:04 pm 
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Joined: Aug Wed 04, 2021 5:47 pm
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Location: Stone Mountain, GA
I've got a B&K 1077B (Rev E) with a vertical instability problem. The vertical deflection will vary at both the top and bottom on the screen. You can see the raster on the CRT in the analyst jump as the raster on the TV under test's raster jumps.

I have tried the following things with no improvement:

1. Replaced Vertical (13GF7) with a NOS tube.
2. Subbed both height and vert. lin. controls with fixed resistors.
3. Literally replaced all of the resistors and capacitors in the vertical section.
4. Replaced all of the power supply electrolytics.
5. Resoldered all connections on the sweep PC board.
6. Cleaned all tube sockets with De-Oxit.

The only things I haven't replaced or substituted are the vertical output transformer, the yoke or the two 560 ohm resistors across the vertical yoke windings.

The only waveform that looks suspicious is the cathode of the vertical output stage. It jumps up and down in sync with the raster. It's voltage reading is fairly steady, varying just a fraction of a volt. The grid waveform and plate waveform look steady.

I have observed that the jumpiness diminishes somewhat as the unit warms up completely after about 10 minutes, but never becomes rock steady.

I'm open to any and all recommendations.

Thanks.

Tom


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Thu 05, 2021 12:03 am 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 3605
Location: Dallas, TX
That seems like a "puzzler".
It could be the vertical transformer or the yoke. The small change in the cathode voltage may reflect a change in loading.
You might try looking at the transformer secondary voltage. I would check the schematic to see if the scope ground would cause a problem. If your scope is a dual trace that you can do a differential measurement with, that might be best. Also make sure the scope probe(s) can handle the voltage.

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It's not the Destination, It's the Journey.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Thu 05, 2021 8:22 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 1565
Location: Belrose, NSW, Australia
I was just about to suggest the same thing!

Two traces on a CRO, one monitoring the yoke current if possible by inserting a 1 ohm resistor (assuming one side is grounded?) and the other looking at the plate of the oscillator, going forward to the output satge. Compare what happens when the sampled yoke current dips. A DSO is useful here because you can capture the change when it happens and analyse it at leisure.

I have known a couple of cases of a similar fault being caused by an intermittent shorted turn in the vertical output transformer. One of these was VERY hard to track down because it was in an HMV F4 which uses negative current feedback from the yoke. This tended to "hide" the fault.

Back when this was made they didn't have polyester insulated winding wire so such faults were more common.

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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Thu 05, 2021 12:40 pm 
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Location: Sunbury, Ohio 43074
Sounds like perhaps an old carbon resistor "warming up". Or the yoke lol. Hope that helps :mrgreen: Unfortunately the only sure fire way to troubleshoot an intermittent yoke would be a substitution. Are you anywhere near Columbus, Ohio? I've got about 4 of these things waiting for me to decide what to do with them. One will be restored.... someday. The others... ??

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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Thu 05, 2021 2:09 pm 
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Location: Stone Mountain, GA
Barry, I'm in Stone Mountain, GA near Atlanta. Quite a trip to Columbus. I had a 1076 with the same vertical output transformer which was non-working which I gave to a teenager to tinker with just before I started working on this problem.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Thu 05, 2021 3:51 pm 
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Location: Stone Mountain, GA
On second thought, I don't think it could be the yoke. If a yoke gets shorted turns, the display becomes a trapezoid. When the vertical sweep decreases, the top and bottom lines are perfectly straight - no slanted lines like in a trapezoid.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Fri 06, 2021 12:21 am 
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I don't have the schematic, but I assume that when you are measuring the cathode voltage is is across a cathode resistor. So what you are seeing then represents some random fluctuating current in the load. Probably, the most likely place this would occur would be in the vertical output transformer, where the copper wire is badly soldered to the flying leads, though it could be in the yoke.

You can calculate the primary load peak current (that occurs at the end of scan) by dividing the peak cathode voltage you see on the scope by the cathode resistance, assuming there is no bypass capacitor. If there is one, simply insert a low value resistor like 10 Ohms in the ground leg of the cathode RC network as a current sense resistor.

Once you know that peak current value, disconnect the output tube's plate connection for the vert output transformer's primary (don't forget to extinguish the CRT beam before that as you will have a bright horizontal line) measure the voltage between the transformer primary connection & cathode connection on the tube socket, calculate a power resistor say 5 or 10W rated to connect across these terminals to give a similar peak current value to your previous measurement.You might have to allow for the DCR of he transformer primary. In other words run a DC current via the output transformer primary, and look again across the cathode resistor or current sensing resistor for any random fluctuations in the current. This will at least rule out the transformer's internal primary connections. (probably the 13GF7 stays in place as you have a series heater string? not sure, if not you can just unplug it)

You can also do similar tests with the yoke, with an appropriate DC current, similar to the peak value it experiences in use during scanning, to try to determine if it is the yoke or transformer that is giving the trouble.

The idea of course is to use test DC currents as a diagnostic, but you want those currents to be similar to what they are in use, otherwise it could either mask the problem or overheat the windings.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Fri 06, 2021 5:44 am 
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Location: Belrose, NSW, Australia
Hugo's suggestion of replacing the transformer primary with a resistor is spot on.

Unplug the CRT base or otherwise extinguish the beam, though.

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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Fri 06, 2021 9:37 am 
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Joined: Jan Thu 16, 2020 12:29 am
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irob2345 wrote:
Hugo's suggestion of replacing the transformer primary with a resistor is spot on.

.


I was suggesting replacing the output tube with the resistor, so as to check the transformer primary.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Fri 06, 2021 9:57 am 
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Location: Sunbury, Ohio 43074
Is there a "vertical hold" control or circuit in the 1077B? Might want to check into that.... it could be what's causing the jumping.

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https://www.bbtvtestequipment.com


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Fri 06, 2021 4:11 pm 
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Joined: Aug Wed 04, 2021 5:47 pm
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Location: Stone Mountain, GA
I checked out the vertical pulse generation early on. I used the "line" setting on the triggering and the vertical pulse was rock steady which makes sense since the vertical pulse is derived from the HV winding of the power transformer. I used dual trace to see if there were any time variation between the vertical pulse and the vertical output cathode signal. Again, they were in lock step.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Sat 07, 2021 2:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Beaver Falls, PA. USA
That could be a transformer problem. I once had a Philco console chassis in for repair, which had a periodic ticking sound in the audio. All components and tubes were good, and substitution revealed that the output transformer was the culprit.

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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Sat 07, 2021 4:46 pm 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
i think it is a transformer problem also.
I didn't look at a schematic until now. The output tube cathode resistor is bypassed with a electrolytic so that would smooth out any waveform variation.
The basic vertical timing is derived from the 60 Hz power line, so syncing the scope on "line" makes sense.
You can monitor the grid waveform on the vertical output tube, if it is steady there you know the problem is after that point, leaving the output tube circuit, the transformer and yoke. Since you replaced the tube and surrounding capacitors and resistors that leave the transformer and yoke. A yoke problem would probably cause other symptoms that pretty much mean it is the transformer. Of course it could be some oddball rare problem like the wiring insulation breaking down but odds are it is the transformer.

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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Wed 11, 2021 7:46 am 
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Joined: Jan Thu 16, 2020 12:29 am
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Notimetolooz wrote:
i think it is a transformer problem also.
I didn't look at a schematic until now. The output tube cathode resistor is bypassed with a electrolytic so that would smooth out any waveform variation.
The basic vertical timing is derived from the 60 Hz power line, so syncing the scope on "line" makes sense.
You can monitor the grid waveform on the vertical output tube, if it is steady there you know the problem is after that point, leaving the output tube circuit, the transformer and yoke. Since you replaced the tube and surrounding capacitors and resistors that leave the transformer and yoke. A yoke problem would probably cause other symptoms that pretty much mean it is the transformer. Of course it could be some oddball rare problem like the wiring insulation breaking down but odds are it is the transformer.


I agree with this line of thinking.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Thu 12, 2021 9:53 pm 
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Joined: Aug Wed 04, 2021 5:47 pm
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Location: Stone Mountain, GA
Success!

I tried the procedure Acornvalve recommended to test the vertical output transformer. No instability at all. I even tried mildly nudging the windings to see if that caused any instability. Rock Steady.

I put the vertical circuit back together. The instability was still there. I started nudging and tapping things on the deflection board. I noticed that the instability was worse when I wiggled the horizontal oscillator tube. Hmmm. That doesn't make sense. I go back and resolder all of the PC connections near the Horizontal osc. tube. Still there. I get out my magnifiers and start looking at every PC run and solder joint. Then I see it. Hidden under a component from a nearby terminal strip I see what looks like a break in a PC run. I check for continuity and it was open. I scrap a little of the solder mask away from either side of the break and repair the break. The run connected the boost B+ to the 390K vertical charging resistor. The instability was gone.

Many thanks to everyone who contributed to this thread.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Sun 15, 2021 2:32 am 
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Joined: Aug Wed 04, 2021 5:47 pm
Posts: 6
Location: Stone Mountain, GA
Thought I would pass one several things I learned while working on the 1077B.

Bean Current. B&K specifies 75 uA. Mine was setting at slightly over 100 uA. Reducing it to 75 uA improved the spot size and sharpened things up somewhat.
Sync Level. I never could get the display on the TV set to have enough brightness or contrast like I could with my old 1076. B&K specifies 0.7 V p-p for the 1077B and 0.5 V p-p for the 1076. I settled on the 0.5 V setting and now have good brightness and contrast.
Working on the vertical circuitry. If you are working on the vertical circuitry and need to keep the CRT from coming on, just use a clip lead and connect it across the CRT's filament. Both ends of the CRT's filament are readily accessible underneath the chassis. No bright thin horizontal line on the CRT.


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 Post subject: Re: Vertical Instability On B&K 1077B TV Analyst
PostPosted: Aug Sun 22, 2021 11:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1807
Location: Phoenix, Arizona
One thing you should do first is remove the bottom cover and re-solder every tube socket and power resistor, and anything else you can reach. Ground lugs.

I have worked on a bunch of these and never had a bad yoke or transformer. Not hat you may be the first.

Electrolytic capacitors should be replaced.

Jim


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