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 Post subject: General Electric 821 Chassis Horizontal linerity issue
PostPosted: Jun Fri 24, 2022 2:15 am 
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Location: Arvada, CO, 80004
I am fixing a GE 821 for a regular customer of mine. He initially brought it to me for a white bar in the picture. It turned out that the horizontal drive was turned too high. Now where the horizontal bar is, is a stretch of the left (Photo was take sideways. I took a scope view of the issue. Here are some photos as well. The R is stretched. It is probably the H signal, and all of the caps were replaced. But I don't know what else to try.


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 Post subject: Re: General Electric 821 Chassis Horizontal linerity issue
PostPosted: Jun Fri 24, 2022 3:19 am 
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Generally stretching on the left side is due to insufficient damping, a low emission damper diode can do it.

The drive wave to the output tube g1 grid has some effects on the linearity, but only about the right hand 2/3 of the scan and as you know if the drive is too high, it can put a fold over on the current when the plate current starts and you can get that vertical white line effect (that is the beam tracing back on itself a few times).Your measured drive waveform doesn't appear to match the schematic so you may want to check into that.

The left hand 1/3 of the scan is due to damped current. In tube sets, the linearity was generally adjusted with a series resonant circuit which modulated the supply voltage of the plate primary circuit with a parabolic like wave, just off the side of the circuit you posted, where C322 is, is the modulation voltage applied to the LOPT primary, you could check that on the scope, but generally this had fairly minimal effects on the left sided linearity, it largely affects the center, but if that L-C network is off it can affect the linearity elsewhere.


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 Post subject: Re: General Electric 821 Chassis Horizontal linerity issue
PostPosted: Jun Fri 24, 2022 3:47 am 
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Thank you! I has going to do a ringing test on the flyback/yoke. I did not know that a weak damper tube could cause that. And yes, They signal looks nothing like what it should. However, in other schematics, it looks normal. I has going to feed a horizontal signal, just to check it out as well. I was debating on whether the signal or the yoke/flyback LC network was off.

And for the schematic, that one was off a GE 805 from Rider, which uses a similar chassis to the 821. Here is the Sams of the Horizontal for that.


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 Post subject: Re: General Electric 821 Chassis Horizontal linerity issue
PostPosted: Jun Sat 25, 2022 1:28 am 
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Location: Belrose, NSW, Australia
Once you set the Hor Drive to just eliminate the drive line, the drive waveform has little effect on the linearity.

A good starting point is to monitor the hor OP tube cathode voltage (if it has a resistor in it) and adjust the hor linearity coil for minimum current. That will generally get you close to best linearity, a small tweak either side of the minimum voltage (and hence current) point will generally be all you need. Try not to go over about 10% of minimum.

Doing it this way gives best hor op tube and flyback life.

If you then need to adjust the width, go back and check the linearity again. Width will also affect the tube current.

Oh yes. If there is a cathode resistor it will be bypassed with a capacitor. Unless it's a film cap, replace it.

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 Post subject: Re: General Electric 821 Chassis Horizontal linerity issue
PostPosted: Jun Sat 25, 2022 5:19 am 
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Location: AUSTRALIA
Location: Queensland.
Mr. Highlander wrote:
They signal looks nothing like what it should. However, in other schematics, it looks normal.


The signal you measured on the scope has basically the correct form to be a good grid drive for a horizontal output tube. It is a trapezoidal shape which is a mixture of a sawtooth (during scan time) and a rectangular wave(during flyback). The idea being that the grid gets taken very negative at flyback to ensure the tube is properly cut off. Normally this wave is created by placing a resistor of a typical value around 4.7k to 10k in series with the sawtooth forming capacitor. Part R99 on the last circuit you posted. It goes by the name "peaking resistor".


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