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Model 4 Screen Brightness Issue

hideehoo

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Joined
Aug 4, 2015
Messages
44
Location
Prior Lake, MN
Once the CRT on my Model 4 warms up, this is what my screen looks like, even at minimum brightness.

IMG_20160228_164107.jpg

The front brightness 500K pot tests OK, and it appears minimum brightness happens at the zero ohm end of the scale. Sprayed it with contact cleaner and even jumped the contacts to force zero ohms, and no change in brightness. Video isn't shaky at all in either mode, so a C210 adjustment doesn't address this issue.

Thoughts?
 

Petrofsky

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Jun 4, 2012
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169
Location
California
The front brightness 500K pot tests OK, and it appears minimum brightness happens at the zero ohm end of the scale. Sprayed it with contact cleaner and even jumped the contacts to force zero ohms, and no change in brightness.
Has the connection to the brightness knob come apart back at the video board? That would mean you would get infinite resistance (and therefore maximum brightness) regardless of where the knob is positioned or whether you shorted the wires going into the knob.
 

hideehoo

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Joined
Aug 4, 2015
Messages
44
Location
Prior Lake, MN
Look on the video board for an "INT-BRT" (Internal brightness) control and turn it down a bit.

I have the RCA board, not seeing an INT-BRT pot either on the board or in the schematic (Model 4 Technical Reference Manual). The three pots I do see control size and hold. The TCE board does show INT-BRT as VR303.

IMG_20160301_002319.jpg
 

hideehoo

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Location
Prior Lake, MN
Has the connection to the brightness knob come apart back at the video board? That would mean you would get infinite resistance (and therefore maximum brightness) regardless of where the knob is positioned or whether you shorted the wires going into the knob.

Brightness pot is definitely still hooked up. If I turn in up from minimum brightness (zero ohms), it gets much, much brighter.
 

retrogear

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Welcome from another Minnesotan !!! Yes, probably a bad cap in the blanking or bias of the CRT. Do we have a schematic that matches?
I could try and help narrow down the possibilities ...

Larry G
 

hideehoo

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Prior Lake, MN

retrogear

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Very likely C517 and / or C518 on the lower right of the schematic coming off a pin of the flyback transformer. A boost voltage is created by them (I think it says 80V source - hard to read). That provides the cathode and grid bias for the tube. If one of those caps is weak, the 80V will drop the cathode below the blanking threshold so you get retrace lines (the white diagonal ones in your photo) and increases the grid / cathode bias to brighten the tube. I'd wager money on it but I'm baptist so I don't gamble :)

PS - I don't think it would be the flyback because that would lower the high voltage and de-focus the tube and your focus looks very good.

Larry G
 

hideehoo

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Aug 4, 2015
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Prior Lake, MN
Replaced all the electrolytics on the board, now the brightness at minimum is even higher than before. I used general purpose (instead of low ESR) Nichicons since a little Googling seems to indicate low ESR caps in CRT applications could introduce unwanted oscillations.
 

retrogear

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Bummer. If you look on the left of that schematic, connector J101 pin 4 ties to point E304 which at the top goes to the G1 of the tube. I'm guessing J101-4 goes to the brightness control. Can you measure the DC voltage at that point and see if it varies with turning the brightness control and what the voltage range is?

PS - while you're at it, take a DC reading on pins 2 and 5 of the CRT. Guessing pin 2 would be like 50-60V and 5 would be 10-20V ...

Larry G
 
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