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Caution with Trimpots

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Tip from Tom Herman:  One of the adjustments listed in the service manual is for one to adjust the Screen (R,G,B) VR trimpots (these are the three knobs behind the speaker grille) until the guns start to raster.

Remember that the service manual is written for the service techs or calibrators.  So unless you know what you are doing, it is highly recommended that you do not touch these three knobs.  Tom explains below:

"DON'T TOUCH the "Screen VR" knobs, this is the coarse control for gray scale/ color temperature. It will mess it up & is nearly impossible to get it back again, with unaided eye!

By "just start to raster" they mean:

  • have a program or test image displayed
  • initialize the various CRT "Cut" values in Service mode
  • look into the CRT lens
  • adjust screen VR until the the slightest hint of light output just starts to be visible on the CRT (typically the edge of the CRT starts showing some light output first)

Then, one needs to do a full gray scale calibration from scratch."


If you do happen to make the mistake of working with the trimpots, a fellow enthusiast offers the steps below to restore it as best one can short of getting an ISF calibration:

"You can reset the pots by:

  • Reseting the xCUT and xDRV parms to 40H
  • Switch to video-3 (make sure no source is plugged into video-3) and stick your nose against the screen.
  • Now turn each pot in turn until the raster just dissappears. You need to be very very close to the screen to see this.
  • Use a ramped grey scale or similar pattern to set the xCUT parms to get the blacks looking black and the xDRV parms to get the whites looking white.
  • Clean your nose marks off the screen."

This seems to work as a temporary measure.

Tom Herman, however, makes the comment about the "restore" procedure below:

"While the previous answer is the "right" answer, it is oversimplified; in practice making "blacks look black" and "whites look white" is a tedious excersise even when instrument assisted. 

Whereas you can get the set "watchable", only a proper gray scale adjustment is going to restore the set to accurate rendition (6500K +/- 300K)"


Some Tosh sets have the trimpot orientation changed/different from the service manual.  If you have a set that have the new trimpot design and you don't look closely at the pots, you could mistakenly turn the wrong know and mess up your system.  Find out more....