From: “Dan Meeks”
To: TekScopes@yahoogroups.com
Subject: RE: [TekScopes] Re: TDS754C for $1000 on Ebay

Fixed!

Thanks to Hakan for pointing me in the right direction.

For future problem-solvers, here is the story:

TDS754C arrived with dim screen and also the screen was too large in both dimensions. It was wrapping slightly around the edges of the tube. I could adjust the brightness using the trimmers on the left side of the high voltage power board (these trimmers are available with the outside cover removed, just in front of the fan). But even with the brightness back up, the screen still overlapped. The brightness adjustment didn't affect the screen overlap either.

There is a 75V and -75V rail on the high voltage board that is derived from the same flyback as the high voltage. +75V is available at J60, a wire leaving the PCB, just in back of the CRT socket. On my board, the 75V rail measured 67.9V, or about 10% low. Turning the HV adjustment (R58, on the right side of the board near the edge) had no effect at all.

I have another HV board from a TDS744, and ohming around U45 (the PWM generator that controls the HV) didn't help - it looked the same as the old board.

However, U45 was indeed bad. Replacing that IC (MC34060P) brought the 75V rail up to 76V (and I adjusted it down to 75V). The screen is much brighter (I turned the brightness back down), and the screen size is normal.

Here is a summary of the controls on the HV board, for future reference (note that your board may be different so no guarantees here):

HV adjust: R58
Horiz Size: R172
Horiz Position:
Vertical Size: R270
+21V adjust: R186
Vertical Position: R402 (available next to the fan)
Contrast: R404 (next to the fan)
Brightness: R403 (next to fan)
Contrast: R404 (next to fan)
Trace rotation: R401 (next to fan)

Hopefully that will help someone in the future.

Dan