I've been trying to run through the calibration process for the 475A that had a blown Z-axis amp, and I've run into trouble right off the bat. I was unable to get a visible spot on screen at the +15 V intensity level as indicated in the service manual. I had to crank the intensity up nearly to maximum, and then the trim pot (R1375) for grid bias had to be cranked to one extreme to see anything on screen.
I tried trimming up the power supply, even though everything was within spec (everything was a little low, but not out of spec, so I trimmed the +50 V rail up to +50.18V, just shy of the upper range in the service manual. All the other low voltage rails now look spot on.
I tried the same process on two other 475s (non-A) and was easily able to get a visible spot at the +15 V intensity level, as well as adjust the grid bias for a visible spot across a wide range of the grid bias trim pot.
Next, I followed the diagnostic procedure in the service manual for the Z-axis circuit on page 4-18. I set up the scope as indicated, lifted on end of CR1343, and measured TP1364 to be +6.05 V (as indicated in the SM). When I press BEAM FIND the voltage jumps up to +29.3V, which is higher than the SM's indicated +25 V. Next I checked the voltage across VR1362, which is +6.23 V, which is the indicated value. Next I removed Q1362 and checked the voltage at the base of Q1352 which measured +1.34 V, and the voltage at the base of Q1344 which measured +0.67 V, both as indicated in the SM.
After that I lifted one end of CR1341 and measured the voltage on the collector of Q1338 while turning the INTENSITY knob through its full range of travel; the collector voltage measured from +7.6 V to -1.1 V (the later being a little short of the indicated -1.3 V from the SM). I also checked the voltage at the base of Q1332 which measured -1.75 V (as indicated). I then removed Q1332 and disconnected J1333 and J1334, and measured the collector voltage on Q1338 again as I turned INTENSITY through its range; again +7.6 V to -1.1 V
So I'm seeing a slightly higher than expected voltage when pressing BEAM FIND at TP1364, and a slightly narrower than expected range of voltage at the collector of Q1338, with the "top" of the range falling about 200 mV short of what the SM indicates. I'm not sure that any of this explains why I can't do the grid bias adjustment as indicated in the manual.
Just for good measure I also swapped the CRT from one of the 475s that I was able to successfully perform the grid bias adjustment on with the CRT in the 475A (they happened to be exactly the same part number, even though the SM for the 475 says that the CRT should be PN 154-0677-00, what I found in my donor was 154-0677-10, which is the correct part for a 475A), but the trouble did not follow the CRT.
I guess I could still check to see if the trim pot (R1375) appears to be in spec (if it isn't it would explain a lot), but I'm otherwise at my wits end.
Any help or advice would be appreciated.
-- Jeff Dutky
