I've been working on my 7603 Mainframe oscilloscope and just finished replacing diodes and some caps in the Intensity and Focus DC Restorer sections of the HV board. Here's a link the an earlier message:
/g/TekScopes/message/146928. After parts replacement Intensity and Focus seem pretty good to this newbie's eye :-)
Now, I'm concerned about Q1152 on the Z-Axis board being almost too hot to touch. The PCB in this area is now brown from years of running hot. The Tek # for this transistor is 151-0406-00. I've checked it and its 'brother' Q1132 with one of those inexpensive ZIF component testers and the hFE on Q1152 shows 207 and on Q1132 it's 136. This doesn't match the device's spec sheet, but I'm sure these testers don't correlate to any published values.
From reading over a few similar posts on this group I checked to make sure R1152 measured 3.9K Ohms and verified the base voltage of Q1152 is at 103V. I got the impression Q1152 does run warm to hot normally. Do you think It's OK to swap Q1152 and Q1132 to see if the heat issue follows the components? They are the same part # and operate in almost identical sections, Z-Axis Amp and Auto Focus Amp.
I haven't done much with board except replace R1146 and R1126, the two 1 Watt, 20k Ohm carbon resistors with newer metal film ones elevated a bit to improve heat dissipation. I also replaced the Astigmatism trim pot, R1193, just because it smelled as if it had been 'cooking' for quite some time.
As stated in the earlier post, the LF power supply voltages are 'pretty darn close' to spec. I do not have a way to measure HV at this point.
I have yet to go through checking all components on the board for obvious out of tolerance values. I've been hunting around for a heatsink that will fit, but I don't think I have one handy at the moment.
I look forward to your thoughts on ways to possibly lowering the heat dissipation of this device. I did read that this transistor can be replaced with a 2N5401. This transistor has an epoxy TO-92 case. Heatsinking these transistors is difficult. It seems like using it as a replacement would create more troubles.
Thanks!!
--Al
