I would appreciate if somebody who has Data Precision 8200 voltage calibrator could check if the power transistors on the display PWB get hot. There is no need to remove the display, just touch it on the back side where the transistors are located (big tinned dots).
I have recently purchased DP 8200. It had problem with few segments on the display but otherwise seems to be operational. I removed the display PWB to replace the bad driver, 74C374, and I noticed that the PCB areas around each of the power transistors that are driving anodes of the 7-segment display are brown. I powered the unit after the chip replacement (I used 74HC374 that I had in stock). Everything worked fine but these transistors get very hot - 78C if all 7 segments are on, little bit cooler (around 65C) when only 2 segments are on. When box is closed, things will get even hotter - there are no ventilation holes.
I did a few quick measurements and results are shown on the attached schematics. Supply voltage on the collectors of Q201-207 is about 8.8V. Voltage drop on these darlingtons is about 4.5V. Voltage drop on 10 Ohm resistor connected to individual segments (R201-R208) is about 1.7V, that means current through each segment is 170 mA. So, the max. power dissipated on these transistors when all 7 segments are on is about 0.75W ( 4.5V * 170mA * 7 segments * 1/7 duty cycle)!
10 Ohm resistors are also dissipating lot of power, 0.29W in the worst case (1.7V ^ 2 / 10 Ohm) and get very hot too.
I feel uncomfortable that things are running this hot. Everything seems to be working properly but I don't understand why it was designed to stress these components so much. Any thoughts?