Linear Current Amplifiers, hFE, and Parts Substitutions for Vector Monitors, Part II: Results
As the article I linked previously states, “Many engineers tell me that in their 20+ years of design they have never needed to know a device’s hFE value.”
However, in the case of the power output stage of a vector monitor that can push up to 8 amps, according to the manual, it turns out that current gain is important!
The original GO8-003 used a 2N6259, with the following specs:
- 2N6259: hFE 15 min, 60 max @ 8A.
We’ve tested all of the following:
- MJL21194: hFE 25 min, 75 max @ 8A
- MJL21196: hFE 25 min, 100 max @ 8A
- MJW21196: hFE 20 min, 80 max @ 8A
- MJW18020: hFE 8 min, 16 typical, ? max @ 10A (maximum not specified)
We already knew that the MJL21194 worked, because it’s been running in Flynn’s Arcade for over a month. Based on this, I gave the MJL21196 a 99% chance of working, because it’s essentially identical. I gave the MJW21196 about a 95% chance, based on my reading of the rest of the specs, and the fact that its hFE is still higher than the 2N6259. But I only gave the MJW18020 about a 50% chance of working: based on the G08-003 circuit, I believed that it would likely have insufficient current gain (hFE) to reach the edge of the screen in all circumstances.
And indeed, upon firing up the test unit, we saw this:
Look closely, and you’ll see that the top of the screen, where the scores are, is collapsed. Not quite into a single line, but into a very small space. So why did it only collapse the top of the screen?
As mentioned previously, “It is not unusual to find a 2 or 3 to 1 ratio in device hFE.” I haven’t pulled the transistors out of the test unit to measure them…but the ranges above, which show a 3-4x ratio between minimum and maximum, and a 2x ratio between minimum and typical, lead me to the obvious conclusion: the two transistors handling upward vertical deflection had the lowest hFE, too low to fill the screen.
This is amongst the reasons why you can’t just substitute any BJT power transistor with sufficient voltage and current capacity into a vector monitor. You have to dive deeper into the datasheet, because the GO8 pushes its output transistors to and beyond their limits, and parameters that many full-time analog engineers brag about not knowing become absolutely critical.
(As of this writing, December 2022, we’re still testing the G08-003 Transistor Upgrade Kit: we have to get a testing cycle in with the alternate transistors before we can confidently build them and ship them to our customers. We expect them to be available for retail sale in late February or March 2023.)