Re: 5440 PS transistor selection

Harvey White

You might want to check on this, but the 5000 series plugins and the 5400 series plugins were not supposed to be interchangeable. I think you certainly couldn't use a 5400 series in a 5000, and I *think* that the reverse is true.

They are not the same scope series, from what I know.

Fat blurry traces indicates that the supply voltages are off, the HV is likely strange, etc.  Not sure, but I'd do a very close read on the manual.


On 6/15/2021 1:25 AM, Dave Peterson via wrote:
To wrap this case:

I ordered a set of replacement MJE800 equivalents from Mouser for 6-bucks-n-change. Got a few nice Pomona jumpers with micro-grabbers to justify the shipping.

Walter suggested offline that my problem was more likely a short downstream. Guess what: Walter's a good guesser. The issue turned out to be a shorted 1uF tantalum decoupling cap on the main interface board on the -30v supply. As a reminder: the blazing hot MJE800 was the -15v supply. Odd, but lesson learned: interacting regulators may do unexpected things.

This was part of a electrolytic recapping and 152-0488-00 replacement. Not necessarily bad, but one of those, "while I'm in there I might as well ..." exercises.

With everything reassembled the PS came up fine on the bench with a variac and meters wired up to monitor ac and dc voltages and currents. Everything came up with no problem and supply current stayed down around 150mV. Regulated voltages were a bit over-range, but unloaded.

After reassembling the scope I was able to run through the calibration procedure. +/- 30v supplies came down to in-range and adjusted to within 10mv. Other supplies are now in spec. I now have a nicely working 5440 scope.

However, and I haven't figured out why yet, the original purpose of this scope was to run a 5CT1N curve tracer. The curve tracer works nicely (very sharp traces) in a 5111A, but in the 5440, which otherwise works at full BW with clean traces, produces fat ugly blurry curve tracer lines. WTF? A puzzle to be solved. Rather frustrating after all the work.

One final lesson learned in this exercise: plug-in curve tracers are relatively low power small signal devices. The Q880 that I thought was shorted was just under driven by the 5CT1N. It can only go up to 20mA Ic, but the MJE800 Darlington transistors drive 4A with several hundred mA Ibe. The plug-in can only put out 1mA steps at most. And being Darlingtons they also have an elevated Vbe, which seemed to not function at all at low Ibe steps. What I interpreted as a shorted collector-emitter was just the first step driving full vertical range. More careful examination of the MJE800 datasheet and curve tracer settings showed the transistor actually never even got out of saturation. To a 5CT1N it does kind of look like a short. Beginners, eh?

Thanks to all who helped, and hope my lessons learned help others down the road.

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