Re: 5440 PS transistor selection

Dave Peterson

Thanks Harvey,

I did discover the plug-in incompatibility when I tried installing the 5440 plug-ins in the 5111A. Jeff Dutky pointed me to the pertinent section of the Tek Wiki that describes this.

In that I also found that the 5111A plug-ins are "forward compatible" and work in the 5440. I was a tad surprised to see the 5A26 from the 5111A gave readouts on the 5440. While tuning up the 5440, on a lark, I used the 5A26 from the 5111A and the 5A48 from the 5440 to prove I could get four channels showing at once with 5 readouts. The time base is a delayed 5B42, but can't do "dual" A/B sweeps. (That'd be 8 sweeps! :P) But it did prove the scope could do the 4 channels. FWIW. I don't have much experience with these scopes, so I don't even know if a dual delayed sweep time base is available or possible. I rather doubt it. So I don't think there's a lower-right quadrant readout to be had. I digress. Readouts are a whole other thread!

The calibration procedure in the 5440 manual is rather brief, but it does include a HV voltage check, which was within spec. The 5A48 need some very slight tweaking to bring CH1 & CH2 into agreement and get the step attenuator adjustments spot on. Once it was tuned it did very well with edge compensation and provides very nice sharp traces across the volts/div range. Noise, of course, shows up below 5mV/div. But that's to be expected (see my other posts about my large FM interference from local broadcast tower).

The horizontal cal seems a bit deficient, and the time base should be calibrated too. It also has some knob damage to the delayed time/div portion that I'll need to fix. And some switch maintenance. It'll need to be partially disassembled to get at them. These issues are minor and don't appear to affect the horizontal performance. The timing accuracy and stability are good.

Perhaps I need to check the 5CT1N supplies in the center slot of the 5440? Could it be a supply issue in the curve tracer? The cal does have one swapping the vertical plug-in back and forth between the left and right slots to verify vertical position and gain. And I ran through the volts/div ranges with a PG506 in both slots. Both producing accurate and sharp traces. But that doesn't guarantee the 5CT1N is happy, does it. The shorted tantalum decoupling cap was right next to the center slot, for what that's worth. I replaced it with a 50v tantalum with the same 1uF value.

I'll have to run through a 5CT1N check-out in the 5440, now that you've brought that up Harvey. Thanks for the suggestion.

On Tuesday, June 15, 2021, 07:22:50 AM PDT, Harvey White <> wrote:

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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