Here’s the Sencore MU150 tube tester after complete rehab. Consistent with many other of these models, all resistors were drifted at least 10-20% high and caps were high-impedance. The entire PCB had to be shotgunned — which is not as hard as you would expect. There are a few strange values, but most things can be done with parts on hand.
After the scare with the shorted rectifier diode almost cooking the power transformer (it was literally boiling, sputtering wax out the sides), I added a 750mA fuse on the hot AC line, along with a grounded power cord. The multisection can cap had to go (it was leaky), so there are two small radial electrolytics taped together inside the old can holder. PCB is basically all new parts.
It’s easier to list what got to stay than what got replaced. Remaining original parts: sockets/switches/mechanicals, neon bulbs, pots, a few diodes, transformers, meter, and the 0B2 regulator tube.
Calibration is also not as hard as everyone makes it out to be, assuming you have a decent scope and can make a couple of test jigs. I suspect the reputation these units have of being “high-maintenance” has to do with the fact that many of the purchasers on the used market are “tube gurus” as opposed to actual technicians. For anyone with a basic knowledge of electronics repair and some standard equipment, one of these can be restored and recalibrated to like-new condition in an afternoon.
There’s some speculation about the difference between the MU140 and MU150 models, apart from some relocated front panel controls. On the schematic, the only differences are that R6 is split from a 10M resistor into a 4.7M and 5.6M in series, with a 220n cap from the junction to ground; and there are ferrite beads on many of the socket wires. Protection diode CR12 in the later models appears to be a 1N816, which has a higher Vf so that the meter can more easily reach full scale without the diode conducting. Anything with at least .66V Vf @ 1mA should work fine here.
Get the MU140 manual (will work fine for the MU150 too) here.