DgatWood - the 81's hum is different than the hum in the 73 or 84.
It's the fact that the 81 has improperly spec'd transistors.
Now, once the transistors are replaced there will probably ALSO be inducted hum that'll need to be fixed too. But the transistor hum is so loud it's doubtful you'd be able to hear the inducted hum.
Zmix troubleshot the issue and posted this:
While that may have been a problem with Zmix's unit, I'm fairly certain that this is not what is wrong with mine. A transistor oscillating should not go away as long as that path through the circuit is active. By contrast, all the fader does is adjust how much of the signal passes through the inductor in the feedback loop. That shouldn't be enough to trigger this. The behavior I'm seeing is 100% consistent with hum induced in the inductors, all the way down to the complex interaction with nearby resistors causing nulls in the hum near each end.
Further, the capacitors in the power supply keep the circuit at a usable operating voltage for probably half a second after the power switch goes off, and for that first tenth of a second, it's very close to 24VDC. I can hear the input in the output for about half a second. In spite of this, the hum goes away instantaneously when the power switch goes off. If this were caused by overheating of the transistor, it would gradually drop as the input voltage dropped. I'm just not hearing anything like that. The only rail that falls off quickly enough for this to make sense is the 12V rail, but that only powers lights and op amps for clip indicators---nothing in the signal path.
Based on that, I think the problem can only be either induced hum in the inductors or ripple in the power supply. I don't have a scope, so all I can do is check for AC with a voltmeter. I'm showing about 0.02VAC on the +24V rail. All the other rails read 0 as expected. That might well be just noisy enough to cause the problems in question, and if so, it would completely explain why moving the transformer outside the case didn't solve the problem for me.
The next thing I tried was stacking two units with their tops off upside down on top of one another to see if adding a second one increased the hum. No change. Switching off the power switch causes the hum to go away instantly with the second unit on top of it and powered. I got the transformer of the second (humming) unit pretty close to the real transformer. Bearing in mind that when testing to see if repositioning the built-in transformer made a difference, I moved it much farther out than the transformer in the second unit was, it seems clear that if the other hum-inducing transformer doesn't cause hum when placed in a similar position, it isn't induced hum I'm hearing... unless it is being induced by load on the 12V rail.
That was easy to test. I unplug the 30V center-tapped secondary that feeds the 12V rail. I lost all the lights and clip indicators, but everything else worked. It still hums very badly in that configuration. Therefore, I agree with Zmix's assessment that most of the hum is not induced.
I tried switching to use the 24VDC supply that fed a not-significantly-humming 73's EQ section, but got the same hum. Since I swapped the only path that 24VDC can take into the EQ boards and since the main preamp stage is obviously not affected by this hum, that implies that I'm seeing a power ripple that occurs on all of the supplies, but only affects the 81 to that extent for some reason. Such a supply ripple could also explain why the 73 hums slightly at high gain.
So at this point, I'm trying to figure out why I'm seeing a supply ripple. My gut says I should replace the LM317 variable regulator with a 7924, snip out R2, and replace R1 and C11 with a jumper.
The same can be said about every metal screw in every switch on the front panel. There's some serious induction going on there.
