Looks like current day Foss units are all potted up. How about yours ?
At that time frame, I would suspicion the power to be DC to the compressor and the involved electronics in acquiring that supply voltage is using a ferrite based power transformer core.
Thus, the efficiency aspect, being acquired in that supply, converting at a more optimal higher frequency.
How many of those power transistors are being used ?
Can we have board / unit photos ?
Is it using any larger capacitance electrolytic capacitors?
Are you finding any bad power transistors . . . . .typically with bad collector to emitter junctions?
A " WORKHORSE" high power TO-3 cased transistor, ending its days of design in the '80's was the
2N3055. IF . . .they were not being used / switched at too high of a frequency.
Looks like Motorola's only usefully numbering was the stamping of that one transistor on the 17th week of 1982.
A FIERCE failure development in your situation is for the thermally conductive paste / slurry being placed as a film between aTO3 case and it's metal heatsink.
Day one . . .all is fine. . . . . .until, after time . . . .heat let's the silicone oil weep out and migrate laterally. That then leaves a dried out powder of beryllium or tin oxide as being a, NOW . . . THERMAL INSULATOR. . . . . . fried. TO 3's.
Fill us in . . .
73's de Edd
Good eye.
(2) are in use, as an inverter drive....mounted on a standard 100w heatsink we have all seen scads of. I will post a PCB component side picture when I desolder the fuse holder (panel mount), so I can get a clean shot. No huge electrolytics in view....I doubt the motor drive inversion function needs that as load is highly constant by design and doesn't mind the dirties.
Haven't been able to get at the C&E to test yet-board has to be massively desoldered to access, but the external test pretty much nailed the transistor failure: drive output to compressor had a 5amp agc fuse in series that blew. Disconnecting the compressor and refusing, no fuse blow.....connection to a known good compressor (the device has two for fail safety-excellent conceptual design)-fuse blows. So the failure is in the power circuit output it would seem, and that is the darlington power transistor pair likely as not.
The TO3's are mounted with insulator pads, not grease-and connection is tight, (appears low thermal interface R-but have yet to test). But the heatsinks were mounted horizontally (and by design had no airflow to the backs. Don't look at me-I know better-I didn't so design or place them haha-mechanical engineers who have no business in this dept. did that). They were covered in dust of 44 years or so w/no cleaning. The failure occurred at low supply voltage applied for >60days. Of the two compressor systems, the one with the highest duty cycle is the one that failed. So I am guessing by way of the heaviest ever transistor loading, given the inductive nature of the sole and dedicated motor winding load. Lot of factors just sync up and say: today was that day for the transistor to no longer to play with the abuses stacked up. Replacement MJ4030's are now on order, as the flags to that part provided thru forum assistance got me pointed there.
I will keep you posted as the mystery unravels.
