Thanks for the feedback. I like your suggestion..... it makes sense. I built a duplicate circuit on a breadboard, but couldn't get it to do anything. One problem is I can't find equivalent Darlingtons. I used similar TIP darlingtons but the circuit doesn't do anything.

What model gyro are you using? I've got a 400Hz 3-phase MCU demo system I'd like to test a small aircraft gyro on.

https://forum.allaboutcircuits.com/...r-for-3-phase-115v-supply.181424/post-1697032

 

What model gyro are you using? I've got a 400Hz 3-phase MCU demo system I'd like to test a small aircraft gyro on.

The gyro is a brand new J.E.T. model AI-904 (M/G). It operates on 28vDC, and converts it to 3-phase 28V 400Hz for the motor. Primary current draw on startup is around 600mA, and rapidly settles down to around 250mA once up to speed.
Someone applied 120v 400Hz AC to the unit.
I promised the owner I'd look into repairing the inverter board (70's technology; how hard can that be, right?). It's completely toasted, and while I have found the inverters and the U2T101's, the PNP Darlingtons are impossible to find. He gave up and the gyro is now mine.

I looked at your circuit. Did you ever get a price & quantity on the transformers? I'd be interested in a number of them (4?) for another project.

If you want to test the gyro, let me know. Right now it's sitting on my bench, gathering dust. FYI- thru a great deal of trouble and bribery I managed to get the detailed manufacturer's service manual & schematics on this and similar units, including ones that run on various AC single and 3-phase inputs. If you're interested in any of the circuits, let me know.

 

Post your schematic (not the one in post #1) for review.

Not sure what you mean.....

 

As I understand things, the schematic in post #1 is not your original work; it is someone else's design that you are copying a part of. As such, there might be issues with what you actually built.

ak

 

The gyro is a brand new J.E.T. model AI-904 (M/G). It operates on 28vDC, and converts it to 3-phase 28V 400Hz for the motor. Primary current draw on startup is around 600mA, and rapidly settles down to around 250mA once up to speed.
Someone applied 120v 400Hz AC to the unit.
I promised the owner I'd look into repairing the inverter board (70's technology; how hard can that be, right?). It's completely toasted, and while I have found the inverters and the U2T101's, the PNP Darlingtons are impossible to find. He gave up and the gyro is now mine.

I looked at your circuit. Did you ever get a price & quantity on the transformers? I'd be interested in a number of them (4?) for another project.

If you want to test the gyro, let me know. Right now it's sitting on my bench, gathering dust. FYI- thru a great deal of trouble and bribery I managed to get the detailed manufacturer's service manual & schematics on this and similar units, including ones that run on various AC single and 3-phase inputs. If you're interested in any of the circuits, let me know.

Thanks for the info.
I've got a local civil aviation airport with a couple of shops for aircraft that I might be able to salvage something from. So far nothing on the transformers.

 

As I understand things, the schematic in post #1 is not your original work; it is someone else's design that you are copying a part of. As such, there might be issues with what you actually built.

Correct! I built the circuit on the breadboard copying the schematic directly.
There is 28v between the collector of Q2 and test point M, but the rest of the circuit is inactive, and no oscillations. I don't have the original Darlingtons, so I used similar TIP Darlingtons. Whether that is the reason for the lack of activity, I don't know, not being fluent in analog circuits.