[SOLVED]Motor transistors keep dying on me

[map2112]

Is the 100V AC supply line always on/there or only switched on with the FF/Play/REW for the appropriate motor? All of the 3 x AC and 3 x AC 0V lines for each motor appear to come from the TB-1 control unit so it’s maybe only energising them as required.

The 100VAC supply is always present regardless of the modes. They come directly from a transformer in the PSU (3 isolated 100VAC secondaries). There is a schematic of the PSU several pages back in the thread.

 

[sahib]

While Emil is wrestling with that, here is some old "time bomb" Rifa snubber cap failure examples from the flash power packs and remote control receivers that I have dismantled recently. Equipment were just over 40 years old.

 

[map2112]

Yes, @map2112 , the motor current of the supply side is small and the reel hub is easy to stop with one hand, but there's still significant voltage that should not be there as there is no transistor in place for the supply motor. It will indeed be interesting to see what happens after I replace the dual cap. I should get the replacement caps on Monday.
I might try to disconnect the 47uF cap and see how that affects the motor. I will also replace the diodes like @RoadrunnerOZ suggested. Replaced 4/8 diodes yesterday before leaving the studio with a severe headache. Writing my thesis right now so I have less time to fool around in the studio, but I have to find some tape machine hell hours next week.

Is it too late to do your thesis on motor control circuits?

 

[EmilFrid]

That looks horrific. I have come across this before. Rifas are more smoke machines than caps nowadays

 

[EmilFrid]

@map2112 haha, that would be a very crappy thesis.

Happy for everyone hanging out here. Helps me troubleshoot but also brings me some good humour

 

[RoadrunnerOZ]

The 100VAC supply is always present regardless of the modes. They come directly from a transformer in the PSU (3 isolated 100VAC secondaries). There is a schematic of the PSU several pages back in the thread.

The schematic for the motor circuits shows the AC supply coming from the TB-1 controller, not direct from the power supply which also shows a connector TB-1 - it does say “ to reel motor” on each one but is it direct? Maybe not so I’m assuming here that there might be an AC control board TB-1?? Is there a schematic for a board of that name?

 

[EmilFrid]

I wil have a look tomorrow!

 

[radardoug]

I had another look at the schematics, and it does not look like the 100V comes from separate windings. Although the first drawing implies it, the second drawing only shows one 100V feed into the card. And I dont see how it would ever have worked.

 

[RoadrunnerOZ]

The power supply diag shows three independent 100V windings - post #109 - going to connector labelled TB-1, the motor diagram - same post shows the three motors - has 100V pairs x 3 labelled to Control Unit TB-1

 

[map2112]

Back on page 6 of this thread there are the 3 drawings that show the PSU, the reel motor module and a “functional” wiring diagram.

There are 3 separate windings 100VAC (1), 100 VAC (2) and 100 VAC (3).

100 VAC (1) feeds the Supply Reel motor via pin 11 on the Reel Motor Control Module.

100 VAC (2) feeds the Takeup Reel motor via pin R on the Reel Motor Control Module.

100 VAC (3) feeds the Capstan motor via pin 11 on the Capstan Motor Control Module.

 

[map2112]

Actually I think I had the two motors confused: it looks like the “functional” drawing implies that:

100 VAC (1) feeds the Take Up Reel motor via pin 11 on the Reel Motor Control Module.

100 VAC (2) feeds the Supply Reel motor via pin R on the Reel Motor Control Module.

 

[RoadrunnerOZ]

AC1 on R, AC2 on 11. Question is if the 100V coming in on R or 11 is always present or does it get switched on elsewhere, coming into the motor control module - or does the module control everything - it would appear that the 100V to the rectifier/transistor circuit is only on in a function mode and without the transistor in place the snubber RC 50Ω/47uF feeds the motor which then runs without any torque.

 

[map2112]

I think it’s always present on the bridge, but the transistor across the bridge is controlled by the mode logic.

 

[RoadrunnerOZ]

However the transistor feeding the bridge has a snubber across it so when the transistor is off or out of circuit the snubber is still feeding the motor and as EmilFrid says this voltage switches off when the mode goes to stop. So the 100V feeding into the bridge must be switched on and off somewhere

 

[RoadrunnerOZ]

Maybe in the motor control unit the AC 100V is controlled/activated there

 

[map2112]

Maybe in the motor control unit the AC 100V is controlled/activated there

I think the third drawing that Emil posted a few pages back in the thread is essentially what is on the Reel Motor Controller board.

The 100V goes from the transformer directly to one side of the bridge (at the junction of D2 and D4) and the motor connects to the other side (at the junction of D1 and D3) and then to the 0V return on the transformer.

Any DC current that flows between the top of the bridge (junction of D1 and D2) to the bottom of the bridge (junction of D3 and D4), which is what the transistor acting as a variable resistance does, gets translated into the motor current.

Either the transistor out of the circuit,
unless the capacitor in the snubber is leaky or shorted (which it may be) it shouldn’t pass enough DC current to enable current flow through the reel motor.

 

[RoadrunnerOZ]

The motor is deriving AC from the transistor passing current for both negative and positive going cycles - if the transistor is out of circuit the motor should not run - but then add that pesky RC across it that seems to make it run - I wonder if this would happen with the transistor in place. Still the problem is really the transistors blowing and the likely cause of that would be either the run caps or the diodes in the bridge and also a reverse breakdown in one of the diodes D1 or D4 would also run the motor.

 

[map2112]

I agree, a bad diode in the bridge may indeed be the culprit.

It looks like the take up motor is working so the components on that side seem to be fine so that might help by comparing measurements between the “good” and “bad” side.

I think the two motors could be swapped (by switching which is plugged into with side of the control module) to help determine if the supply motor or it’s capacitor are bad.

With the transistor removed one could connect a resistor to the collector - emitter contacts in the socket to run the motor. Since there is already an emitter resistor in the circuit you could probably just temporarily jumper the collector - emitter socket to run the motor (and save any more innocent transistors until everything is sorted out)

 

[RoadrunnerOZ]

Seeing that the 47uF exploded it’s likely that it may have taken out one or two of the diodes

 

[EmilFrid]

Hello boys and girls, and sorry for my abscense! Busy writing about empty signifiers but revisiting the thread during a write break. It is correct that the Ac goes through motor windings and becomes present at the bridge. I do not understand why my supply motor gets enough voltage to run while not having a transistor and while the take up is running, and only then. It seems, on the take up side, like the transistor pulls the current while engaged and pulls nothing while off, which is to say: I see voltage on the motor once engaged but none in stop mode.

I will replace the rest of the diodes and putting in new run caps tomorrow