Maybe via the run cap? There are 2 caps in the one case?
[SOLVED]Motor transistors keep dying on me
- Thread starter EmilFrid
- Start date
Help Support GroupDIY Audio Forum:
That’s weird - did you actually replace the diodes? This could happen if a diode is going short under load or high voltage even though they check ok on a meter may not necessarily mean they aren’t breaking down.
400 or 600V diodes?
Perhaps the snubber circuit (C111 and R104) that are across the transistor collector to emitter (which is presently removed from the circuit) could be passing enough current to make the bridge energize the supply motor?
Most likely the motor current is pretty small. If you try and hold the supply hub does the motor stop (minimal torque)?
It will be interesting to see if replacing that dual section motor start cap fixes things.
Most likely the motor current is pretty small. If you try and hold the supply hub does the motor stop (minimal torque)?
It will be interesting to see if replacing that dual section motor start cap fixes things.
The motor could run via that 47uF cap but what is not explained is why the transistor(s) blow - unless there’s a blown diode in the bridge setup or the run caps are leaky or both.
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.
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.
The snubber follows the same path as the transistor - in parallel - if you take out the transistor it will feed the motor but there will be no drive torque. The voltage comes via that cap. The diodes, transistor and run caps should all be replaced together then tested. From the flow diagram you posted #158 if you include the 50Ω resistor and 47uF capacitor across the collector and emitter of the transistor you can see the supply path - if you disconnected the 47uF the motor would stop with no transistor in place, but investigation down that path doesn’t show any cause of the transistor blowing unless the cap is breaking down and shorting and spiking the transistor via the 50Ω - but I believe you’ve already replaced that cap?
Yes, but I still find it odd that the motor gets energized only when any mode is engaged. Shouldn't there be voltage regardless of whether take up motor is spinning or not, if it gets energized through the snubber? Or am I missing something?
That's correct. I replaced those caps since one of them exploded (which, now that I think about it, may or may not have something to do with the current issue as well)
That's correct. I replaced those caps since one of them exploded (which, now that I think about it, may or may not have something to do with the current issue as well)
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.
That cap exploding could have killed the transistor and some diodes as well as possibly the run cap
Don’t forget the transistors 
There are two other run caps as well C102 and C104
