Motor transistors keep dying on me

[map2112]

There is some interaction between these two motor circuits that isn’t quite obvious.

Maybe stepping back a bit would help for a moment; So what started this whole issue in the very beginning? Was the machine working fine and then just malfunctioned? Did it happen after some maintenance or other repair event?

Maybe substituting the two bulbs as loads will let you “see” what’s going on.

Somehow we need to help isolate the interaction between these two circuits.
Maybe you could disconnect the output drive from the two op amps (lifting one side of R32 and R33) and try and control each motor circuits (with the light bulbs as loads) independently.

You will get there!

 

[EmilFrid]

Thanks for your kind help and encouragement. This has really taken a toll on me. This issue started after my transport logic unit broke down. I replaced the whole unit and all the logic now checks out good. What happened during the breakdown of the transport logic -- or at least that's my hypothesis -- was that it ruined the inverter and opamp on the reel motor control card.

Yes, it is the interaction we need to figure out. Could i add a bulb in series with the collector of the power transistor? Putting it in series with the motor in some other way is very tricky.

 

[map2112]

We would really want to take the motors entirely out of the equation. Couldn’t you simply unplug the motors and connect the bulbs between J107 pin 1 and 4, and J108 pin 1 and 4?

As to the original problem, are you sure the “new” transport logic unit is identical to the failed one, and nothing got miss wired as part of this repair? Is this like a remote control panel or the one that is part of the transport?

 

[sahib]

.......

How about if you temporarily replace the two motors with two 220V incandescent bulbs as loads. Maybe it will let you troubleshoot this while taking the motors out of the equation?

Also Ian’s earlier idea about installing a bulb in series with the motor could be another approach to try after that?

I was about to suggest that with dummy resistors if Emil had some.

Ian's idea is also good but I would take the motors out of the system altogether.

 

[EmilFrid]

Ok, I'll see if I can find a good way to hook up bulbs. Yes, this new one is identical and it sends out the correct logic to the reel control card. And everything is wired correctly. It's part of the machine as a 19" rack unit. Now to find a bulb or two!

It's still weird that almost all the failures are base-emitter shorts. I haven't really stumbled upon that before.

 

[EmilFrid]

Yes dummy resistors is a good idea as well. I'll dig around in the workshop and see what I find

 

[map2112]

The only reason I mentioned incandescent bulbs is that they are generally common and somewhat forgiving, and you can visually “see” what might be going on with the circuit without connecting test equipment to monitor the loads. I would think we should see the brightness of the bulbs changing as the torque commands are being issued.

For example the torque on the supply “motor” should be low when in the fast forward mode and the play mode, but high in the rewind mode. The bulb would indicate that.

Just an idea.

 

[EmilFrid]

i found two 220V bulbs in the studio's live room. Added it between the two points Map suggested, but i can't get any current going through it. Have we read the schematics wrongly? To me pin 1 and 4 should work

 

[EmilFrid]

The only thing, aside from ground, that both transistors has in common is C103. Yes, it is a can with two separate capacitors, but perhaps something as gone wrong there? Or am I completely wrong here?

 

[map2112]

Still not sure why the bulbs on pins 1 and 4 of the motor jacks didn’t work, but that capacitor could definitely be the “smoking gun”; any possibility it has an internal short between sections or the wires have been connected wrong could be what it tying together the two motor circuits and getting those wild voltage readings.

Are there 4 separate wires connected each with its own push on terminal or lug that could have been cross connected at some point in the troubleshooting?

 

[Evertide]

i found two 220V bulbs in the studio's live room. Added it between the two points Map suggested, but i can't get any current going through it. Have we read the schematics wrongly? To me pin 1 and 4 should work

The motor is disconnected ?

 

[EmilFrid]

The wires are soldered directly to the cap, but it's connected correctly. I will measure for shorts!

@Evertide Yes

 

[EmilFrid]

Ok, so another odd thing. When having one transistor mounted and measuring the collector of the other (disconnectet) one, something happens. With no transport mode engaged it shows 400 slowly sinking to 150V. If I put the transport in any other mode, rw, ffw, play, the voltage STAYS at over 400V as per the picture added. This can't be right?

Edit: it didn't sink to 150 but stayed at 400, while it jumps to (and stays at) 450 if any mode is engaged

 

[map2112]

Yes this is very odd. Could be those motor capacitors

 

[EmilFrid]

Yes, i can't really see any other options at this point. They're probably a bitch to source. I will look around online. But until i get them i will pretend like they are NOT the problem and look elsewhere as to exclude any other possible issue. Also, i still don't know how this fits in with the emitter-base breaking down and not the more common collector-emitter.

 

[Evertide]

Ok, so another odd thing. When having one transistor mounted and measuring the collector of the other (disconnected) one, something happens. With no transport mode engaged it shows 400 V

What is the AC Voltage over the bridge ?

Within the bridge the transistor is seeing an DC pulse. Outside , for the motor it is a variable resistor...

With 100 Volt AC as power, there should not be over votltage.

C 103 , just use 1 separate 2/250 capacitor...

 

[EmilFrid]

I will have a look at that tomorrow when i go back to the studio.

Yes, you're correct. Don't know what i was thinking. Too much tape recorder confusion today.

 

[map2112]

You might be able to find some 2uf non polarized caps used as motor start or motor run capacitors in an old air conditioner unit or something to use temporarily. I don’t think the value is too critical at least to run a test.

 

[map2112]

In fact you can probably just disconnect them and see if the storage voltages and interconnections go away.

 

[EmilFrid]

I found 2uF run caps online and ordered them.

Yes, that is a good idea. Will do tomorrow. Going to bed soon