radardoug
Well-known member
The 400 volts, mentioned previously, is a RED FLAG CLUE!!!! With a 100 V rms secondary, you should not be able to generate 400 volts. All the clues point to a coupling between the two motor amplifiers.
[SOLVED]Motor transistors keep dying on me
- Thread starter EmilFrid
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sahib
Well-known member
Emil, you are fighting too many battles at once.
Reading 400V against what reference? To circuit ground or AC 0V?
The only common connection to both transistors is the circuit ground. C103 has nothing to do with circuit ground. It sits on the AC only (as do C108 and 110).
The reason why the bulbs did not turn on, I have never tried it before but (even at transistors full turn on) is 100V sufficient to glow a 220V bulb.?
Remove both motors and their associated capacitors from the circuit.
Have only the power transistors.
Connect dummy resistors as the motors. If the transistors survive and you are able to measure 100V across the resistors then the control side of the circuit is fine and we can then bark up the motor branch of the tree.
Reading 400V against what reference? To circuit ground or AC 0V?
The only common connection to both transistors is the circuit ground. C103 has nothing to do with circuit ground. It sits on the AC only (as do C108 and 110).
The reason why the bulbs did not turn on, I have never tried it before but (even at transistors full turn on) is 100V sufficient to glow a 220V bulb.?
Remove both motors and their associated capacitors from the circuit.
Have only the power transistors.
Connect dummy resistors as the motors. If the transistors survive and you are able to measure 100V across the resistors then the control side of the circuit is fine and we can then bark up the motor branch of the tree.
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I definitely am...
I'm reading 400VDC to circuit ground, as the incoming 100VAC is rectified and the presented at the collector. So I basically measure collector to circuit ground.
Yes, but I also don't see any voltage on any pins of the that plug in to the motor. I've measured all four of them in every possible configuration but it's as if the motor has to be connected in order for voltage to be present there (?!)
Thanks for the advice. I will get some heavy duty resistors and remove the motors from the circuit.
Yes, this is perplexing and it's even more strange that the 450 VDC stays at 450 when any mode is engaged. I'm going to start with removing the motors from the circuit and move on from there.
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Johnny1234
Well-known member
I hate to chime in so late in this topic, but the resistor/capacitor combination named CR1 and CR2 are also used in the otari 5050 series and they fail all the time with their old age.
sahib
Well-known member
You previously had the same but this time it was discharging as I believe the motors were in circuit. Are you reading this after disconnecting the motors?
As I mentioned before on start up the charges on the capacitors around the motor sum, which in return charges C111 (and CR1). However, at this stage of the troubleshooting I would not pay attention to it.
The only pin relevant on that plug is the one the motor is connected to. You are also measuring AC at this point with reference to AC0V. So, are you saying that when the transistor is on you are not reading 100VAC? However, again we will see that with the dummy loads. Under normal operation you should be able to read 100V across the resistor.
Than you too.
I am reading this with only one transistor connected. Before, when I measured 300V slowly discharging to 150 volts, there were no transistors mounted at all.
Yes, i measured AC between pin 1 and 4 and got nothing. But as you say, I should get some dummy loads and hook it up before we do anything else.
Yes, i measured AC between pin 1 and 4 and got nothing. But as you say, I should get some dummy loads and hook it up before we do anything else.
Disco Volante
Well-known member
What sort of values are we looking to find for dummy loads? I have a bunch of largeish Rs lying around I could contribute.
This thread is very entertaining, like a true whodunnit murder mystery!
This thread is very entertaining, like a true whodunnit murder mystery!
Disco Volante
Well-known member
What values? It would be fun be able to help..
Disco Volante
Well-known member
40W would be a full-load condition. Probably 10W would be ample power for testing purposes.
Of course, Old Man Ohm is always right, which is why we need to pick a reasonable value ;-)
Of course, Old Man Ohm is always right, which is why we need to pick a reasonable value ;-)
Johnny1234
Well-known member
need to measure impedance of the winding at 60 hz.
Disco Volante
Well-known member
This is just about up there with some of Zizek's more cryptic titles...
sahib
Well-known member
Mr Murphy is at work.
Can you post a better pic of the cal procedure section of the manual, the one you posted is dark and incomplete - it seems from the little I could read that the takeup reel motor and supply motor are governed by some sort of sensing to provide the back tension from the supply reel motor for fast forward/play and vice versa for rewind.
Better if you have a pdf of the whole manual.
The voltage across the motor stator and rotor coils will be higher than the static AC voltage into a resistive load due to back emf from the motor windings and the run/start capacitor. Each motor has its own 0V AC so any reading of voltage needs to be done from 0V 1, 2 or 3 for each motor not from system ground.
If any reel motor bearings are stiff and binding this could cause excess load on the other motor. I assume you’re testing with tape loaded?
Better if you have a pdf of the whole manual.
The voltage across the motor stator and rotor coils will be higher than the static AC voltage into a resistive load due to back emf from the motor windings and the run/start capacitor. Each motor has its own 0V AC so any reading of voltage needs to be done from 0V 1, 2 or 3 for each motor not from system ground.
If any reel motor bearings are stiff and binding this could cause excess load on the other motor. I assume you’re testing with tape loaded?
Here are a couple of pics.
No I don't have any tape loaded and the motor shaft spins freely so no issues with bearings.
Since the machine works with only one transistor I unplugged supply motor while fast forwarding on the take up motor, measuring the connector where the motor should sit and voila – it reads 80VAC and slowly discharges but only while the other motor is spinning. I'm getting closer and closer, Watson...
No I don't have any tape loaded and the motor shaft spins freely so no issues with bearings.
Since the machine works with only one transistor I unplugged supply motor while fast forwarding on the take up motor, measuring the connector where the motor should sit and voila – it reads 80VAC and slowly discharges but only while the other motor is spinning. I'm getting closer and closer, Watson...
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Ok. It looks like the supply reel tension is only applied in Repro mode. It’s possible the bridge rectifier(s) may be cactus causing the transistor to blow when they pass the current to drive the motor - have you checked/replaced the diodes? Also if the run cap is breaking down this can cause a real problem. I recently had an Akai with issues where the tape would slow down to a halt in FF, REW or Play due to faulty run/start caps and would draw heaps of current. Run voltage at the motor was around 185V for 100V AC supply on startup.
Yes, the diodes are all good. I've ordered new run caps and will have them tomorrow I think. It's reasonable to believe them to be bad. Not changed since 1979
But I noticed now that there is an odd interaction I wrote about above. I will even post a video of it. No transistor on the supply reel which should disconnect the motor completely and yet...
But I noticed now that there is an odd interaction I wrote about above. I will even post a video of it. No transistor on the supply reel which should disconnect the motor completely and yet...
