Motor transistors keep dying on me

[EmilFrid]

@CJ the motors spin very easily without friction

 

[sahib]

Just a consideration. I could not find any data on TFK but ST has the same emitter/base voltage spec (7V). May be you should try with ST.

 

[EmilFrid]

@sahib yes, that sounds like a good idea. Will get those. It's strange though that all the transistors I've tried have blown including the original 2sc1114. This is so puzzling.

 

[sahib]

If ST also blows then there is definitely something somewhere. Of course as it is always the case in repair it is something very simple when you find it.

 

[EmilFrid]

@sahib yeah I truly hope it's something simple. Starting to get frustrated

 

[auddoc]

Hi!

I have an issue with my Teac 90-16 that is driving me insane. The transistors driving the take up and supply motor keep dying. I today managed to rewind (without load) for a minute or less before yet another transistor died. Take up is even worse. Few seconds. Starting to feel a bit hopeless as I don't really find anything out of the ordinary. Collector measures 150 VDC and the control voltages feeding the bases of the transistors read 1-4 volts depending on mode. No shorts (except for the one emanating from a broken transistor), I just tried measuring the voltage feeding the bases to see if there was any spikes but couldn't find anything.

Anyone know how to fix this, or at least present a different perspective as I'm starting to feel blind?

Thanks View attachment 139846

Given the symptoms, I would guess you may have a motor fault, as in drawing too much current from said transistors. Off the top of my head I don't remember, what the typical current/ voltages should be. but the fact that this symptom repeats suggests the motors may be at fault. Could be something as stupid as the bearings needing to be cleaned and lubed. This is a function of miles and age. Always follow a process of elimination when troubleshooting this sort of stuff. Especially when the symptoms repeat themselves. Gregg

 

[CJ]

Another possibility is that the motor control circuits might have problems, usually there us a slow start circuit so you don't stretch the tape, if thus were bad then you might be getting full power at startup which might damage the transistors so look to see if the motors start slowly at startup.

 

[EmilFrid]

@auddoc yes, might be the issue but I find it odd that both fail at the same time. I oiled the bearings when I bought the machine and the motors spin very easily and completely silent

@CJ that is an interesting point. Will have a look at that. Will be busy recording for a few days so might not be able to look into it until the end of the weekend

 

[auddoc]

I did warranty service for Teac/ Tascam for about 22 years in the past but I don't recall servicing motor drive issues on that particular model. I would agree about bearing issues. If they spin freely and make no noise, the bearings are probably ok. Capstan and pinch roller bearings good as well? Probably smart to eliminate all the possible mechanical causality first. Are the drive transistors original part numbers as well? Same ratings?[email protected] After mechanical cause elimination, might be useful to go through all the drive components with a DMM and see if anything is pooched there. Do you have a schematic you can send me?

 

[EmilFrid]

@auddoc oh ok, that is great! Yes, the capstan works as it should. With drive transistors you mean the ones driving the power transistor? I'm not sure about the English terms. But in case you mean that, they are the same as they've always been and they measure fine. But the power transistors have been changed many times during the past month. I even beefed it up and got more heavy duty transistors but they, too, failed.

I posted schematics in the thread, but if the pics aren't good enough I could post better ones tomorrow.

 

[auddoc]

Sweden is 240VAC, correct? Is your machine multivoltage and maybe set for 120VAC.? That would possibly cause damage similar to what you described. I would even check the current-limiting resistors, caps and yes the driver transistors on the motor drive PCB. Perhaps in-circuit and out of circuit.

 

[EmilFrid]

@auddoc yes, it's set to the correct voltage as it has functioned flawlessly until now. I checked the current limiting resistors and they showed the correct value. Maybe the inverter ic is messed up? Maybe I should look into that

 

[auddoc]

I wonder if the surge suppressors are pooched (CR1 and 2. what is the actual part number in the BOM? numbers on schematic looks odd). Or the caps in the RC filters across the final transistors. Do you have a scope to see what the 100V AC waveform at the motors actually looks like and measures when both the motors are disconnected? If both motor transistors go at the same time, it must be something common to both drive circuits, perhaps the IC's in the middle of the schematic (IC2)

 

[EmilFrid]

@auddoc i will have a look at part numbers when I return to the studio tomorrow. I have all the documentation there. And I have a scope so will definitely make some measurements.

Yes, I actually changed the op amp ic to rule this issue out – no difference. I also made sure to socket it for ease of service in the future. I also ordered a new inverter so will try to swap that one out as well.

 

[EmilFrid]

And also I replaced the caps in the RC network as one of them exploded

 

[auddoc]

I would scope the outputs with the motors disconnected- they are probably on a connector, I would guess. With no load, the transistors might not commit suicide.

 

[EmilFrid]

I will disconnect the motors and measure. It sounds like a good plan. Anything in particular that I should look for?

 

[auddoc]

Which cap exploded? The one in the output snubbing filter?(C111 and R104?) or somewhere else?

 

[auddoc]

I am assuming it should look like a normal-ish sine wave as the schematic notes are 100VAC across the motor drive lines. You may want to float the Gnd on the scope just to be safe the first time.and if there is equal AC on both motor lines to each motor.

 

[EmilFrid]

Yes, C111 or C112 (as they're just connected point to point under the chassis it's difficult to say which one of those two exactly), so I changed both of them for two that can handle s little higher voltage