Motor transistors keep dying on me

[radardoug]

Have you double checked that there is no connection between the transformer windings that supply the two motors? Your symptoms sound like exactly that fault. Those windings need to be completely separate. If one transistor hangs in there fine, it points to a wrong connection between supplies.

 

[EmilFrid]

@radardoug thanks, I will have a look tomorrow. I hope there's no catastrophe waiting for me

 

[CJ]

What kind of insulator mounting kit are you using?

Are you ready to kill somebody yet?

Don't worry, the more pain, the more joy.

 

[JohnRoberts]

It's always the last thing you fix....

JR

 

[EmilFrid]

What kind of insulator mounting kit are you using?

I've tried a few different ones. I don't know what they are called in English but it's "glimmerskiva" in Swedish, which seems to be the standard. Haven't had issues with them before.

Are you ready to kill somebody yet?

Yes, myself! This is really taking a toll on my self-esteem as a repair guy. I usually manage to fix stuff

Don't worry, the more pain, the more joy.

Thanks, I will try to keep this in mind.

I'm writing a thesis all week so might not be able to do anything with the machine until Friday. But maybe we'll have time to make some fault hunting plans during the week. I'm very grateful for everyone chiming in.

 

[EmilFrid]

Update: so today, between writing on Althusser and drinking copious amounts of coffee, I fault searched the logic for the take up motor. Turns out it was just a faulty opamp. I'm thinking, maybe this opamp going rouge could be the reason for the transistors blowing? Will order a bunch of transistors and also check the windings like Radardoug told me to do. Right now there are some TO-3's on sale in my regular shop for only 0,4 euro a piece but they're rated 160v and with the nominal voltage in the machine at 150v I think I'm safer getting some badass deflection transistors for expensive prices again...

 

[JohnRoberts]

Faulty logic driving power transistors can tell them to both turn on at the same time often with bad results.

This was a known problem with early class D audio power amps, where they would shut down whenever they detected low power supply voltage to prevent stupid logic states.

JR

 

[EmilFrid]

I hope this is the root of the issue.
One thing that is nice with these machines (as opposed to the piece of **** fostex G16 I cut my teeth on) is that there is no obsolete microchips in them, making them repairable, even if the problems are hard to pin down in such a complex system. If anything ever goes bad in the system control unit I think I'll just swap all the logic chips en masse as they're all socketed

 

[EmilFrid]

Ok, so here's s strange thing: the opamp and the logic chip breaks when I have the transistors installed and press rewind/ffwd. But they seem to only break when the transistors are connected. Something tells me there is some sort of issue with the ground. What do you guys think?

 

[JohnRoberts]

Check grounds and power supplies including low voltage rails.

Did you check the transformer windings like radardoug suggested?

JR

 

[EmilFrid]

@JohnRoberts thanks, will do immediately.
Yes, I checked and found no issues.

 

[EmilFrid]

Checked the reel control board now. D11 and D14 had stared conduction on reverse. Let's see if I can find some more utterly confusing stuff.

 

[JohnRoberts]

diodes can fail from reverse over voltage spikes, and overheating (literally melting).

 

[EmilFrid]

The reverse voltage hypothesis could work with the fact that it is usually the base-emitter junction failing. Maybe I should try to put a powerful diode on the base or emitter and see if that makes a difference? In that case I'd be closer to thee root cause