[SOLVED]Motor transistors keep dying on me

[RoadrunnerOZ]

Excellent news - full torque on both motors?

 

[EmilFrid]

Yes, full torque and it also responds to calibration of the transport

 

[EmilFrid]

I hope it continues to work. After this hell I don't dare to jump just yet

 

[RoadrunnerOZ]

Ah well - you’ll just need to do a ceremonial dance in the woods at midnight with three black feathers in your hair and chanting some mumbo jumbo. That usually helps

 

[EmilFrid]

Haha some gris gris gumbo yaya.

I'm hoping it will hold up know... This far, thank you so much for the assistance - all of you - and here's to a healthy tape machine

 

[map2112]

Great! This has been an interesting journey, and probably frustrating for you at times.

The rectifier bridge motor control circuit is quite ingenious; I actually built a small prototype on the bench when you sent the schematic just to see how it worked!

Good luck with the machine!

Mark

 

[EmilFrid]

Thanks so much for your input throughout the thread. This has indeed been quite the journey. But I think we can say that the dual run cap was the issue. In the future id love to switch to dc motors. But that's a project for the distant future.

Yes, it is a really smart solution. Glad my schematic could be of use.

 

[sahib]

So I was an idiot and wired up the run cap wrongly. Now it works, no more dead transistors, BUT the take up motor is quite weak. I guess time to change the other caps as well

Whaaat?????

Good job I am not there.

You would be subjected to recording 16 bars of C9b5 on an out of tune guitar for this.

Good news. Weldone.

 

[EmilFrid]

Haha, stuff happens when you mess around at 4 in the morning. Thanks a lot and thanks for helping me on the way

 

[EmilFrid]

View attachment VID_20241211_181942_01_01.mp4

 

[sahib]

Haha, stuff happens when you mess around at 4 in the morning. Thanks a lot and thanks for helping me on the way

Easily done. Great stuff. Back to being a happy running machine.

Thank you too.

 

[RoadrunnerOZ]

The rectifier bridge motor control circuit is quite ingenious; I actually built a small prototype on the bench when you sent the schematic just to see how it worked!

It takes a bit to get your head around the idea of using a bridge rectifier as an AC output device controlled by a DC governor. It’s very clever.

 

[EmilFrid]

Is it the same way those neve diode bridge compressors work?

 

[RoadrunnerOZ]

Not really - a rectified audio signal controls the transistor feeding one leg of the bridge which then in turn controls the AC audio fed through by shunting it to ground like a volume control - the AC audio signal doesn’t pass through the diodes in the bridge. In the motor control circuit the AC to the motors passes through the diode network - different system.

 

[Brian Roth]

Not really - a rectified audio signal controls the transistor feeding one leg of the bridge which then in turn controls the AC audio fed through by shunting it to ground like a volume control - the AC audio signal doesn’t pass through the diodes in the bridge. In the motor control circuit the AC to the motors passes through the diode network - different system.

Yep! In my "childhood" in this biz, I first recall seeing that design in the Ampex MM1100 multitracks. Earliest versions were (in)famous for blowing up the Delco branded power transistors. Big clue....power up the recorder with no tape loaded and one (or both?) of the reel motors would run full torque.

Bri

 

[Johnny1234]

Yep! In my "childhood" in this biz, I first recall seeing that design in the Ampex MM1100 multitracks. Earliest versions were (in)famous for blowing up the Delco branded power transistors. Big clue....power up the recorder with no tape loaded and one (or both?) of the reel motors would run full torque.

Bri

Yep, I always tell people with the 1200 to make sure right side idler arm is dis engaged and to the right.

 

[Brian Roth]

Yep, I always tell people with the 1200 to make sure right side idler arm is dis engaged and to the right.

IIRC, the dying motor drive transistor problems were greatly reduced by the time the MM-1200 machines were introduced. Nevertheless, it was still a problem for years. Somewhere I have a copy of a series of mods for the 1200's collected by.....the Record Plant studio????

Bri

 

[inkster]

View attachment 141294

Do you use the DBX? I service one of those for a client and he had me bypass the DBX system entirely cause he preferred how it sounded out of circuit.

 

[EmilFrid]

On this machine I don't need the dbx unit. The noise floor is ridiculously low. But on my 85-16 I use dbx on everything except piano and drums. I don't know why I get less tape hiss on the 90-16. Perhaps better bias oscillator?

 

[Disco Volante]

It takes a bit to get your head around the idea of using a bridge rectifier as an AC output device controlled by a DC governor. It’s very clever.

I believe the ReVox B77 is built on the same principle...