Fostex B-16 supply reel running wild

ethyrvalve

Norman Nanoweber
Hi all,
I'm looking at someone's B-16 right now. Its supply reel spins at full speed/power (clockwise) as soon as it's powered on. The tension arms and transport buttons have no effect on it.
When I trick the tape end sensor and tension arms into thinking tape is loaded I can make the transport functions work for the capstan, pinch roller and takeup reel motor. With the supply reel motor transistor connector unplugged, it pretty much works like a normal tape machine (except that it obviously can't rewind). I can even see that the supply reel's brake is working (disengaging for play/fast wind).
The owner told things gradually started acting funny and it was only when I first looked at it that it started going haywire.
I already replaced the motor drive transistor and it didn't fix the issue.
All the PSU voltages test very close to the schematic values except the +26v rail (which is used for the reel motors) measures about +29v. About a year ago, another tech recapped the power supply, replaced a diode bridge and also replaced that same motor transistor. The components match the schematic except he did slightly downrate a 3300µF cap to 3000µF.
Just wondering if anyone has any experience with these machines and/or clues?
Thanks!
 
"Fostex B-16 supply reel running wild "

So then it's like a cage-free B-16. :D

I think the issue is with the sensors at the arms. With them down, the reels should not run.
I know when I had the G-16...I saw very small relay switches inside, that would "click" on/off as the arm was moved up/down.
The B-16 being an earlier model might be somewhat different, but I would think the basic operations are about the same.

Do you see any small relay switches in and around the arms?
 
“Relay switches” you mean like micro-switches? Those are usually the tape end switches or at least tell the transport logic when tape is unloaded.

Ethyrvalve, do you know if the the 3300uF cap that was replaced with a 3000uF part is a filter cap?

I’d be looking at the reel servo board...start by exercising any connections between tension arms and the reel servo board. Re-flow solder joints between those components too. Either the messages aren’t getting from the tension arm to the reel servo circuitry, or the reel servo assembly can’t “hear” the tension arm.
 
Have you checked the voltage on the base of the motor drive transistor? I'd then trace that back to where it comes from on the motor controller PCB.
 
Thanks for the advice everyone. James, your hunch led me in the right direction: Q41 (which is directly connected to the motor drive transistor) ended up having a shorted junction.
 
So it's fixed? Yay!

Yup, thanks! I needed to replace 2 transistors and a capacitor. These decks can be a bit intimidating to service at first but it gets easier after you break things down into blocks. I've now got a bit more respect for those 80s Fostex engineers.
 
Glad u got it working. I just spent a half hour on The Bottle Garden site reading about yr gear and that Studer link story. Very cool place you have there. Next time I'm in Montreal I'll look it up.
 
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