Studer B67 Mk II fixed.

This time for real. The SN75462 replacements got here yesterday and I popped on in IC3’s socket and the machine works. The sensor lock in STOP and the motors are both responsive. Here, see for yourself:

There are a few things I need to figure out for next time I look under the hood:

Does J3/4-6 provide the +24V to the solenoid?

EDIT: Yes. Look at the schematic. J3/7 is connected to the +24V through the power ON/OFF switch. The other end of the switch goes to J3/4-6. So one end of the solenoids is fed directly from the +24V rail.

Does K-EDIT deliver exactly the same voltage as what comes from the +24V rail, or is it something like ~25V (the Zener diode “trick” so that it looks like the output of IC3 is ~25V instead of 5V)?

EDIT: The fast answer is that K-EDIT gets exactly what the other end of the solenoid is fed, because of KVL. See here for the longer answer. There’s no zener “trick” so to speak.

Does the voltage at IC 4 pin 1 equal to the voltage on pin 7? That is, does it work like some kind of a comparator? See previous post and notes I left in the manual.

 

Studer B67 Mk II problem fixed*!

(*almost)

I was in the middle of writing a progress post and then I went to probe around IC3 in the transport and realized that IC3, was put in the other way around. That is, pin 1 is where pin 8 should be, etc. I flipped it around and it is now fixed! Here’s a more detailed explanation of what I’ve done in the past few weeks:

First a recap of the problem: Upon turning the machine ON, only one motor will be responsive to the transport controls. For instance, pressing PLAY would make the take up motor spin but not the supply. Then if I spun the roller on the right clockwise, the take up motor would turn off and the supply would turn on and start spinning. Spin the roller counterclockwise and now the take up would run and the supply motor would shut down. Even weirder was that disconnecting the J5 connector from the pre-divider board caused the machine to operate correctly.

So when I picked up troubleshooting the machine again I decided to go over the counter’s schematic to try understand better what’s happening with QP-DIR1 and QP-DIR2. To be honest, I’ve done that before, but this time I also graphed the waveforms along the way from where the signals come into the board, and their way to becoming Y2-FORW and Y2-REVS. I also keep these drawings with the manual for future reference. That didn’t tell me much other than that when the roller on the right is spinning clockwise, Y2-FORW goes HIGH and when it’s spinning counterclockwise it’s going LOW and Y2-REVS goes HIGH. I also realized (might have before as well) that the pre-divider carries these signals FROM the counter TO the transport.

Advice on the tapeheads.net reel-to-reel forum directed me to the tension sensors, so I started learning the circuit comprised of ICs 4, 1, 6, and 2. That made sense to me because that circuit is responsible for generating the pulsating signal that controls the motors. My understanding of how IC4 is used is that it is some kind of a comparator. So depending on YAN-TT1 and YAN-TT2 it’ll go positive or negative. That’s how the lower half of IC4 (YAN-TT2) was acting and that made sense. However, the voltages from YAN-TT1 weren’t enough to cause the comparator to work the right way. Instead it went from +12V to something like -2V. (By the way, that’s still how it is!) I thought that since this voltage is dependent on the displacing of the tension arm, that maybe its mechanical settings are out of wack.

So I took out the left sensor and adjusted it per the manual. I put it back in the machine and now both left and right sensors were locking in place. This one made me scratch my head, but it led me to start figuring out what’s controlling the solenoids of the two sensors. This stuff isn’t mentioned in the manual, so it took a bunch of probing and continuity tests to realize that the solenoids are fed the unregulated +24V and the respective outputs of IC3 (SN75462). I mentioned that to a friend and he said that IC3 goes bad often, so I put in an order for replacements. Meanwhile I went to check the voltages at the inputs and outputs of IC3 to see if it’s working right and that’s when I realized that it was put on backwards. I flipped it and now not only are the sensors not locked, but BOTH motors are responsive to the transport controls!

However, the sensors should lock when the machine is stopped. A quick check of the voltages in and out of IC3 showed that it’s not functioning right – it’s a NAND gate but when both inputs voltages are high (IC7 pin 7 is HIGH meaning the machine is stopped, and then a signal derived from pin 7 also HIGH) I get +24V at its output, but it should be 0V. It’s a good thing I ordered some SN75462 so I’ll drop a replacement in and see what’s up.

Studer B67 running/spinning motor problem – fixed!

Straight to the point: The casing of the take up motor’s power transistor was making contact with the top chassis. I uninstalled it off, and there was a piece of fuzz on one of the transistor’s legs going all the way up to the casing. I cleaned it up, reinstalled the transistor, tested that there’s no continuity between the transistor’s casing and the chassis, and turned the machine on. The motor wasn’t spinning anymore! Can’t say if it was the fuzz that caused it, but reseating the transistor fixed it.

So how did I arrive at that? I’m not yet fully proficient in electronics, so what’s following here might be incorrect, but my prodding and testing was guided by my friend who’s a Studer tech, so maybe not!

Anyway, in my last post I detailed the measurements I took on both motor control boards. The readings on the supply board are normal, the readings on the take up board weren’t, particularly, the voltage between pins 5 and 10. On the other hand, the voltage on pins 7 and 10 was within the tolerances of what would come from the transport, so that reduced the problem to a short that happens somewhere around the motor (as opposed to the transport). I started checking for continuity at the transistor socket, and I noticed something peculiar. On the take up board, I had perfect continuity between pins 5 and both the transistor’s casing and the chassis. Same thing with pin 6. Pin 5 is the collector and 6 is the emitter, and it didn’t make sense that they should be at the same voltage. Also, that’s not what I got for those pins on the supply motor’s board. My friend suggested to check if the casing of the transistor is touching the chassis, and that was it.

I should mention, however, that now there’s a different anomaly. I turned the machine on and followed the manual’s initial checkout procedure (p. 2/17, section 2.6). When I pressed play, and only the take up motor was responsive. I turned off the machine, tested some connections on the supply board, turned the machine on, and now the supply was responsive but not the take up. Turned it off again, tested some connections on the take up, turned it on, and now the take up is running and not the supply. What the hell? Hopefully this one will be easier to chase down.

Voltage measurements on a Studer B67’s motor control boards

The motor control boards are the little ones that have the Rifa capacitors that are known to leak. I wrote about recapping these here.

My friend who’s been helping me figure out what’s wrong with this machine told me to take some voltage measurements on the motor control boards, so I did. I still don’t know what they mean, but I will soon. For future reference (for my own sake and others’), here is what I measured:

Take up motor control board:

Points 3 & 4: 107.7 VAC
Points 4 & 2: 107.6 VAC
Points 4 & 1: 188 VAC
Points 5 & 10: 0.30 VDC
Points 6 & 10: -0.012 VDC
Points 7 & 10: .042 VDC
Points 12 & 14: 189.8 VAC
Points 11 & 13: 106 VAC

I think the voltage between points 5 and 10 is a problem, because it’s deviating from the supply motor voltages, but also I was told it should be high DC voltage.

Supply motor control board:
Points 3 & 4: 100.2 VAC
Points 4 & 2: 100.3 VAC
Points 4 & 1: 95.4 VAC
Points 5 & 10: 139.2 VDC
Points 6 & 10: 0 VDC (this one was exactly zero)
Points 7 & 10: 0.0 VDC (Again, on the nose).
Points 12 & 14: 6.8 VAC
Points 11 & 13: 4.5 VAC

The voltages between points 12 & 14 and 11 & 13 differ, but I think that’s because the take up motor is running at full speed.

I think I’m getting close! I will update this post with what should be the correct voltages when I know them. EDIT: the correct voltages are what showed up on the supply board.

Did some Studer work today. Nothing major, but trying to get stuff out of the way as far as the spinning motor.

I didn’t mention it before, but a few ICs on the logic board have this really weird coating/oxidation. It didn’t come off easily with alcohol or DeoxIT, and I’m not even sure it was oxidation, but my French friend who’s a Studer wiz suggested to get a fiberglass pen to remove this stuff. It totally worked! Here are two chips next to each other, one before I cleaned it with this pen, and one after:

IMG_20151210_232338

The lighting in this picture is bad, but you can tell the difference.

I cleaned the legs off with alcohol, and got a few drops of DeoxIT in the sockets and moved the ICs in the sockets. That should take care of any oxidation.

I also replaced the stop button’s bulb, not that it makes any difference, but I thought I’d follow up on it.

Anyway, turn on the machine and the takeup motor is spinning out of control again. The stop button doesn’t light up and that is really grinding my gears. It seems like the right voltage is feeding into it (~27V).  I don’t know if this is related to the motor spinning. I’m pretty sure the stop button was lighting up before. Haven’t tried a different bulb yet.