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Author Topic: Studer A80 Mark IV Slowing down  (Read 1704 times)

radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #15 on: April 11, 2017, 05:25:09 pm »

No, client hasn't sent me the parts yet, so I haven't done anything. I'll let you know when I do.
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #16 on: April 11, 2017, 05:29:36 pm »

That transistor is very low gain, but otherwise good.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #17 on: April 11, 2017, 05:41:37 pm »

My datasheet for the RCA411 shows it has a gain of 10 while the BUV23 has 15 would more gain be better ? None of the BUV types that I could find go above 20 Hfe.
   I was putting the tins back on the machine today and just for the hell of it I put a screw driver on the RCA411 mounting screws and was able to snug it up about 3/4 of a turn. Only one of the others could I turn at all and that was 1/4 turn and not being heavy handed at all. Of course the machine did not slow down at all today?
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #18 on: April 11, 2017, 08:37:04 pm »

That gain should be OK then. Also that was at 4 A I think, so much higher than the A80 would run it.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #19 on: April 15, 2017, 02:59:06 pm »

While waiting for parts... I took some measurements at the transistor RCA411 case to ground (capstan transistor).
With tape loaded but not playing I get around 50 volts. When I first hit play it drops to about 48v.
then it slowly begins to drop. During a slowdown it drops to 22.5v and the machine slows down.
Once warm it drops to about 28 and rises to 48 and it moves around quite a bit between there. These measurements were all done very close to the middle of the reel same amount of tape per side.
With the machine not playing if I pinch the capstan shaft the voltage drops. On the other hand with tape playing and measuring the voltage at the 411 for a reel motor when I try to slow down either motor the voltage goes up on it's transistor. Which is what I expected in both cases ?
    Just trying to jog some more ideas out of my brain and make some sense of it without just shot gunning parts. Any ideas ?
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #20 on: April 15, 2017, 04:30:34 pm »

Dropping voltage on that transistor should mean the motor drive is working harder. But I did have a thought about whether the problem was in the diode bridge that precedes this transistor. And it sounds like that could be the problem in your case. If the voltage across the 411 is dropping it looks like it is trying to do the right thing, but maybe the diode bridge is failing? Hard to meter that, maybe pull the bridge, put each diode on a power supply under load in constant current mode.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #21 on: April 15, 2017, 07:39:06 pm »

Thankfully I ordered some of those from Mouser as I kept looking at that bridge but not knowing exactly what it was there for... On mine instead of the BY159 called out on the schematic it has a b250c/800 which just has higher ratings and is still available. Also found some BC140 transistors from Newark. And have some BUV23 transistors on the way. Hopefully changing one at a time I'll get to the bottom of it. Today my machine slowed down constantly to the point where I had to give up mixing a song. Whatever it is, may be about to show itself, not too unpleasantly I hope.
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #22 on: April 16, 2017, 04:20:17 pm »

Diodes going zenery in a bridge is not unknown, and that is quite a small bridge. I'm going to try and get my clients card here so I can check the bridge too.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #23 on: April 17, 2017, 06:21:23 pm »

Got the bridges today soldered one in around 10:30 a.m.. Worked all day with no slow downs.
But, since that happened when I put in the new contactor, I'll believe it after about a year without any slowdowns  : )     I have the BC140 and BUV23 transistors on the way. Eventually we'll get it.
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #24 on: April 17, 2017, 09:25:35 pm »

I've got the board on the way so I can test the diode bridge. Did you have anyway to test the diode bridge? Best way is a constant current supply, put it across a diode, push the current up to a good value, and see if the voltage across it increases.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #25 on: April 18, 2017, 05:58:48 pm »

After yesterday I tried not to get excited. Good thing today my A80 slowed down so often I could not get anything done. By Thursday I should have both of the transistors in question, in hand.
  I noted that the BC140 on the card .372 capstan control was very hot. Normally it is warm/hot to the touch. I had to take my finger off of it today.
      I am unable to test the bridge under load. I did the normal diode test which I expected it to pass and it did.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #26 on: April 19, 2017, 03:32:54 pm »

Got the parts today soldered in the BC140 machine slowed down. Put in BUV23 transistor machine slowed down.

O.K. things I've done on .372 card
All op amps (4) Changed for new 1) 1455 3)LM301AN
Electrolytic Cap 22u 40v (1)
D9 Diode bridge
BC140 transistor
Checked with magnifier for obviously burned/bad components.

Outside Circuit board

New Phase shift 8uf capacitor
New BUV23 transistor (in place of RCA411s)
Had capstan motor rebuilt .

What about rectifier card 1.080.355 wouldn't a problem on this card show up at the test points
on the stabilizer card 1.080.370 ? Those voltages are all good.

Going to change Q5 Q6 on .372 capstan control card.


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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #27 on: April 19, 2017, 04:25:15 pm »

changed Q5 & Q6  no change and wave looked the same on the scope
Will now do Q7,8,9.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #28 on: April 19, 2017, 05:02:18 pm »

That didn't do it. I don't have a vari-speed hooked up to the machine. But, I wonder on the 1.080.372 card schematic it shows where the vari-speed hooks in. Could any of those components cause a pseudo vari-speed, if it were to begin to fail. Like C24,C11, R58, R59. Maybe R28 in series with the pot R27 ?
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #29 on: April 19, 2017, 08:51:49 pm »

One thing I have just noticed is the zener diode D7 which is supplying 11 volts to lots of stuff. That could be a candidate. Also, presumably it is to some degree temperature dependant. Maybe get in with a can of freeze spray?
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