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Author Topic: Studer A80 Mark IV Slowing down  (Read 1078 times)

Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #30 on: April 24, 2017, 06:55:36 pm »

I could not test that Zener so I replaced it. No change. Machine is still slowing down.
 What is the purpose of R49 the 10 ohm 5.5 watt resistor in series with one leg of the RCA411s (BUV23) transistor ?
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #31 on: April 24, 2017, 08:54:01 pm »

Basically that limits the max output current. Probably makes it easier for the preceding transistor to drive it I tested the diode bridge on my board, its good.
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #32 on: April 24, 2017, 08:59:11 pm »

I just tested that 10 ohm, its good. I'm starting to wonder if this circuit is a bit marginal. One thing you could try is to swap the driver transistor Q10 with a grunty Darlington transistor. That should make good and sure that the drive is there.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #33 on: April 25, 2017, 07:57:38 am »

Could you recommend an available part number ?
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #34 on: April 25, 2017, 01:57:20 pm »

ZTX605 ? The gain is 2000 @ 1 amp. The BC140 15-30 @1 amp.
This gets a bit beyond my scope. This machine has run well since 2009 so 8 years without a slowdown. It has had other minor issues with the record/reproduce boxes that I have been able to fix. But speed wise it has been rock solid. Tell me if I understand the circuit correctly.

The capstan motor spins creating pulses in the tacho head which are fed into card .372.
There is a timer section that creates a reference frequency of 1600hz these two numbers are compared some how and a speed adjustment is made so they match. I wonder if the problem could be in this area rather than the drive section. My square wave looks like this at 1600hz.
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #35 on: April 25, 2017, 04:33:35 pm »

Something like a TIP120. Very grunty. Your 1600 Hz looks fine. One thing if you could measure. The voltage at the collector of the RCA 411 should drop to 3 volts when it is trying to correct. Can you measure this voltage when the machine is slowing down? If this voltage is not down to 3 volts, it means the drive side is not working right.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #36 on: April 25, 2017, 07:18:45 pm »

I will check it. Thought I had something when I pulled that 10 ohm resistor, it had a nice black spot on it from R46 270 ohm right up against it. Changed them both out still slowed down. Oh well,
this thing is busting my chops !
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #37 on: April 25, 2017, 07:22:45 pm »

Studer faults can be a bitch! Keep at it! If you can do the measurement it might help. I'm going to throw the board up on the bench at some stage and see what I can find.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #38 on: April 27, 2017, 12:01:48 pm »

It does not go down to 3 V it drops to around 22.5. If when the machine is playing tape normally I grab the left reel hold down and stop the machine for a second it only goes down to around 25 v.
With the machine not playing, but tape loaded, if I manually pinch the capstan between my fingers it only drops to around 25. Also, on the schematic where it shows these voltages it says the nominal is 80 volts. I never see above 50 volts. Tape loaded, playing or not. Without tape loaded (capstan not spinning) it is at about 135 volts.
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #39 on: April 27, 2017, 04:38:36 pm »

So manual says it should drop to 3 volts. That indicates to me that it is not driving hard enough.
Also, check when it goes to 22 volts, whats the voltage across the R49, 10 ohm, If it is close to 22 volts, then thats the problem. You could try reducing this resistor to 8. 2 ohm. Does the motor spin freely?

If the voltage across R49 is lower than 22 volts, then  gain of the output or the driver transistor is the problem. I would try the Darlington driver which will give you more gain on the driver. Output transistors are generally a bit low gain anyway. Also there are a number of tantalums around the board, check them all for shorts. Its a bit fiddly, but also do a multimeter check on all the resistor values, watching for where they are in circuit.
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radardoug

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Re: Studer A80 Mark IV Slowing down
« Reply #40 on: April 27, 2017, 04:44:17 pm »

One thing I hadn't considered is the motor being stiff from dried up bearings.
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #41 on: April 27, 2017, 04:56:19 pm »

First thing I did when this problem started was send all three motors to Athan for rebuild.
So all new bearings and lube and spinning nice. Also replaced 8uf phase shift capacitor.
Just replaced 5.6v Zener D8 but haven't tried it yet. Will check voltage across that resistor. 
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #42 on: May 07, 2017, 05:00:08 pm »

Took a reading across R49 while playing ( No tape loaded, Spooling motor control card removed. )
2.8 volts, when it slows down it goes up to 3.2 volts. This looks right it shows 0-3 volts there on the schematic.
   Just got in some parts and am going to take to shot gunning items on the .372 card one at a time until I find it. Starting from the right side of the schematic "driver" and moving to the left.

Of course, always looking for other better ideas ! 
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Mikeyod

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Re: Studer A80 Mark IV Slowing down
« Reply #43 on: May 07, 2017, 06:33:11 pm »

Pin 6 IC3 is supposed to be 5.5 volts. It is 6volts until the machine slows downs then it goes up to 11.5 volts.

Pin 6 IC4 should be   .5...18 volts nominal 8.4 volts. I'm getting 16 when running normal rising to 19 when the machine slows down.
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