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R/E/P => Bruno Putzeys (Designer) - Dave Hecht (Master Tech) => Topic started by: Mikeyod on March 28, 2017, 01:43:06 pm

Title: Studer A80 Mark IV Slowing down
Post by: Mikeyod on March 28, 2017, 01:43:06 pm
Hi guys,
I've got an A80 2 inch 24 track Mark IV and it has begun randomly slowing down.
After about a half hour the problem is gone and it runs fine.
What I have done/checked.
Stabilizer voltages all check out fine.
All motor caps have been replaced.
All motors have been rebuilt by Athan corp. in Jan. 2017
Replaced timer IC on capstan control board
While viewing the 1600 Hz signal on a scope with frequency counter the frequency drops to around
1300-1375 Hz the second I hit the stop button the frequency comes right back up to 1600Hz.
When the freq. drops the tape tensions on both sides remain normal 300 on left 450 on right.
However if I turn the left tape tension sensor manually and lower the tension down to 100 the frequency comes back up to 1600Hz but falls off again as soon as I release it. All voltages on the stabilizer card hold steady when the machine slows down. Like I said after about half hour I can go about my business as usual and it will not slow down.
Any thoughts ? Thanks in advance.

Mike O'Donnell
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on March 29, 2017, 06:40:57 pm
Have you replaced the Frako caps on the cards in the card frame? They will probably start smoking soon if you haven't! I had an A80 with speed problems, this cured it. But I still have a problem with 30 ips I haven't sorted.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on March 30, 2017, 10:04:50 am
Yes, all of those caps have been replaced.
I tried running the machine from cold with the auto-locator unplugged to see if this could help narrow it down but the same result. After half hour it cleared up. I'm assuming something is warming up then functioning properly ?
   Could the LY4 contactor be a cause of this ? Looking at Mouser to see if it is still available.

Mike
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on March 30, 2017, 07:52:49 pm
Are you running high speed or low speed?  If high speed, check it on low speed.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on March 31, 2017, 10:45:31 am
This is a 15/30 i.p.s. machine that I run mainly at 15 but I will try it at 30.
I took the contactor apart, the contacts were very fried. I am cleaning them up to see if it helps.
Meanwhile I ordered a new one if it does help.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on March 31, 2017, 12:04:57 pm
After filing away at the contacts on the LY4 contactor I have used the machine for the last 50 min. starting from cold and the machine did not slow down at all. Another thing I noticed is that the auto-locator does not miss the mark by several seconds and then jog itself into place. Of course this could all be coincidence and it will slow down tomorrow ;). I'm guessing if this is the cure then a brand new contactor will really do the trick, as these were real pitted and I had to take some stock off those contacts. Keeping them perfectly flat is difficult with a file and why bother really when a new one is on the way. We'll see.

Mike
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on April 02, 2017, 02:24:03 pm
Further points of interest;
I have used the machine four times now after the first time I filed the contacts on the contactor.
The second time the machine did not slow down at all again but it began to miss the mark auto-locating. It would miss by about 1 second at first then it got to about 6 seconds.
The third time it started right away missing the mark by 6 seconds and about halfway through a song it slowed down. But, for the first time it sped back up to normal. I shut the machine off and pulled out the contactor and a couple of the contacts were blackened. A little bit of 220 sandpaper and they were clean. Put it back and it immediately hit the mark auto locating and no slow downs.
  Today ( fourth time ) the machine did not slow down but has started missing the mark locating. It started off with 1 second but quickly got worse. I'm sure that by filing the silver off of these contacts they are just getting dirty fast.They are thinner and the contact area is no longer perfectly flat causing them to arc.   I'm thinking this might be it.

Mike
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on April 02, 2017, 08:32:40 pm
Where is that LY4 contactor? Is that the braking circuit one?
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on April 06, 2017, 07:08:13 pm
Yeah, that's the one. It looks like in the schematic it supplies brake current in one direction(Power amp, Direction logic, End of tape brake amp, Brake amp) and drive current( on the schematic it shows F-M1 I thought feed motor 1) in the normal position. Unless I'm not seeing it correctly which could be. A poor connection in the normal position could cause instability?
  Since I changed it. It went four days without slowing down. Today however, it slowed down momentarily sped back up to nominal and then no further trouble. That would be a pain if this were a red herring. Or maybe only part of the problem.
    RadarDoug do you say this could have no bearing on the slowing down ?

Mike
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on April 07, 2017, 01:30:21 am
Didn't you say your tensions were good when youhad the slowdown? This being the case, I dont think the contactor is involved, as it is on the two spooling motors. Which means the problem is in the capstan circuitry.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on April 07, 2017, 08:55:58 am
That is true. But, I had to lower the tension by rotating the left hand tape tension sensor which also controls the reel motor current. Also, pressing stop the 1600 Hz would immediately return.
  I'm just trying to go through this logically because there are so many things on these machines that interact. I've changed all the ICs on capstan control card .372 because they are socketed and I had some, no change. All motors rebuilt by Athan corp. No change. Cleaned contactor, good for two days new contactor good for four days with only one slowdown. So if the contactor has nothing to do with it this is one pesky problem.
      Are there any other suggestions as to what it could be. That one slowdown shows there is still something. Could the RCA411  be the problem ? I would think that transistor either works or it doesn't ? Any help very appreciated !

Mike
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on April 07, 2017, 04:34:30 pm
It looks like you have a similar problem to my machine. Note that the fact that when you press stop and get 1600Hz immediately is because the load comes off the capstan and the drive circuit can supply. I am suspecting either the output transistor or the drive transistor. The owner has a spare motor, and I am going to set the whole lot up on the  bench so I can work on it. Its too hard to do it on the machine.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on April 07, 2017, 06:32:46 pm
I guess that would be the BC140 on the capstan control card and the RCA411 on the back of the machine ? Do you have any part numbers for suitable replacements ? Maybe for the BC140 a 2N3107. A 411 replacement seems a bit more difficult.
   Let me know what you find !

Thanks,

Mike
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on April 07, 2017, 07:32:10 pm
I'm a big fan of transistors used for vertical deflection in televisions, because they are made for high voltage and spiky conditions. I used to use these to replace MDA transistors in Ampex MM1200s. BUV numbers I think? Have a look through whats available from Mouser etc. Go higher in current and voltage and you cant go too wrong.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on April 11, 2017, 04:21:53 pm
Hi Doug,
Any discoveries with the bench testing ?
Looks like the BUV23 should be a good replacement for the RCA411

Mike
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on April 11, 2017, 05:29:36 pm
That transistor is very low gain, but otherwise good.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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?
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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 ?
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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.


Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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 ?
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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?
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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 ?
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on April 25, 2017, 07:57:38 am
Could you recommend an available part number ?
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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 !
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on April 27, 2017, 04:44:17 pm
One thing I hadn't considered is the motor being stiff from dried up bearings.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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. 
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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 ! 
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod 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.
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on May 25, 2017, 11:13:00 am
Hi Doug,
Did you see these last two posts ? Were you able to learn anything from the card your client sent about this issue ?

Still trying to figure this one out too !!    : 0)

Mike
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on May 25, 2017, 04:53:24 pm
I think you are onto something there. Pin 6 of IC3 should go high with slowing down, that would be correct.
But if pin 6 IC4 is already high, it would indicate that stage is trying too hard, and then runs out of steam. So the problem is following IC4. Try R48, the zener D8 (sus), and Q10.
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on June 05, 2017, 07:40:34 pm
Mikeyod, any joy on this??
Title: Re: Studer A80 Mark IV Slowing down
Post by: Mikeyod on June 06, 2017, 09:40:26 am
Hi Doug thanks for asking !
I changed all those components but no luck.
I felt like I needed to look elsewhere for the problem. So, I measured the voltages at the spooling motor transistors. The right hand voltage was always 5.8 volts which to me meant it was working as hard as it could all the time. After changing the diode bridge and the 68 ohm current limiting resistor it came up to about 12 volts, only at the beginning of a reel. When I changed the diode bridge and resistor for the other side things really started happening, the machine tensions went all over the place. After resetting them I had 25 volts on the right side. Today I'm going to change the 10 ohm resistors that are right on the RCA411 spooling motor transistors and see what happens.
  The manual says for 1.080.384 card. Stopped 126 volts, I get 165. Play 65 volts, I get 25 volts.
Rew/Ffwd 6 volts, I get 5.8 volts. So, I'm trying to make heads or tails out of this.
What do you think ?

Thanks, Mikeyod
 
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on June 15, 2017, 06:06:37 pm
Hmm, from your symptoms, I would still say that it is the capstan card that is at fault. I haven't got around to measuring the same parts on the one here yet. Interesting that things improved greatly when you changed the reel bridges. Can you try running the old bridges under load on a d.c. supply? Some of these things will measure fine on a meter, but fail under load, with higher than normal saturation voltages.
Can you revisit the capstan card and see if you are getting right voltages there now? 
Title: Re: Studer A80 Mark IV Slowing down
Post by: radardoug on June 16, 2017, 11:54:46 pm
Mikey, thinking about it it could be a tension problem, at higher speed the motors need to turn faster so will need more drive. Do you have a tentelometer? Check tension at stop, then 15, then 30. If the takeup tension is lower at 30 that would point to a problem with the drive circuit on that side. I am going to Auckland next weekend and will check on the Studer there. The capstan card checks out fine here, all values around that output are correct. There is one Frako cap on the power supply on the board, mine metered sort of OK but I changed it anyway. It is 20 volt decoupling so I dont expect it would have been the issue.