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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 15th Sep 2021, 2:08 am   #1
Linnovice
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Default Humming Studer A807

Hi all. I own a Studer A807 MkII which is in very good working condition. It does have one very annoying problem though. In play or record it develops a very pronounced hum. It is not a steady sound, more an oscillating hum but I do not think its the capstan motor. If I lift the tape tension arm up slightly the hum disappears but as soon as I let it down the hum returns. (There is quite a lot of tension there.) If it try and adjust the height of the tension arm with the trimmer it makes no difference to the noise. I've checked all along the tape path for something rubbing or sticking but its all running perfectly and in ff and rw there is no noise.
Would anyone have any idea what could cause this? Since I've had the machine the noise has been there but new hearing aids is making it more obvious . Could it be a component in the tensioning, is there any adjustments on this board?
I have been told that these machines were made to last and usually were operated in machine rooms where any ancillory noises didn't matter. I can't believe that Studer would be that slap-dash in their approach though.
It is really irritating as the machine records and plays beautifully (apart from the hum that is ).
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Old 15th Sep 2021, 8:31 am   #2
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Default Re: Humming Studer A807

Is the hum coming from the physical deck or is it mixed in with the audio output?

As a tape plays, are the tension guides/arms sitting in the correct places as per the service manual? Yes the tensions will be adjustable but if they are significantly out it is for a reason. Just readjusting the tensions as per the manual may be all that's needed, but it may also only temporarily conceal symptoms rather deal with the root causes.
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Old 15th Sep 2021, 8:41 am   #3
DMcMahon
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Default Re: Humming Studer A807

If a recording is done and played back on another machine is the hum heard on that machine, this may help to prove it is purely mechanically related.

David
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Old 15th Sep 2021, 9:08 am   #4
Linnovice
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Default Re: Humming Studer A807

Morning Tim/David, thanks for replies.
It is definitely electro/mechanical. Recordings are good and play perfectly on other machines without any trace of hum.
I have adjusted the tension arm as per the manual and it is riding at the correct height according to operation. ie. Play/Record, FF/RW, Library Wind, Shuttle. All are correct.
As I said earlier. If I lift the arm (approx 10mm) the noise disappears. I don't think it has to do with position as I can't adjust it out with the trimmer. I think it is more due to tension but I do not think there is an adjustment for that.
Hopefully someone will know better
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Old 15th Sep 2021, 11:12 am   #5
TIMTAPE
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Default Re: Humming Studer A807

On my old Revox A700 if I adjust back tension to much higher than recommended, I do hear a growl. As the supply motor rotates, the growl warbles somewhat.

Does the manual state what the normal back tension should be in grams or does it just state where the guide should sit as a distance from a reference point on the fascia?

With the machine switched off and no tapes loaded, using a simple spring gauge attached to the roller and pulling upwards towards the supply platter, you should be able to measure the static tension on the (hidden) return spring at various points in its vertical travel. If the static tension is correct at the correct distance, that would normally ensure correct back tension as the back tension comes from the elongation of that internal spring.

Does the roller move smoothly with finger pressure up the slot, with increasing tension? No binding or restriction? Where in the slot does the roller guide sit when correctly adjusted as per manual? Is it roughly in the middle of the slot? A long shot. Is it possible someone has replaced this spring with one too strong?

Silly question, does the supply guide spin freely?

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Old 15th Sep 2021, 11:41 am   #6
Linnovice
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Default Re: Humming Studer A807

Yes, the supply guide does spin freely as well as sliding up and down within its slot. I've just checked the manual and it appears there are no adjustments as to grammage and pressure but, there are a couple of adjustment pots for setting different voltages. I will have look at those and report back.

Just off to get my truss out, this thing weighs a ton!!!
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Old 19th Sep 2021, 4:14 pm   #7
Linnovice
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Default Re: Humming Studer A807

Hi all, back again. I’ve spent a few hours today re calibrating the tension arm electronics. It’s fairly simple to do and I think it’s improved the hum situation (I won’t really know until I get it back in my music room). I did have a problem with one of the trimmers though. It requires a setting of +0V DC but to get anywhere near that was a pain. My DMM was flicking all over the place from +34V DC to -22V DC. I managed to get it settled on +0.002V and left it at that but I’m now wondering if it could be a faulty trimmer.
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Old 19th Sep 2021, 4:16 pm   #8
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Default Re: Humming Studer A807

It could be a faulty or dirty trimmer, but the ones used in Studer machines were better than those used in Revox’s.
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Old 20th Sep 2021, 5:04 pm   #9
Linnovice
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Default Re: Humming Studer A807

Whilst in my workshop I’ve taken the opportunity to completely re calibrate from scratch. It’s been on all day, the hum is still there but much reduced. I’m feeling optimistic that recalibrating the tensioning has improved the noise level. I should have it back in the listening room tomorrow so will find out.
Takes quite a while to program 8/9 tape types in three speeds and both iec/nab. Worth it in the end but right now my head is full of cotton wool!
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Old 27th Sep 2021, 3:26 am   #10
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Default Re: Humming Studer A807

Hi all. Well, I’ve just spent the best part of a week trying to find the source of the hum and now am completely stumped. All of the electronic settings have been checked and adjusted and are all now in spec. The suspect trimmer has been changed and nothing else on the board is out of spec. I’ve stripped out the spooling motors, checked, cleaned, reseated and there is nothing amiss there. I’ve stripped the brakes, changed the drums to ones with fresh linings, lubricated the pivots and recalibrated them. Apart from the occasional light ‘rub’ (which will go as they bed in) they are fine. I’ve even taken out the motor control board and as far as possible with it out of the machine everything looks/measures to be in spec. It’s not the capstan motor (with a stethoscope on the casting that supports the motor there is no hum coming from there).
So that’s it. I’m stumped! Is it possible that it may have something to do with the motor windings or some other electrical component, picking up the mains oscillation? It’s in that kind of frequency.
It records/plays perfectly. The mechanics all work well and are set up as per manual.
I’m sitting here in the dead of night recording the bbc. Sound levels are low and the Studer is happily humming away and making a perfect recording! ******!!
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Old 5th Oct 2021, 12:37 pm   #11
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Default Re: Humming Studer A807

I recently had this problem too and coincidentally also a customer of mine. I advised him to check the 100pF capacitors in the power supply and make the DC voltages exactly symmetrical.
This was his answer:
From Hub to me.
This wasn't really the solution.
I then measured the half-sinuses on the motors with the scope and it was very deformed on the winding motor.
This was due to the control, so I replaced C15 and C16 on the transport PCB and the problem was solved.

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From me to Hub,
That's great, persistence wins. In my case, those C15 and C16 were already replaced (C15 on leg 10 of TCA 561 and from the TCA to Q17 over the base of Q17)
I've looked at the capacitors you mention but they are pretty new. Still I replaced them with better capacitors and... the noise is almost gone. A very soft sound can still be heard, which also disappears when the arm is raised slightly.
I've increased the capacitors by 1uF but that doesn't do anything, it gets louder rather than softer. Actually I should have tried to replace the capacitors with a lower value but then I had to solder everything out again. On the other side is a capacitor of 1uF and that may be a better value for the right as well.

From Hub to me,
Did you measure the half-sinuses on the motors with the scope? If such a sawtooth is visible there, you can find it further back in the circuit.
In my case it was the capacitors around the TCAs
Adjusting the belt tension regulation is also slightly better with the help of a scope than just adjusting the height of the arms.
Described in the service manual
Greetings Hub

Thanks Hub,

Jack Marotick
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