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Author Topic: Studer A-80/RC recording issue?  (Read 7129 times)

Offline audiolab1

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Studer A-80/RC recording issue?
« on: April 25, 2009, 11:23:28 AM »
Hello everyone,

A few years ago I was lucky enough to pick up a 1/4" Studer A-80/RC in nice shape for a song.
It had a problem with the capstan servo which made it look like the capstan motor was dead.
A few days of diagnosis and a $0.60 IC later it was working very nicely.
I have only used this deck for playback but recently decided to try making some recordings.
Unfortunately the first recording test showed nothing going on the tape on one channel.
I traced this to a bad relay on one of the record amplifiers which I have just replaced.
Now I get recorded signal on both channels and it sounds very good even though I have not calibrated it yet as I am waiting for a test tape to arrive.
In order to quickly assess recorder frequency response I recorded a CD with sine frequencies at steps from 20-20,000 Hz.
On playback of this tape I noticed a couple of things which may or may not be normal.
At high frequencies, above 8kHz or so, there is noticeable fluctuation on the VU meter of one channel only.
I guess a good check for this would be to swap the record amplifiers and see if the condition moves with the amplifier.
Also, I noticed some low level rumbling, popping, or crackling (though crackling seems to suggest high frequencies!) when I come close to the speakers. As a further test I played back a blank tape and the output was completely clean with only a little hiss present. I then recorded with no signal to the input of the deck and this VERY low level low frequency rumbling could again be heard in both channels. Could this have somehting to do with the bias oscillator or is it normal?
Or could it suggest the deck requires demagnetizing?
I am not sure I could detect it on musical program but I am wondering if there is anything in the system I should look at.
Thanks for any advice!

Andrew

Offline ironbut

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Re: Studer A-80/RC recording issue?
« Reply #1 on: April 25, 2009, 01:34:39 PM »
Hey Andrew,

Like I've said before, I'm no Studer expert.
The low freq. rumble you hear could very well be mechanical rather than electrical. One way to narrow this down is the try your experiment again with the tape running at different speeds. If the rumble changes frequency, it's mechanical. If not, it's probably electrical. If it seems to be mechanical, try listening to your machine with a stethoscope particularly around the capstan. It could be the bearing.
Other things you should do is clean and check all the rolling surfaces the tape passes over. A little bit of play in a bearing will cause cogging and it amazing how these little things that seem very isolated from the heads will be audible. I doubt if the noises you're hearing will be solved by demag-ing but if you haven't done it in a while, go ahead and do that too. You really should have all the cleaning/demag. done before you use a test tape. They need to be as accurate as possible to be useful and any oxide build up or residual magnetization will damage it.
One word of warning. If the high frequency fluctuations are coming from the same channel as you replaced (if the problem doesn't follow the card) avoid recording until you have a better idea what's going on. It may be something upstream of that card that whacked out the original.
I'll leave it at that and let the real Studer guys take over.

Oops, just remembered, check the connectors (molex) to be sure they're nice and clean. A pin that has a bad connection over time will get dull looking and corroded.
« Last Edit: April 25, 2009, 01:57:50 PM by ironbut »
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Offline Ki Choi

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Re: Studer A-80/RC recording issue?
« Reply #2 on: April 25, 2009, 02:37:17 PM »
I would second that on demaging the heads as one of the first things to do.  Use the heavy duty Hand-D-Mag if available.  Magetized heads would certainly produce rumble like sound but if you say all your tapes have been playing back properly, it may be the machanical issue.

How many hours do you have on your runtime-meter?

Ki
Ki Choi

Offline audiolab1

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Re: Studer A-80/RC recording issue?
« Reply #3 on: April 26, 2009, 10:41:30 AM »
Thank you ironbut and Ki for your responses.
I'll try to answer all of your questions in order.
The first test I performed was trying the same "record silence" test at 15ips.
The rumbling sound is identical in level and pitch, suggesting that this is record section issue and not a mechanical issue.
I think the repro section is not the problem as there is no rumbling noise when I play back a fresh section of blank tape.
Could this also suggest that a demag will not help? It seems that the rumbling is being recorded onto the tape.
The only reason I though it might be oscillator or power supply related is that the same type of rumbling can be heard in both channels. It seems unlikely that the same problem would manifest itself in both record amplifiers, but I guess it's possible.
My next step will be to measure the record bias current on both channels to see if it's stable.
I can also check the oscillator frequency it make sure that it is also stable.
I'll report my findings once I have completed the measurements.
Ki, I can't remember exactly how many hours are on my machine, but I think it was somewhere around 5000.

Andrew

Offline mikel

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Re: Studer A-80/RC recording issue?
« Reply #4 on: April 26, 2009, 12:13:32 PM »
regarding demag on Studers; Fred Thal, the tech who redid my A820, strongly cautioned not to demag the A820. he told me that the A820 was designed with non magnetic pieces and that demaging would have a negative affect.

i have no idea about how this info relates to the A80; but Fred was quite specific about the A820.

my Ampex fefurbisher, Jeff Gillman, did encourage demag on the ATR-102.

mikel
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Offline reelnut

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Re: Studer A-80/RC recording issue?
« Reply #5 on: April 26, 2009, 02:04:14 PM »
It is my understanding that a worn erase head will cause rumble. You need a magnifying glass to look at the poles of the head. If the iron is not very smooth, but appears grainy-looking, you have a problem.
John Hanek (dedicated amateur)- A810, X2000R, X1000R, A3440, GX636. Played thru Yamaha CX1, MX1, YST-SW150 (2), S55 (2).

Offline audiolab1

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Re: Studer A-80/RC recording issue?
« Reply #6 on: April 28, 2009, 03:25:13 PM »
Thanks for your responses so far everyone!

Reelnut, I checked the erase head surface and it looks very smooth, so I doubt it's the problem.

Here are my findings:

 I started with the "Stabilizer" card. This essentially is the regulated power supply for the all of the audio and oscillator electronics. 16V AC windings from the power transformer are then regulated into +/- 12V supplies with the required stiffness.
I checked the output voltages of this card and found them to be very close to spec. (+11.8V and -11.6V) I used the provided adjustment pots to make both of these exactly 12.0V and watched the levels closely for 30mins for any changes. There were none. Next I measured AC ripply voltage on both the +12V and -12V supplies and found it to be very low at about 8mV. Just to double check I attached a scope to the supplies and looked for ripple and could see nothing within the noise.
So far so good.
Next I looked at the bias oscillator and attached a frequency counter. The 149kHz measured was within spec of the 150kHz target.
I then attached a scope to look at the bias signal and found a clean sine wave with some small ripples at the peaks and valleys of the sine wave. I don't know if this is normal or not but it does not look like it was noise pickup from the measurement setup. This measurement was done with no input signal to the deck.
Next up was a look at the record amplifiers.
I stared by looking at the record bias points on these cards and found a slightly "noisier" version of the sine signal I found from the erase bias test point on the oscillator card. Some small ripples could be seen on the sloped sections of the sine wave, but in this case it might have been noise pickup from the test leads which run close to the oscillator card itself.
Then I applied some music signal to the inputs of the deck and observed the "pulsing" of the bias waveform as the AF signal was now mixing with the bias signal.
Finally I disconnected the relay output points on both channels, this effectively removes any AF from reaching the record head. Now with only the bias signal reacing the tape I recorded and played back a few minutes of tape. Once again the very low level burbling could be heard if I increased the output level of my system slightly and put my ear to the woofers.
Again I am wondering if this is normal but knowing the pedigree of this deck and the Studer engineers I highly doubt it.
I guess my next step should be to try replacing all of those 30+ year old electrolytic caps on all of the audio and oscillator cards. There's also a pile of tantalums on these boards that should probably go too.
Maybe that's the problem?
 
Andrew

Offline docb

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Re: Studer A-80/RC recording issue?
« Reply #7 on: April 28, 2009, 07:20:13 PM »
Sounds like you are talking about bias rocks. You might try adjusting the bias using a 20Hz -30Hz tone and adjusting for lowest noise rather than the usual overbias method of using a 10kHz tone and dialing for a few dB down past the peak.

Here's neat article about the process:

http://www.blackmersound.com/includes/analog_tape_bias.html
Dan "Doc B." Schmalle
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Managing Director - retired, The Tape Project

Offline ironbut

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Re: Studer A-80/RC recording issue?
« Reply #8 on: April 28, 2009, 08:44:12 PM »
You should check the caps anyway. Some of the old ones (gold colored) can be trouble. Here's some good info on that.

http://www.analogrules.com/a810.html
steve koto
 Sony scd 777es(R. Kern mods)> Vpi Aires>Dynavector XX-2mkll>Bent mu>CAT ultimate>CJ premeir 140>Magnepan 1.6qr(Jensen xover)Headphone Eddie Current Zana Deux>AT ad2000,HD800 ,Metric Halo ULN-2 (battery powered),
 HE Audio Jades