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Author Topic: Revox A77 - calibration problems.  (Read 16536 times)

Offline oceansmoving

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Revox A77 - calibration problems.
« on: August 29, 2013, 07:46:05 AM »
I recently bought a Revox A77 but ran into problems when trying to calibrate the record side on one of the channels.
All record calibration is done with a test tones at -20db (155mv).
1k for rec level, 10k for bias and 12k for rec level, all according to the manual.
Problem is, I cannot get enough level at the eq calibration.
Even if I set the bias at PEAK, (test to get maximum high frequency output) the EQ won't get above -2dB with the pot at max, in relation to the overall rec level of 0dB. (Figurative reference level so you'll undestand the issue)
And, of course the more I overbias from PEAK the less HF max level I can get.

Channel II is no problem to calibrate, lots of room do get the EQ and bias just right.
The pots, 7 1/5 bias pot in particular seems a little glitchy.

The nominal and variable resistance seems to be within spec, but I will probably replace them anyway.
I'm not sure it will solv my problem though, what do you think might cause this?
« Last Edit: August 29, 2013, 07:50:03 AM by oceansmoving »
Joakim Jensen
Gothenburg, Sweden

Offline Tim

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Re: Revox A77 - calibration problems.
« Reply #1 on: August 29, 2013, 12:00:25 PM »
The A77 left channel is infamous for having not enough HF signal.  This is because one can't easily see the left track area of the head (play and record) on that particular deck and that means that oxide deposits often are left on the head surface after cleaning. I see it all the time in my repair business. Clean you heads again making sure the left track area (top area) is spotless. Use a dental mirror to inspect, or better, remove the headblock for better visual and cleaning access (3 screws).
« Last Edit: August 29, 2013, 12:02:50 PM by Tim »
Tim Leinbaugh
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Offline oceansmoving

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Re: Revox A77 - calibration problems.
« Reply #2 on: August 29, 2013, 01:08:52 PM »
Great tip! There was definitely some oxide left there that I had missed.
Will see if it makes any difference after the weekend, thank you!
Joakim Jensen
Gothenburg, Sweden

Offline oceansmoving

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Re: Revox A77 - calibration problems.
« Reply #3 on: September 05, 2013, 01:02:33 AM »
I got some additional HF-response by the cleaning, thanks for that!
Unfortunately, the problem now is the low end. At peak 100hz ~ 0dB but the more I overbias the more lows build-up I get. But, bias at peak would mean that on CH2 (which has very good HF response) I would not be able to get the HF down within spec even with the EQ pot at minimum.
This means I have to compromise with the amount of bias to get a somewhat even response.
This is the result of around 1-2dB overbias:

-------------- Left/Right Energy Analyzer Results -----------------




FRQ (Hz): Left (dB): Right (dB):



22 -38.5 -40.6
65 -38.1 -40.9
108 -41.2 -43.6
151 -42.9 -43.9
194 -45.7 -47.2
237 -47.2 -49.2
280 -47.4 -49.1
323 -48.5 -50.0
388 -45.6 -47.9
474 -46.4 -47.9
560 -46.2 -48.7
646 -47.0 -48.7
732 -46.7 -50.0
818 -48.2 -50.4
904 -48.8 -50.2
991 -48.7 -49.5
1120 -46.7 -47.5
1292 -47.4 -48.6
1464 -48.0 -47.9
1637 -48.4 -49.6
1809 -49.1 -50.0
1981 -49.5 -50.3
2153 -49.9 -50.7
2326 -50.8 -51.4
2498 -50.9 -51.2
2670 -51.6 -51.7
2929 -48.8 -49.8
3273 -50.2 -50.0
3618 -50.0 -49.5
3962 -51.6 -51.4
4307 -51.1 -51.5
4651 -52.8 -52.4
4996 -52.9 -52.1
5340 -53.4 -53.1
5685 -53.4 -52.3
6029 -54.3 -53.3
6374 -53.8 -53.1
6718 -54.9 -53.9
7235 -52.0 -50.7
7924 -52.2 -50.1
8613 -52.1 -50.9
9302 -52.8 -51.2
9991 -52.3 -51.5
10680 -52.0 -51.3
11714 -49.5 -48.2
13092 -48.8 -47.6
14470 -47.7 -48.1
15848 -47.2 -48.7
17227 -46.4 -48.3
18605 -46.1 -48.9
19983 -45.8 -49.5
21361 -47.9 -51.4

I want to keep the key frequencies 1k, 12k and 16k within +- 1.5dB.
But, as you can see = pretty heavy LF bump. If I were to overbias more, say ~ 4db, I'd still be able to get a nice HF response with the EQ, but even more low end.
6dB, as RMGI advises for SM911 tape = huge LF bump and so much HF roll-off that I cannot even compensate for it with the EQ.
« Last Edit: September 05, 2013, 01:04:37 AM by oceansmoving »
Joakim Jensen
Gothenburg, Sweden

Offline Tim

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Re: Revox A77 - calibration problems.
« Reply #4 on: September 06, 2013, 11:37:43 AM »
It's not possible that bass response (20 to 1K) changes radically with typical bias levels as you claim it does.  That means your measurement methods and measurement equipment (computer-based) is likely flawed.  Only use an analog sine-tone generator and analog AC millivolt/DB meter for setting bias, EQ and levels.

http://www.newark.com/tenma/72-505/audio-signal-generator-frequency/dp/66F3575?in_merch=true&MER=ALT_N_L5_ElectricalAndElectronicTest_None

http://www.ebay.com/itm/TEST-EQUIPMENT-LEADER-LMV-181A-AC-Millivoltmeter-Wide-Bandwidth-Audio-Volt-/151115625692?pt=LH_DefaultDomain_0&hash=item232f3174dc

An addition problem (other than your inaccurate measurements) could be the condition of the heads.  They could be damaged from long-term oxide deposits (tape residue left on heads for years can cause corrosion).  They could still be dirty.  They could have been misaligned by someone and they could be worn-out.

5-6 db overbias with SM911 tape @ 7.5 ips @10K should give you a flat HF (1 and 10K the same level) after adjusting record EQ. It should also give you a good (< 3% ) distortion level at peak record level (+6 VU). Be aware that turning the bias pots CW does not always mean you are increasing bias to the record head. On some decks increase is CCW (I can't recall right now how the A77 is oriented).

 
« Last Edit: September 06, 2013, 11:23:22 PM by Tim »
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Offline oceansmoving

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Re: Revox A77 - calibration problems.
« Reply #5 on: September 09, 2013, 06:35:28 AM »
It's not possible that bass response (20 to 1K) changes radically with typical bias levels as you claim it does.  That means your measurement methods and measurement equipment (computer-based) is likely flawed.  Only use an analog sine-tone generator and analog AC millivolt/DB meter for setting bias, EQ and levels.

http://www.ebay.com/itm/TEST-EQUIPMENT-LEADER-LMV-181A-AC-Millivoltmeter-Wide-Bandwidth-Audio-Volt-/151115625692?pt=LH_DefaultDomain_0&hash=item232f3174dc

http://www.newark.com/tenma/72-505/audio-signal-generator-frequency/dp/66F3575?in_merch=true&MER=ALT_N_L5_ElectricalAndElectronicTest_None


I don't really see the problem using DAW, a good sound card and good software for calibration.
The frequency response is enough and even if the AD FR isn't 100.00% flat a DA/AD loop and good measuring software will tell you how to compensate. Not that that's the case with my setup.

Quote
An addition problem (other than your inaccurate measurements) could be the condition of the heads.  They could be damaged from long-term oxide deposits (tape residue left on heads for years can cause corrosion).  They could still be dirty.  They could have been misaligned by someone and they could be worn-out.

5-6 db overbias with SM911 tape @ 7.5 ips @10K should give you a flat HF (1 and 10K the same level) after adjusting record EQ. It should also give you a good (< 3% ) distortion level at peak record level (+6 VU). Be aware that turning the bias pots CW does not always mean you are increasing bias to the record head. On some decks increase is CCW (I can't recall right now how the A77 is oriented).

Yes, turned out rec head was badly misaligned. This has been corrected but I haven't been able to recalibrate everything yet so I've yet to see whether this caused the weird low end bump or not. Thanks for the help.
Joakim Jensen
Gothenburg, Sweden

Offline High and Outside

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Re: Revox A77 - calibration problems.
« Reply #6 on: September 09, 2013, 01:38:01 PM »
This thread reminds me of why I quit fooling with consumer decks in the first place. They just didn't make them with adequate adjustability. Once you get used to working with studio machines, which have all the adjustments you need, and adequate range on them, you get spoiled being able to get the machine dialed in properly, and no longer want to live with the compromises that the consumer decks force you into.

And I believe I was working on an A77 when I finally threw in the towel.

One of the bad engineering practices commonly recommended when pretending to calibrate these decks is to use bias trimming to get the frequency response closer, and I see that's just what you're trying to do. Sure, changing the bias will change the frequency response, and it has been recommended in countless HiFi magazines from the '60's, but think about it for a second: it's the backwards way to approach it. You wind up adjusting bias away from the optimum bias point in order to compensate for the lack of a proper frequency response adjustment.

Setting the bias for its own sake is one of the most important and critical adjustments in a recorder. The optimum adjustment point for bias is the point that yields the lowest distortion and the lowest modulation noise. Modulation noise is something you don't run into very many places in audio other than magnetic recording, so you may not be familiar with it. It isn't tape hiss, which is constant. Modulation noise is noise generated only in the presence of signal. It's one of the least appealing parts of magnetic recording, and in my opinion we should work hard to minimize it. That's why I think it's foolhardy to adjust the bias away from its optimum point just because the manufacturer failed to include a proper trimmer on the EQ circuit.

Also, let me suggest a more accurate method for bias setting: use a 1KHz tone. At 15 or 30 ips, bias 1/4 dB over the peak. At 3-3/4 or 7-1/2 ips, bias 1/4 dB under the peak. It's a long story, too long for this post, but you can find some of the story in the papers so generously provided on the MRL site.

And on another note: Tim generally gives very good advice on this forum, so I'd recommend not dismissing his suggestions out of hand. Whenever we are using measurement instruments our first job is to check the measurement setup and make sure it is really measuring what we think it's measuring. It's trickier than we think--at least until we have fooled ourselves a few times, then we gain a respect for this requirement.   
Paul Stubblebine
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Offline oceansmoving

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Re: Revox A77 - calibration problems.
« Reply #7 on: September 10, 2013, 12:08:57 AM »
One of the bad engineering practices commonly recommended when pretending to calibrate these decks is to use bias trimming to get the frequency response closer, and I see that's just what you're trying to do.
I know that, and I don't WANT to do that, it's just what I HAD to do to get somewhat descent results. But I also know it shouldn't be necessary, that's the reason I made this thread!


Quote
And on another note: Tim generally gives very good advice on this forum, so I'd recommend not dismissing his suggestions out of hand. Whenever we are using measurement instruments our first job is to check the measurement setup and make sure it is really measuring what we think it's measuring. It's trickier than we think--at least until we have fooled ourselves a few times, then we gain a respect for this requirement.

This was not my intention, it was I who asked for help, right?
I'm just a little tired of the perfuctory bashing of digital measuring methods, without the technical explaination!
Just give me the techincal reason for not doing this digitally and it's fine, for some reason you rarely get that though.
The whole "Don't buy ANYTHING analog, and most of all DO NOT try to do anything yourself unless you have spent thousands of dollars on a whole setup of analog measuring gear". You get that in EVERY forum, 9 of 10 members and just discouraging. Then 1 in 10 will come along and ask "Tell me what you know and what gear you are using and then we'll see if it will cover what you are trying to do". Basically just not assuming you're an idiot. Most of the time it turns out that it will work just fine.

Thank you for a lot of good input!
Joakim Jensen
Gothenburg, Sweden

Offline oceansmoving

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Re: Revox A77 - calibration problems.
« Reply #8 on: September 12, 2013, 02:40:11 AM »
Trimmers have been replaced and setting are much more stable now.
Azimuth adjustment and frequent head cleanings also gave me a much flatter frequency response. I'm gonna recap also but so far things are sounding much better!
Another thing I noticed yesterday though, when recording I get a little bleed through fr?n the direct input even when monitoring the repro. When recording a snare track for examble you can hear the hit in the background before the tape signal.
It's barely audible but still, is this a problem or as designed?

It's like the signal is bleeding through from the preamps or rec head or something and it happens the first time I record onto that tape so I guess it can't be print through either.
« Last Edit: September 12, 2013, 05:43:12 AM by oceansmoving »
Joakim Jensen
Gothenburg, Sweden

Offline oceansmoving

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Re: Revox A77 - calibration problems.
« Reply #9 on: September 18, 2013, 01:33:29 AM »
Recapped now. I also noticed some tangent on the rec head to fixed that and even re-aligned the head height because it was a bit off.
I know experts say you shouldn't touch it but I was carefully observing the signal and found a sweet spot that worked good both sound wise, and I could visually see that the head was much better in line with the taoe and not scewed in any way. Then re-aligned and recalibrated and she sounds really good now. BUT, then I noticed another problem :( I don't know if this can be fixed or if I should just sell it again:
Tape is not in contact with the bronze guide between play head and pinch roller. Laquer on the nut is intact, so I don't think THAT have been touched, I think it's the vertical alignment that's off,  a little tilt to the left (looking at the machine standing) which I guess should be OK to mess with (screw that holds guide piece to the block? Though, I miss around 2-3 mm to get tape contact and I can't imagine getting it by just tilting it? But then again, neither head or pinch can be horizontally moved right, so it can't be anything else?
« Last Edit: September 18, 2013, 06:58:16 AM by oceansmoving »
Joakim Jensen
Gothenburg, Sweden

Offline Tim

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Re: Revox A77 - calibration problems.
« Reply #10 on: September 18, 2013, 10:38:36 AM »
 Revox decided there was no need for the tape to contact the brass spacer part of the guide, so what you are observing is normal. The guide flanges will guide the tape.  You can change the guide penetration by loosening the bracket screw and moving the entire assy forward if you want (so that the tape contacts the brass spacer). That, however may change the play head tape wrap angle, so don't over do it.  Or just leave the guide alone.

When adjusting any head height, it is important to turn all three alignment screws (height, tilt and azimuth screws) by exactly the same amount.  If this is not done, the head tilt (zenith) will change from it's proper factory-aligned position.  Even small head tilt errors will cause problems (premature head wear one of them). Azimuth should be re-adjusted/re-checked after any head height adjust.
« Last Edit: September 18, 2013, 03:57:25 PM by Tim »
Tim Leinbaugh
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Offline oceansmoving

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Re: Revox A77 - calibration problems.
« Reply #11 on: September 18, 2013, 11:02:41 PM »
Thank you Tim. Well, I don't see any instric value in tape contact, just want to make sure there's nothing wrong with my deck.
Besides, in other forums I have been told it should, hence the confusion.
I tilted the entire post a little to the "right"( looking at the front as in the picture below) as I think it wasn't entirely straight but still no contact with the tape so I don't think it's even possible on my machine. If Imove it any more to the right it will bump into the capstan axis. Of course one problem that I'm fearing, if you say that it should be possible to get contact, is that the capstan axis might be slightly bend "downwards". I sure hope not..
Before-picture attached.
« Last Edit: September 18, 2013, 11:05:45 PM by oceansmoving »
Joakim Jensen
Gothenburg, Sweden

Offline Tim

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Re: Revox A77 - calibration problems.
« Reply #12 on: September 19, 2013, 12:46:20 PM »
I've never tried to readjust the guide so that the tape contacts the guide post/brass spacer, so that may not even be possible.  Like I said, the tape does not need to contact the post and Revox did not intend tape to contact the post. Did anyone tell you the tape needs to contact the post, or is that just your assumption?

The person on Tapeheads only said he was "pretty sure" it does, which means he has not actually observed the tape contacting the guide post. It was just a guess on his part.

I would just leave the guide alone and not do anything to it.  If the factory lock paint (on both the bracket and height screw) is still intact, that means it's correct and should not be tampered with.
Tim Leinbaugh
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Offline oceansmoving

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Re: Revox A77 - calibration problems.
« Reply #13 on: September 23, 2013, 02:06:45 AM »
Exactly, it's obviously important to make sure I look a little stupid for asking, but then it changed to "pretty sure".
It is what it is, I will leave it as it is and it seems like it shouldn't be in contact as you say :)

One more thing, the tape lift lever is a little wobbley during rw/fw which makes me a little nervous that it's in contact with the play head, resulting in extra wear. I guess it's due to vibrations, but for now I hold it down a little to keep it steadier.
I don't know if it's possible to do anything about this? When I look at the mechanics it seems that I can't "tighten" it without possibly interfering with the pinch roller arm movement.

Joakim Jensen
Gothenburg, Sweden

Offline High and Outside

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Re: Revox A77 - calibration problems.
« Reply #14 on: September 23, 2013, 09:07:50 AM »
Hey Oceans,

This is another reminder: the rule here is that we use our real names. The posting policy is in a sticky that's the very top thing on the main page. Please either sign your posts or edit your profile so that your name appears in your posts automatically. Thanks.
Paul Stubblebine
Managing Director, The TapeProject