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Old 20th Aug 2017, 9:28 am   #1
monaro0162
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Default Setting record and playback levels on the A77

Good morning all,

I have been restoring a couple of Revox A77 MK4s for a while now and have achieved good results. I can set the head alignment, the bias, the eq; adjust the levels, set meter calibration pots, and the replay levels on the playback side so that I get a great sounding machine where the recording sounds almost indistinguishable from source. I have followed the procedure in the A77 manual and repeated it a few times to make sure that the results are the same. However, I’m not sure that the overall levels are set correctly, because I am noticing some difference in audio levels when I swap recorded tapes around between machines. The part of the procedure in the manual is not very clear (to me at least).

The A77 VU meters to not operate in playback, they only operate during recording. So, to my understanding, I need to make sure that the replay level on the machine, using a calibration tape, is set correctly first, then I set the record levels to match.

The calibration tape I have has a 1khz recording on it, recorded at 0dbm, with a recording density of 257nWb/m. What output voltage should this give on the line out sockets on the A77? I can measure using a scope to show p-to-p values, or an rms voltmeter. The volume control should be set at maximum? During record, if I set the recording levels and make a recording so that I achieve the same output voltage as per the calibration tape, what should the VU meter be showing? 0db?

I appreciate that this is a complicated subject; I’m looking for a simple answer if possible, so as I can get the final calibration completed. What I’m trying to do is get the recording levels laid to tape set to ‘an industry standard’ (debatable I know) so that recordings can be switched around between different machines, and different manufacturers machines, and will playback at more or less the same output level, and indicated VU level.
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Old 20th Aug 2017, 10:52 am   #2
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Default Re: Setting record and playback levels on the A77

To add, my simplified diagram of the circuit layout attached.
Voltages in green as shown in the block diagram in the service manual.
Are these nominal values, as well over 2.5v is possible on the output without clipping?
The red lines indicate the signal path when in recording mode, so as 'on the fly' monitoring can be done, with the VU meters active. They are not active on playback.
The 257nWb/m tape density figure *may* correlate with the 2.5v line output level? Maybe that is just my wishful thinking and it is merely co-incidence!
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Old 21st Aug 2017, 12:55 pm   #3
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Default Re: Setting record and playback levels on the A77

I think Revox altered the magnitude of the output levels during the evolution of the A77 versions. Perhaps others can confirm.
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Old 21st Aug 2017, 1:32 pm   #4
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Default Re: Setting record and playback levels on the A77

See section 2-4 ;
http://aes-media.org/historical/pdf/...stl-manual.pdf
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Old 21st Aug 2017, 5:59 pm   #5
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Default Re: Setting record and playback levels on the A77

Thank you SteveCG and Restoration 73. I'll give it a good read when I have some time to spare

In the mean time, thinking this through again...

The actual line out level on the sockets is irrelevant, as this can be adjusted up to almost 6v before clipping occurs, by a combination of turning the volume up, and by adjusting the replay level trimmers on the playback amps. It's the flux density of the recording laid to tape, which I am trying to 'standardize'.

When playing a calibration tape, calibration would be easier if the meters on the A77 were active during playback. But they are not, they are only active on record.

So you have to work backwards, playback a calibration tape with a recording of a known density or level, note the output voltage level, then set the recording side of the machine so that the level matches, then set the meters during record to represent this known level?

The information on the calibration tape states 257nWb/m at 0dbm.
Which doesn't correspond with the table in the tape manual which quotes 185nWb for 0db. 257nWb/m is about +2.7db according to the second table. It's a long time since I did logarithms at school, and I'm aware that decibels is a scale, rather than a unit of measure, but I'm still not quite there on setting the levels.

If I can get a recording made on both machines at an indicated "0db" on the meters (and recorded at a respectable flux level) to playback at the same level on both machines, I'll be happy. At the moment there seems to be a difference in output level between machines when I swap the tapes around. I'm not sure if this is due to the recording side, or playback side of the machines.

I just want to confirm that the recording side is calibrated properly, and that the correct levels are being laid to tape on both machines.

Last edited by monaro0162; 21st Aug 2017 at 6:05 pm. Reason: edit
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Old 21st Aug 2017, 7:57 pm   #6
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Default Re: Setting record and playback levels on the A77

There is a missing element here, and that is that VU meters are set up with a "lead" of 4dB to allow for their sluggish transient response. This is, incidentally, by design -a true VU is specified to take 300mS to reach 0VU from rest. In other words, for a steady tone at -4dBm, a VU should read 0dB.
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Old 23rd Aug 2017, 7:06 am   #7
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Default Re: Setting record and playback levels on the A77

dBm is an iffy unit to use as it is a power ratio, IE 0dBm is one milliwattt and has no direct relationship to impedance or voltage, but is usually specified when terminated by a 600 ohm load.

dBu might be a better here, +4dBu is 1.23v and AFAIK the equivalent of +4dBm into a 600 ohm load.

The Revox manual I have doesnt help much, being mostly uninteligible and doesn't give you much idea how to set the VU meters. It says "AF generator 500hz full modulation" What? Then says "output V to remain constant - +/- 1dB when output switch, switched. If OP doesn't stay constant follow 6.5.4 6.5.4 says when setting record level "input level - set sig gen 500hz IP level about 40mV" !! 40mV hardly moves the VU needle. I set mine I think from memory at 1v ish, but then I had no test tape.

I got really frustrated with this stupid manual, it sends you round in circles with vague hints. I'd be most interested in how you resolve this.

Andy.
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Old 24th Aug 2017, 1:54 pm   #8
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Default Re: Setting record and playback levels on the A77

Read through out this thread:

A Question For Revox A77 Specialists - UK Vintage Radio Repair ..
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Old 25th Aug 2017, 8:32 pm   #9
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Default Re: Setting record and playback levels on the A77

It seems that I'm not the only one who's had difficulty setting the correct levels. I think that the manual is very good, with the exception of the part on setting the levels which i agree is vague. Some of the mV levels mentioned are barely above the noise floor on the signal generators that I am using! i did try laying out the ops in an excel spread sheet but yes have ended up in circles. There's what could be a useful hint in the post from 2006 quoted it mentions the 0.775v 'standard' somewhere may be I need to add this in to my procedure somewhere. Does anyone have the calibration procedure that someone made, mentioned in the 2006 post?

I should have some time to do a few more calibration runs next week, I will keep you posted. Really keen to get the calibration, the last part of the process, complete, on these machines, that I've been working on, on and off, for several months now. And just to add to the fun, one of them is actually a Dolby machine
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Old 25th Aug 2017, 8:47 pm   #10
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Default Re: Setting record and playback levels on the A77

775mV is 1mW (0dBm) in a 600 Ohm system which is the reference level for telephony and a lot of other audio things.

If your signal generator doesn't go low enough, then you could make an attenuator easily enough at audio, they only get difficult at radio frequencies..

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Old 28th Aug 2017, 5:58 am   #11
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Default Re: Setting record and playback levels on the A77

You could try using an online tone generator if you have a half decent soundcard on your PC but around 40mV seems a low level to set the meter and 40mV what? 40mV peak/P-P or RMS?

Let us know how you get on. A.
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Old 29th Aug 2017, 12:28 am   #12
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Default Re: Setting record and playback levels on the A77

Here's the level setting routine that I came up with:

Establish the density at which you wish to record to tape.
I used a calibration tape quoted at 257nWb/m.
This figure represents how hard the tape is magnetised or driven during recording.
The figures vary for different brands of tape.
257nWb/m is going to represent a recording VU level of 0db.
Attach a dual channel oscilloscope to test point A on both playback amplifiers.
Play the 1khz @ 257nWb/m part of calibration tape.
Monitor both A test points for a signal of ~500mV on both channels.
No level adjustment exists up to TP A, so any differences observed in signal level or phase will be due to the heads or head alignment. Adjust the azimuth of the playback head so that there is no phase difference between channels and so that the level on both channels is the same.
A reference playback level has now been set for 257nWb/m at 0db.
The record side of the recorder now has to be set so that it lays a signal to tape at the same 257nWb/m level, with the VU meters adjusted to show 0db.
Load a blank tape for recording.
No level adjustments exist on the input amp.
Turn both CH1 and CH2 record level controls to maximum.
Set the balance control to the central position.
Press Record CH1 and Record CH2 buttons in.
Set both selectors to AUX.
Supply a 1khz input signal @ 25mV to the line in phono sockets.
Set the machine to record mode with the tape running:
Whilst monitoring test points A on both channels, adjust the record level pots on the record amps so that the same 500mV as noted during the playback test earlier is achieved on both channels.
Then adjust the meter calibration pots on the record amps so that the meters show 0db.
The record side of the machine has now been set to match the level on the calibration tape.
Now we move to adjusting the playback levels out of the line out sockets and the headphone socket.
With the machine still in record mode and running:
Set the volume control to maximum.
Attach a dual channel oscilloscope to the line out sockets.
Set the input selector to input.
Trace on the scope will be showing the 25mV khz input signal, amplified by the input amp and drive amp up to a level of several volts (CH1 and CH2 record level controls remain at maximum, volume control remains at maximum)
Adjust the replay level controls on the playback amplifiers so that the levels are the same and both channels are balanced.
Switch the input selector to tape (iec/nab) and observe any differences in level. The level from the tape may be higher or lower than from the input. Ideally they need to be the same. Equalise the levels by adjusting the replay level controls on the playback amplifier.

That’s it. The machine should now be laying recordings to tape at 257nWb/m at an indicated 0VU, the same as the calibration tape, and there should be no difference in volume levels when switching between input and tape.

Obviously the EQ and bias have an effect on the levels, and need to be adjusted too. I haven’t included these here for simplicity. (Also the record head azimuth needs to be set, to match up with the playback head). I set all the pots for the bias and eq to the mid-point before starting, then did this level adjustment routine, then did the bias and eq, and then repeated both procedures a couple of times. I am now within 1 db when I swap tapes around between machines, and the volume, balance and record level controls are now all in the same position, before the volume had to be set higher on one machine. I have the same brand of tape on each machine and both machines playing back through a mixer. When I monitor live recordings and switch between the two, I really can’t tell the difference. Result!

I just hope that the calibration tape that I have is good, because a lot depends on it!
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Old 29th Aug 2017, 12:32 am   #13
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Default Re: Setting record and playback levels on the A77

What I do.
First I set the recording levels full open even as the output in Stereo and the balance in the middle. Than I put a signal, 1000Hz, so loud that I get 0,7 Volt on the output. (0dB) Than I trim the VU meters to 100%. P504.
Second, I start the test tape and I trim the output at 1,5 Volt (it's + 6dB) P801
Thirth, The testtape have also a pink noise and that i used for set the playback azimut. In mono with my ears, I listen to the best sharp noise.
Fourth, I take a Maxell tape for adjust the recording, level, bias an EQ. (in Stereo)
Fifth, 1000 Hz at -20dB input measered on the output (I used a second VU meter (RTW), and I adjust the recording level (P503) at the same as the input. So for and after is the same. Input - NAB no difference.
Then, 10.000 Hz I turn clockwise slowly the bias trimpots (100KOhm is better) so that the bias first rise and than fall down ca. 5 dB. Than 14.000 Hz and I adjust the EQ to -20 dB.
Step 5 must repeat it 2 or 3 times.
Sixth, I record pink noise and listend mono and adjust
the recording head for the best result so the azimut is the same as by the reproduse head.
After this the setting is correct and you do it in stereo the channels are the same adjust.

When you will see moving the VU meters by play back.
Set the source on 1-2 and 2-1 and the recording levels about 8. And see the are moving.

Jack Marotick from the Netherlands, I repair Revox recorders and more
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Old 29th Aug 2017, 6:19 am   #14
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Default Re: Setting record and playback levels on the A77

Thanks, that's very clear, nice one.

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Old 29th Aug 2017, 7:37 am   #15
Ted Kendall
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Default Re: Setting record and playback levels on the A77

Yes, that's it, pretty much. One playful little complication on the standard A77, but not on the Dolby version, is that if you switch from Tape to Input in record, the level recorded to tape drops by 1dB, because the monitor amp loads the record feed down slightly. The Dolby machine has a different internal level structure and better managed impedances at this point, and the problem does not occur.
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Old 29th Aug 2017, 7:51 am   #16
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Default Re: Setting record and playback levels on the A77

Test point A is at he top of P801 on the playback amps.
mV levels quoted are p-to-p as observed on a scope.
On the Dolby machine, the signal routing is somewhat different.
The 'record calibration' pots beneath the front flap are in the circuit as well, and vary the signal levels sent to the input of the record amplifier. Again I set them to the central 12 o'clock position before starting. They are both sitting at about the 1 o'clock position now. The meter cal pots on the record amps have no effect. You have to use the pots on the playback processor.

I may have to re visit the Dolby calibration set up that I did. I set it up 'parrot fashion' as per the info on the HCCC site, before fully understanding the level setting procedure. The gain adjustments on the record and playback processors make no difference to the levels observed, with a steady 1khz test tone, at least. I guess that with Dolby the gain is dynamic? As set though, the Dolby is working. Definitely reduces tape hiss to inaudible levels during quiet passages in the music on the tape, with no suppression of high frequencies evident during louder passages. During quiet passages, if I flick the Dolby switch in and out quickly, due to the three heads I can momentarily hear a boost and cut in hf and hiss levels, so it seems to be working
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Old 10th Sep 2017, 10:16 pm   #17
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Could anybody could help on the Calibration procedure, specifically the Dolby version;

The routine I posted referred to the standard A77 version. I can repeat the procedure and each time the result is as wanted - no difference in levels when switching between source and tape. 0db on the VU meter indicated during recording equates to the same output level observed from the 257nWb/m calibration tape, used as the 0db reference setting. The VU meters are not active on playback on the standard A77, they are on the Dolby. On the Dolby they work differently because of the way that they are integrated into the Dolby circuitry. So I drew out another diagram to try and make sense of it. Significant difference is the addition of the record calibration pot in the process.

I ran my routine as mentioned. The L calibration pot in the circuit has to be set back a little from maximum, and the R calibration pot has to be set to maximum, to get 0db on the meters, and no difference between source and tape. All good.

So just out of interest I decided to run the Dolby calibration process, as per the Revox instructions. When set to CAL, with the right channel calibration pot on maximum, I can only get an indication of minus 2db, the left channel will go up to 0db if the calibration pot is rotated to maximum. Almost as if the output of the internal 800hz calibration oscillator is not set high enough. But P103 is already set on maximum.

So if I run the revox calibration routine, after my calibration has been carried out, this results in slightly louder output level and higher VU readings on playback, not the expected result of equal levels. So I’m wondering where the difference is and how to equalise the levels, once and for all. I haven’t touched the Dolby pots since I set it up as per the HCCC site a few months ago. Should I now be looking at adjusting the gain and fet pots within the record and playback processors again?

Other than that, the machine sounds fantastic. No discernable difference between source and tape.

I have the operation supplement for the Dolby version, but what I really need is an English version of the Dolby service manual. Only have A German version. Does an English translation exist?

Almost there, almost there...
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Old 7th Oct 2017, 10:03 pm   #18
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Default Re: Setting record and playback levels on the A77

Many thanks to everybody who offered advice on this excellent forum.

I re did the calibration and Dolby setup routines a couple of times, and narrowed the difference that I noted down to about 2db when using the 800hz internal calibration oscillator. It's been a lot of fun ending with a great sounding machine.

During this adventure I've acquired 4 machines (about 3 too many..) so this one has now gone to a new owner, to re-coup some of the costs I incurred when buying test equipment. The Ferrograph RTS 2 is a great test set
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