topoxforddoc

Setting up a R2R tape deck - part 1 - Repro (playback)

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After Lodgesound (Stewart Emmings) passed away last summer, I was asked if I might pass on some of his knowledge in due course. I was a complete tape novice before Stewart taught me the basics. I'm sure that there will be some on this forum, who will know far more than I do.

Stewart worked in broadcast for his whole career, at the BBC, C4 and then at the BFI, where he was in charge of video and audio restoration. He and his best friend Mike restored the sound tracks and pictures on Hollywood film re-releases, TV programmes and numerous other material.

In a standard 3 head machine the tape passes over the erase head, then the record head and finally the repro (replay) head in that order. That means that the repro head monitors the signal, which you have just recorded onto the tape. Hence when lining up a tape deck, you need to set up the repro head first.


Equipment needed: you need a test tape, PPM meter, oscilloscope (and a tone generator, plus a new tape - for the record set up, which I'll describe in another post later)

Before starting clean the heads and tape guides with a lint free wipe/cotton bud with a small amount of isopropyl alcohol. Demagnetise the heads, if the machine hasn't been demagnetised in recent memory.


The first step is to check the repro head azimuth against a test tape. This is really important, as incorrect azimuth cannot be corrected in later adjustments. Incorrect azimuth leads to poor sound quality, esp losing high freq leading to muffled sound.

1) You need a test tape and a scope to set up repro head azimuth, plus a small allen key to alter the repro head azimuth if needed

Load the test tape. Connect up the R2R outputs to Ch 1 & 2 of your scope and set the scope to read X-Y (Lissajous). Play the 10 k tone and look at the scope - a perfect azimuth should give you a sharp focused line at 45 degrees (should look like this /) - if the azimuth is incorrect you will have an oval trace - adjust the azimuth on the repro head until you get a perfect 45 degree line


Once the azimuth on the repro head is correct, you now adjust the repro output levels to give you a flat frequency response on replay.

2) Adjust the repro levels on playback (repro) head.

Load the test tape. Connect up the R2R outputs to an accurate meter, preferably a PPM meter (much better than VUs, which were known in the BBC as "virtually useless"). If you use the internal VUs on the machine, you may find that these are not accurate enough.

Play the test tape 1k test tone and adjust the repro level on Ch1 (using a tweaker or small screwdriver on the repro control panel - read your instruction manual to find out where this is) until you get the correct reading on the PPM meter (usually PPM4 = minus 4VU = 0dBu). Do the same for Ch 2.

Repeat the process as above for 10k tone on the high freq repro level adjustment on the control panel.

Repeat the process again using a 100 Hz tone on the low freq repro level adjustment on the control panel.


The repro side of your machine should now be lined up properly with a flat frequency response (if you want, you can repeat these steps on a series of other test tape frequencies and check it with a sweep test tone). 

I'll do the record section on another day.

Charlie

PS I specifically didn't make any reference to particular test tapes, as it depends on your machine, the tape speed of the machine and what reference flux you want to set your machine up for. Choosing a test tape is a whole chapter in itself. If you are interested, then the bible on this is the MRL reference guide.

choo&u.pdf

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As someone who was heavily into tape in my early career, this is very useful information for anyone coming into tape now, before all these skills are lost.

My only comment to the above is that even PPMs aren't really good enough for use in lining up a tape machine. The correct instrument is an audio millivoltmeter. 

The problem with a PPM is that each division between PPM 3,4, 5,6 is 4dB which is far too wide to be useful for lining up. A proper ANSI VU meter is actually the better instrument provided what the tape machine has is a proper ANSI VU meter. This has 1dB divisions either side of 0VU, so much easier to see if the levels are correct.

Machines of the Studer /Ampex level will have ANSI standard VU meters, semi-pro machines like the ReVox or Ferrograph don't.

If you're serious about lining up tape machines, then it's well worth investing in an all-in-one meter such as the Ferrograph RTS-2 which includes signal generator, audio millivoltmeter, distortion factor meter drift and W&F meter all in one box. 

Oh, and one other thing, never assume that the tape machine has been demagged recently. We used to demag the machine every time we brought a test tape near it, even if it had been done earlier that day.  Demagging also applies to all one's tools. A magnetised allen key or screwdriver can do an awful lot of damage to a test tape by magnetising the heads or tape guides. 

S.

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Serge,

Thanks. I gather that the topic of metering is a bit controversial. I have ANSI VU meters on my Sony APR5000s, Studer A820 and Otari MTR12. However, Stewart told me that the broadcast industry in the UK lined up all their machines to BBC spec using PPM, as lining up to exactly PPM 4 was more accurate than lining up to -4dB on a VU meter. I take your point about using an audio millivoltmeter, which would be 0.775 V at PPM4/-4 dB on VU.

I do use the VUs for setting up bias on a tape formulation, as the PPMs are too wide to permit accurate level of over bias on a tape.

Finding an old Ferrograph RTS2 is not easy, but maybe I'll just to go hunting.

Charlie

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18 minutes ago, topoxforddoc said:

Serge,

Thanks. I gather that the topic of metering is a bit controversial. I have ANSI VU meters on my Sony APR5000s, Studer A820 and Otari MTR12. However, Stewart told me that the broadcast industry in the UK lined up all their machines to BBC spec using PPM, as lining up to exactly PPM 4 was more accurate than lining up to -4dB on a VU meter. I take your point about using an audio millivoltmeter, which would be 0.775 V at PPM4/-4 dB on VU.

I do use the VUs for setting up bias on a tape formulation, as the PPMs are too wide to permit accurate level of over bias on a tape.

Finding an old Ferrograph RTS2 is not easy, but maybe I'll just to go hunting.

Charlie

The UK broadcast industry did indeed use PPMs for line-up, for two main reasons. Firstly, it's what they had, and they were available everywhere, so easy to find. Secondly, if you're only lining up to a fixed mark, like PPM4, then yes, a PPM isn't difficult to use. However, when lining up a tape machine from scratch, and depending on what test tape is used, the line-up may not be PPM4 (0dBu) but something else. PPM4 only applies to a certain flux off the tape, and a certain replay gain so isn't universal. 

Also, lining up to a fixed mark applies to replay gain. Record line-up especially finding the correct bias point which for Reel-to-Reel machine was usually 2dB over-bias, reading 2dB down on an unspecified level on a PPM isn't as precise as with a VU or indeed, a millivoltmeter.  The real problem with VU is that its dynamic performance under programme conditions depends on what the programme is, what the dynamic range is, and as such is indeed Virtually Useless. The VU meter was invented for telephony, where it was necessary to understand how loud speech sounded rather than what the peak level was. This meter was carried over to recording out of familiarity rather than usefulness. Broadcasting has always used some form of Peak meter, as transmitters, both AM and FM (and now digital) have a maximum level over which they can't go so what the peak level is must be measured rather than how loud something is.  American broadcast practice used VUs in studios, again due to familiarity, but often with Peak lights that warn when the peaks are getting close to maximum.

S. 

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Serge,

I use PPM meters for setting up a fixed replay level at PPM4. On record, I would normally use the PPM meters to set the record level at PPM4 using the test tones. But I would then use the on board ANSI VU meters to set the bias, as the markings on PPM are too wide apart at 4dB and one can't really interpolate the gaps accurately.

Charlie

There'll be another post on record set up later

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On 10/02/2018 at 10:42, topoxforddoc said:

PS I specifically didn't make any reference to particular test tapes, as it depends on your machine, the tape speed of the machine and what reference flux you want to set your machine up for. Choosing a test tape is a whole chapter in itself. If you are interested, then the bible on this is the MRL reference guide.

choo&u.pdf

Just read the Guide. Dear me, that takes me back! I used to be able to recite the introductions to both the Ampex and the Badische Anilin und Soda Fabrik AG test tapes.  

Tape EQ standards were probably the best example of the mantra, 'As standards are a Good Thing, let's have another.' 

When a machine was labeled with IEC or NAB equalisation, which IEC or NAB, at what speeds, and did it apply both to record and playback or playback only, with record being fixed to one or the other? 

We used to have the sort of CD vs LP debates over whether it's best to bias on the peak or overbias. Of course it depended at which speed and using what tape, but it kept us out of the pubs. 

I could even remember the correction factors for using a 7.5ips (19cm/s) tape at 15ips (38cm/s)and vice-versa without having to look them up.

Happy days, well, sort of!

S.

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When I worked for Ferrograph I used these to set up the decks - we did a lot for the BBC.

https://www.ebay.co.uk/itm/Ferrograph-RTS2-ATU1-Audio-Generator-Millivoltmeter-Wow-Flutter-Distortion-meter/152874053881?hash=item239800f0f9:g:9KwAAOSwZ8ZW-HkN

Wish I could afford these now just to play with. 

Serge, don't you still have one? 

Edited by mickbald

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20 minutes ago, mickbald said:

When I worked for Ferrograph I used these to set up the decks - we did a lot for the BBC.

https://www.ebay.co.uk/itm/Ferrograph-RTS2-ATU1-Audio-Generator-Millivoltmeter-Wow-Flutter-Distortion-meter/152874053881?hash=item239800f0f9:g:9KwAAOSwZ8ZW-HkN

Wish I could afford these now just to play with. 

Serge, don't you still have one? 

I do, I actually have two, one of which I've robbed for spares to keep the other one working. The big rotary switch that selects the millivoltmeter sensitivity is somewhat fragile.  

Although I have more accurate measuring equipment using software and a PC, the RTS-2 is the thing I use most. Just so convenient. 

S

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Hi Henry,

Thanks. All well in Cheltenham. I have sent you an email about the RTS2/ATU1. Happy to come by and collect at some point later (I'm not going to the Audiojumble)

Best wishes,

Charlie

3 hours ago, toprepairman said:

Hi Charlie, hope all is well with you.

I do happen to have a spare set RTS2/ATU1 which could be prized out of my hands for half the  price of the one listed on 'bay.

Main concern would be shipping as would not want to risk any damage to the meter movement.

Are you going to Tonbridge?

Henry

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