Setting up a R2R tape deck - Repro (playback) line up

[topoxforddoc]

After Lodgesound (Stewart Emmings) passed away last summer, one of the mods 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 know 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

[topoxforddoc]

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.

http://www.mrltapes.com/choo&u.pdf

[337alant]

A friend of mine asked me how to calibrate the Tascam BR 20 so this is associated specifically to the deck but the sequence is typical to all

BR 20 cal
All of the adjustment pots are behind the lower front panel behind the VU meters so remove the 2 cross head screws in the very bottom left and right of the front panel, this panel should swivel up and you should see the row of pots along the bottom.
Set both the Cal switches to CAL for input and output don’t use the variable pots for any of this
For all adjustments use a small flat non magnetic screw driver
Note I only use IEC and 320nWb/m if you use tapes of 250nWb/m then calibrate the replay levels for that

1. Input / Output level check, Its important to do this first so you know your meter levels and input / output levels are set correctly
Ref Level is 1.23VAC (TRMS) +4dBm @ 1Khz
Set your signal generator to IK at an output level of 1.23v check the output of it with a TRMS meter on AC ( this is the equivalent of 3.05V PP)
If OK plug your Sig Gen into the XLR input and select input on both channels, this should read 0Vu on the meters and also 1.23V at the output of each channel
If not adjust with R308 for output and R307 for the meter levels.

2. Replay levels with a MRL tape reference tape’s for each speed 7.5 & 15”ips & 320nWb/m
7.5” -Ref level is 320 nWb/m for 0vu,
so using the 1K signal,
select repro level and adjust with R301 for 0vu
Select Sync and adjust with R302 for 0vu
Note you can skip to Repro Asimuth at this point as the HF and LF has to be reset later anyway
Using the 16K signal for High Freq
Select Replay and adjust with R305 for 0vu
Select Sync and adjust with R306 for 0vu
15” – Ref level is 320 nWb/m for 0vu,
16K signal, select Repro and adjust with R303
16K signal, select Sync and adjust with R304
Note if you have a 400hz signal on your cal tape the 16K and 400hz should reproduce the same level

3. Replay Azimuth- Connect an oscilloscope to the XLR outputs and adjust for AC levels from both channels or/and XY mode
Select repro and playing the MRL 1K test tone adjust the tape head screw B so that both channels have the highest levels and are perfectly in phase, overlapping the sign wave helps check alignment and phase, if you use XY mode then you should have a perfectly flat diagonal line starting from lower left to upper right. Continue checking with 10K & 16k to fine adjust phase alignment.
Repeate steps 2 to adjust all repro levels
4. Record Bias.
Mount a blank tape of 320nWb/m and connect the Signal generator with the reference levels output of 10K to both channels input to the XLRs, Hit record and play and note the levels this should be 0vu, I normally knock the input out of CAL and adjust both channels back approx. 4dB.
Turn the bias pot anticlockwise and the signal should rise keep turning until the signal peaks, note the peak point and keep turning until it drops 10 db. Then turn the Bias pot clockwise an note the peak point then over bias by the amount advised by the tape manufacturer
5. Record Azymuth
Input a 10k signal at the ref level and select record / play and select repro
Follow the sequence in section 3 but this time adjust the record head screw B so that the record head is exactly aligned with the playback head
6. Record levels
Speed 7.5ips
Switch the input back to CAL
Input a 1k signal at the ref level and select recrd / play and select repro
Adjust the ref level to 0vu with R310
Input a 16K signal and adjust to 0vu using R312
Using the Signal generator run through a series of input signals from 40hz to 20khz and tweak R310/R312 to get the best overall frequency response
15ips
Do the same as above using R313 for level/ and R311 for high speed 15 ips
If you have problems getting the FR within 2 dB set record level the 0vu at 1k then switch out of CAL and adjust the meter back to -10dB then run through all of the frequencys and adjust for the best FR all at -10
I use the MRL tape and these tapes for 7.5 & 15ips
https://www.ebay.co.uk/itm/TASCAM-Al...QAAOSwc-tY5RLu
https://www.ebay.co.uk/itm/TASCAM-Al...QAAOSwAANY5ROc
Which are excellent for fine tuning at -10dB

hope this helps
Alan

[topoxforddoc]

Alan,

Thanks for your expert insight, as always. I know that you, like many others, use an audio millivoltmeter to set up levels, rather than the BBC method of using PPM meters. Very handy to post the more purist way of doing this.

Charlie

[337alant]

Charlie
Thanks also to you for your original post, its a very interesting subject
Your absolutely correct that the recording and broadcasting industry use PPM meters, I like to think that they did this to standardise the recording and playback levels throughout the industary but the PPM standards are not always consistent between the UK, Europe and the USA, there is an interesting table in this article that shows the differences in the standards
https://en.wikipedia.org/wiki/Peak_programme_meter
To most of us with R2Rs who don't have access to a recording panel and PPMs its best to stick to the machine and Tape manufacturers levels in dB.
Most VU meters are not absolutely accurate so its best to check your output with a dB or TRMS AC volt meter.
Also be aware that tape saturation can also give false levels this varies between machines, tape types and tape speed, at 7.5 ips its advised to check through the frequency range at -10dB, this isnt easy to do with some VU meters so monitoring the output with a dB meter gives a more accurate reading
Anyway an interesting subject which I am constantly learning new techniques to get the best out of my machines.
Its always worth studying the manual for your machine to check for the correct reference and operating levels and if people are interested in adjusting their machines for use with modern tapes then I wouldn't recommend they do any adjustments unless they completely understand what they are doing and have a minimum of the following MRL ref tapes for each tape speed, a signal generator with adjustable gain, a TRMS or dB meter and an oscilloscope.
The Udemy Brad Leigh course is also a good place to start
https://www.udemy.com/analog-audio-t...and-alignment/

Alan

[337alant]

A couple more points that make a diffrerence;-
-- The tweaker must be a none magnetic type , I personally use a plastic one or an alloy blade type
--For the Azimuth on the tape heads these need to be adjusted with a screw driver or Allen key so I use my de-magnetiser on them before using them near the heads
-- Its also important to fully clean and if necessary demag the tape transport as well as lubricating the capstan, pinch roller and tape tension rollers this does make a difference to wow & flutter and speed if dry

Alan