topoxforddoc
10-02-2018, 10:00
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
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