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Author Topic: Alignment of direct tape-head output modified decks  (Read 4573 times)

Offline AZ_Gary

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Alignment of direct tape-head output modified decks
« on: November 25, 2008, 10:03:07 AM »
Those of us that have modified our machines to bypass the internal reproducer amplifiers and take the tape-head output directly to an external amplifier are faced with a real challenge in regards to alignment. In my case I have modified both a RS-1506 and a MX-5050BII-2 to feed into a Seduction modified for NAB and IEC equalization.

Learning about tape deck maintenance is enough without the added problems. I'm not talking about demagnetization, cleaning and lubrication or tension adjustments. It would appear that we are limited to maintenance of the physical head alignment. Without the VU meters most alignment procedures for reproducer and record alignment don't seem possible. I was able to do an azimuth alignment of the playback heads using a scope connected to the Seduction outputs. A key adjustment seems to be setting the SRL level upon which successive adjustments depend. Can any of the gurus here give some pointers on how we can maximize our alignment and adjustments on these modified machines?

Thanks,
Gary
Gary Marklund
Sun City, AZ - USA

Offline docb

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Re: Alignment of direct tape-head output modified decks
« Reply #1 on: November 25, 2008, 11:56:16 AM »
A pair of VU meters offers no great advantage over a scope for playback alignment. In fact they can fall short because they can't help you much with azimuth adjustment unless they offer a way that you can sum or null the left and right signals. A dual trace scope can show you left/right comparative levels just like a VU meter will show, and in X-Y mode it can show you exactly what is going on with phase between channels.

The first tone on our alignment tape is 1kHz. For a Seduction, connect each channel of your tape preamp to an input on your dual trace scope and set the vertical amp to about 200mV/div. For a Tube Repro you will want to use about 1 volt/div or higher if you use the balanced outputs. Run the 1kHz tone and adjust the three head adjustments - height, tilt, and azimuth - in a cyclical fashion to get the largest and at the same time most even output signal on both channels. You typically cycle thru the adjustments two or three times to get it dialed in just right.

Next, switch your scope to X-Y display format. Now adjust the azimuth screw to give you a Lissajous figure that is a 45 degree line that goes from lower left quadrant to upper right quadrant. That puts the two channels in phase at 1kHz.

Next go to the 10kHz tone, switch back to the standard dual trace or "YT format" and check levels. They should be about the same as the 1kHz levels. Then switch to X-Y format and once again tweak the azimuth screw to give you a 45 degree sloping line Lissajous figure to put the channels in phase at 10kHz.

You can do this once again with the 15kHz tone to really fine tune the azimuth. Depending upon head condition and cable length the 15kHz tone is sometimes down a bit from the others. Also some machines may not give a super stable Lissajous figure at 15kHz, so you just have to average out the setting to what looks the most stable.

That's the basic alignment touchup. Next on the tape are lower frequency tones of 90Hz 60Hz and 30Hz. If you see problems with levels there they are usually due to "head bumps" - resonances in the circuit, and they are not necessarily something that is easily altered.

The next track is an asymmetric 1kHz sine wave. This will allow you to see if the signal coming into your scope is in positive or negative polarity. With this tone you can figure what the signal polarity is at each stage of your system by looking at this signal at the output of the tape preamp, line preamp and amp. The wave will show a pronounced positive peak when in positive polarity and a pronounced negative peak in negative polarity. You may see it flip as you look at the different outputs thru your signal chain.

The last track is a series of pops that can be used with poppers like the Cricket. You hold the Cricket in front of your speaker and it will tell you if the pop is pushing the woofer forward or pulling it back. You can then switch the polarity of your speaker cables if necessary to get it to pop forward.
« Last Edit: November 25, 2008, 01:36:00 PM by docb »
Dan "Doc B." Schmalle
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Managing Director - retired, The Tape Project

Offline AZ_Gary

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Re: Alignment of direct tape-head output modified decks
« Reply #2 on: November 25, 2008, 02:53:03 PM »
Thanks, Doc. That is exactly the information I need.

Gary
Gary Marklund
Sun City, AZ - USA