[ProAudio] Dragon modifications (was FeralA - Recordings released encoded with Dolby A)

Wed Feb 12 12:43:55 EST 2020

Hi Richard,
As I recall (it's been 35 years), we calculated the scale factor by making precise measurements of the mechanical parts between the ribbon and head. The LVDT transducer itself is, of course, easily calibrated. The complete project file is likely rotting in a file cabinet somewhere at Capitol right now - sigh!  I never explored the remote connector, but it would be relatively simple to protect it from static and any other voltage "insults" with some low-cost semiconductors available now (low Vf schottky clamp diodes plus <10 mA high-voltage JFETs in series - it's my current favorite "bullet-proofing" for most digital I/O ports).
And thanks for the correction about actual head construction. The Dragon is truly an amazing machine. My home system includes a more modest BX-300, but it likely needs rubber part replacement and a performance check-up since it's been in storage for a couple years due to my recent down-sizing move.
Best regards,Bill

-----Original Message-----
From: Richard L. Hess via ProAudio <proaudio at bach.pgm.com>
To: Bill Whitlock <engineer_bill at verizon.net>; proaudio <proaudio at bach.pgm.com>; mm1100 <mm1100 at yahoo.com>
Sent: Wed, Feb 12, 2020 12:04 pm
Subject: [ProAudio] Dragon modifications (was FeralA - Recordings released encoded with Dolby A)

 Hi, Bill, 

 That is fascinating! 

 One very minor nit: the comparator only works on the right channel of the currently playing side. It is actually a six-channel-ish play head. The left (outer) channels are normal and the right (inner) channels are split--series-connected for the audio.

 The LVDT is a wonderful solution. 

 Do you happen to have the displacement of the azimuth adjuster ribbon to arc-minutes conversion factor data?

 I have made a modification to the Dragon as I find with many problematic tapes, the auto system doesn't work as well as one would hope. This modification is simple. I cut the wire that is the ungrounded feed to the azimuth motor. I run it through a spare pole on the Timer Play/Rec switch so that when that switch is in the Rec mode, the auto azimuth is defeated and the motor's feed is switched to a pin on the Gizmo connector. Since the Gizmo connector has a bipolar power supply, I simply extend a 4-pin DIN cable to a small Hammond diecast box with an (on)-off-(on) [(on) means momentary] switch with a series resistor. So, I can remotely pulse the azimuth motor forward and back while watching the stereotool.com azimuth display. 

 I do find tapes that are outside the range of the electrically adjusted azimuth so I manually adjust the azimuth with the electrical version centred and then reset the manual adjustment later (or use an MR-1 and manually adjust).

 The other modification I've thought about for a while (but haven't done) is to replace the remote connector (which I'm afraid to use since the logic chip could be damaged by static and I hear it's unobtainable) with a 5-pin DIN connector and run that to the head preamp outputs before the CMOS switches so that audio from both sides of the tape are brought out simultaneously from that wonderful Nakamichi play head. My intent here would be to use a standard DIN - 4 RCA cable and RCA to TS adapters and use the four instrument inputs on my nearby RME Firefaces to provide the additional gain and do whatever else is needed in software. But I don't have enough call for four track HiFi cassettes. I can use my DATATape CMS1000 tape players for court reporting type of stuff (they do 15/32 to 7.5 in/s), but are not as HiFi as the Dragon.

 And, I previously mentioned always using the outboard Dolby 422 when needed rather than the inboard decoder so that I can easily adjust the playback level into the decoder to calibrate it. Someday, maybe, my colleague John Dyson will do a Dolby B/C decoder...I hope.

 Cheers,

 Richard

 On 2020-02-11 10:31 p.m., Bill Whitlock via ProAudio wrote:

Not wanting to digress too far, but your mention of the Nakamichi Dragon again reminded me of my years at Capitol. Capitol was unique in the cassette and LP business because they were so highly vertically integrated. They manufactured their own tape, molded their own cassette shells (or C-zeros as they were called) - and even their own lacquer master discs. Part of QC for the cassette shell molding operation involved using modified (in my electronic development lab, which was not located in the "tower" BTW) Dragons to measure azimuth accuracy and consistency. The auto-azimuth mechanics was fitted with an LVDT linear transducer which then drove a large zero-center analog meter reading +/- 10 arc-minutes full-scale. The shells were generally excellent except when someone shortened the cool cycle on the injection-molding machines, which caused the shells to warp slightly. And, for those unfamiliar with the Dragon, the azimuth-adjusting mechanism was driven by a closed-loop servo that measured phase difference between signals from the two "sub-heads" on each track. It was a wonderful tool for QC. 
  We also developed a super-low noise LP player to measure noise floor of lacquer master discs to help with lacquer additive formulation. Another player measured the increase (and once-around variations) in tracking force caused by electret charges on the surface of vinyl records created during the pressing operation. The staff rheologist (plastic expert and formulator) used it to test new "anti-static" vinyl additives. 
  My 7-year stint at Capitol was fascinating because of their heavy investment in R&D - but, by the nineties, they'd pretty much become just another record label. 
  Bill Whitlock Whitlock Consulting Ventura, CA

 -----Original Message-----
 From: Mike via ProAudio <proaudio at bach.pgm.com>
 To: proaudio <proaudio at bach.pgm.com>
 Sent: Tue, Feb 11, 2020 5:23 pm
 Subject: Re: [ProAudio] FeralA - Recordings released encoded with Dolby A

 Just a side note about Dolby cassettes:

  Corey Bailey Audio Engineering<proaudio at baileyzone.net> wrote:

  > IIRC, it was Dolby B. Could have been "C" because, at the time, I used
  > whatever was built into the record deck & was compatible with my car
  > stereo so that I could switch the NR off.

  I reviewed a Marantz cassette deck last year, looking for something I 
  could recommend to the home-archivists who want to digitize the closet 
  full of cassettes of their band's shows from 30 years ago, but who no 
  longer have a means to play them and wouldn't spend the bucks on a 
  restored Dragon. It has a Noise Reduction switch (for playback only), 
  but no B/C switch. I asked their tech support about it, assuming that it 
  was Dolby B, and the answer was that it works for either B or C.

  I didn't have a good way of testing how good of a job it did on either - 
  the best I could do was to compare playback in the Marantz with that of 
  my TASCAM 112Mk2, but with several other significant differences between 
  playback on the two decks, I couldn't really evaluate the NR decoding.

  It turns out that what's in the Marantz (and also in an almost identical 
  deck from TASCAM) is that it's not really Dolby, because there are no 
  more Dolby decoder chips available, and that's what was used in all the 
  cassette decks. This decoder circuit was something that was developed (I 
  don't know by who) to approximate the Dolby playback - one size "fits" 
  all - though I suppose not Dolby S.

  -- 
  For a good time call http://mikeriversaudio.wordpress.com
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Richard L. Hess                   email: richard at richardhess.com
Aurora, Ontario, Canada           http://www.richardhess.com/
http://www.richardhess.com/tape/contact.htm 
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