CLK_ADJ

THE LATHE, aka: LS-76, and the LJ-10 and LJ-12 tape machines
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Fonotec
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CLK_ADJ

Post by Fonotec » 13 Mar 2020, 19:10

660_table.jpg
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The turntable-speed selector switch on the control panel of THE LATHE selects which pull-up resistor's 5-V signal will be removed (by grounding it) from which NAND gate input on the tt servo drive board so as to invoke the correct speed-reference frequency from its 432-kHz master clock (crystal), and, by having more than one deck of 'poles', the switch simultaneously makes additional selections, including which resistor to signal 5 V to on the Variable Pitch card so that the 555 oscillator circuit on it will make the correct frequency for the sampling of audio peaks to happen 25 times per turntable revolution for all selectable angular velocities...

{The four-deck switch also selects the turntable-speed compensation network for banding and expansion, as well as which electro-magnetic relay to energize on the feed servo board (so that base pitch corresponds to the correct turntable speed).}

The basic 555 astable multivibrator circuit is illustrated on the page of an online 555 frequency calculator.
Astable_calculator.jpg
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On the LS-76 disk computer, the 555 frequency is established by having a 620-Ohm resistor as R2 (shared by all speed settings), and a selectable fixed resistor (changing resistance for each change of tt speed), plus a 5-kiloohm trimpot (combined), as R1. The capacitor, which is also shared for all speed settings, is specified as 4.7 µF. The fixed resistor used in R1 is selectable via the turntable-speed switch, but the same trimpot is used by the selectable fixed resistors for all three turntable-speed settings.


(The updated board's tt speed can be, either, '33', '45'. or, for CD-4 cutting, 'half-speed', which can be either half-speed '33' (i.e., 16+2/3 RPM), or half-speed '45' (i.e., 22+1/2 RPM.), but the Variable Pitch board's fixed resistor for 1/2-speed is for 16+2/3 RPM and would have to be lowered in resistance to work best with 22+1/2 RPM. (The original LS-76 'OSC' board was designed to drive a larger platter which is 17+1/4" in diameter, suitable for hosting a 16" transcription disk's oversized lacquer master, which is no longer made by any lacquer master manufacturer.))


R60 is a 16.9-kΩ resistor for the '33 FEED CLOCK' frequency network.
33_CLK.jpg
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{The audio peaks sent to the disk computer (Variable Pitch and Variable Depth boards) are acted upon up to 5 times per revolution if an expansion beyond the base pitch would be needed so as to avoid over-cutting. The copy of the program audio that goes to the lathe is in advance of the cutting signal by a full turntable revolution. If the automation audio has a peak requiring expansion of the base pitch that persists for multiple turntable revolutions, such as when cutting a continuous low frequency (e.g., 200 Hz) or loud (i.e., 1 kHz at 7 cm/sec peak lateral velocity) sine wave (during calibration or, possibly, the mastering of a test record), the required expansions will happen only once per revolution, as a 'pulse' of acceleration, not unlike hitting 'Return' on a manual typewriter.}

For 25 samples to be evenly spaced during a full turntable revolution at 33+1/3 RPM, the reference frequency, corresponding to the frequence of sampling, needs to be 13.888... Hz. This is because one revolution at 33+1/3 RPM takes 1.8 seconds. [25 / 1.8 = 13.888...] For 45 RPM, the reference frequency needs to be 18.75 Hz, since the turntable revolution at 45 RPM only takes 1.333... seconds. [25 / 1.333... = 18.75]

On #656, I checked the 'CLK' test point (F) on the Variable Pitch board using a frequency counter. It indicated that the reference frequency at '33' was over 14 Hz. The '45' feed clock signal, however, was already at 18.75 Hz, so, I couldn't tweak the 'CLK ADJ' trimpot in order to fine-tune the '33' feed clock signal from 14+ Hz down to 13.888 Hz without inadvertently changing the feed clock frequency for 45 RPM.

{I read that a 555 astable multivibrator circuit may deliver results that deviate from the calculated values of the arranged components by as much as 20%, which would mean the '33' feed clock might have been making anything from 11.573 Hz through 16.66 Hz and be within the tolerance of the generic circuit. L. J. Scully cherry-picked very accurate resistor values for the fixed resistors (within 0.059% of target), but when parts get changed over the years, out in the field, such as when a capacitor, or the 555 IC chip, itself, is replaced, the empirical results are unpredictable, and this procedure corrects for such offsets from the design targets.}


{Adjusting the feed clock requires extending the Variable Pitch board with two mated Shortyboardz so that it's pushed out of the cage far enough for one to reach the 'CLK ADJ' trimpot and test point, F, on the back of the board. (See image, below, of one such extender board (originally made to extend the PCB inside a channel of an Ampex ATR-100 input/output assembly (a.k.a., 'penthouse VU meters').))}

Image


So, in order to avoid having to adjust the clock every time I switch turntable speeds, I had to add a fixed resistor to the '33 CLK''s R1 network. Rather than putting a soldering iron on the circuit board unnecessarily, I decided to add the helping resistor to the turntable-speed selector switch contact for wire 202-C that goes to V.P. edge connector pin D and on to the 16.9-kΩ fixed resistor and 5-kΩ trimpot. (The rear of the control panel where the contacts of the turntable-speed selector switch are found can be accessed by lifting the 24" x 7" x 3/8" panel out of its frame. Two snakes are tied to the rear of the panel which fan out as multiple wires going to the switches, meters, and pushbuttons, as well as the LPI Display board. The snakes' other ends go inside the steel weldment and to the card cage, below. There's just enough slack on the snakes to allow the control panel to be flipped over and repositioned in the same frame in front of the A-table.)
Composite_Controls.jpg
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Pin_D.jpg
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First, after powering down the lathe, I unsoldered wire 202-C from the back of the switch and clipped to it one of the leads of a Shallcross decade resistance box.
Image
The box's other lead was clipped to the contact on the turntable-speed selector switch for '33 FEED CLOCK'. I then powered back on THE LATHE and read the frequency counter display as I adjusted the decade resistance box switches until I saw 13.888... Hz.

The target resistance appeared to be 863 Ohms. To my surprise, I found a resistor in the studio shop that measured 836 Ohms. So, I thought I'd try it. It actually worked perfectly, causing the frequency counter to display 0.013888 kHz at the '33' turntable-speed switch step (when trimpot R62 was deflected identically to its value of resistance that causes the frequency counter to display 0.01875... kHz at the '45' turntable-speed switch step). I was willing to add a 27-Ohm resistor to 202-C's helping resistor, but it wasn't necessary. I was a little surprised to notice later that morning, before I drove home, that my car's trip odometer (an orange dot matrix display) read 836 mi. (!)


I performed the identical procedure on lathe #660. It required 430.1 Ohms for the '33 FEED CLOCK' switch contact. I used a 300.1-Ohm resistor which I found, first, added to a cherry-picked (from 20 different 1%-tolerance, '130-Ohm' metal film resistors) 130.0-Ohm resistor for that. Two of their four (combined) legs were twisted together in a Western-Union splice and soldered and covered with heat-shrink tubing, shrunk with a heat gun after soldering in place their other two (combined) legs - one, to the '33 FEED CLOCK' switch contact and, the other, to wire 202-C.
contact_33.jpg
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I looked at the 13.888 Hz signal from the 555 test point for both lathes on an oscilloscope (with a probe compensated for square wave reading on that scope) and saw no overshoot or noise, The duty cycle is as the calculator indicated, about 97% 'on'...

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