2668-B PC

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

2668-B PC

Postby Fonotec » 08 Sep 2016, 03:30

The LS-76 Display Processor circuit board uses two, 8 bit, parallel output A/D converters to generate the numerical representation of the feed. The 7-segment led driver chips on the LPI Display card in the Control Panel take the data-selected, 8 bit, parallel signals from the Display Processor board and convert them to display patterns that we recognize as "digital" numbers (from 90 to 600), or make a blank display (when Feed is off). When illuminated, the three, red-digit leds are showing the carriage pitch, in "LINES/INCH."

One A/D is for generating the bits used to create the display code for the base pitch. It generates these bits by digitizing a copy of the base pitch signal that is going to one of the Feed Servo board's uA748 op amps (trimmed by the "base pitch" selector pot on the Control Panel). The other A/D's outputs are switched on by logic whenever the feed expands. It digitizes the expansion signal, so those bits help generate the display code for when the carriage moves faster than base pitch. (The expansion signal is obtained from the summed output of the base pitch op amp and two other (uA748) op amps on the Feed Servo board - one, for manually-selected expansion current, and another, for automated variable pitch expansion pulses.) By having one A/D converter dedicated to digitizing only the base pitch, the present setting for that can be referenced at any time that Feed is on, by flipping a switch, even when expansion is taking place.)

400_LPI_detail.jpg (35.21 KiB) Viewed 844 times

I like to keep inventory of NOS discontinued parts for the vintage studio gear so that I can minimize down-time when service is wanted. Since I have only been able to find broken examples of ADC-89A8B from the obsolete market, I decided to depot the module (made by Datel Systems, Inc., who also made the ADC-Econoveter and DAC-98BI modules, used in THE LATHE), so that I could clone some working models for inventory.

Here's how it looks before being depotted:

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Here's the PCB (2668-B PC) that was found inside:

2668-B_back.JPG (50.21 KiB) Viewed 851 times

The depotted circuit can be viewed in further detail on two examples of 2668-B PC, which I reclaimed from their diallyl phthalate casings, using a Dremel rotary cutter.


As you can see from the depotted examples, removal of the plastic casing with the rotary saw leaves the board mostly intact, however, some small sections of track near the edges often get pulled off by the high speed blade, since the plastic casing is in intimate contact with the circuit board, leaving no gap between the inside of the casing and the surfaces of the PCB. Therefore, I opted for PCB cloning, rather than repair, as the simplest way to end up with perfect modules. Wherever a small section of track or a through hole pad is missing, the cloning engineer can correct that easily with the digital imaging software used for generating the drill map and track layout, à la Photoshop. For starters, a "tan line" remains where the trace used to be, and by depotting two examples, I ended up with an unambiguous roadmap for a complete and working board, since the slight damage to each board was random, and, therefore, unique - so, the uncertain areas of the first board were explained by the same areas that were undisturbed on the second board.

After getting Gerber files made from these boards, down in Richardson, Texas (at Micro Technology Services), I hired Sunstone Circuits (in Mulino, Oregon) to manufacture some PCB clones with the solder mask option. I recently packed this first board and tested it in a custom jig, made from a Radio Shack, Copper-plated through hole breadboard and a tomato stake (as legs) - both of which were sawed with a Black & Decker jigsaw.

The board takes two 7493 (4 bit binary counter) DIP ICs, one 7474 (D-type flip-flop), and one 7400 (NAND gate), which I put in the four, screw machine sockets. I'm glad I socketed the chips, since the two, 4 bit, binary counters that I tried first would pass the logic checker test but would not work correctly in this circuit. The gold legs were of no benefit. When I tried other 7493s, the circuit turned on and made the right patterns on the oscilloscope for all parallel bit outputs.

(The analog test signal I used was from the Neumann console oscillator. Until I changed the ground connection for that oscillator's return to 2668-B PC's Power Ground pin (rather than its Analog Ground), whenever the sine wave entering the Analog input was either muted by turning the oscillator off, with the switch, or with the oscillator's gain pot turned, fully CCW, the LSB pin would emit a heavily-attenuated copy of the Krohn-Hite oscillator's square wave (4 kHz), used for this test as the A/D's clock signal, which was (still) going into the SC pin.)

Out of curiosity, to see if I could reconstruct the digitized signals, I then made a test jig for the 8 bit, parallel input D/A converter, DAC-98BI, which the 6 bit, parallel output A/D converter, ADC-Econoverter (see "Depotting," in this site's Exotics section: viewtopic.php?f=15&t=124), signals to on the Variable Pitch and Variable Depth boards. In those circuits, since ADC-Econoverter only sends 6 parallel bits to DAC-98BI, by way of six, 5 bit shift registers (7496), the D/A's two least significant bits are grounded. (Green circle in drawing, below, indicates grounded input bit pins.)

{By digitizing the control audio (on the Variable Pitch and Variable Depth cards) and sending the bits through shift registers and immediately reconstructing them (as analog) in unipolar mode, the lathe automation ends up with scaled pulses of control bursts for the feed motor current that are synchronized by a turntable velocity compensation clock for the feed, on board the Variable Pitch card. Switching the turntable speed selector switch to a given turntable angular velocity's step changes not only the reference sub-frequency modulus logic for the TT Servo Drive board's 432 kHz crystal oscillator, but also the reference frequency tuning network resistors for the 555 feed timer on the Variable Pitch card. Since the automation is designed to store peak audio control pulses 25 times per revolution (and act on those stored values 5 times per revolution), the 555 feed timer is clocked at 13.888... Hz for 33+1/3 rpm, since 13.888... x 1.8 seconds (the time required for one revolution at 33+1/3 rpm) = 25. For 45 rpm, the 555 feed timer is automatically re-clocked to ~ 19.23 Hz, using the same 4.7 uF tantalum capacitor, across the threshold, discharge, and trigger pins, but with different resistors selected in parallel. A trimpot in the network allows fine-tuning of the time constant. Once it's set for 33+1/3 rpm, it's already trimmed for the other selectable platter speed compensations The timer's output is connected to the Clear inputs (pin 16) of the six, 5 bit shift registers that are in between the A/D and the D/A. A one-shot (9602) synchronizes the Start Convert pins of the A/D converters on both automation boards.}

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When the newly-cloned 8 bit ADC and an original 8 bit DAC were interconnected between the two test jigs (directly), I was able to confirm on an oscilloscope that the (40, 200, and 1,000 cps sine wave) test tones which I digitized, emerged, as analog, again, in unipolar mode, from the DAC's analog output.


Picture, below, shows the first clone PCB (2668-B PC) which I packed with mostly new, and some NOS, components - shown without any DIP chips in the screw machine sockets or gold pins in the pin holes. Identical to the originals, except that the clones can be serviced, since they aren't potted.

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Re: 2668-B PC

Postby Fonotec » 13 Oct 2016, 18:59

Today, I finally finished packing two more DATEL ADC-89A8B clones, after having to wait many weeks for some 0.1% tolerance, 1/4 Watt, film resistors of the right values for the 8 bit, parallel output, A/D converter's circuit board (2668-B PC). I just tested them in the homemade test jig and saw that all 8, bit-output pins are working on both of the new clones.

These two boards will now join the first two (tested and confirmed) clones in the 100-year old Mosler wall safe that's in the studio repair shop. I hope I never have to use these, of course, but it's nice to know that there is hope, against the day...

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