The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Tag: Repairs

If it used to work, it can work again

HP 7475A Plotter: Pen Carousel Geneva Drive
A note arrived from someone who obviously couldn’t pass up an orphaned HP 7475A plotter:

The plotter I received works beautifully, except that the carousel doesn’t rotate. I found a YouTube video showing a 7475a running with the cover off, and there’s a little plastic piece – it looks like a teardrop – that advances the carousel, and is apparently part of the carousel motor assembly. Mine is missing that piece …

The keyword is Geneva drive, a wonderfully simple technique to convert one rotation of the stepper motor into 1/6 turn of the pen carousel, with no need for fancy sensors.

The (unofficial) HP Computer Museum has All The HP 7475A Documents and the Plotter Service Manual shows All The Parts. And, of course, I’ve written a bit about my adventures with an old 7475A.

Back in the day, you could get the entire Pen Carousel Housing Assembly w/ Motor (PN 07475-60175) as a unit and the Carousel Motor Only (PN 3140-0687) as a separate thing, but not the Geneva drive wheel:

HP7475A Carousel – Geneva drive cam

The cam’s drive wheel end (in inches, because early 1980s):
- 0.25 thick overall
- 0.10 thick plate under pin end
- 1.09 OD – rounded end
The pin sticking up from the cam:
- 0.154 OD (or fit to slot?)
- 0.16 tall (above base plate)
I have no good (i.e., easy + accurate) way to measure the distance from the motor shaft to the pin, but I doubt it’s critical. As long as the pin doesn’t quite whack the hub end of the slot, it’s all good:

HP7475A Carousel – cam driving

The 0.10 plate + 0.16 pin height don’t quite add up to the 0.25 overall measurement, but that’s certainly measurement error. I’d round the pin length downward and carve the drive from a 1/4 inch sheet.

A 3D printed part would probably work, apart from the accuracy required to fit the D-shaped motor shaft. Perhaps a round hole, reamed to fit the shaft, carefully aligned / positioned, with epoxy filling the D-shaped void, would suffice. A dent in the round hole would give the epoxy something to grab.

I’d be sorely tempted to use an actual metal / plastic rod for the pin, rather than depend on a stack of semi-fused plastic disks. The pin must withstand hitting the end of the “missing” slot during the power-on indexing rotation, because turning the carousel isn’t quite a non-contact sport. Normally, though, it enters the end of the slot without much fuss:

HP7475A Carousel – cam engaging

The blocked slot sits at the bottom of that picture, with a small locating pin sticking upward just above the circular feature at the end of the arm: we’re seeing the negative of a plug inserted into the original injection mold.

With a bit of luck, another HP 7475A plotter will fascinate everybody within hearing distance!

[Update: It lives! ]
2017-06-28
LM75A vs. SOIC Adapter: Mirror Imaging

After hairballing an LM75A I²C temperature sensor to verify at least one of the eBay lot worked, a bag of SOIC-to-DIP space transformers arrived, so I soldered up another LM75A:

LM75A on DIP8 adapter – top

The SOIC chip pattern sits at right angles to the DIP pins, which took some getting used to.

The slightly defocused wire connecting pin 4 (on the IC) to pins 5, 6, and 7 (on the PCB) selects address 0x48.

So I flipped it over, soldered four wires (+5 V, GND, SDA, SCL) to the numbered pins on bottom of the board, made up a little header for the other end, wired a socket strip on the crystal tester board, plugged it in, and … nothing worked.

Turns out that the other side of the board carries a TSSOP pattern, which I’d neatly masked off with a snippet of Kapton tape, surrounded by eight numbered pins. Of course, those pin numbers correspond to the TSSOP pattern facing you, so they’re mirror-imaged for the SOIC pattern on the other side.

Soooo, the proper wiring for the SOIC pattern as seen from the TSSOP side has the pin numbers exactly bass-ackwards:

LM75A on DIP8 adapter – bottom

The insulation looked a lot better the first time I soldered the wires to the PCB. Honest.

Anyhow, when correctly wired, the LM75A worked as it should:

LM75A Temperature Sensor – installed

It’s snuggled chip-down against the top of the 125 MHz oscillator can, with a dab of heatsink compound improving their thermal bond and a yellow cable tie around the foam holding them together. The socket header is wired pin-for-pin to the DAC I²C socket directly above it.

The OLED temperature display shows 28.250 °C, because the oscillator just started up in a cool basement. It’ll eventually settle around 39-ish °C, where its output should be pretty close to the 125 MHz – 344 Hz value hardcoded into the source.

Oh, that’s a 3 mm amber LED next to the relay can: much less glaring than the white LED, no matter what it looks like here.

2017-06-27
Handbag Strap Rivet Repair

One of the leather strap anchors on Mary’s giant haul-everything-to-a-concert(*) handbag pulled its rivet through the canvas fabric:

Handbag – pulled-through rivet

We knotted the strap around the zippered opening and completed the mission.

Of course, it wouldn’t have pulled through if they’d splurged on washers, but noooo too expensive:

Handbag – intact rivet – inside

Some rummaging produced a pan-head M3 screw of suitable length:

Handbag – repaired – outside

A slightly battered acorn nut was a special treat for the inside, with another washer to keep me happy:

Handbag – repaired – inside

That was easy!

(*) At Tanglewood, where they don’t strip-search you on the way in, tow-behind coolers seemed de rigueur, and a good time was had by all.

2017-06-25
Disaster Tourism

Riding around the block after a nasty storm showed far more than the usual number of leaves on the Dutchess Rail Trail:

Wappinger Tornado – Rail Trail near Titusville Rd – 2017-06-01

I spotted several trees down on both sides of the trail approaching Maloney Road, with another large branch across that access ramp:

Wappinger Tornado – Maloney Rd Rail Trail ramp – 2017-06-01

You might be able to see the large tree down across the trail on the far side of the road, up the slope.

Maloney Rd had many downed trees:

Wappinger Tornado – Maloney Rd 1 – 2017-06-01

With chainsaw chips and flare ash piles everywhere:

Wappinger Tornado – Maloney Rd 2 – 2017-06-01

From the National Weather Service:

The National Weather Service in coordination with Dutchess County Emergency Management officials, have confirmed a brief touchdown of a tornado on May 31. The tornado path began near the intersection of Maloney Road and Route 376. The tornado traveled due east along and just north of Maloney Road for approximately 1.25 miles before dissipating. Damage included numerous snapped hardwood and softwood trees and the roof lifted off a shed.

Both of Mary’s gardens suffered beatdowns, with the Vassar Farm plot pretty thoroughly pulverized by marble-size hail; she’s not in a good mood right now.

The DPW crews had plenty on their to-do list, but that branch was gone a day later.

Update: The top of the barely visible tree in the second picture just kissed the trail fence, but a much larger tree smashed both fences on its way across the trail:

Wappinger Tornado – Rail Trail S of Maloney – 2017-06-04

If you need some firewood, maybe you can make a deal …

2017-06-05
Beckman DM73 Circuitmate: Back From the Dead!

Prompted by the condolences on the death of my Beckman DM73 Circuitmate, I brought the carcass back to Squidwrench, took it apart, annoyed the switches, and re-soldered the battery connections:

Beckman DM73 Circuitmate – restored

I worked minute dots of Caig DeoxIT into the switches, without disassembling them, with magical thinking guiding my technique. One of the battery connections seemed suspect, but we’ll never know.

In any event, it beeps happily when turned on (*), the LCD behaves properly, it’s no longer pressure-sensitive, and life is good! It surely needs calibration, but that’s definitely in the nature of fine tuning.

Thanks for nudging me into Doing The Right Thing™.

(*) Including the double beep with the AC/DC button held down.

2017-06-02
Beckman DM73 Circuitmate: RIP

I’d added Mad Phil’s trusty Circuitmate to the tool kit I carry along to Squidwrench:

Beckman DM73 – new ground clip

Over the last few months it became increasingly erratic, eventually got to the point where slight pressure on the case would blank the display, and finally didn’t turn on at all. Yes, I replaced the batteries.

So I took it apart:

Beckman DM73 Circuitmate – case open

Nothing seemed particularly broken and, even after resoldering all the joints, it continued to not work at all:

Beckman DM73 Circuitmate – PCB

If you want to try your hand at instrument rehabilitation, let me know.

[Update:It’s back from the dead !]

2017-05-21
Generic AD9850 DDS Modules: Beware Swapped D7 and GND Pins!

Compare this picture:

AD9850 DDS Module – swapped GND D7 pins

… with any of the doc for the generic AD8950/51 DDS modules you’ll find out on the Interwebs. This snippet from the seller’s schematic will suffice:

AD9850 module schematic – cropped

Here’s a closer look at the 2×7 header in the upper left corner:

AD9850 module schematic – J5 detail

Don’t blame me for the blur, the schematic is a JPG.

Compared it with the board in hand:

AD9850 DDS Module – swapped GND D7 pins – detail

Yup, the D7 and GND pins are reversed.

Some careful probing showed the silkscreen is correct: the pins are, in fact, correctly labeled.

Should you be laying out a PCB in the expectation of using any DDS module from the lowest-price supplier, remember this high truth: Hell hath no fury like that of an unjustified assumption.

Fortunately, I’m hand-wiring the circuit and caught it prior to the smoke test.

2017-05-18