Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Someone with a jammed Amazon laminator inadvertently dislodged the switch wiring, so I took a few more pictures to help. Note: I see absolutely no reason to assume any two laminators will have the same wire colors, but the overall functions should be the same.
The top set of three switch terminals control the overall power to the laminator:
Amazon Laminator – switch wiring
The center terminal comes from the unmarked (no ridges) wire in the line cord. The two outer terminals are connected together with a short jumper from the terminal nearest the motor, with a longer black wire to the wire nut binding other black wires.
The bottom set of terminals select the temperature:
Amazon Laminator – switch bottom contacts
The white wire on the center terminal goes to the wire nut holding the other white wires and a black wire (!) going to the middle of the three thermostats on the extrusion. The black and blue wires on the outer switch terminals go to the thermostats on the aluminum extrusion to the heater.
Verily, it is written: There’s nothing like a good new problem to take one’s mind off all one’s old problems.
Some suggested 151-1032-00 replacements obviously won’t work, such as Tekwiki’s 2N5397 single JFET. Bonding a pair into a single heatsink might suffice, but two separate cans generally aren’t identical enough for the purpose.
Curiously, Tekwiki also lists the 2N5911 as a 151-1032-00 replacement, which (being an actual dual JFET) looks more promising. This agrees with another cross-reference, although the “Sim[ilar] to” suggests considerable caution.
The actual Tek 151-1032-00 can in its heatsink, oriented with the tab at the top (just visible to the right of the heatsink fin):
Tek 151-1032-00 – top view
Testing one side (with the tab on the left):
Tek 151-1032-00 test side A
And the other side (tab still on the left):
Tek 151-1032-00 test side B
A picture being worth a kiloword:
Tek 151-1032-00 – measured pinout
The drain and source over on the left side seem to be swapped compared to the 2N5911, although both gates are on the proper pins. This being a JFET, the source and drain may be electrically identical and it’s possible the tester labelled them backwards. The only way to be sure Tek wasn’t tragically clever is to poke around the PCB to figure out which pins connect to which other components.
So take a picture of the component neighborhood around the Q230 sockets:
PXL_20220105_210538214
Overlay it with a similar picture of the solder side, suitably reversed / recolored / transformed to match:
Tek AM503 – 151-1032-00 area – X-ray traces
The copper-side traces aren’t complete, as the red coloring marks only traces under the soldermask and omits bare solder-coated traces. Some traces on the component side run invisibly under parts. If I were doing it for money, not love, I’d pay more attention to the details.
Devote some time to tracing the traces and labeling the parts:
Tek AM503 – 151-1032-00 area – part IDs
Then doodle out the actual connections:
Tek 151-1032-00 – part connections
R246 shows Q230B lives in the left side of the can, because it’s connected between the B gate and B source pins, and confirms the tester swapped the B source and B drain pins. Whew!
R236 connects the B drain and the A source, confirming the pinout matches the 2N5911.
Comfortingly, the A side gate goes to all those other parts as it should.
So a 2N5911 will drop right into the Q230 socket with the proper pins going to the proper places. Whether it’s electrically Close Enough™ to the Tek spec, whatever it might have been, remains to be seen, but a good transistor circuit won’t depend too much on the actual transistor parameters.
The fact that changing R220 also changed the noise should have pinpointed the noise source, but such things are always more obvious in retrospect than in real time running. This post should help me start the next debugging spree a bit further up the learning curve.
The AM503 signal path includes a pair of … unique … differential amplifier ICs made by Tektronix back in the early days of integrated circuitry:
Tek AM503 – U370 U350 detail
The picture has the signal flowing right-to-left through U350 and U370, starting with the Q310 dual NPN in the metal can and the Q315/325 PNP pair (both over on the right side near the cable).
Anyhow, the differential output of U370 shows the noise across pins 6 and 8 (yellow and magenta):
Tek AM503 – diff output U370.6-U370.8
Again in retrospect, pins 9 and 5 would have been a better choice.
The white line is the difference between the two pins and resembled the scope output in the bottom trace well enough to satisfy me.
The differential input to U350 on pins 16 and 14 also shows a distinct similarity to the output noise:
Tek AM503 – diff input U350.16-U350.14
It’s essentially impossible to snap a scope probe around those IC pins, but merely extraordinarily difficult to securely grab the tails of the pin sockets extending beyond the solder side of the PCB.
Finally, a look across R317, the emitter resistor between the halves of Q310:
Tek AM503 – diff input – R317
That was enough to finally convince me the problem lay upstream of Q310.
Ruling out the DC level pot required balancing another AM503 atop this one to plug its cable into the PCB, which showed same output noise.
Hat tip to Sherlock Holmes:
“When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.”
Because Q230 is socketed, I pulled it out and popped it into another AM503, whereupon the noise followed the transistor:
Tek AM503 – three amps – GND
All three AM503 amps are set to GND / DC LEVEL CAL. The cyan trance is the formerly noisy amp (now with a good Q230), the magenta trace is the formerly good amp (now with the bad Q230) , and the green trace is the best of the three AM503 amps (untouched, for well and good reason) in my collection.
One of my Tek AM503 current probe amplifiers (SN B064098) suffered from DC offsets in the AC / GND / DC modes, to the extent that zeroing the GND (more formally known as “CAL DC LEVEL”) offset wouldn’t keep the other two baselines on the scope screen. Kibitizing with another AM503 owner with a different problem clued me to apply a change made in later units: replace the 1 kΩ resistor at R220 with a 470 kΩ resistor to reduce the source impedance changes between the switch positions:
AM503 – R220 change
For the record, R220 sits parallel to the attenuator shield above and to the right of Q230 (in the black clip-on heatsink):
Tek AM503 – R220 detail
The new resistor somewhat reduced the offset problem, but also dramatically increased the noise level I’d been studiously ignoring, to the point where the AM503 output was unusable:
Tek AM503 – three amps – GND
The rule of thumb is that it’s always a connector or, perhaps, a similar metallic contact in the signal path. The AM503 has a breathtakingly aggressive switched attenuator covering the 94 dB range from 1 mA/div to 50 A/div:
The switches are cam-driven bifurcated gold-plated spring fingers contacting gold-plated PCB pads under that aluminum shield:
Tek AM503 – Attenuator Contacts – detail
The spring-loaded thing to the right is R206, the first 50 Ω 2× attenuator in the form of thin-film elements fired on a ceramic substrate. The two switches put C218 into the signal path in AC mode.
You (well, I) clean the fingers by very gently pulling a strip of lens cleaner moistened with isopropyl alcohol through the closed contacts:
Tek AM503 – Attenuator Contact Cleaning
The pale blue cylinder is the attenuator cam roller extending across the PCB behind the front-panel knob. The two switches bypass C218 in DC mode and connect R220 to ground in GND mode.
Clean gold-on-gold contacts are about as good as it gets and those things looked absolutely pristine. After wiping the contact connecting R220 to ground had no effect, it finally penetrated my thick skull that the problem wasn’t in the attenuator contacts and had to be downstream in the amplifier and filter chain.
Reseating all the cable connectors and jostling the (socketed!) semiconductors also had no effect.
Could one of the semiconductors have gone flaky after four decades?
For the first time in a loooong time I (had to) set up remote desktop sharing, starting from an existing SSH login through a single-port pinhole in an immutable router firewall.
The remote PC runs Xubuntu 20.4 LTS and I verified it already had x11vnc installed. If that’s not the case, make it so.
In order to share / control the desktop of a different user (hereinafter known as kay), I must SSH into that PC as kay. My SSH session uses public key authentication and kay has no need for outbound SSH, so just use my PC’s public key in kay‘s authorized_keys file. On the remote PC, where I am signed in as me:
cd ~
sudo mkdir /home/kay/.ssh # kay does not have a public key
sudo cp .ssh/authorized_keys /home/kay/.ssh # so just copy mine
sudo chown -R kay:kay /home/kay/.ssh # transfer ownership
sudo chmod go-rwx /home/kay/.ssh # set proper permissions
Why You Shouldn’t Use Heat Pumps in the Northeast US
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Mini-Lathe Tailstock: Alignment
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Homebrew Magnetizer-Demagnetizer
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Arduino Serial Optical Isolator
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Mysterious Noise in Toyota Sienna Minivan: Fixed!
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Baofeng UV-5: Squelch Pop Suppression
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bCNC Probe Camera Calibration
432
Demolition Card GTA 5-10-9
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Multimeter Range Switch Contacts: Whoops!
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Realigning Tweezer Tips
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Schwalbe Marathon Plus and Michelin Protek vs. Glass Chip
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Kenmore Model 158 Speed Control: Carbon Disk Replacement
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Kenmore Electric Dryer: Power Resistor Replacement
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Old Kenmore Sewing Machine Foot Control Repair
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Closing the Dmesg Audit Firehose
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Blog Page Views
That adds up to 200 k page views from 122 k visitors, for an average of 1.6 pages / visitor, down slightly from last year. For a variety of reasons, I wrote only 242 posts over the course of the year, so more folks read only the single post matching their search terms.
To give you an idea of how awful online advertising has become, WordPress shoveled 817 k ads at those readers, slightly more than four ads per view. Given the toxicity of online advertising, I just started paying $50/year for a “personal” plan to get a few more gigabytes of media storage, which also let me turn off the ads. Most of you won’t notice, as you already run ad blockers, but it will calm the results for everybody else.
Fortunately, losing the $250 / year income from those ads won’t significantly affect my standard of living.
The Smell of Molten Projects in the Morning 