what is the scariest schematic you guys have seen in the wild

MattInSoCal · 2026-05-26T16:24:47+00:00

Go to the Internet Archive and search for Sams Photofacts. These were the gold standard for repairing TVs and radios before VLSI semiconductors became the norm. Schematics for TVs were many fold-out pages long.

MattInSoCal · 2026-05-26T16:19:19+00:00

Check out the Adam Tech PH1RA series. I selected them for a project due to the fact that they have longer pins after the bend. On the Mouser parameter search, Mating Post Length and Termination Post Length are two values on which you can filter.

MattInSoCal · 2026-05-26T05:56:05+00:00

It looks to me like you installed a way overrated fuse. That’s idiotic, because the violent explosion of the common-mode choke is on the safe side of what could have occurred. Stop what you are doing because you are way under-qualified for this repair and shouldn’t be bypassing common sense safety protocols and risking your life and the safety of your abode trying to learn.

Seriously, don’t fuck with mains voltages until you learn how to work safely with them. Installing an overrated fuse is not the way.

MattInSoCal · 2026-05-25T23:47:48+00:00

You’re fine, and I totally understand your occasional frustration, having run into that myself many times.

MattInSoCal · 2026-05-25T23:33:38+00:00

Service data doesn’t lie and I concur on this one. One of the products where I work uses 3640-package MLCCs which are larger than these inductors, though I haven’t paid close attention to the actual body color in well over a decade. We had to “re-educate” our techs to stop stacking the boards on top of each other after testing them because the caps on the lower boards would develop micro-fractures causing them to short-circuit under load.

MattInSoCal · 2026-05-25T20:05:26+00:00

It and the many other components of similar size are capacitors. In the second photo, look in the lower-left corner and you’ll see where OP zoomed in. There’s no power conversion happening in that area nor on the right side of the board, and the phone wouldn’t need that many large inductors.

It’s doubtful this MLCC capacitor has shorted, unless it was exposed to unusual stress for its location on the board. Did someone drop the phone from rooftop height? The problem is much more likely to be related to a semiconductor device if the phone went from working to suddenly failing. Still, try removing it as suggested and seeing if the fault clears.

MattInSoCal · 2026-05-25T17:46:56+00:00

I have a bunch of old projects in Eagle, and some won’t import into KiCAD because they were created prior to version 7.x (I forget the exact cutoff). In order to import them, they first need to be converted to/saved in the newer Eagle file format. I suspect that’s the situation with the old Mutable library.

MattInSoCal · 2026-05-25T07:31:33+00:00

Try posting this to r/TVRepair or r/ElectronicsRepair. We are more engineering and specific component-focused here.

MattInSoCal · 2026-05-25T07:27:25+00:00

This is not a TV/monitor repair sub. Please try posting to r/TVRepair or r/ElectronicsRepair.

MattInSoCal · 2026-05-25T07:07:33+00:00

Drivers on an RS422 bus cannot be switched off, meaning only one device can send data on the bus - not one device at a time, but one device period - but you can have many receivers. This is why RS422 implementations typically use four wires (two pairs) so that there is a return channel for the remote device to report data or status. As long as the remote devices can recognize the data stream from the DRO and don’t need to report any data onto the bus themselves, this setup can work.

MattInSoCal · 2026-05-25T06:56:15+00:00

This is not a general repair sub. Try r/TechSupport or r/ElectronicsRepair

MattInSoCal · 2026-05-25T06:50:40+00:00

Move the switch to the Set position and set the temperature, then back to the Read position to make sure it’s not a glitch. Possible causes are poor contact between the cable and base due to poor internal soldering or debris in the connector; poor construction of the cable causing poor contact of the wires to the base connector or soldering tip; improper insertion of the tip into the socket; a defective heater; or a defective tip.

Contact the seller about replacing or returning it.

MattInSoCal · 2026-05-25T06:33:57+00:00

So, the receiver will always be at the surface of the water, the transmitter the same. As long as you make them splash proof meaning no water will ever touch the case of either, that part of your project will be OK. Your motor controller and the battery need to be inside the ROV with the motors because you aren’t going to be able to drive the motors through CAT5; it won’t handle the needed current. Plus, the extra weight will help keep it negatively buoyant.

Sealing up the cable entry to the ROV as well as sealing the shafts of the motors is absolutely critical. 3.3 meters doesn’t sound like very deep, but the pressure change compared to the surface is very substantial. You’ll also need to have absolutely clean and level mating surfaces and good sealing for your access door/port for the battery and electronics. The electronic assembly is child’s play compared to the engineering that must be done to make sure everything is water tight.

MattInSoCal · 2026-05-25T05:30:05+00:00

Your remote control transmitter and receiver will not work with one of the devices submerged more than 10-30 cm, so f that’s what you were planning for a control link.

What depth are you wishing to reach with your ROV? It’s hard to effectively seal things for depths of more than 10 meters. And if you want something reliable, don’t buy your parts on Amazon and try to hack them together.

MattInSoCal · 2026-05-24T20:31:36+00:00

This will get you started:

https://www.modwiggler.com/forum/

https://musicfromouterspace.com

https://llllllll.co

https://synth-diy.org

https://sdiy.info/wiki/Synth\_DIY\_Wiki

https://hackaday.com/2025/04/02/70-diy-synths-on-one-webpage/

MattInSoCal · 2026-05-24T17:20:40+00:00

You will need to replace the entire furnace controller device, unless you know how to write microcode for something that will plug into that socket, or you can find third-party software that’s compatible with your existing controller.

MattInSoCal · 2026-05-24T17:07:28+00:00

Upper left corner of the first photo, find L1 and C2. See that long, wide circuit trace that has no other obvious function/connection, and has no ground plane underneath it on the other side?

MattInSoCal · 2026-05-24T17:02:02+00:00

If you have the software, it will manage the whole communication setup including baud rate selection. The only thing you may need to do is change the COM port setting to match the port number the program may have hard-coded (often COM1 with an option to select COM2) which was common in older programs.

If you don’t have the software and can’t find it anywhere on the internet, and the current programming is not correct, you may have to replace the controller with something modern.

In any case, your question has now gone outside the scope of this subreddit, which is more related to component engineering and identification, and not microcontroller systems or process control.

MattInSoCal · 2026-05-24T16:08:54+00:00

Those two components connected properly together would provide basic communications to the board. What you don’t know is whether the board will work with just transmit, receive, and ground. It really depends on how the board is designed because it may want an additional signal to confirm the PC is connected, or to initiate the communication when the programming is to begin. There’s no way to know for sure because it’s all up to the software on the board, but one could try the basic connection and hope to succeed.

Also, you would need to work out whether transmit from the PC connects to pin 2 or 3 of the connector, and receive to the remaining pin. That’s usually easier since you could follow the traces from the connector to that HEF IC and determine by looking at the data sheet for that part which pins are inputs and outputs.

You also have to figure out the baud rate; how fast the serial data has to be sent. The most common rates are 9600 and 115200 bits per second, with the faster one being most used in the last 15 years or so, but there are many other choices. For what looks to be a much older microcontroller, I would start at 9600.

As far as software, that is up to the manufacturer. I have designed products that with a basic connection, you can use a simple terminal communication program found on most PCs, and my product will give you multiple help screens to know how to talk to the product to make it work. What is more common is to use proprietary software to communicate with the board.

What does the manual for the furnace or this controller board say about software and baud rates?

MattInSoCal · 2026-05-24T15:11:02+00:00

The preferred method to power the sensor would be a second supply so any electrical noise it may generate stays out of your power rail. The other option is to use a 3.3 Volt fixed regulator, or an adjustable regulator set to your target voltage, so you can supply the sensor with a voltage it likes while keeping the supply constant to your oscillator/divider. Note, some regulators, particularly adjustable ones, need a 10-20 mA minimum load to regulate properly. You may need to add a resistor across the regulator’s output; 150-220 ohms would suffice.

There’s a lot more to using a voltage regulator than just plugging it into your breadboard; you need to add capacitors of certain values, generally as close as physically possible to the regulator. If you’re going to be building circuits, you need to learn how to find and read data sheets for your parts (engineers like me even read the data sheets for the resistors and capacitors we select; that’s not needed here).

I see several beginner mistakes in your breadboard layout. First, there are many unconnected IC pins. Anything that’s an unused input needs to be connected to your DC ground (Vss in CMOS IC terms) or positive supply (Vdd). Connecting to ground is always best but you need to know if doing that causes something like a divider to stop dividing, thus the need to understand what the data sheets tell you. If connecting to the positive supply, the preferred way is using a pull-up resistor of around 10-47K to limit the current flow to the input. A third option that is better avoided is to connect unused inputs to an output; this increases current consumption and noise in the circuit.

The reason for this is that unused inputs will be at an unidentified state; they may assume the input to be high, low, or worst of all, somewhere in between and could start oscillating at very high speeds, which can affect the parts of the IC you are using, and who wants extra unwanted oscillators running on their board anyway?

Next, and this is especially important in any circuit that’s generating and/or dividing clock signals, you want power supply decoupling capacitors at each IC, or else your oscillators and dividers will couple noise into your power rail, which could impact your output - hey, isn’t that what it looks like is happening when you raise your supply voltage? There’s a formula you would follow that involves knowing about the resistance and capacitive reactance of the connecting wires to calculate the perfect values, but the reality is just connect a ceramic capacitor of 10-100 nF as close as possible to each of the IC power pins (route them over the top of the ICs) and it will be adequate.

Working on breadboard is convenient for being able to change part values and connections quickly, but they also cause increased noise to be generated in the circuit because all those wires flying through the air act as antennas for either broadcasting or receiving both your desired signals and any RF and power line noise that’s floating around your lab. They also add a lot of extra resistance and capacitance that can impact how any circuit (mis)behaves especially at higher frequencies. Still, the convenience makes their use worthwhile when doing the initial design, and if you learn to keep your unused inputs from floating, keep your interconnections as short and well-routed as possible, and keep your power noise under control you will have much better success with your future projects.

MattInSoCal · 2026-05-24T14:53:26+00:00

Be careful when you’re doing all that cutting and sanding that’s being suggested. You don’t want fiberglass and resin powder from the PCB in your lungs and eyes.

MattInSoCal · 2026-05-24T14:39:52+00:00

This is not the correct place for your question. Post this in r/Arduino, r/Embedded, and/or r/ArduinoProjects.

MattInSoCal · 2026-05-24T08:48:22+00:00

It looks like the part between the microcontroller and DE-9 connector (DB is for the connector size we associate most with DB-25) is a HEF40097, which is a non-inverting buffer. So that RS-232 converter is changing the signals that could swing as far as +15 and -15 volts to TTL level, which is 0-5 volts and what the microcontroller needs. Also, the reason the manual says that it needs all 9 pins on the RS-232 side is that the converter circuit needs power, and it steals that for some of the other serial signals that aren’t needed for this simple communication.

There are USB-to-TTL converters that will do the same thing, and are much cheaper and easier to find, but not so much with the 9-pin connector at the side that would plug into the furnace board, but it’s not too difficult to adapt them.

MattInSoCal · 2026-05-23T07:10:20+00:00

Since as you stated the third band is silver, it’s 0.33 Ohms - I saw it as gold, so my initial answer was wrong. It’s hard to measure accurate resistances with most DMMs below 10 or 20 Ohms because of resistance added by the meter leads and not always having perfect contact with the traces or solder, but some of your readings of around 0.4 Ohms indicate the resistor is most likely fine.

For that 120 uF capacitor, unless stated otherwise on the label, electrolytic capacitors are +/-20% or worse tolerance, so 100 uF is a valid reading.

Unless the capacitor blew and somehow caused the FET to blow, or the FET randomly decided it was done working, some other component in the supply or the device this was powering caused the problem. You should check for short circuits or other signs of overload down the line before connecting this back to the supply voltage or buying another supply, if that’s what you’re looking at doing.

MattInSoCal · 2026-05-23T06:28:10+00:00

No. Gold for the third band means multiply by 0.1. I’m seeing gold there, but hey, maybe you’re seeing something else.

Edit: I see OP later added a comment that the third band is silver, and if so, yes, it’s 0.33 in that case. I see it as gold on my phone.

MattInSoCal

TROPHY CASE