[dfox]

Using various interface-ish components only for their electrical parameters is neat trick that is very common in certain applications. In this case the lightbulb is slightly weird but probably serves to shape the 600V pulses in some way and also to protect the switching transistor should the grid outputs get shorted to ground. Looking on the schematic there are also neon lamps across the outputs to protect the CRT from overvoltage (which you will find in essentially any modern CRT monitor).

Another such neat trick is in most simple photographic flashes: the neon bulb which indicates that flash is ready also provides feedback to the flyback charging supply by increasing it's load when correct voltage is reached.

And then there is typical CRT degaus circuit, which produces the required damped oscillation waveform by running mains voltage throught two thermally coupled thermistors.

Edit: Horizontal deflection circuits of CRT TVs and monitors are usually full of neat tricks: shifted hysteresis inductors, deriving various voltages from the flyback and output multiplier, synchronisation of main SMPS switching frequency to horizontal retrace... One can view CRT VGA monitor as one big switchmode power supply with video amplifier bolted onto it (and many CRT monitors are actually mechanically built this way, with RGB video going directly from the captive VGA cable to amplifier on the neck board)

[WalterBright]

My old Dodge instruments need 5 volts, so there's a mechanical device to convert 12V to 5V. It involves heating a bimetalic strip that opens and closes a circuit quickly, i.e. buzzing, which produces 5V.

Some people have decided to fix this by using an electronic voltage circuit to get a clean 5V from the noisy 12V supply. But the instruments just wouldn't work right.

It seems the instruments relied on the 5V supply being jittery. With a smooth voltage, the mechanical instruments would get stuck.

Watch a pilot in an older airplane. He'll tap the gauges, too, to get an accurate reading.

[Johnythree]

Actually the reason is that the instrument itself also uses a bi-metal strip and it relies on the ratio between the two strips to cancel out voltage fluctuations.

It's very elegant, but if you take out one, it goes haywire.

[zkms]

> My old Dodge instruments need 5 volts, so there's a mechanical device to convert 12V to 5V. It involves heating a bimetalic strip that opens and closes a circuit quickly, i.e. buzzing, which produces 5V.

Are you sure that this is not a https://en.wikipedia.org/wiki/Vibrator_(electronic) that would be powered by a solenoid and not just a bimetallic strip?

[WalterBright]

I think you're right.

[zkms]

No.

You were actually initially right, I searched for that actual part and it is indeed a bimetallic strip along with a heater (that turns off when the strip moves away from a contact) and not a device with a solenoid/relay.

Apparently whatever it supplies is tolerant of that sort of interruptions in its power source -- I imagine that it would operate on a quite lower frequency than a "vibrator"-type power supply where magnetic force moves the contacts (and elasticity returns them).

[WalterBright]

I read about this probably 10 years ago, my memory was clearly a bit faulty on it. I didn't want to take the device on my Dodge apart just to verify it :-).

The thing is, the instruments require the noisy 5V.

You can buy kits with an electronic 5V source, and also to replace the old guts of the instruments with modern ones. That retains the original look, with modern accuracy and performance.

[kens]

Nice examples. By the way, those symbols on the schematic aren't neon lamps, but spark gaps - simply pointed PCB traces separated by about a millimeter. Presumably these are to safely discharge any excessive voltage buildup.

[dfox]

In this case that probably suffices as there is single intended video source.

Probably all multisync monitors I've seen have actual neon lamps and I believe that these neon lamps are in fact crucial for the "multisyncness", because they provide time for the control electronics (which given the package and pinout seems to usually be 8051) to settle after mode change without blowing out the CRT.

For a long time I've considered all the warnings of the "wrong video mode may damage your display" kind as nonsense. Then about ten years ago I had blown two 12" POS monochrome CRT monitors by unintentionally running them at 1024x768@75Hz, there was correct image for about a second followed by loud bang.

[dfox]

By the way I was thinking about all the multisync monitors I disassembled and got the impression that in all of them horizontal deflection was driven by class D amplifiers (there was half-bridge with switching MOSFETs, IRF640 and such). Does anyone on HN know whether that is correct assumption?

[phire]

Hmm...

If regular CRT televisions behave the same way, that means at some point in time it would have been possible to create a "jamming signal" that transmitted a signal with the wrong refresh rate and blew up all the TVs in the area.

[mjg59]

Regular CRT televisions weren't multisync - at best they'd be able to deal with both 50Hz and 60Hz inputs, but they wouldn't attempt to sync to anything else.

[phire]

Multisync means the CRT will succeed at syncing to multiple specified frequencies.

There is nothing stopping non-multisync monitors (including TVs) from trying to sync to an out of range signal and depending on the design they might damage themselves trying.