• First Image

    Paul Kocyla • 07/31/2026 at 16:25 • 3 comments

    There it is. It was hard to capture. What works is a global shutter 120fps capture with averaging.
    Still a bit distorted here, but come on, it's working already :)
    I'll play around a bit for beeing able to elaborate and improve on that.

  • TV Build Finished

    Paul Kocyla • 07/17/2026 at 12:41 • 0 comments

    Those old synchronous motors sometimes struggle with sync-ing due to worn out bearings or a wrong running capacitor value. I found out that the motor almost locks best with 4uF, but it slipped a pole around 4 times per second.
    A video suggested to raise the voltage a bit with a Variac to help the old lady sync, and it helps!

    I cleaned up the cable connections and have done the final assembly.

    So it's working now. Next step is the PC interface that generates a suitable video signal, so I'll be able to actually put something onto the screen! The input signal in this video is just the mains AC taken from a tiny wall brick transformer.
    I will use the same transformer to synchronize the transmitter.
    The disk is perfectly locking to the mains frequency now.
    The motor has four poles, so there is a 25% chance it will lock into the correct phase. However, if it doesn't, you can tap a button to let the motor slip and lock into the next phase. It's been done like that in the 1920s as well. You should have a stable picture after at most 3 short taps.

  • Motor Sync Problems

    Paul Kocyla • 07/16/2026 at 19:30 • 0 comments

    MechaTV electronics is working properly. However, I had some trouble with the motor that did not synchronize to the 60Hz mains frequency.

    First: You need the right capacitor for the second winding. The type plate specs what kind fo cap you need, but there is NO information whatsoever on the net (chats included) that can tell you anything about it. I guesstimated by measuring the winding resistance, taking into account all the other motor parameters.

    After optimizing, the motor tried to lock to 60Hz, but pole slips happened 4 times per second. The motor just could not pull into the main frequency lock.

    Fortunately, I knew what to expect, and raising the voltage helps a lot with those vintage synchronous motors.
    After a short period of desperation, I got the variac delivered and raised the voltage a bit: Now it is finally syncing.

    Seen a video about that. When the motors are that old, or if the bearings are not perfect, or if there is a "flywheel" load on them (which the Nipkow disk is !), you got to expect those errors.

    Nevertheless, an old motor might do as expected when raising the voltage a tiny bit to overcome those aging effects - and it does :)

  • Neon Bulb Modulation

    Paul Kocyla • 07/05/2026 at 11:17 • 0 comments

    The power amplifier stage is working. I am modulating the neon bulb by applying AC coupled sine wave to the grid of the power amplifier tube. Self biasing works well.

  • Crater Bulb

    Paul Kocyla • 07/04/2026 at 13:43 • 0 comments

    It wasn't easy to find the right neon bulb for that project. They are mostly too small.
    Those nice, big beehive lamps, that amateurs used in the 1920s, are not available on Ebay anymore.
    There are bulbs with large electrodes, but they are not that bright because of the coating, they are more decorative.

    I have two of those crater bulbs in the picture. This one was already broken, so I opened the base to check for the internal resistors. There are two inside, and they both have to go.
    I want the tube to draw current through the bulb, and they would be just in the way - reducing contrast.
    The tube itself will have enough internal resistance to prevent the current from running away.

    This one is either from the 1930s or a more modern replica (resistors look younger), but it is so far the closest I can get to a 1920s type. Technically very comparable, though.

  • Copper Oxide Rectifier

    Paul Kocyla • 06/27/2026 at 00:36 • 0 comments

    This thing finally arrived, and it works OK.

    One thing: I mention in the video that it's got five plates. That's not true, those big rectangular plates are just heat sinks and taps, in between are more plates!!

    The rest is in the video, I made some quick real talk:

  • Filament Supply OK

    Paul Kocyla • 06/25/2026 at 23:24 • 0 comments

    Yes, the battery solution works :)
    As I don't have the copper oxide rectifier yet, I used modern diodes instead. Two diodes and a center tapped secondary winding of +/- 6.3V supply.
    Without the battery, there is a nasty hum, which is even nastier after amplified by the HUGE factor of 8 :-D
    Yeah, it was good back then. With the battery, the hum is gone - at least gone enough for the goal here.
    The battery would look different in the 1920s, more like that:

  • Filament Supply Trap

    Paul Kocyla • 06/22/2026 at 12:36 • 1 comment

    Good news and bad news:

    Good news is that the balancing resistors nicely eliminate the 60 Hz hum to an acceptable level. Bad news is that I get around 0.5Vpp of hum at 120 Hz.

    I was wondering what it was, crosstalk, bad ground design, etc. 
    Amplification works super silent with a direct 5V DC supply, so I know that the rest of the circuit is very clean. But even with the 'humdinger' resistors, this 120 Hz hum reappears despite filament AC voltage beeing a very clean sine wave. This is a known problem that vintage Hi-Fi guys, who wanna use period correct partts, struggle with until today.

    The true reason is that those old tubes have a very thin, fragile filament without much thermal mass.
    The filament is basically "blinking" at 120 Hz (when fed with 60 Hz AC), modulating the current of the tube.
    I wasn't aware of that, because I'm only used to indirectly heated tubes that don't have this issue.

    I found out that this was one of the reasons why mechanical TVs came so late, because indirectly heated tubes were not wide spread in the era of battery radios.
    However, there was a solution. Copper oxide rectifiers existed since WW1, and they have been used in the 1920s in battery chargers and battery eliminators (radio power supplies).

    Perfect. They are low voltage, high current devices, can be perfectly used to rectify 12V AC to over 5V DC. I ordered two on Ebay for $20 and $60 (the only ones available today that won't cost over $300). One has 5 plates, one has 16 or so, but I can prolly remove some to avoid too high voltage drop. Current rating should be OK to light two 5V/0.25A filaments.

    Together with a cap/choke the supply should be ripple free, at least good enough for not seeing hum bars on the image.

    Update: I tried to full-wave rectify with modern components, using diodes and a 100uF cap - and guess what: SAME PROBLEM :-D 

    OK, further research revealed that the battery eliminators back in the day used either massive coils with low resistance to solve it, or they used messy electrolytic cells as "filter batteries".
    The cleanest so far were small 6V lead acid batteries that were just charged in the background through a resistor and acted like a giant filter.
    I will try this next. I will try whatever helps. The easiest way would be to use indirectly heated tubes, but NO, I will not give up yet. Challenge is to stay in the lower 1920s and feel the pain they had...
    I found a picture of an early battery eliminator, and yes, it's got battery jars in there to solve exactly that problem. To be fair, I am not quite sure if those jars in the image are the actual battery cells or a messy electrolytic rectifier. Just read that this cheap form of rectification was a thing, too - updates will follow...
    This has to be the reason why indirectly heated filament tubes have been invented when the power grid was getting available everywhere - it's cheaper than those dirty workarounds. But OK, I'll do the dirty workaround because why not:

  • Filament Supply

    Paul Kocyla • 06/20/2026 at 23:13 • 0 comments

    The two amplifier tubes UX-201A and UX-171A both need a 5.0V filament supply.
    The filament is directly heated, because the tubes come from an era where radios have been mostly battery powered. Indirectly heated tubes appeared in the mid 1920s, but I got the other ones already. I chose tubes that have been produced in the millions, so they are still available. And they have this nice pear shape and use those screw-on-board sockets.
    I wanna use them, because they come from the early 1920s and look stunning.

    As the filament is the cathode of the amplifier circuit, AC voltage would introduce tons of hum. There are three common solutions to that problem:

    1) use a clean, rectified DC voltage
    2) use a "humdinger" pot for every tube
    3) use an AC filament supply with a grounded center tap

    I decided to use option 3 to avoid bloating up complexity by adding additional adjustments and parts.
    My transformer has two 6.3V supplies, so I made a symmetrical -6.3V - 0 - +6.3V supply from them.

    As the tubes need 5.0V each, I used two equal 15 Ohm resistors for each supply leg to drop the voltage for each tube. It seems to work - let's see if it will be hum-free.

  • Wiring Bias Supply

    Paul Kocyla • 05/28/2026 at 00:35 • 0 comments

    Wiring up the bias supplies: Two negative voltages, around -4.5V and -25V, using an old trick from the past: back-biasing.
    Instead of returning from chassis-ground to the transformer, you run through a resistor network to get negative voltage drop across the resistors. It's possible, because the main load is a two stage class-A amplifier, so there is a predictable average current running through it.
    Fluctuations are taken care of by buffering the voltages with capacitors. Grids should not draw current in nominal operating mode, so you can use very high resistor values (like 500kOhm-1MOhm) for biasing. For stabilizing the voltage, run it through a 100k resistor to a capacitor.
    Way back then, even small caps like 30uF were HUGE. That's why that resistor trick was VERY helpful, cause you can use an 1-2 uF cap and get same stability.

    I am using big form factor capacitors on purpose. Those foil caps are available off the shelf and come close to what would be available back then regarding the form factor. You can get a 2-5uF cap for that voltage that's smaller than your fingernail, but that would have been science fiction in the 1920s, so I bought caps with same capacity but more higher voltage, as they are as chunky as the caps of same capacity way back then.
    Why I don't use genuine parts? Those are pretty much all degraded and non functional by now. IF I get a functional one by chance, I'll replace the modern part :) If time permits, I can stuff a modern one into an antique casing.

    I am using modern resistors, as I don't know the final values yet. If the project proves worthy of beeing more original, I will replace them with 1920s parts. However, they are super rare, hard to find and quite expensive.
    To make it more like early 20s, I could wind resistive wire onto a ceramic tube - will prolly do that :)