Commodore 64: Using an ancient BW TV

4Aug/1626

Commodore 64: Using an ancient BW TV

Using the Commodore 64 on the main TV produces a really great picture over the composite cables, but using it via a converter to VGA or via RF is a little dicey. I like the idea of 1:1 picture when using composite, there's no need to covert the signal. I also don't always get to use the main TV, so I went hunting for a suitable display for the Commodore 64.

Turns out that last weekend I was in luck. Whilst rummaging at the local Trash and Treasure I stumbled across a Samsung LCD TV (RF, VGA, Composite and Component in!) and then... the holy grail... a tiny, portable, black-and-white CRT Television! Check out that hideous battery pack.

It happily allows you to scan the VHF/UHF airwaves; unfortunately there aren't any signals broadcast in this spectrum anymore. Actually, at the low end of VHF I got a local radio station, but no picture. It has the option for an external antenna, but this used a mono 3.5" audio jack.

Hooking it up to the C64

The external antenna jack was easy to work with. Opening the TV (runs on DC voltage, but BE VERY CAREFUL WITH CAPACITORS NEAR THE TUBE), I inspected the circuitry and found that the in-built telescopic antenna was also wired into the jack. This makes sense: plugging in the external antenna disables the internal antenna. The jack functions as a nice routing switch, choosing between sources when the jack has a cable plugged in.

With this knowledge, I chopped up a 3.5" audio cable that I had spare and worked out which wire was ground. As expected, it turns out that the shielding was ground and the very tip (the white audio wire, red is unconnected) made contact with the RF input pin and disabled the internal antenna. With this, I then cut an RCA lead and joined the relevant cables. The C64 has a single RCA-style port for the RF output. Plugging the wire together, I then started scanning the airwaves.

I vaguely remember, from back in the day when tuning in a brand new Nintendo Entertainment System, that most consoles output a frequency somewhere near UHF channel 60. I happened to start at the 'top' of the UHF band, but after winding the dial to the other end I had a picture!

There was further tuning on the side. Contrast, brightness and V-Hold allowed me to get quite a clear display in black and white, of course.

The fun part now was taking a quality picture of the tube. Shutter speeds are usually way too quick to see the full image... the camera can easily beat the scanline. I therefore slowed the camera down. Using this theory, I also had fun and sped the camera up...

Hah... nice... very easy to see how the tube works. That Horizontal scanline illuminates a bunch of dots/pixels and does it fast enough to resemble a whole picture to the slow human brain.

Testing a game

My first cartridge was acquired from Game Over? in Amsterdam. This is 'Rat Radar Race', a game I'd never heard of. It was purchased because I didn't want to leave the shop empty-handed and, for quite a while, I've wanted to test out how cartridges work.

I plugged it in and turned on the machine. I was amazed to find that it booted straight into the game. Very seamless and very fast! With floppy disks and tapes you actually had to enter BASIC commands to boot. This method is much nicer!

The best part? Audio! The RF cable was dodgy, but functioning quite well. The picture and hideous audio was being output rather well. I tried the game for a bit: You're a mouse, there's three of you... you navigate a maze by holding down the arrow of the direction you wish to turn next. Scaling the maze, you pick up cheese. If you hit a cat or a fellow 'blind' mouse then you fail. The theme music is actually the 'three blind mice' 'melody'.

RF Modulation/Demodulation

Just like an old dial-up modem, the RF mechanism for getting your console to display on your TV is inefficient. RF was meant for radio waves; the goal was to be able to transmit images over long distances. The tuner in the TV is therefore capable of tuning in to differing frequencies, producing different channels on your display.

This is overkill if your console is sitting right next to the TV. There is no real need to convert to a lossy format, only to make the TV find the signal and convert it back to a displayable format. Hence, TVs later added extra inputs for 'direct' signals. After RF came composite, SCART, Component, VGA, DVI, HDMI, etc... The Commodore 64 can actually produce a composite signal (as I was using on my other TV), so I wonder how hard it would be to provide a direct composite input into this little TV?

Bypassing the RF input and providing Composite

Turns out that this is totally achievable. The job of the 'tuner' circuit in the TV is actually to produce a composite video signal to rest of the video circuitry. The main question is: Where do I inject the composite signal from the C64 without destroying the TV, the C64 or endangering myself?

Our first step is to inspect the circuitry and determine what ICs are used. Next we'll check out the datasheets and then try and work out a method for signal injection. The signals we are talking about are available 'naked' on the back/front of most audio/visual components, so shorting them out is possible in the real-world and therefore shouldn't damage our equipment if we happen to do so. The main issue is when you wire up the signal to a power rail or other high-voltage feed... such a process wont end well!

I would first recommend that you review a few BW TV Schematics to understand what basic components are used. Ralph K has a great article: TV and VCR tuner modules which has a schematic for a tuner that shows how the fundamental components are connected. The tuner IC actually seems to be a mirror of the one used in this TV... either way, it still helped with the circuit tracing.

A quick scan of the circuit board in this TV shows that it was built on a suite of Samsung chips. There's a KA2133 - 1-Chip Deflection System that provides the synchronisation for the tube. There's high voltages down in this area, so be very careful around the large capacitors!

Up under the tube is a KA2101 - Linear Integrated Circuit (TV Sound IF Amplifier). Not exactly what we're after, but we'll need this after we get the video fed in.

Top left of the board is the RF 'tuner'. Actually, the left-most box is a de-modulator. It does the opposite of what the RF modulator inside the C64 does. Unfortunately, it's not as simple as the modulator; it needs to be fed in variable parameters to determine the exact frequency to demodulate at. The C64 merely has a fixed set of parameters to modulate the signal.

You can see the wires running from the front panel into the space between the demodulator and, what I believe is, the tuner next to it on the right. This tuner is also shielded, but from the underside of the board I can see it has an IC in there. With a good torch and a little more disassembly of the chassis I was able to read the model of the IC. It is a KA2912 - Video IF Processor for BW TVs. Bingo. That datasheet also shows that Pin 3 is the video output.

The underside of the circuit board also has the pin numbers for the IC. How awesome for us? And for the assembly line lemmings who constructed it. Thanks to PIN DETAILS OF IC A-Z, BASIC ELECTRONICS AND ANTENNA : KA2912, we can see that it actually outputs a composite signal!

Injecting an external composite signal

From here, we're going to do damage to the circuit board. The first step is to bare the trace running to pin 3 by scratching the protective coating away. I've used my pocketknife to do this.

Next we need to actually cut the track. Use the sharpest tool you have and scratch the track at a perpendicular angle, slicing a gap into the board. Make it a little wider than 1mm. A flat-head screwdriver can be used once the initial cut is in place. Now that you've got pads to solder to, tin the areas that are bare. Make sure that you don't have any solder bridging the gap! At this point I then turned on the TV to test it. No picture? Perfect! The signal from the UHF/VHF tuner has been severed.

At this point, as a test, we're going to hook the composite RCA plug directly into this track. It's better to find out as early as possible if we've got the track or location wrong. If there is no picture when you do this, then you'll need to dig further into the datasheets and determine a better location to cut in the signal.

I hooked up the trusty C64 and ... it worked! Well.. nearly... the picture was scrolling and buzzing awfully. Turns out that there are two GROUNDs in on the circuit board. There's the 'signal ground' and 'supply ground'. If you, as I did with a paper-clip, ground the incoming signal to the supply ground then you'll get a shit signal. I then tried grounding to the other signal ground and got a much clearer picture. It was still rolling though.

What could be the problem? I quickly rotated the V-HOLD trimpot and had no luck... picture still rolling. At this point, I should have stopped ... breathed ... rolled the trimpot slowly... and tested it properly. But I didn't... instead I went and re-adjusted EVERY pot on the board. In fact, I didn't keep an initial record of what they were all set to and COMPLETELY de-configured the TV. I then spent another night re-configuring based on guesses from the photos I took for this blog. Painful... There were also fragile wires around the tuner circuit of which I managed to break free of the circuit board; these then required resoldering and reinforcing. Finally, I was nearly back to a functional TV again, the final issue being that a trimpot I replaced was grounding against the tuner RF shield! After fixing all these mistakes, I had a rather reliable picture.

Now that this was certain, I went about inserting a switch that selected between the antenna plug or the composite input. This was a DPST switch, as I wanted to switch the mono audio as well.

Now that I had a quick way to switch between both inputs, I plugged in both the RF signal and the Composite from the C64.

For the standard BASIC screen, shown above, the picture was very nice on composite (first picture) and quite blurry on RF (second picture). Either way, they both showed well.

Above is the display difference of a game cartridge. It actually seems that the cartridge is changing how the C64 outputs the picture! The first picture is composite and seems to be over-scanning? The second is RF and is happily rendered within the bounds of the picture tube. I wonder if that's PAL vs. NTSC or some other timing issue. Or maybe because I happened to alter all the trimpots and de-configured the TV. I'll keep digging.

Audio

The same trick was then applied to the sound channel output by the KA2101 - Audio amplifier. Documentation, other than the datasheet, on the KA2101 wasn't so easy to find, so I searched for similar devices. Turns out the MC1358 is a clone (let's not get into which one was created first) and there are quite a few circuits available online as examples of audio amplifiers.

If you look here at TradeOFIC, you'll see a stereo amplifier. We don't need 2 channels, but we do need to know where to inject our composite audio signal. On the mid-left of the diagram, you'll notice that they have an input select that cuts the line from Pin 8 of the MC1358 and splices in audio from an external plug. I love it when it's this simple! There is a capacitor on the other side, so we'll check our circuit and cut in after that if we have one too.

The circuit was cut and the wires were hacked on. I then fed via the external audio input via the same DPST switch that I used for the audio. I only need to switch one wire as ground is common (remember to use SIGNAL ground!) and the audio is mono, so there is only one audio and one video wire. Having a TPST switch would've allowed for 'future expandability', but I cannot see myself installing stereo speakers into this little beast.

The best part? It seems that feeding a proper audio signal in with the composite video stabilised the video signal! Look at that crisp picture!

Looking back...

This was not an easy task... this post was written around 4 nights after I started pulling apart the TV. Take your time with old electronics. The case was brittle and the circuit board had been repaired and re-soldered.

One thing I didn't mention above: I had to replace one shotty capacitor and I destroyed a trimpot. Don't forget that flexing wires around will break their solder joints... so expect to re-tin the majority of contacts under the board. Go nuts and replace/re-tin ANYTHING that looks suspicious!

Capacitors are a standard item to replace. Cell batteries are usually next in line for leaking. You're bound to find all sorts of issues with vintage items. Good luck!

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