Commodore 128 / Commodore 64 Shortboard RF Module Replacement

I have been unhappy with the regular 40 Column video output on my Commodore 128. Watching a video by TheRetroChannel on Youtube, I saw his RF Module replacement. I feel technically that is not the right thing to call these types of boards, they “replace” the RF Module, but they are not “Replacement RF Modules”. You loose the “RF” Output, this really isn’t an issue as not many people would likely want to ever connect up the Commodore 128/64 by RF to an old TV tuned to Channel 3/4.

He released the board designs on Github as open source projects. There are two versions the C64 Longboard and the C64 Shortboard/C128 versions. For the Commodore 128 I needed the short board version, so I ordered them from JLCPCB. This is specifically the board that fits the Commodore 64 Short Board and Commodore 128 as they share the same type of Modulator. He also made a Commodore 64 Long Board version, they are basically the same but the Long Board version is a bit larger pcb to fit the Long Board properly.

https://github.com/TheRetroChannel/C128-C64-Shortboard-RF-replacement

The boards have various options on them. I have populated everything except the C64 Hard Reset section. This is for a Commodore 128 after all, and it already has a reset button. That isn’t a Hard Reset though, but by the time I install this that also won’t be an option. It is indicated it may not work on the C128 though I don’t know if that is the case or not.

I put on the 500 Ohm Trimmer Pots rather than the default resistors. I had the exact parts in stock, and I have a fair quantity of them, I purchased them for some project, maybe even when I was working on the earlier RGBI Adapter builds years ago. I did set them to match the set resistors as a starting point. The center and “right” pin have to be set to the baseline value, 75 Ohm and 180 Ohm I believe. I don’t know if I will have to do any adjustments on them or not (they were perfect at those starting values), but I had them and it made sense to me to use them. I probably have more of those Trimmers than resistors of the correct values anyways. This whole board was populated with parts I had in stock, the Audio jack was salvaged, but everything else is new. It was neat having a project I had everything for.

It was a strait forward build, everything is labeled. The two capacitors are labeled on the bottom of the board instead of the top, that did have me almost putting them in the wrong locations. It is easy to transpose the positions when flipping something over. I know he mentioned he made is so that the parts would cover up most of the silk screen markings. When assembled it does look pretty nice too. The only other thing I did check which pins on the Trimmers needed the proper baseline resistance set on them, but that was easy. I picked the White boards as I thought it would look nice when installed as well. It won’t clash with the color of the C128 board, or look like some poor attempt to color match it.

I am going to start with the normal Chroma/Luma paths. I will test that everything is working properly there, then I plan to switch to the External Chroma/Luma lines. The whole reason I am doing this modification is to try to improve the poor video quality I get from the VIC 40 Column video output. My Commodore 64s have far superior Video Output to the C128.

I really don’t need the SVideo and 3.5mm Audio Jack output. I have my RGBI Video Adapter which already has level adjusted SVideo and the SVideo jack (which is why I had a spare SVideo jack in my stash of parts), plus the Audio Jack on it. It won’t hurt to have them. It was unclear as to if the Chroma/Luma output on the Commodore AV Port was still active, but looking at the Schematics and board itself it is still connected.

Again, it was an easy build. TheRetroChannel does say the hard part of this mod is removing the RF Modulator module from the C128/64 board. He is correct, I have removed three of them, and well it is not something I look forward to.

I desoldered the factory RF Modulator, and stuck in the new unit. I fit it without soldering. The pins helped hold it reasonably secure so it wasn’t sliding around. So I did a test fit, and put the board in the case to get it lined up to the openings properly.

You can see that it is crooked in relation to the board. This is due to the alignment of the holes in the case having the opening for the “switch” lower than the “RF” port opening. Once I had it set where I wanted it, I carefully removed the boards from the case. I then tacked some of the pins with a bit of solder. I fitted it back inside the case again to make sure it didn’t move. Then I removed it from the case again and finished soldering it in. It was not difficult to align the board, as the pins held it fairly firmly in place as there are 12 pins they gave enough friction to not have it flop around while I was lifting it out of the case or flipping it over to solder.

It is in and the pins are all cut down properly. It was time to test it. I wanted to get some pictures of the output before switching out the modulator, but I forgot.. I fully remember it was awful in comparison to both of my Commodore 64s even in SVideo output. I was hoping I had some pictures of testing the SVideo output when I built the new RGBI adapter. I didn’t take pictures of the SVideo output. It was still awful at the time..

There is a jumper on the board to enable the Composite video, I believe the Chroma line to it. If you are just going to use SVideo, having it disabled is to slightly improve the output. I forgot to install it and ended up with the first screen above, basically no color except the “noise” around the text. The second shot is the Composite after putting the Jumper on. The last being SVideo output. The SVideo is much cleaner with no noise around the text. Even the Composite is a huge improvement over having the RF Modulator installed.

I wish I had some pictures of the Before. It is dramatic in this case. I was going to do the Chroma / Luma Bypass, those two pins on the lower right of the board. Without doing that, the video is comparable to my Commodore 64s. I feel it isn’t worth it at this point. I don’t want to bend out the VIC’s Chroma and Luma pins from the socket and solder wires to them. The improvement as it is was totally worth it. The two trimmer potentiometers are left set on the 180 and 75 ohm settings, I didn’t see a reason to adjust either at this point. I figure your probably safe to put in the standard resistors for those unless you want to go all out and tweak it to perfection. The same wit the Chroma / Luma bypass. You can still see Jailbars on the display, they are far better and overall the image is much sharper and cleaner.

I have recently build one of these up and installed it in a C64 Shortboard. For that one I did populate the Hard Reset section, but I didn’t put in a button or wire it in at this time. I used the regular resistors instead of the trimpots. I modified the Audio Jack to sit lower on the pcb, the jack I used is nearly identical to the one shown here. That helped the board sit more level in the C64 than on the C128 here. There are multiple pads for the Audio Jack, I was thinking maybe it is compatible with a slimmer jack type, but I didn’t want to risk it. I do have a slimmer jack that looks like it may have fit. The slimmer jack doesn’t seem as well built though, so I used the style I had used previously. You can lookup the more recent post showing that board. It is the same though, except sitting a bit more level due to the slightly lower mounted audio jack.