SyncFix64 v1.1

Following the first prototype of the LM1881 based SyncFix64 I made a few minor changes, improving the schematics and layout. Then I was ready to order the first batch of properly manufactured PCBs. The boards took a little over two weeks for production and shipping.

This was also my first try at ordering a v-cut PCB panel and I’m quite satisfied with the result considering the resulting low price per unit. I assembled one of the boards tonight and the device is working properly. So I assume the whole batch should be fine.

The project files for the updated version 1.1 of the SyncFix64 are available on Github and here is the list of required components:

C1,C2,C3 0805 100n
Q1,Q2 SOT-23 BSS138
R1 0805 100r
R2 0805 680k
R3 0805 10k
U1 SOIC-8 LM1881
U2 SOT-223-3 AMS1117-50
C4,C5 1206 22µF

Update: There are German assembly instructions available for download now.



SyncFix64 Prototype

After the encouraging results of my first attempt to fix the composite signal from a C64, so that the cheap TFT monitor could display it, I shared the idea on Forum64 to double-check and get some feedback. Consensus seems to be that the circuit at least won’t hurt the video source. Since, in addition to that, it seems to be working for me, I decided to design a board for it in KiCAD.

The goal was to make it so small that it would fit inside the display’s case alongside the display controller, so I went for all surface mount components. To get immediate results, I created a single sided layout and etched the board myself using the toner transfer method.

Soldering the components to the finished board proved a little challenging, cramped as it was, without a solder mask layer. But it turned out fine and it fits inside the case easily. The display had two separate input lines to begin with, so now I’ve got one fixed for my old C64 and a second “regular” one to choose from.

Update: The KiCAD project including schematics and a preliminary board layout is available on Github now.



Cheap Displays for Old Hardware

A couple of months ago I dug out my old Commodore hardware again and started tinkering with it – the goal being to finally try out a bunch of mods, hacks, and builds that I missed back in the day. To start with, I bought a cheap TFT display for around 13€ to connect to the C64 and C128.

It was meant to sit on my work bench to e.g. quickly test a naked C64 board. It wouldn’t matter if the video quality wasn’t great. But after I made the necessary adapter cable and hooked everything up, I was a little disappointed to find out that the display wouldn’t show anything other than a black screen interrupted by an occasional flicker. I verified that the display itself was working by connecting it to a Raspberry Pi and I double-checked the cable I had made.

Doing a little research, I learned that the video signal produced by the C64 is actually not that great and that problems with modern displays are quite common. I got curious and used my (also very cheap) Hantek 6022 oscilloscope to take a look. The first thing that caught my eye was that the Commodore signal looked rather weak when compared to that from a RasPi.

C64 Composite Video

C64 Composite Video

Raspberry Pi Composite Video

Raspberry Pi Composite Video

So I thought if I amplified the signal, maybe I would get the TFT to display it. I did a little search on how to do this and stumbled across Raphaël Assénat’s page where he describes a problem very similar to mine. The interesting part is that his first impulse too, was to amplify the signal. But later he found out that the signal strength didn’t seem to matter much, it was the unorthodox v-sync sequence produced by the Commodore that confused his display. So he added a LM1881 sync stripper and an AVR micro controller to replace that sequence. Unfortunately, that contraption is very sparsely documented.

C64 Composite and VSync

CH1: C64 Composite, CH2: LM1881 VSync Out

Since he mentions that just “removing” the sync sequence already resulted in a somewhat stable display and that amplification might not be necessary after all, I figured I might just try to get away without the AVR and the amplifier and  I came up with this reduced circuit:

To my great delight and surprise, this yielded instant results! I hadn’t really hoped that this simple improvised circuit would be enough to make my display cooperate.

Of course, there are a few issues and to-dos left:

  1. Ask a few people who actually know this stuff for their opinion and for some advice on the actual components and their values.
  2. Make sure this does not damage the video source in any way.
  3. Revise the circuit accordingly.
  4. Make a nice board with proper wiring and real connectors.

Do you have any thoughts or advice on this? I would love to hear in the comments!