{"id":548,"date":"2016-02-06T02:41:24","date_gmt":"2016-02-06T02:41:24","guid":{"rendered":"http:\/\/int03.co.uk\/blog\/?p=548"},"modified":"2021-03-08T18:07:50","modified_gmt":"2021-03-08T18:07:50","slug":"vectrex-buzz-fix-enjoy-the-silence","status":"publish","type":"post","link":"https:\/\/int03.co.uk\/blog\/2016\/02\/06\/vectrex-buzz-fix-enjoy-the-silence\/","title":{"rendered":"Vectrex Buzz Fix… Enjoy the Silence"},"content":{"rendered":"

One of the famous features of the Vectrex is the annoying buzz from the speakers. After a while, it was just too irritating and I decided to try and fix it. Some purists say that the buzz is an authentic part<\/a> of the Vectrex experience, but I’m sure the engineers who designed it didn’t intend it to buzz \ud83d\ude42<\/p>\n

Unfortunately, the LM386-3 audio amplifier (pictured below) is mounted on the power board, right next to the wires which drive the CRT yoke magnets. The amplifier is fed by a poorly screened audio cable which runs to the logic board. The screened cable is only connected to ground at one end (the power board). Consequently, it picks up a lot of interference…<\/p>\n

\"vectrex_power_board_removal3\"<\/a><\/p>\n

The usual fix is to install some better quality screened cable<\/a>, and disconnect one of the ground leads (the theory being that this eliminates a ground loop). However, the new cable may not completely eliminate the noise, because it doesn’t address the issue of the amplifier being located on the power board, and I didn’t like the idea of disconnecting ground. Another approach is to add a pre-amplifier<\/a> to boost the level before it reaches the LM386.<\/p>\n

I decided to try a different approach, by installing another power amplifier, well away from the power board and magnets. Originally, I planned to buy an LM386 chip and assemble an amplifier on strip-board. However, I then discovered you can buy a ready made module built around the LM386 for about the same price as the components:<\/p>\n

\"lm386_module\"<\/a><\/p>\n

These modules cost about \u00a30.99 on eBay, and are widely available. I couldn’t find a schematic, so reverse engineered it (below):\"lm386_circuit\"<\/a><\/p>\n

For comparison, here’s the original amplifier schematic for the Vectrex:<\/p>\n

\"vectrex_audio_amp\"<\/a><\/p>\n

The main difference is that the Vectrex has pin (1) on the LM386 not connected, which sets the gain to 20. However, the new LM386 module has a gain of 200, due to the capacitor C1 bridging pin 1 and 8 on the module. With this much gain, we might still get the buzz, and we could also get clipping\/distortion.<\/p>\n

Fortunately, this is easy to fix by removing R1 from the module (actually R1 is a 0 ohm link, perhaps provided for this purpose). This disconnects the capacitor between pin 1 and 8 of the LM386, reducing the gain to 20, the same as the Vectrex.<\/p>\n

The module has two screw terminals for the output, so a couple of short wires in these allow the existing speaker connector to push fit:<\/p>\n

\"lm386_module_pins\"<\/a><\/p>\n

Next, I removed the Logic Board and replaced the audio cable with a higher quality foil screened cable, soldering it to the ground and wiper of the Vectrex volume potentiometer, on the component side of the Logic Board:<\/p>\n

\"vectrex_new_audio_cable\"<\/a><\/p>\n

Although the original Vectrex LM386 is powered from the +9V rail, I decided to power the new amplifier module from the +5V rail on the Logic Board, to avoid having to run a +9V power line from the Power Board. To do this, I ran a twisted pair of red\/black wires from the +5V and GND rails nearby the AY3-8192.<\/p>\n

The audio cable, 5V and GND are installed in a 0.1″ crimp housing to fit the 4-pin head on the module. The finished assembly is shown being tested outside the Vectrex below, before it was finally installed inside the machine:<\/p>\n

\"vectrex_audio_module\"<\/a><\/p>\n

To mount the board, I used a self-adhesive cable tie base just inside the cabinet above the speaker. The module is cable-tied to this base.<\/p>\n

The end result is a massive improvement, completely eliminating the buzz!<\/p>\n

Here’s a photo of the final installation:<\/p>\n

\"\"<\/a><\/p>\n

As shown above, the 5V power comes from a supply junction between the AY-3-8912 and the 6522, and the GND comes from a ground plane via near the top right of the 6522.<\/p>\n

I recently stumbled on this nice video (in German) where someone has tried this same modification:<\/p>\n