Vectrex Buzz Fix… Enjoy the Silence

One of the famous features of the Vectrex is the annoying buzz from the speakers. After a while, it was just too irritatingĀ and I decided to try and fix it. Some purists say that the buzz is an authentic part of the Vectrex experience, but I’m sure the engineers who designed it didn’t intend it to buzz šŸ™‚

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…

vectrex_power_board_removal3

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

I decided to try a different approach, byĀ 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:

lm386_module

These modules cost about Ā£0.99 on eBay, and are widely available. I couldn’t find a schematic, so reverse engineered it (below):lm386_circuit

For comparison, here’s the original amplifier schematic for the Vectrex:

vectrex_audio_amp

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 theĀ capacitor C1 bridging pin 1 and 8 on the module. With this much gain,Ā we might still get the buzz, and we couldĀ also get clipping/distortion.

Fortunately, this is easy to fix by removing R1 from the module (actually R1 isĀ 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.

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:

lm386_module_pins

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:

vectrex_new_audio_cable

Although the original Vectrex LM386 is powered from the +9V rail, I decided to power the new amplifierĀ moduleĀ 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.

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:

vectrex_audio_module

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.

The end result is a massive improvement, completely eliminating the buzz!

Here’s a photo of the final installation:

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.

I recently stumbled on this nice video (in German) where someone has tried this same modification:

29 thoughts on “Vectrex Buzz Fix… Enjoy the Silence”

  1. Hi there,

    I am trying to ‘cure’ a buzzing Vectrex myself in the moment
    but have problems so far: it works when my audio cable is
    in a certain position and the audio cable only grounded
    on the amp side. But it’s really finicky. And I’ve tried
    probably a dozen different placements/changes so far.
    So my foremost question is: did you experience something
    similar ?

    1. I tried moving the existing cable, and couldn’t eliminate the buzz. I think the existing cable is quite poorly screened, and picks up interference from the yoke magnets. Also the existing amplifier is on the same PCB as all the power electronics that drives the yoke magnets,.
      I completely replaced the existing cable with some higher quality screened audio cable that has a metal foil screen (I think it is made by Van Damme), and also replaced the existing amp with a separate amplifier module. Having done this it seems to be pretty much immune to interference, and the placement of the cable doesn’t seem that critical now.
      I don’t think moving the cable alone would be enough. Replacing it with a better quality cable would probably help. I can highly recommend the method described above with the power amplifier board, it was cheap and easy to do and the results are amazing!

      1. Ah, I forgot to mention that I am already using another amp, just like you šŸ˜›
        But I am still using the original audio cable and connector. Since
        the position of the audio cable is still very important I’ll try a (better)
        shielded cable next, then. Reading your reply I still have to ask:
        ‘the placement of the cable doesnā€™t seem that critical’, does this
        mean that you do hear something when moving the audio cable
        around ?

        1. Ah, I see!
          If I remember rightly, on mine the original audio cable was only grounded at the power board (where it connects to the original amp), so when that connector is disconnected from the power board, there was no ground screening of the cable at all.
          My new cable is grounded at both ends, and runs fairly flat to the PC. I’ve also twisted the power and ground cables together.
          It doesn’t seem to pick up any interference, regardless of where you put the cable now. I mean to say, I don’t remember hearing anything at all when I move it around.
          If you are still having issues, you could try running the audio cable under the PCB also.
          My bet is that using a better audio cable grounded at both ends will probably fix your issue.

  2. Success !
    I finally found the time to give it another go and got it to work:
    I’ve basically used the described setup here with one difference:
    when I removed the logic board I hooked up my shielded cable
    not to the component side of the pcb but on the underside directly
    to the Vectrex potentiometer and grabbed 5V/gnd on the soldering side, too. There’s plenty of space there for all cables
    and pcb and the entire logic board is between the tube and
    power board and the new cables/amp.
    Now there’s only a very small noise coming from the neckboard, nothing from the audio as it should be šŸ™‚

    1. Great! pleased to hear you got it working nicely šŸ™‚
      Likewise, all I could hear after the mod is the very faint noise from the neck coils (i.e. “coil noise”) but silence on the audio. Perfect!

  3. Thanks for publishing this, I have ordered the parts and will be attempting it on my noisiest Vectrex.
    There is, like you say, an argument to be said that the buzz has some historical interest though. Could you please give some advice on the best way to install a switch to allow a user to flip between the old amp and the new?

    1. Excuse the delay in reply, I’ve been travelling and just got back.

      I think once you’ve made the modification, you may not want to go back to the buzz šŸ™‚

      However, if you do want to be able to switch between the amplifiers, one option would be to use a “double pole double throw” (DPDT) switch to allow the speaker to be switched between the two amplifier outputs. My thinking is that switching the output side (rather than the input) would avoid the risk of introducing noise at the input side of the new amplifier. The switch would be connected so that in one position, both speaker wires are connected to the new amplifier, and in the other position both speaker wires would be connected to the old amplifier.

      Hope this helps, but let me know if you have any questions!

  4. I think I’m going to give this a shot. I performed the shielded cable mod by itself but it didn’t seem to make much difference.

    Would it be possible to add a picture of exactly where you pulled the 5v and ground?

    Also, did you leave the original amp in place?

    Thanks!

    1. Hi Tom,

      Likewise, I found that shielded cable alone was not enough to eliminate the buzz, and that’s why I moved the amp.

      I don’t have a photo of the 5V/GND connection, but could take my Vectrex apart again to get one… no promises, but I may find time this weekend.

      The original amp components are still in place, but there are no audio or speaker connections to the old amp (they are connected to the new amp). Reinstating the old amp would just be a matter of swapping some connectors over.

      Regards
      James

  5. I finally had a chance to install a cap kit and the audio mod. I pulled the 5v and ground from the solder side of the logic board from a couple unused through holes near the volume pot.

    No more buzz! I did notice that the volume doesn’t go quite as high as stock, I’m guessing from using 5v instead of 9v. My next attempt might be using 9v if the wire routing required isn’t too crazy.

    Thanks for the idea and details!

    1. Hi Tom, I’m glad you managed to fix the buzz!
      The volume is indeed reduced because of the 5V supply, but I decided it was plenty loud enough for me! From memory, I think the 9V supply is on the neck board, so it might mean running some longer cables to bring that to the amplifier. If you decide to do that, I would try running both ground and 9V and twisting them together, to hopefully avoid picking up noise.

  6. hello,

    james, could you explain the solder points you used a little more detailed please? and which parts did you removed from original circuitry exactly?

    Markus

    1. Hello Markus
      The audio cable is soldered to two pins of the volume control (R326) as pictured above, and the other end goes to the new LM386 module (the old audio cable ran from EP107/EP108 to connector J302).
      The existing speaker connector P103 is disconnected from the existing PCB jumper J103, then connected to the output of the new LM386 amplifier module instead.
      To power the new amplifier, I soldered two wires, one Black wire to the GND plane (easy to find) and Red one to one of the 5V supply lines. There are lots of places you can find 5V on the logic board, and the easiest way is to examine the Vectrex circuit diagram. What you need is the “+5V DIG” power rail. This originates from J204/P204, and runs all over the logic board. Many of the ICs on the logic board have their VCC pin connected to +5V DIG, so you just need to locate a convenient point on the PCB where you can access one of the tracks carrying +5V.
      You can double-check by switching off all power completely, then use a multimeter to test for continuity between +5V on J204 and the point you plan to draw the +5V from (this is a bit safer than probing around the PCB with a meter when it is powered on).
      From what I remember, I found a track carrying +5V somewhere on the top side of the PCB nearby the AY3-8192 chip.
      The best place to look is in the Vectrex Service Manual which has circuit diagrams (schematics) for the logic board.
      Hope this helps
      James

  7. Hi James,

    Just wanted to Thank You for your Buzz Fix! I just added it to my recently acquired non-working Vectrex, which I restored, and the Fix works like a charm!

    To others who wish to try your mod, some of the eBay sellers of this LM386 amp board have ad pictures of boards that look the same as the one in the mod, but actually ship different boards. I had this experience with one seller, the board did not have the variable resistor or the large electrolytic and had R1 but no C1. This board did not work properly, too much gain with and without R1. I was successful with another seller in gettting a board identical to the one pictured.

    I grounded both ends of the audio cable attached to the recommended pot terminals and found that its routing wasn’t critical. I don’t hear any buzz from the speaker. My audio cable was from a fairly thick salvaged audiophile cable with double shields, foil and two internal wires.

    I was too lazy to hunt for +5V from on the logic board, I spliced into the +5V wire in the bundle that snap-connects to the logic board (i.e. plug P204 that connects to jack J204 on the LB). Trace the wires from this plug back to the Power Board and the circuit board is labeled for each wire.

    Last, I had to give my on/off volume control pot a good cleaning with contact cleaner.

    Thank You for the knowledge and inspiration!!!!

    1. Great – I’m really glad to hear this helped! Thanks for the additional information above, I’m sure that will be a big help to others attempting this mod. Many thanks!

  8. Hello !

    Thanks for this mod.

    I have done the first mod from console5 before with the opamp circuit then I switched to this one. Should I remove the opamp circuit ?

    It works fine with both however I notice some distortion on some sounds I don’t know if it’s normal due to 5v instead of 9v… also when I connect a ground line on the logic board I get lots of noise so I don’t connect it…

    1. I think the console5 mod is a pre-amplifier, which boosts the signal. It’s possible this might then be too high a level and you could get distortion (clipping) in this case, so you might need to remove the pre-amp (in any event, the pre-amp isn’t really needed with this LM386 mod). First thing to check though: did you remove R1 from the LM386 module? The LM386 module I bought had the gain set to 200, so I removed R1 to reduce the gain to 20 (same as the Vectrex). If yours is still set to 200, this could also cause distortion.
      The 5V supply versus 9V supply just results in reduced volume. Distortion is usually due to the input signal or gain being too high (so the signal gets “clipped”). Hope this helps!

      1. yes I have remove the R1 as you recommended…

        I just wanted to avoid messing with the cap and the resistor which has been partially removed from the pcb to add the preamp. Distortion is not really that bad. It just sounds different from the original, less clean imho, it could be my imagination…

        Also I thought the preamp could compensate the 5v vs 9v issue

        1. I can understand why you don’t want to mess with the PCB again. It just occurred to me that you actually don’t have to remove it, you could bypass the pre-amp by connecting the LM386 input to the input of the pre-amp instead of connecting to the output šŸ™‚
          Basically, the gain of the LM386 module is 20, if your input voltage level multiplied by 20 exceeds the supply voltage to the LM386, you will get “clipping” (which means sounds will be distorted, because sine waves get “squared off”). Using the pre-amp doesn’t compensate for the 5V vs 9V issue (i.e. although the pre-amp does increase the volume, it risks causing distortion because the output voltage is limited by the supply voltage). If you have access to an oscilloscope, it would be fairly easy to check for this.
          To prevent clipping you could either 1) remove the pre-amp, 2) bypass the pre-amp, 3) increase the supply voltage to the LM386 to 9V, 4) reduce the gain of the LM386 further.

          1. Thanks James.

            You mean I remove the cable that goes from audio connector on the logic board, then use a cable between preamp input to lm386 input ? sounds better to me to get rid of that preamp if it’s useless, maybe I shouldn’t be afraid to revert back now. What about using a voltage regulator to turn 5V to 9V to avoid getting noise from the power board ?

            I also have one interrogation, when I try to link the ground from the LM386 to the logic board I always get lots of noise, so I didn’t connect this at all. What is going bad ? Is there a specific place to connect GND and avoir noise ? Same for +5v, I use the point on the wiki for the preamp…. Not easy to see where it points on the pictures of your tutorial.

            Can you please send a link to buy good insulated cables for audio in + shielding ? I used an RF cable from the first mod, and man it was a hell of a struggle, solder flows like water on the shielding, not compliant at all… almost messed the pcb because of that…

  9. Hi James. Just removed the preamp and sounds better now, less distorted. Next step would be adding a 9v voltage regulator.

    1. Great, I’m glad that cleared it up!
      The good news is that you don’t need to add another voltage regulator, you could use the existing +9V unregulated supply from the main power board.
      You would need to run a long power cable from the power board. It looks like you can access the +9V rail at the cathode of Diode D101 or D104. Take a look at the schematic on P30 of the Vectrex Service Manual.(https://console5.com/techwiki/images/a/a7/Vectrex-Service_Manual.pdf)

          1. Actually, thinking about it…. the input voltage level to the amplifier is the same, and the amplifier gain is the same, so the output voltage from the amplifier will be the same, hence the volume will be the same. As long as the supply voltage is high enough to avoid clipping, it probably won’t make any difference.

            So I think we can conclude that there is no issue running it from the +5V line after all…

  10. that’s what I was thinking, sound fine enough to me and satisfied with the mod… however I have some volume drops from time to time, sometime sound just disappears for 1 second or less, then comes back with more volume, had this on 5V and 12V, have you ever noticed something like this ?

    1. This is not something I’ve ever seen myself. My best guess is that it could be a bad solder joint or poor connection of some kind, resulting in an intermittent electrical connection. It’s also possible it could be the volume potentiometer (if that is worn/dirty/damaged).

      1. the only place I could have messed with is the cap/resistor while installing the first mod, and maybe the rf cable junction however, it sounds fine enough to me, I won’t touch this anymore šŸ™‚

        thanks for this output

        have you ever thought about a mini jack out mod ?

Leave a Reply

Your email address will not be published. Required fields are marked *