• Work Log

    Keri Szafir03/29/2023 at 16:29 0 comments

    A customer asked for someone who'd fix a bad humming issue on this '86 ZX Spectrum +2. I've never worked on these, and am getting into tinkering with vintage computers and will go for any opportunity to do that, so I came...

    The machine was in a well-preserved state, working, but with an audio problem. I did my research on issues and mods for this particular model, and it turned out that the manufacturer changed the transistor type to 2N3904 without updating footprints on the PCB silkscreen layer, so given that these had a reversed pinout, they were soldered in the wrong way. Easy fix. BTW if you don't have a desoldering gun (super useful!), you can clean the holes with a stainless steel needle like I did.

    I also replaced all the electrolytic capacitors just in case.

    And now, the mods...

    First of all, the machine originally had jumpers LK1, LK4 and LK7 in the standard rather than "Peritel" (that's how Euro SCART was called in France) position, meaning that theoretically it wouldn't support SCART without switching them... I left LK1 and LK4 in place, with composite video and vertical sync on the connector, handling both the standard monitor (are they even around?) and SCART connection. I changed LK7 to LK8, cutting off the brightness signal from pin 3 and putting audio signal there instead. Nota bene the cable itself also has to be modded, if it comes from Retro Computer Shack and is meant for Spectrum 128K & +2. It has a network of resistors and diodes summing the RGB signals and putting the voltage on the brightness line on pin 3. Because these diodes and resistors were also originally built into the computer I worked on, I could reroute pin 3 to audio and get rid of the diode/resistor network in the plug, simplifying it (but losing interoperability with other Spectrum computers), freeing the external 3.5mm jack connector for other uses such as plugging the computer into a mixer etc.

    Next modification is rerouting the FM modulation for audio (C31  mod) from composite video luminance input on the TEA2000 composite video encoder to UHF transmitter input, separate from composite video.

    And now the most important mod: de-humming the tape deck...

    The problem lies in extremely high gain of the tape read amplifier (a common emitter stage followed by an opamp, with capacitive reactance rather than resistance dictating the gain, with some filtering involved. It picks up any hum, even though the device is powered from a filtered DC power supply, and the tape deck gets regulated +5VDC. I wouldn't modify the amplifier without taking a risk of losing functionality, so I went a different route: cutting power off when it wasn't needed. I used the leaf spring motor switch for the amplifier as well, removing the orignal R306 - 56Ω, adding a 100Ω for the amp instead, switched by the leaf spring contact. The power filtration needed some boost, that's why I replaced C315 100µF with 470µF. After this mod, there is noise on the audio input when the tape drive is in use (ie record, playback or rewinding), and it's dead silent otherwise. Since this Spectrum will be used with a memory card interface, it's a huge improvement without affecting any functionality.

  • Speccy Fixes and Mods Video

    Keri Szafir03/29/2023 at 16:00 0 comments