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VR machine problems

Discussion in 'General Electronics Discussion' started by Simon_m74, Jul 25, 2016.

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  1. Simon_m74

    Simon_m74

    31
    1
    Jul 25, 2016
    Hello,

    I have a VR machine that I am restoring, but i have an issue with what is called the `Format board`.

    The main system is an Amiga 3000, that connects to the Format Board for the following reasons:

    Via the parallel port, the Amiga activates a relay that turns on the main PSU
    This PSU powers :
    • The format board`s electronics
    • The Tracking hardware
    the Format board controls:
    • Video amp, switching and protection (fed from custom GFX cards on the Amiga, to the headset, via the format board.
    • The audio mixer : Amiga audio, CD audio and microphone are all mixed, and fed to the amps:
    • The amps, one for the headset, one for the main external speakers.
    • Analogue Joystick trimming.
    Right : I`ll try to explain the operation, before I explain the problem:

    The Amiga`s parallel port, once its powered up the Format Board, then controls the following:
    • Video Switching: This is switching of the external video output, between Standard Amiga mode (for diagnostics/editing) and Game video. This can be done at any time by a keyboard shortcut, but is usually in Amiga mode during the boot up, and then switches to the Game once its loaded.
      • It does this using MAXim 453 dual input video amps, fed a signal from a `flip-flop`(I think)
    • Audio Mixing/Volume : The 5 audio channels (CD L/R, Amiga L/R, Mic) each feed into a DAC. The parallel port (via the flip-flop) controls these DAC`s and therefore the volumes, by depending on the digital input, it amplifies the analogue output in reference to the analogue input.
      • The 5 audio channels are now mixed, into a single L & R, and fed into a pre-amp for the headphones, and a Main amp for the external speakers, both of which have the own main volume over-ride and bass/treble controls.
    • Flashing Lamps: On the machine, there are three lamps, for (in some games) to choose an option. These lamps are controlled again from the parallel via the flip-flop, to be either On, Off, or Flashing, done by a 555 timer for flashing, and simple logic gates to determine on, off or flashing.
    • Joystick Trimming : this is an `internal system`, and does not use the parallel port. Its just simple trimming pots for tuning the analogue joysticks, before sending the signal to the AMiga`s joystick ports.
    Ok, Thats what it Should Do....
    BUT......

    • Its not video switching (from SW or keyboard Shortcut)
    • Flashing lamps are in a random state - but mostly off (and not controllable by the diagnostic SW- although they should be)
    • Audio is VERY BAD. Quiet, very distorted, high pitched `whistles` at times.

    the Audio was the main problem, so I have bypassed the DAC system, and fed the Amiga Audio direct to the Amps, It seemed to work for a while, but its not now.

    The Audio output from the amiga is tested as Fine!
    The Parallel port, I assume to be ok, as its powering up the Format board without fail each time.
    I have tried more than one Format board - and I have the same problem!

    Its been suggested that the main Amp`s are faulty, but if thats the case - why would the headset audio be faulty too ?

    Does anyone have any ideas?

    I am based in Leicester in the UK.

    Thanks

    Simon
     
  2. Sunnysky

    Sunnysky

    488
    126
    Jul 15, 2016
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