Quantum/Amplifone HV Blues

Hi ...

I'm in the process of repairing an Amplifone HV board from a quantum.

The game itself is fully working when the HV board from a Star Wars is
fitted but I'm unsure of the what is happening on the games own HV board.

The board has been recapped, W1 has ben replaced with a solid wire
link. New regulators and a BU406D have been fitted. Since the red HVT
was obviously dead due to the HUGE bulge in it I removed it and for testing
purposes I fitted a 100w lightbulb as a dummy load.

Powering the board up with the dummy load I found that I had +24.5v and
-25.8v at the regulators. The lightbulb glowed indicating that I'm getting
some form of output across the input of the HVT. On previous encounters
with Amplifone HV boards this has indicated that all was fine.

Looking at the 20Khz signal coming from the 555 I had a nice square wave of
about 10v p-p at pin 3 of the 555 and about 0.9v p-p going into Q1. This
seems fine, however the input to the BU406D is not right. I changed Q1
and Q2 but it's still the same. I'm getting an odd square wave ... see
ASCII art below:

____ ________ _______
        | | | |
        | _____| | _____|
        | | | |
        | | | |
         - -

On my scope it seems to be a square wave where most of the negative going
portion is clipped to zero.

At pin 9 of the HVT input (to the MC1)

      / | / | / |
    / | / | / |
/ | / | / |
        | / | / |

        |______/ |______/ |

On my scope it seems to be a sawtooth wave generated when the square wave
is positive

Lastly on 1 of the HVT input (to the BU406D)

  ___ _____________ _____________
        | | | |
        | | | |
         - -

On my scope it seems to be negative going pulses when the malformed square
wave is going negative.

Anyway my question is ... is this normal. I'm sure that I've never seen
wave forms the same as this on previous HV boards and I don't really want
to risk fitting a new Wintron till I'm sure the board is OK.

(let me know if the ASCII art is no good after Emailing and I'll toke some
photo's of the signal my scope is putting out)

Thanks in advance for any help.

Dave Langley
www.robotron-2084.co.uk

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Received on Wed Apr 23 04:53:37 2003