Apr 302019
 

A friend of mine sent me a couple of Vanguard pcbs which didn’t sync at all on all monitors I tested.

The only difference I saw was that his pcbs were Centuri versions while my pcb which synced good was SNK version.

Centuri versions on the video board have a factory mod with some flying wires which are not documented on the schematics and which are related to the sync circuit.

 

SNK version has no flying wires on the video board:

offcourse undoing the factory mod on the century board fixes the sync problem but I wanted to learn something more.

It seems that Centuri versions were modified to use the positive sync monitor supplied in their cabinets and probably they also improved the sync

signal in comparison to the original circuit.

Anyway , if you want to keep the mod but invert the sync for common monitors or TVs, you have only to solder the wire which goes to pin 3 of the 74LS05 marked in red

to pin 4.

 

Vanguard repair log

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Feb 242018
 

Bought this classic as not tested but luckily it was working except the missile sound was completely missing.

Looking at the schematics I found this

 

 

In the game there are only two missile sounds.

Shot A is your ship missile sound, while Shot B is the one from some enemies

IC37 is an LM3900 op amp which is very unreliable and infact after changing it , the missile sound was restored

Satan Of Saturn repair log

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Jun 282017
 

Got this game cheap for my collection because the seller stated that it has very noisy sound

When powered, the game had a constant white noise while music and the other sound effects were working correctly.

The sound problem was not due to the amplifier or bad caps but it was produced by the logic of the game.

To make a long story short, this game has a similar circuit to what Space Invaders use to produce analog sounds.

It uses a SN76477, which is a programmable sound chip which has a white noise function.

On Space Invaders and in this game, the chip continuosly output the white noise which is switched OFF and ON using operational amplifiers and it is modulated with capacitors and resistors to produce different sounds.

I downloaded the schematics from the US version ( called Zarzon ) where it is shown how the chip is connected to the LM3900 OP AMPS

 

 

Instead of trying to understand which of the two chips are broken, I piggybacked a good one on both chips until

the white noise disappeared.

Problem was fixed by changing the LM3900 Nr.47 on the schematics