Hardware issue - beam turns off

[flip]

Hi,

 

I've been trying to fix a buzz-model Vectrex with an odd problem: with certain games, the beam seems to turn off. Annoyingly, the first test pattern on the test cart also does this. Other games runs perfectly normal. I've recapped the machine, but that hasn't solved the issue.

 

I am guessing the problem is in the Deflection Detection/Beam Limit circuit but before starting to swap random components, I'm wondering whether anyone has come across this - and how they've solved it?

 

Cheers,

 

Philip

[Rolo]

Not yet. Ahm, I'm wondering, what is really happening: Is the beam turning off, because of a malfunction in the deflection circuitry or

is the machine hanging, due to one of many possible reasons and therefore the deflection detection correctly switches off the beam?

Does this always happen under reconstructible circumstances, right at the same moment or randomly?

[flip]

Thanks for the reply. It's reproducible. When test #3 (linearity test) comes up, the screen goes blank. Oddly enough, some games play fine (Minestorm for example), while Berserk for example seems to experience the same problem. Pressing a button doesn't recover - only switching off or reset does.

 

The main voltages (-5, 5 and -13v) remain stable when the screen goes blank. I'm looking at the 555 and the two potentiometers (R525 and R526) there, but they are hard to reach... I'm pretty sure the problem is on the 'power' board rather than on the logic side...

 

Thanks for thinking along,

 

Philip

[Rolo]

Just a few raw thoughts:

• Problems with the timer would affect the H.V. Is that the case? Is the intensity test of the test cartridge displaying as expected? How about focus?
• If deflection would not work as supposed, I would expect the game to continue in the background: sound should go on and eventually the title screen should come back sometime, since that displays without problems at game start. Why would the logic board stop, if deflection was not right? It did not see any kind of feedback involved. Probably I have to study the schematics more thoroughly.
• There is a beam cut-off test on the test cartridge. Is that working?
• How about a bad ram chip with faulty memory cells, crashing the system under certain conditions? I had this before.
• A partly defective rom chip? Is it a masked type?

Edited August 19, 2015 by Rolo

[flip]

Excellent points - especially the one about the game continuing. I hadn't considered that - there doesn't appear to be a loopback that halts the logic if the beam is cut... I guess the same goes for the timer issues. Problem with the tests is that I can't get past the title screen of the test cart. The first test (listed as #3 in the service manual) fails and I can't get to the next ones...

 

I had replaced a number of chips on the logic board hoping to resolve the issue, including the ROM. That's an EPROM, which tests fine in my programmer. The two ram chips were also replaced with new ones, though I realise that that doesn't rule out they could be faulty... Looks like I'll have to go back to the logic board to see whether anything weird is going on there, possibly in the DAC part...

 

Philip