Tempest Vector Analog Help

crayzkirk

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I have a couple of spare Tempest boards I am working on and when I hook my oscilloscope to the X/Y loops I get a real mess. Not readable and not recognizable. The boards will run in test mode however I cannot tell what it is reporting. The X output is stuck at +10v on one and +4v on the other. I traced the circuit back to the DACs however the input to the OpAmps are basically zero. Is there a way to scope the output from the DAC? I get no reading with a DIMM or scope on the input to the OpAmps for X/Y from the DAC however the outputs look a lot like the individual channels. It looks like the output is bad at the first stage where the DAC is sampled and held in the capacitor. I have checked the inputs to the DAC and none appear to be dead or stuck using a logic probe.

Given that the DACs are not cheap and I don't want to start just shotgunning the board hoping I will find the right chip. I want to understand what is going on and how to fix it. The X BIP does nothing however the Y BIP does causes the output to change in the Y direction. Which chips are most likely to fail? DAC, OpAmp, switch or multiplier? Let's not even mention the silicon VDR which seems to be impossible to find.

I have managed to fix one board set however it had physical damage to the R/W line from the CPU. It was fairly simple to trace back from the LS244 to the CPU with a logic probe.

Thanks in advance,

Kirk S.
 
I have a couple of spare Tempest boards I am working on and when I hook my oscilloscope to the X/Y loops I get a real mess.
Ordinarily at this point I'd want to hookup a Fluke 9010 to check the ram, check the rom signatures, and verify that the CPU is actually running game code properly. All too often with that description I'd expect to see some ram/rom error causing it to run "so far" until some failure occurs with a ram/rom then the cpu is resetting.

Sure it could be an analog problem but that's not the assumption I'd jump to. All of the chips that you mention are possible to fail but you should find some way (sounds, etc) to prove to yourself first that the game code is running properly. Once you know that it's not just resetting itself then you might be better isolated to an analog-section problem.
 
Thanks... One board is not resetting, pin 40 is solid on the CPU. The other will reset once it gets out of test mode so I need to figure out that one (I swapped the Pokey chips around and it acted different so that is a possibility). I have a spare harness, control panel, transformer and AR-II which I am using as a test bed. The game will run and not reset, the output is just garbage. I have pulled and verified all EPROMs against their known images. The player 1 start led is bad so I just put one in the wire connectors and there are no blinks indicating RAM failure. I can ground the slam switch and the screen will change.

I bought a fluke 9010A however it does not have the RS232 option. And I found out how much people want for a 6502 pod. Yikes! Besides, that seems almost like cheating...

Can the output of the DACs be measured with a scope? If so, how?

Update: checked each output with the scope and could see how changing Y BIP would cause the signal to look similar to X BIP when turned all the way down. X BIP would do nothing at all. Visual inspection shows X BIP pot lead broken off. Could feel break with fingernail Pushing lead on brings screen back correctly. Will replace and add to spare parts pile. Passes all other tests although needs to be checked with a real monitor.

The real question is: why am I not checking the common stuff and jumping to the "it must be the biggest/most expensive chip" on the board conclusion?
 
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Because we all do that. Had a board go wonky and it was a bent pin on one of the ICs on the underside of the board hitting the ground line. Channelmanic found that. Felt like an idiot.
 
Here's a list I try to follow.

Visual inspect, replace broken pots
Voltages
Fluke Rom/Ram
Then dig into the analog section..

It's like the old saying, is it plugged in? Is it turned on?

Always start with the simple :)
 
The DAC outputs "current", and a scope measures voltage. The first opamp basically converts the current to voltage.

The board thats resetting outside of test mode probably has a bad rom.
 
Thanks. Voltage vs current makes sense. I read the writeup on the life of vector generators (again) so I get a little of it. I was digging into the other board and found that when I flexed it, the output changed. R146 cracked, will replace although it still has problems running the game. I will use a known good aux board so I can eliminate that as a cause of problems.

It looks like the adjustment pots can give a fair amount of information as to the functioning of the different parts of the analog section; it is just going to take some time to understand what it is saying.
 
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