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Old 9th Sep 2016, 5:58 pm   #1
acollins22
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Default Vectrex repair

Hello Folks,

This recent thread has prompted me to dig out my Vectrex and have a go at fixing it.

The story so far is that I bought it about six months ago as not working but not investigated. I took it to an old school TV repair shop for them to have a look at and a skeleton pot was replaced on the power board but they couldn't get further than that. My suspicion is that they didn't really want to.

So I've now got it back and I'm trying to fix it.

I know how to use a 'scope, a DMV, soldering iron etc. but I don't know how to go about the repair so any advice as to the order of play would be gratefully received.

The symptoms are...

With power on I see nothing on the screen, there is no power on beep (if there should be one) and no "high level" clues.

However, there is faint hiss from the speaker and crackles when I turn the volume knob. The tube end glows and I have -5V, +5V and -13V on J204 of the logic board.

What would be the best thing to check next?


Cheers,

Andy.
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Old 9th Sep 2016, 6:17 pm   #2
TonyDuell
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Default Re: Vectrex repair

OK, the power supply OK, the EHT oscillator is doing something. The fault is most likely on the digital PCB.

Check that Rst/ (pin 39 of the 68A09?) is high. Check for a square wave of about 1.5MHz on the E clock line (pin 34 of the 68A09). Those indicate that the CPU is running.

If you like lucky dip servicing/stock faults, replace the 2 RAMs IC204 and IC205 with known-good ones. They're 2114s which are probably the most unreliable IC ever.
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Old 9th Sep 2016, 8:51 pm   #3
acollins22
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Default Re: Vectrex repair

Thank you for that Tony.

Pin 34 has 1.5mhz on it as you suggest.
Pin 39 looked odd. but I looked at the schematic and RST is on 37.
Pin37 is Low...

There is a pull-up resistor (R201 22k) to 5V so I think that the RST line must be being pulled low by something. IC207, the 6522 shares the RST line and I had a PET a while ago where a faulty 6522 pulled the RST. I had completely forgotten about that whole episode until now (funny how we forget stuff). The other possibility I can see is C224 (0.01uf). If that has gone short circuit, that could explain it I suppose.

I swapped out the 6522 with one from the spares box but no improvement. :-(

Am I thinking along the right lines here?

More investigation needed...


Thanks,

Andy.
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Old 9th Sep 2016, 9:41 pm   #4
TonyDuell
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Default Re: Vectrex repair

Sorry about the pin number for RST/, the schematic in the manual I downloaded has a couple of unclear parts.

The reset line seems to go to the 68A09, the 6522, the AY-3-8912 (which is much the same as the more common AY-3-8910 sound chip but with only 1 8bit I/O port). It has a pull-up as you mention. There is also an electrolytic capacitor (Looks to be C228, 22uF) and of course the reset button (via R208), and some 0.01uF capacitors sprinked around that I doubt are the problem.

My first suspects would be the pushbutton switch (is it stuck pushed in?) and that 22uF capacitor.
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Old 10th Sep 2016, 10:33 am   #5
acollins22
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Default Re: Vectrex repair

Thanks Tony.

I have made some more progress...

I went to the unit this morning to have another look and plugged it in as usual. RST/ still low.

I pushed the Reset button a few times and the sound of the machine changed. I don't mean intentional sound from a running program but the general hash I hear through the speaker.

I pressed it again and the MindStorm screen came up!

I put the original 6522 back in and the machine won't start regardless of how many times I press the button or how long I wait and so I conclude that it has failed but it's not the only thing.

In order to get the game to run I have to power the machine on. Leave it for about 10 seconds. Press the reset button. Tadaa!

So it seems that the reset circuit is still not right and I'll get on to that later, and check the capacitor when I get a bit more time.

As a side note. The power switch on the front is faulty in a curious way. Using it to turn off the machine just mutes the sound and reduces the screen brightness. It doesn't kill the power :-(

Side note 2. There are all kinds of hash coming out of the speaker and also the screen has a bit of a wobbly. I think that points to dried out capacitors.


Thanks again for the help. It's great to see the thing starting to come back to life.


Cheers,

Andy.
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Old 10th Sep 2016, 7:08 pm   #6
Outrun_uk
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Default Re: Vectrex repair

Noisy speaker could be poor grounding, Vectrex are noted for this!
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Old 10th Sep 2016, 8:09 pm   #7
cmjones01
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Default Re: Vectrex repair

Does the reset circuit involve an electrolytic capacitor connected across the reset switch? For some reason they are a very common failure point on arcade game PCBs, causing exactly the symptoms you describe - occasionally booting, but not usually. They go leaky or short circuit and replacing them always cures it.

Chris
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Old 10th Sep 2016, 11:01 pm   #8
TonyDuell
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Default Re: Vectrex repair

Yes, there's C228 (22uF electrolytic) across the reset switch. I would suggest checking it.

The power switch (on the back of the volume control) switches the transformer secondary to rectifier connections. Maybe it has failed. That should not be hard to sort out.
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Old 11th Sep 2016, 6:01 pm   #9
acollins22
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Default Re: Vectrex repair

There have been a few developments.

The switch had been wired to only switch one phase. It's not at all clear why as the second side seems to work fine. I've corrected that and the unit switched on and off correctly now.

I've replaced the electrolytic caps on the logic board but the reset problem is persisting. It takes about 30 seconds before I can press the reset button and get a response.

More investigations to come.


Cheers,

Andy.
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Old 11th Sep 2016, 6:19 pm   #10
TonyDuell
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Default Re: Vectrex repair

When the machine fails to respond to the reset button, does the RST/ line behave correctly, or is it stuck low? If the latter it may be one of the 3 ICs connected to it that is misbehaving.

Is it a thermal problem? When the machine is working, have you tried giving the ICs on the digital board a shot of freezer spray to see if you can get the problem to come back?
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Old 14th Sep 2016, 7:04 pm   #11
acollins22
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Default Re: Vectrex repair

It's been a few days but I've had another look.

The reset button works now and the RST/ pin is being held high until the reset button is pressed when it goes low until the button is released. It then floats high.

I'll try see what else I can find.
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