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Old 4th Oct 2022, 12:30 pm   #1
RichardJones
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Default Questions regarding the MB Vectrex display...

Hello. I have a Vectrex console that wasn't working - with only a white dot on the screen. I have had it repaired to the point where the game logic is working with the CPU replaced; but now there's no display - not even a dot on the screen. The gentleman working on it was not prepared to take it further so I thought I'd ask here.
I've read from other posts that ram ic's can be a cause and I've skimmed through the service manual but have limited experience myself.
Could it be that; with no dot there's actually a display with no data? The CRT filaments are glowing when turned on.
I have access to a multimeter but that's about it.
Any suggestions I would welcome. Thanks,
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Old 4th Oct 2022, 3:02 pm   #2
TonyDuell
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Default Re: Questions regarding the MB Vectrex display...

How do you know that the 'game logic is working'? Do you mean you get the startup tune? If so then the RAM must be good to get that far.

As you know, this is a vector display, so it can't generate the high voltages from the horizontal output stage. The deflection is handled by a dual audio amplifier IC, there is a separate high voltage generator circuit. This uses a transformer somewhat similar to a TV line output transformer ('flyback transformer') driven from a 555 oscillator.

Interestingly (and I did not expect this), the CRT heater is fed from that transformer rather than straight from the 12V DC supply. So for the heater to be glowing that ciecuit must be doing something.

Nothing on the screen suggests one of 3 causes :

1) A CRT electrode voltage is missing. Connect the -ve multimeter lead to ground and measure the voltage with the +ve meter lead on pin 6 or pin 7 of the CRT. The service manual suggests +170V here. Similarly check the voltage on pin 5 of the CRT (should vary between about +50V and -30V depending on the setting of the brightness control) and pin 2 of the CRT (should be a bit less than +50V)

2) The CRT is biased ro cut off -- that is that the cathode is always too +ve wrt ground. Note there is a protection circuit (Q503 and associated parts) designed to blank the screen if the vertical deflection signal is missing -- that is that the beam is not moving. This may have failed, or...

3)... There is a deflection problem, the beam is either deflected off-screen or the protection circuit is operating. A 'scope would be very useful to check the X and Y signals on J402 and the Z-axis (brightness) signal on J506.
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Old 5th Oct 2022, 1:13 am   #3
RichardJones
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Default Re: Questions regarding the MB Vectrex display...

Thank You for a starting point. Yes the game itself is working. The fellow(s) that have worked on it have also said there are lines on the very edge of the screen if the deflection circuit is disconnected. What could be made of this?
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Old 5th Oct 2022, 2:10 pm   #4
SiriusHardware
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Default Re: Questions regarding the MB Vectrex display...

For anyone else following or looking to contribute, the service manual is here:-

https://console5.com/techwiki/images...ice_Manual.pdf

Richard mentioned that he has a meter but nothing much else, so presumably no scope. Any faultfinding process will have to be voltage / resistance measurement based, at least initially.
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Old 5th Oct 2022, 3:01 pm   #5
TonyDuell
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Default Re: Questions regarding the MB Vectrex display...

You still haven't told us what you really mean by 'the game is working'. Has the digital board been tested in another machine? Or is it just that you get the statup tune? Or something else.

I am pretty sure the tune uses the AY-3-8912 sound chip rather than via the DAC. The DAC is of course used to produce the deflection and intensity signals for the screen image. So even if you get the tune, there might be a fault there.

Have you checked the -5V voltate rail at the digital board? I think you can get the tune without it, but obviously no screen image.

I realise you don't have a 'scope, but it's going to very hard to trace this fault if you can't check whether or not signals are present.

And what do you mean by 'the deflection system was disconnected'. What exactly was disconnected?
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Old 5th Oct 2022, 4:02 pm   #6
SiriusHardware
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Default Re: Questions regarding the MB Vectrex display...

Although we ideally would like to be able to look at the X, Y and Z axis signals there are a few things which can be done first - to begin with, check that all the fixed voltages (Power supply voltages, CRT voltages) indicated on the circuit diagram are actually present. However don't try to measure the EHT, the very highest voltage, with an ordinary meter and probes.

Although we can't look at the Y-Axis output waveform directly because there is no scope available, the beam cutoff protection circuit takes a sample of that waveform, turns it into a DC voltage and uses it to keep the protection cutoff transistor Q503 turned on for as long as there is activity on the Y-Axis output.

We should therefore be able to see that DC voltage which is derived from the Y-Axis output if we measure across C410 in the protection circuit. If there is no voltage there, there is probably no significant activity on the Y-Axis output.
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Old 5th Oct 2022, 4:52 pm   #7
TonyDuell
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Default Re: Questions regarding the MB Vectrex display...

The first thing I suggested was to measure the CRT electrode voltages at the CRT pins. As commented, they come from a transformer excited by a 555 astable, and this must be doing something for the CRT heater to be lit.

I think we need to know exactly what the machine is doing and what is known to be working.

I get the feeling, possibly unfairly, that the OP was hoping for 'Oh, that's capacitor Cxxx open'. Fact is, there's not an engineer anywhere, no matter how many Vectrexes (Vectrices?) he's worked on, that can do that. This fault could be in quite a few different sections of the machine. We have to know what voltages/signals are right, which ones are wrong. Then we can go to a lower level, and hopefully end up indentifying the failed component.
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Old 5th Oct 2022, 5:47 pm   #8
SiriusHardware
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Default Re: Questions regarding the MB Vectrex display...

Maybe Richard can let us know what he feels his current level of knowledge is, whether he knows how and when to measure voltage and resistance and feels comfortable reading circuit diagrams and matching the circuit symbols and connections up with the physical components and connections in the unit. Once we know that we will be in a better position to pitch any advice offered at the required level.

We also have to bear in mind that Richard is in Tasmania and we are in the UK which will lead to delays in the exchanges, although of course we welcome any input from others, including those who are geographically closer to Richard.
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Old 6th Oct 2022, 3:17 am   #9
RichardJones
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Default Re: Questions regarding the MB Vectrex display...

Thank You for responses thus far. I have basic experience with a multimeter voltages and resistance and diode testing. The game plays the startup-tune and with the controller the fire button is working and the game plays. I have heard from the guys who worked on it that voltages and continuity are all sound so it is a matter of me getting the multimeter out and following this thread when I'm able to. I have the console5 and service manuals printed out. It might take awhile.
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Old 6th Oct 2022, 9:56 am   #10
SiriusHardware
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Default Re: Questions regarding the MB Vectrex display...

So if we say to you something like 'Measure the voltage across C517' or 'Measure the voltage on pin 5 of the tube connector' or 'Measure the voltage on pin 1 of IC401' you'd be able to find that item on the diagram and then find that component or connection within the unit and make the requested measurement?

If so, you are off to a good start. One of the first rules for working on something which has already been worked on by someone else is to listen to what they say, but then verify everything again yourself. Never take anyone else's word for it that all the voltages are OK, etc.

Do you have anything in the way of spare components, assorted resistors and capacitors and so on? Does your meter have a frequency measurement (Hz) range on it?

Last edited by SiriusHardware; 6th Oct 2022 at 10:14 am.
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Old 6th Oct 2022, 11:06 am   #11
RichardJones
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Default Re: Questions regarding the MB Vectrex display...

Hi. I'm able to do measurements across components reasonably well if I can find the -ve. IDK whether ground or an in-circuit -ve is required usually? I can usually find the symbols on a schematic. I have an Archer Radio Shack reference book for guidance. The multimeter we have is from the family business and not my own so I'll have to borrow it or get one myself. IDK if it does frequency - what would be the symbol on it?
I've done a small number of repairs myself. Re-capped an Atari STE PSU, replaced the Schottky diode in an Amiga PSU, Put a PAL oscillator in a model 1 TRS-80 and some CR2032 RTC conversions in Amigas.
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Old 6th Oct 2022, 11:33 am   #12
SiriusHardware
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Default Re: Questions regarding the MB Vectrex display...

It sounds as though you are reasonably comfortable with small unsoldering and soldering jobs, so that's all good. Do you know how the pins on ICs/Chips are numbered?

A meter having the ability to measure frequency (which would usually be designated by a 'Hz' position on the meter dial) isn't vital, but it can sometimes be useful for checking whether repetitive signals / frequencies are present or absent when you don't have a scope.

If in doubt, post a good photo of the dial on your meter so we can see what it can and can't do and that will help us to be more specific when we ask you to make measurements with it.

I've just been on a brief voyage around the circuit diagram and it looks as though GND and circuit 0V are connected together, but that will be easy to verify.

Let us know when you are clear for take-off. (But bear in mind the time difference, so this is going to be a bit like playing chess by mail).
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Old 6th Oct 2022, 12:03 pm   #13
RichardJones
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Default Re: Questions regarding the MB Vectrex display...

Thanks. I have seen online how IC's are numbered as I had to do a bodge wire on the TRS-80 motherboard to change a line from pin 9 of an ic to pin 8 of the same ic and also cutting a trace. That changed the multiplier from 11 to 13 to give 50hz in total. This is how the factory produced PAL machines for Australia I've read. I will have to do a refresher as my memory isn't the best. I might be able to use the multimeter this weekend but I will let the group know when I'm ready Thanks for the advice.
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Old 6th Oct 2022, 12:10 pm   #14
TonyDuell
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Default Re: Questions regarding the MB Vectrex display...

I seem to remember a metal earth braid soldered to the logic board at one end and the power board at the other. You can use this as the ground. Connect the -ve lead of your meter there for all measurments unless one of us says otherwise.
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Old 6th Oct 2022, 12:15 pm   #15
RichardJones
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Default Re: Questions regarding the MB Vectrex display...

Thanks Tony. I'll do that. You've reminded me of the earth strap I installed for a Delco-Bose stereo in a Corvette I owned years ago. Quite monsterous but eliminated the alternator whine at the time.
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Old 7th Oct 2022, 12:15 pm   #16
RichardJones
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Default Re: Questions regarding the MB Vectrex display...

As the multimeter I had access to isn't available atm I've ordered one with Hz frequency capability. Hopefully will be here in a week. I'll get back in touch with the group then. Thanks for the input.
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Old 7th Oct 2022, 3:23 pm   #17
SiriusHardware
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Default Re: Questions regarding the MB Vectrex display...

It seems as though you do enough tinkering with retro tech to justify owning a meter, and to be honest this was likely to be an even slower process if you only had access to a meter which you could only borrow at weekends, so I'm glad you've decided to acquire one yourself.
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