11 replies to this topic

[Rybags]

Looks like I'll be swapping one of my 7800s and the only power wart I have for the system for a 1084 monitor.

So, I'll be looking to make alternative power arrangements for when I want to use the remaining ones.

The thought I had - rather than replace the plug and use another 9V supply, can we just tap in somewhere and supply +5 DC to the machine?
Or is the 9V needed for an alternate use somewhere else in the machine?

What I'd probably use if this is feasible is a USB power wart that I bought - USB plug at one end and some other arrangement at the 7800 side.

[GroovyBee]

I see two problems :-

1) The power on/off toggle switch circuit creates its own unregulated supply from the 9v using a resistor and a Zener diode.

2) The 9v is also dropped to around 6.8v at one end of a potentiometer that controls the colour delay line on both MARIA and TIA.

Measure the voltage on the DEL inputs on MARIA and TIA. If its less than 5v they can be replaced with two fixed resistors forming a potential divider (maybe a trim pot too).

[Rybags]

OK. The alternate is to just aquire a plug end from someone and mate that to a different 9V DC wall-wart.
Might be the easier way to go about it.

Thanks for that.

[GroovyBee]

The power on/off could be bypassed with a switch in the power cable (to save wear and tear on the power connector) if you fed the regulated 5v directly into where the 7805 regulator is located (after removing it). The PCB mounted 7800 power connector could be replaced with a suitable barrel type. That would immediately give you a wider choice of suitable off the shelf supplies. The key to getting rid of the 9v requirement is the voltages seen on the colour delay inputs on the video chips. If they are 5v or less you can work around everything else.

[SlowCoder]

May I ask why you wouldn't want to go the route of an adapted 9v connection? That rout would be the easiest path, only requiring soldering to 2 points. You wouldn't even need to remove the old plug, and if you ever got a new 7800 wart, you could plug it right in.

[SlowCoder]

GroovyBee, on Mon Jul 4, 2011 1:51 AM, said:

I see two problems :-

1) The power on/off toggle switch circuit creates its own unregulated supply from the 9v using a resistor and a Zener diode.

2) The 9v is also dropped to around 6.8v at one end of a potentiometer that controls the colour delay line on both MARIA and TIA.

Measure the voltage on the DEL inputs on MARIA and TIA. If its less than 5v they can be replaced with two fixed resistors forming a potential divider (maybe a trim pot too).

Nice show of knowledge.

[GroovyBee]

That should be 6.4v (its two diode drops). I blame the lack of my morning tea at the time of posting .

[Rybags]

Yes, I think I'll go the simple route - either add an alternate socket or make a plug adaptor.

I guess the power plug used is non-standard and impossible to obtain new - suppose I'll just have to put a wanted thread up.

Or maybe the pitch/size is near enough to something in use today and I could hack something up.

[GroovyBee]

I think you'd be better off with a round barrel type. Remember... Araldite is your friend if it doesn't quite fit .

[Classic Pac]

Don't forget you are going to need at least 9 VDC and 1 amp, in my experience if the 7800 doesn't get at least 1 amp it plain just won't stay on.

[zylon]

I modded one for someone to use a Genesis power cord. That was over 10 years ago and they still have it and it is still working.

[toiletunes]

Rybags, on Mon Jul 4, 2011 7:19 AM, said:

I guess the power plug used is non-standard and impossible to obtain new - suppose I'll just have to put a wanted thread up.

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