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Geoff Harrison 06-04-2008 08:41 PM

Almost ready to power up
I've got the motherboard finished and I'm checking the voltages before I insert the chips and everything's kind of high, but I suppose that's to be expected with no load.


    Should be        What I'm reading
    ---------        ----------------
      +5                +11.13
      +9                +11.90
      -9                  -9.71
    -16                -18.85
    +16                +20.00

The one I'm most concerned about is that 11v reading on the 5v line. How's that ever going to come down to 5v with a 15 ohm resistor across the regulator? Any suggestions before I risk blowing all my chips?

Grant, do you have an unpopulated board laying around that you could check the voltages on?



Geoff Harrison 06-04-2008 11:57 PM

Following up on my own question, I attached a 12v incandescent lamp to the 5v line as a load, and the voltage came down to a much more reasonable level (actually a little below 5, but that's better than 11 :eek: ). I'm going to plow ahead and insert the chips. Wish me luck.


Grant Stockly 06-05-2008 12:53 AM

Geoff is also using normal transformers instead of the switching power supplies until I can ship him a switching supply. Astrodyne canceled my order last April! :mad:

Geoff Harrison 06-05-2008 01:46 AM

I'm up :D

After installing the ICs, I tentatively powered up and the 5v rail was reading 8v. Scary, but it was dropping slowly. I was able to deposit and read values on the front panel, which was encouraging, and after a minute or two the 5v supply was down around 5v but continuing to drop. I turned it off when it dropped down to 3v.

Just for comparison, I powered up my vintage 680 and its 5v supply was rock solid at 5.2. I powered up the replica again and now it too was solidly reading 5.05. Very strange. Any ideas what would have caused that? Capacitors forming up? I've no idea.

Anyway, the main thing is it's up and running.

Thanks Grant for another great kit.


Grant Stockly 06-05-2008 07:25 AM

I wonder if its possible the 7805 was damaged when the power was applied with the bypass resistor and no load?

Can you desolder the large resistor and check the 7805? You should be able to put a 1A load on it without too much of a voltage change.

Grant Stockly 06-05-2008 07:26 AM

I see you are experimenting with VTL!

THAT is amazing code. 768 bytes!

Geoff Harrison 06-05-2008 11:45 AM


Originally Posted by Grant Stockly (Post 1382)
I wonder if its possible the 7805 was damaged when the power was applied with the bypass resistor and no load?

That's an interesting thought, I've read 7805s don't like to see high voltage on their output side.

I unsoldered the resistor and the regulator is putting out a solid 5v now, but I don't know whether its load handling capacity has been damaged in any way. I may swap it out just to be safe.

MITS used that bypass resistor across the regulator on their 4k board too. I don't know much about circuit design, but doesn't that kind of negate the purpose of a regulator? I assume that when the circuit is loaded normally all the current flows through the regulator, and the resistor comes into play only if a heavy load is added (say, a couple of 16k memory boards), but then what ensures that the voltage stays constant?

I guess when you're testing an unloaded board with this kind of power regulation, it's probably a good idea to put some kind of load on it.


Originally Posted by Grant Stockly (Post 1382)
I see you are experimenting with VTL! THAT is amazing code. 768 bytes!

And it runs happily with just 1k of RAM!

I'll put my 16k board in later and try to download basic. It should work, it runs fine on the vintage machine.


rfeecs 06-07-2008 05:41 AM

Since the 7805 can only sink a few mA, it will not be able to regulate without a load because of the 15 ohm resistor. Probably the resistor is added in an attempt to boost the maximum current capability. With almost 12 volts input and 5V output, the load has to sink roughly 450mA before the regulator can function.

As you found, if the load current drops below 450mA or so, the output voltage goes up. Pretty scary regulator design, no doubt to save a dollar or two.

Good thing the 680 doesn't have a low current sleep mode.

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