+5V power supply help

Level42

New member
Joined
Dec 22, 2006
Messages
5,895
Reaction score
49
Location
Ridderkerk, ZH, Netherlands
I am trying to run an original Atari Asteroids PCB in my Asterock cab.

Now, it runs, but only for a short while after I turn it on. I measured the +5V and sure enough it was very low (about 3,7V, I was amazed that it ran at all for a while). When things start heating up after let's say a minute or so, the board obviously crashes).

Now, I tried to up the +5V adjusting the pot but I can only reach +4V.

Note that the power supply works fine at +5.1 Volts with the original Asterock PCB. However, the Asterock PCB uses different type of EPROMs which require +12V and -5V which are supplied separatly.

So, the larger "power hunger" of the Asteroids probably lies in the extra power the EPROMs take.

Anyway....I wonder why the power supply can't get up any more after I have fully turned up the +5V pot.

The Asterock machine uses a similur set-up as Atari (power-brick, and a neat "AR-2" board but the +5V regulation is different. It uses an LM723 as regulator.

Here's the schematic:
web.jpg


Note that the sense inputs are NOT used.

I was thinking maybe the regulation is (too) limited because of the used resistor values around the pot ?
 
Last edited:
do some simple tests:
turn off game. disconnect the game board from the power supply.
1. turn on and measure the 5v rail

2. what is the voltages at pin 11, pin 12 of the lm723 regulator ?
both need to be above 9.5volts, max 40v

3. what is the voltage at pin 6 of the lm723 regulator ?
somewhere around 7.1 volts ? typical range is 6.95 to 7.35

4. what is the voltages at pin 2 (around 5v ?), pin 13 (around 7.9v ?) of the lm723 regulator ?
5. and pin 10 may need to be around 6.7v


may be as simple as replacing the lm723 and the 2n3055 (both should be cheap, easy to get).
 
Last edited:
ALL measurements done unloaded (PCB disconnected) and did not change the +5V pot (so still at "full" power)


1. turn on and measure the 5v rail
9.0V

2. what is the voltages at pin 11 =>14.9V
, pin 12=> 14.5

3. what is the voltage at pin 6 of the lm723 regulator ?=> 6.97V


4. what is the voltages at pin 2 (around 5v ?)=> 9.11V
pin 13 (around 7.9v ?)=> 8.44V

5. and pin 10 may need to be around 6.7v => 9.06V
 
Mmmm, the weird thing is that the voltage is low when I turn the pot fully to the left. Then when I turn it right it goes up (a bit) until it reaches about 4.2 V (unloaded) and when I keep turning right it goes DOWN again...
 
i re-read your first post, and have been comparing the schematic against the defender one (that shows voltages at various points around the LM723.

to save your asteroids game board, you could make up a dummy load to connect to the 5v output using a combination of automotive headlight globes. doing a quick calculation, a 55watt 12v bulb should draw 2.6amps at 5v, so two in parallel should make a nice 5.2A dummy load. hot hot hot!

if 5v regulation is still good with one bulb, but drops with two bulbs, monitor the voltage of the 14v supply to the 2n3055 collector - does that drop too or is it just the output that drops ?

so far the best i can come up with is to start the process of rebuilding the power supply with the semiconductors. the lm723 is central to the functiong of this power supply as it controls the 2n3055, so i would first replace the lm723 and recheck the circuit regulation, then the bd736 transistor, diode D10 and the 2n3055.

you may need to lift the legs on the resistors and check they are still within spec as per the schematic - replace if not within 5%
 
Thanks for the help !

Just wanted to add that I checked all caps (electrolytic one's) with an ESR/Capacitance meter and they all show great.

I'll see if I have an LM724 around. I do still have a 2N3055 I believe...
 
Back
Top Bottom