Shout Out and Repair Log for I,Robot Switching Power Supply

  Hi Everyone...

I spent some time this past week troubleshooting and repairing an
I,Robot Switching Power Supply (Atari # A038074-07 and -17) that was
graciously given to me by Rob (spy.hunter@verizon.net).... Thank you Rob!!!

I figured that since these beasts arent' very common, that I would at
least document the solution. It took me a bit to get to the problem and
I can see how this might be a common problem with all of these supplies too.

This PS has Serial Number UR628 and since it was from Rob, it is not
original to this cabinet.

I set this up on the bench. Attached the 120V supply and turned it on.
Didn't hear anything, no squeal, no clicking. Measured the +5V output
and had 0V. Measured the 145V output and had 0V. Looked like every
output was dead. Since Atari took the time to create TM-261 which is the
Switching Power Supply Service manual, I sat down and did a quick read.
It is a very nice book, especially for 1983 which at the time, Switching
Power Supplies were just coming into commonality. The service manual
gives a great breakdown of the entire supply, a theory of operation for
each section, full schematics and some waveform data for troubleshooting.

The first thing I noticed was that there were lots of cold solder joints
lurking around, at least a dozen, I was hoping that this would be the
problem at hand. I soldered them back up nicely and re-tested, still
dead. Bummer. Time to move on.

The supply functions by rectifying the AC120 to a DC 170 which feeds the
switcher transistors into the primary of the high frequency transformer.
The 170VDC supply was fine and solid. In order for the switching
transistors to run, the 3525 PWM Controller must drive them, this IC
needs a 12V supply to function. Looking at the schems, this supply is
derived from a smaller 60Hz transformer (T1), bridge recifier (CR21-24)
and 7812 (VR1). A quick probe of the output of the bridge showed 0V here
too. That is no good, nothing will work without the 12V.

Measuring the transformer output directly, I did not have the 9VAC that
I expected on the output of the small transformer. I took a look at the
primary winding and measured 120V going in, nothing out. Well, that is
not great. An open transformer, sorta sucks since this is a custom part,
I figured I would pull it out, check the continuity and also figure out
if I could get some other 'wall wart' transformer mounted on there
somehow. Upon desoldering, I found that one of the pins was broken off
right at the transformer so the pin was soldered into the PCB nicely but
this was the issue with the secondary being open. I fashioned a new pin
using some heavy duty resistor lead and had a new solid secondary
connection. I resoldered the transformer in and retested. I had some
startup noise however, it was unfortunately not just startup noise, it
was fairly constant. Sort of a one per second click. This is a common
symptom of a switching power supply that has a problem and as a result,
won't stay oscillating due to a shutdown condition. I started measuring
resistance from the various output voltages to ground, just to make sure
I didn't have a freaky short or something. Nothing odd there. I starting
perusing the manual once more and on my way to the theory section I
found this little nugget on page 12 in the 'precautions' section...

    * The 5-Volt output must have at least a 1-ampere load for proper
      operation. If operated under no load, the supply will generate a
      series of very short pulses.

Hmmm, that sounds familiar. I didn't want to throw a known-good PCB onto
this supply yet and have it fry my board, so I started hunting for a
resistor.

5V @ 1A = 5Ohm Resistor (also 5 Watts, don't want to try this with a
little 1/8 Watt guy)

Found a perfect 4.7Ohm 5 Watt resistor in my spares. Hooked it up across
the 5VDC output and fired it up again. No clicking this time. DC voltage
check.... +5.1V, veddy nice. I followed by checking the other voltage
outputs, also looking good. So, I think this one is fixed.

It would have been quite difficult to determine that the pin was broken
on the transformer without taking it off the board. I think in some
cases someone might have thought the transformer was open (like me) and
thought it was a more complex repair trying to find the custom part.

I haven't thrown this onto my I,Robot yet. I will test it out on a
different PCB first just to be sure, something more replaceable like an
Asteroids PCB rather than I,Robot just in case it spikes (bad I,Robot
joke) or something.

Also, I promised Rob I would post some pics since this happens to be an
I,Robot cabaret of all things. It is an Asteroids Deluxe style cabaret
cabinet but it has a 19" monitor in it and the PCB's are quite crammed
in there. It has a harness for the I,Robot PCB and Switcher. The monitor
is a G07 and the control panel is flat with basics, joystick, fire on
right and left and start buttons. I also have a PCB set with a video PCB
labeled 'Ice World' which has no solder mask. The CPU PCB is a standard
I,Robot labeled PCB but it has hand written ROM labels. This PCB is
something I got from Duncan quite a few years ago and it was not
originally with the cabinet.

either way, I will get some pics here later this week if I have a chance.

---------------------------------------------------------------------------
** Unsubscribe, subscribe, or view the archives at http://www.vectorlist.org
** Please direct other questions, comments, or problems to chris@westnet.com
Received on Sun Nov 7 02:21:56 2010