Atari System1 R20 Burning

Michael Roma

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Atari System1 ARIII R20 Burning

R20 (2.2ohms, 1watt) burned up on my ARIII board so I ordered some new ones and replaced it today. I powered the game on will all boards connected and noticed a burning smell again. I powered it down and pulled the ARIII again. R20 looks a little discolored but measures 2.1ohms across it.

I disconnected the main system board and powered the game back up. Resistor not burning, no smell. I left the game running for approx. 5 minutes while I tested voltage on the ARIII.

I connected the system board (without a cartridge) to the ARIII and immediately started smelling that burning smell and noticed that the solder on one of the R20 legs was heating up and getting soft. I unplugged the game and reinstalled another 2.2ohm, 1watt resistor.

Tested voltage again with only the ARIII connected...

5v was around 2.1v on the top side of resistor and 5v on the bottom (see pic)
10.3v was around 5v
15v was good
22v was 24.1v
-15v was good
-22 was -24.5v

ARIII_Voltages.jpg


Where should I start looking for the root of the problem... The ARIII or system board??? Thanks

- Mike
 
Last edited:
Tested voltage again with only the ARIII connected...

5v was around 2.1v on the top side of resistor and 5v on the bottom (see pic)
10.3v was around 5v
15v was good
22v was 24.1v
-15v was good
-22 was -24.5v

Where should I start looking for the root of the problem... The ARIII or system board??? Thanks

- Mike

For starters, the +22 and -22 volt supplies are fine. These are unregulated voltages and the 22V is only a typical value.

Still trying to find what I did with the assembly drawing for that board and don't know the resistor numbers for the two on the left side that you pointed out.
Are R26 and R27 getting real hot?

Ed
 
R26 and R27 are cool to the touch with only the ARIII connected and running.

Also, the two resistors in the upper left are R28 (bottom) and R29 (top). I can measure +5v across both legs of R28 but only +5v on the lower leg of R29 and +2.1v on the upper leg.

- Mike
 
R26 and R27 are cool to the touch with only the ARIII connected and running.

Also, the two resistors in the upper left are R28 (bottom) and R29 (top). I can measure +5v across both legs of R28 but only +5v on the lower leg of R29 and +2.1v on the upper leg.

- Mike

OK, looking at this in an analytical way:
Resistors R28, 29, 30, 31 and 32 are tied serially between +5V and ground.
Adding them in series - I come up with about 12400 ohms. The current running thru these in series = 403uA (ignoring the tiny current on the Voltage sense pin).

Based on this current - you should see the following voltage drops across the resistors:
R28 => 0.001V
R29 => 3.023V
R30 => 0.403V
R31 => 1.572V
R32 => 0.001V

And, based on these calculated voltages - you should see about 4.999V on lower leg of R29 and (0.403 + 1.572 + 0.001) or 1.98V on upper leg of R29. The resistors are have a tolerance of 5% (or worse) so the 2.1V you see should be fine.

So, the voltages you measure are fine yet R20 is toasting.
Since you have +5V on the output - your Q7 / Q8 circuit is funtioning but not correctly. Q7 and Q8 are tied in parallel to provide your +5V. It sounds like one of the two transistors has a problem. 2N3055 transistors are relatively cheap - replace them as a pair and be done with it.

What voltages do you get on either side of R20, R23 and R25?

Ed
 
Also,
Those 3300uF caps look like they're originals. Since you have the board out for repairs - I would recommend this as a good time to replace the electrolytics on that board.

Ed
 
OK, I didn't read so well.

You said R20 only burns when the main board is plugged in and NOT when the main board is disconnected.

What does the 5V line on the ARIII board do when the main board is connected?
Measure both sides of R26 with the main board connected.
Might still be a bad Q7 or Q8 on regulator board and doesn't rear it's ugly head until loaded.

Ed
 
Atari System1 ARIII R20 Burning

I replaced both 2N3055s and get the same thing... seems to be stable when only the ARIII is connected then R20 starts smoking once I plug the system board it.

- Mike
 
R20 burn

Just a silly question... With your system board unplugged, did you measure across the 5Volt and ground to see if it is shorted and killing your ARIII?
 
Re: R20 burning

This troubleshooting long-distance stuff is tough!
OK. Let me get this straight. You said that you read +5 Volts on your 10.3 Volt (10.6 Volt) line with only the ARIII connected. That means that either you are not getting a high enough voltage at your 10.3 Volt input or that something is severely shorted loading it down that far. The only thing I can think is that the Bridge rectifier or the filter Cap is bad and producing too little voltage at the 10.3 Volt line. This would cause Q7 and Q8 to be on full (ie saturated) trying to pull your +5 Volt line up to par when loaded. (ie when the main board is plugged in) Then you would be dropping 10 Volts across R20, Q7 B-E, Q8 B-E, R23 and R25 causing about 3 Amps current flow and about 30 watts of power dissipation, mostly through R20 and Q7/Q8.
long story short... Pardon the pun... Check Your 10.3 volts with everything unplugged. If it is not at least 9.5 Volts then I would check the wiring to the Bridge rectifier and the Big filter cap. If the wiring is good replace the Bridge and the Cap and see if you get your 10.3 volts back.

KMB
 
Well I finally had some time to revisit this and pulled the xformer out. First off I noticed that someone hacked the wire going to one of the fuses and the fuse holder itself was a little loose.

Also, Is there a way to test that bridge rectifier is good? How about the filter cap?

- Mike

Atari_sys1_power_top.jpg


Atari_sys1_power_bottom.jpg


atari_sys1_power_hack.jpg
 
Well I got the game working now... not sure exactly what the problem was.

I replaced the bridge rectifier, one of the fuse holders, that hacked wire on the transformer and loosened/tightened the connections on the big blue in the cab. When I plugged the ARIII in after that I was getting approx. 14v at my 10.3v test point and all other test points were good. I then power it down, plugged the system board in, applied power, and R20 was stable. I powered it down again, hooked the cartridge up and viola... Indy is back!!!

I am hesitant to say that it was the bridge rectifier as the one I pulled tested good according to this (http://newsgroups.derkeiler.com/Archive/Rec/rec.games.pinball/2005-12/msg12372.html). Are those instructions correct???

Regardless, thanks to everyone for the suggestions... it is due to you that the game is now working!!!

- Mike
 
R20 Woes

Glad you got it going.
(Nice hack job on the transformer wiring. What some people won't do.)
Good find on that stuff. You can see that the quick connector had been regularly overheating. It was probably a combination of the bad fuse holder and various connections. You Rule!

As for actually testing these things: If you have a meter that can read cpapcitance (and that high a value) then disconnect at least one side of the cap and use it across the terminals. (observe polarity) This is a pretty good indicator that it is good/bad. If you have an ESR meter that is better. (and more expensive)
The diode check function, (indicated by a little diode symbol usually on the low Ohms scale somewhere) can be used to read the diode forward voltages for each of the 4 diodes in a bridge. They should all be the same reading, within a fairly small tolerance... otherwise it is bad. If you read them backward it will have a really high reading... just swap probes. The schematic probably shows which terminals are which.

Keep on gaming, Kenneth
 
I am reviving this bc it may help get my Indy back up and running.

Mike were you getting anything on the screen when R20 was initially damaged? Mine displays garbage.
 
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