atari a/r hum: bad diode?

mc300baud

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ok, so i finally tracked this hum down to the a/r board in my Asteroids. i rebuilt it with a kit from Bob to help bulletproof it when i sell the machine. it didn't hum before i did the rebuild but now it does. i can swap in another a/r-01 (one that i haven't rebuilt) and the hum goes away.

the +5 on my humming a/r i can dial in right at 5v but the one that doesn't hum i can't get lower than 5.5v so i'm assuming it probably needs a rebuild as well.

looking over the humming a/r i found this:

ar_diode.jpg


the diode at CR2 looks to have little metal bubbles coming out one end and maybe the side. it also tests with almost no resistance unlike the one on my non-humming a/r. i'm assuming it's probably blown; would this be a likely source of hum? is there anything else i should be looking for that might be causing the hum? i've already checked over all my work and it looks good with no stray solder accidentally connecting traces.

my local electronics shop closed up a few months ago so now i have to go a ways out of my way to pick up parts but if ya'all think this is the likely culprit then i'll do it. i know i could order one from Bob but i don't have anything else i need at the moment and it would seem to be pretty expensive when i add on the shipping.
 
That's CR2 - 1N4001 and it's between +5 volt return and the - sense so it should have nothing to do with the audio hum. It wont hurt to change it as it's a common diode and Radio Shack should stock them 1N4001 | Catalog #: 276-1101 I fixed my hum by replacing the 2 audio amp chips TDA 2002 .It's easy to test, I unplugged the edge connector from my asteroids board, also unplugged the monitor, turned the volume pot all the way up then powered up the game. With power to the A/R board find the 2 pins on the harness edge connector for the sound inputs 1 and 2 and take a bare wire in hand and touch to input 1. you should hear a loud audio hum like the input of a guitar amp. Then touch audio input 2 and you should hear the same sound. In my case one was loud and one was not. One of the Amps was bad so I replaced both. The audio amps are set up in a push pull configuration and if one amp is bad this causes an imbalance which induces the hum. That's what worked for me.

 
Great video! Replacing the audio amps along with a rebuild kit always seems to do the trick for me as well. Look for TDA2003's - these are easier to find and are a drop-in replacement for the TDA2002's.
 
One thing I noticed when I fixed one that was humming, was one of the nylon screws holding the amp to the heatsink was badly yellowed (from it overheating). If you see any that are - chances are thats the culprit.
 
Cool vid! Thanks! Both my upright and cocktail Asteroids machines have a hum so I may have to shotgun replace these amps to see what happens.
 
One thing I noticed when I fixed one that was humming, was one of the nylon screws holding the amp to the heatsink was badly yellowed (from it overheating). If you see any that are - chances are thats the culprit.

Probably should be metallized screws. The only ones that need to be isolated are the TIP32 (+5V reg) and the 7905 (-5V).
 
Probably should be metallized screws. The only ones that need to be isolated are the TIP32 (+5V reg) and the 7905 (-5V).


I have noticed when rebuilding ARII boards that many of the obviously previously worked on boards have nylon screws on all components, as well as insulators. I usually remove the nylon and insulator restoring the board back to an 'as built' condition. The boards have always worked fine for me after these rebuilds.
 
With power to the A/R board find the 2 pins on the harness edge connector for the sound inputs 1 and 2 and take a bare wire in hand and touch to input 1. you should hear a loud audio hum like the input of a guitar amp.

ok, so i just tried this with both of the a/r's that i have, the one that hums and the one that doesn't. i get the same result that you originally got with both boards, one speaker line (i think 1) gives me a lot more hum than the other.

looks like i might be able to make a run by Alphatronics today and if i can i'll pick up the diode as well as a couple of TDA2002's or equivalents and see if that makes a difference.
 
That's CR2 - 1N4001 and it's between +5 volt return and the - sense so it should have nothing to do with the audio hum. It wont hurt to change it as it's a common diode and Radio Shack should stock them 1N4001 | Catalog #: 276-1101 I fixed my hum by replacing the 2 audio amp chips TDA 2002 .It's easy to test, I unplugged the edge connector from my asteroids board, also unplugged the monitor, turned the volume pot all the way up then powered up the game. With power to the A/R board find the 2 pins on the harness edge connector for the sound inputs 1 and 2 and take a bare wire in hand and touch to input 1. you should hear a loud audio hum like the input of a guitar amp. Then touch audio input 2 and you should hear the same sound. In my case one was loud and one was not. One of the Amps was bad so I replaced both. The audio amps are set up in a push pull configuration and if one amp is bad this causes an imbalance which induces the hum. That's what worked for me.


Nice job Sam
 
ok, so i just tried this with both of the a/r's that i have, the one that hums and the one that doesn't. i get the same result that you originally got with both boards, one speaker line (i think 1) gives me a lot more hum than the other.

looks like i might be able to make a run by Alphatronics today and if i can i'll pick up the diode as well as a couple of TDA2002's or equivalents and see if that makes a difference.

Let us know what your results are after you replace the amps.
After I replaced mine I tested the inputs and got the same loudness on both inputs so I knew right off that was the problem. Their's no telling how many Asteroids could use this fix because their are many post's about this hum problem.
 
ok, so i replaced the diode and fixed one bad solder joint on the resistor at R1. i figured at this point i'd plug it in and see if that made a difference and low-and-behold the hum is gone. unfortunately i don't know which of the two fixed it (should've done them one at a time to find out.)

there's still a tiny bit more noise than in my other a/r but no 60-cycle and it's more than acceptable. turned down to a reasonable level or in a gameroom with other machines and it'd be hardly noticeable.

no need to pull the TDA2002's, though now i've got a couple in reserve which probably can't hurt considering how many Atari machines with a/r's i've had over the last few years.
 
ok, so i replaced the diode and fixed one bad solder joint on the resistor at R1. i figured at this point i'd plug it in and see if that made a difference and low-and-behold the hum is gone. unfortunately i don't know which of the two fixed it (should've done them one at a time to find out.)

there's still a tiny bit more noise than in my other a/r but no 60-cycle and it's more than acceptable. turned down to a reasonable level or in a gameroom with other machines and it'd be hardly noticeable.

no need to pull the TDA2002's, though now i've got a couple in reserve which probably can't hurt considering how many Atari machines with a/r's i've had over the last few years.

Glad you got it fixed. The diode CR2 and resistor R1 are both in the 5 volt circuit so your hum must have been somehow coming from the 5 volt line or maybe it was causing some sort of ground problem. Did you test the diode after you removed it. Just wondering if it was shorted out or had opened up. I did read one post where someone said their hum was caused by a bad diode so we should add that to the list of causes for Asteroids Hum.
 
i had almost no resistance either way when i tested it in-line.

i had been looking forward to rebuilding and comparing the other a/r that has a little less noise but won't drop the 5v to under 5.5v, but it looks like i might have the machine gone before i could get a kit from Bob. if i don't sell the machine and i do the rebuild i'll report back.
 
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