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ATR 100: 120hz hum on playback only
Old 26th December 2008
  #1
Lives for gear
 

ATR 100: 120hz hum on playback only

Hi Gurus,

I've got an ATR100 with a hum problem.

There is no hum at all when it's in input. But when I switch to repro I hear the hum.

The hum is the same with the 1/2" and 1/4" head blocks, so the problem is not in the heads or the connections to them.

I had the playback electronics & power supply recapped (the audio electronics in the meter bridge, not the stuff on the cards) and it made no difference at all.

I don't think it's the power supply since there's no hum at all on input, but I could be wrong about how the signal flows...

Any ideas before I embark on an expensive trip to an ATR specialist? The hum is very slight and I'd love to be able to get this fixed in-house. I'm experienced with electronic repair, so I won't butcher it.

Thanks in advance,

Jeff
Old 30th December 2008
  #2
Gear Head
 

Is the hum in both channels?

Don't forget to check the wiring that connects to the female edge connector that the head block assembly plugs into. The connections from the heads themselves to the male edge connector on the head block may be fine, but if that socket has a loose wire it would cause you to have the problem with both blocks.
Old 30th December 2008
  #3
Lives for gear
Where I would look would be cabling from headstack to playback amplifier and the playback amp circuit board. It could still be a problem in the power supply that only shows up in the high gain structure of the playback amp.
Old 30th December 2008
  #4
Lives for gear
 

The hum is in both channels. I think I've checked & wiggled every cable at some point, but I will specifically check the edge connector etc.

The hum is louder with the 1/4 heads, which probably require more gain than the 1/2" heads... maybe the power supply is still suspect even though I had ruled it out before. I'll put it on a scope and check.

Thanks for your suggestions, and any more that might be out there!

Jeff D.
Old 7th August 2020
  #5
Here for the gear
 

Sorry for the zombie thread resurrection, but did you ever figure this out? I'm having the exact same problem. Just had a power supply rebuild too and all cards refurbished by ATR. 120hz in both channels though one channel has more harmonics and the other has some 10k and 20k. It's even present on the outputs with machine off. Then it's only around -120db though so wouldn't have noticed if not for software.
Old 7th August 2020
  #6
Lives for gear
 

Quote:
Originally Posted by blcstudio View Post
Sorry for the zombie thread resurrection, but did you ever figure this out? I'm having the exact same problem. Just had a power supply rebuild too and all cards refurbished by ATR. 120hz in both channels though one channel has more harmonics and the other has some 10k and 20k. It's even present on the outputs with machine off. Then it's only around -120db though so wouldn't have noticed if not for software.

Hi blc, that is quite the zombie resurrection! I did fix the problem, but I don't remember what was done. I usually write notes on the service manual so let me take a look at that in a day or so and I'll get back you here.

JD
Old 7th August 2020
  #7
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Radardoug's Avatar
 

Could be a pin1 problem. Try lifting all the pin 1s.
Old 7th August 2020
  #8
Here for the gear
 

Quote:
Originally Posted by Jeff D. View Post
Hi blc, that is quite the zombie resurrection! I did fix the problem, but I don't remember what was done. I usually write notes on the service manual so let me take a look at that in a day or so and I'll get back you here.

JD
Thanks so much. Would appreciate it greatly.
Old 8th August 2020
  #9
Here for the gear
 

Quote:
Originally Posted by Radardoug View Post
Could be a pin1 problem. Try lifting all the pin 1s.
Gave it a shot but unfortunately no change.
Old 8th August 2020
  #10
Lives for gear
 

Hi blc, bad news: I looked at my schematics with notes all over them, but the fix was not notated anywhere. I had later blown a bias transistor by adjusting the bias with a non-insulated screwdriver which touched the chassis (oops), and the troubleshooting for that is all over the drawings too so I am not even sure which notes go with which fix. Sorry that I am not a better tech :/

I don't remember the fix at all, but if I was to start troubleshooting again I'd analyze the power supply voltages fed to the playback amplifier with an oscilloscope, looking for any 120hz ripple on the DC power supply rails. Sometimes circuit boards will have a capacitor locally to further smooth out power to the board, and maybe one of those is bad? I am just guessing here.

Also, plugging the ATR into the same AC power circuit as your A/D or other receiving device might be a good idea to rule out any ground loop hum (as well as the pin one lift mentioned earlier).
Old 10th August 2020
  #11
Here for the gear
 

Quote:
Originally Posted by Jeff D. View Post
Hi blc, bad news: I looked at my schematics with notes all over them, but the fix was not notated anywhere. I had later blown a bias transistor by adjusting the bias with a non-insulated screwdriver which touched the chassis (oops), and the troubleshooting for that is all over the drawings too so I am not even sure which notes go with which fix. Sorry that I am not a better tech :/

I don't remember the fix at all, but if I was to start troubleshooting again I'd analyze the power supply voltages fed to the playback amplifier with an oscilloscope, looking for any 120hz ripple on the DC power supply rails. Sometimes circuit boards will have a capacitor locally to further smooth out power to the board, and maybe one of those is bad? I am just guessing here.

Also, plugging the ATR into the same AC power circuit as your A/D or other receiving device might be a good idea to rule out any ground loop hum (as well as the pin one lift mentioned earlier).
Thanks for checking. I'll check my other machine to try and isolate the issue and then probably take it in for repair as I'm not not that skilled in those areas.
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