DOD R-431 31 band E.Q. had much 60hz hum, now fixed

[CaptDon]

I made some improvements on the low frequency end of my night club sound system and traced a significant amount of hum coming from the graphic E.Q. I was surprised to find the hum at pure 60Hz. Moving the 120Hz control made no difference. First off, the unit is not chassis grounded and also doesn't have any bypass capacitors to the chassis. The power supply wires to the various circuit boards looked very feeble so I beefed up the grounds with much heavier wire. NO CHANGE in hum level. I then found when I placed my hand near the tiny power transformer (it is magnetically unshielded) the hum reduced so I installed some copper shielding around the transformer and a copper riser between the transformer and the audio boards. BARELY ANY CHANGE. Again I noticed placing my hand in that area reduced the hum. Aha.... The 120v.a.c. hot wire and fuse holder are barely 1/4 inch away from audio hi impedance circuitry. I installed a grounded copper shield between the 120 volt a.c. wiring and the audio circuits. Hum reduced by at least 20Db. There is still a tiny bit of hum coming from within the E.Q. unit, but it is small enough that I can accept it. Feeble and poor design throughout, and yet DOD audio gear is generally ranked well in quality for pro gear. Just wanted to share my story of marginal gear which took some time to figure out.

[AkibUddin]

I made some improvements on the low frequency end of my night club sound system and traced a significant amount of hum coming from the graphic E.Q. I was surprised to find the hum at pure 60Hz. Moving the 120Hz control made no difference. First off, the unit is not chassis grounded and also doesn't have any bypass capacitors to the chassis. The power supply wires to the various circuit boards looked very feeble so I beefed up the grounds with much heavier wire. NO CHANGE in hum level. I then found when I placed my hand near the tiny power transformer (it is magnetically unshielded) the hum reduced so I installed some copper shielding around the transformer and a copper riser between the transformer and the audio boards. BARELY ANY CHANGE. Again I noticed placing my hand in that area reduced the hum. Aha.... The 120v.a.c. hot wire and fuse holder are barely 1/4 inch away from audio hi impedance circuitry. I installed a grounded copper shield between the 120 volt a.c. wiring and the audio circuits. Hum reduced by at least 20Db. There is still a tiny bit of hum coming from within the E.Q. unit, but it is small enough that I can accept it. Feeble and poor design throughout, and yet DOD audio gear is generally ranked well in quality for pro gear. Just wanted to share my story of marginal gear which took some time to figure out.

Can you show with us about your work 

[CaptDon]

Sadly, the unit is reassembled and back in use in the equipment rack. I should have taken some photos. I had always noted a bit of hum in the low frequency folded horn bass bin but recently had the chance to connect the low frequency amplifier to a cabinet loaded with 18" speakers and the hum became very pronounced in a quiet room. This sent me searching for the cause. I though it would show up as ground loops or poor interconnect cabling but was surprised to find what I would call a design flaw in a piece of audio equipment. This particular system is used for bands in small night clubs, American Legion, Veterans of foreign wars, and the 'animal clubs' Elks, Moose, Eagles and the like. Cheers mates!!