Peter P. Posted June 30, 2019 Share Posted June 30, 2019 I'm repainting my bedroom. On Friday, while I was upstairs replacing the outlets in the room and listening to the radio downstairs, all of a sudden the music was overrun with this wicked hum. I listen to a non-commercial station which frequently has a problem with their CD players skipping so I thought they must be having a problem. I ignored the noise for about 30 seconds then couldn't stand it any more. I went downstairs to change the station and lo and behold, the other station had the same problem. Don't tell me my tuner crapped out! Then I switched my amp to another source and the hum was still there-don't tell me my amp crapped out! While checking things out, I noticed the case on my equalizer was REALLY hot. What's going on here?! So I shut the equalizer off and the hum reduced by over half. Then I got the bright idea to bypass the equalizer altogether and when I did, the problem disappeared completely! Now that I've got the hum isolated to my 40 year old Audio Control 520b equalizer, my mind starts to think; now's a good time to buy something new! I didn't see much that interested me, so I started perusing craigslist and eBay, where I even found exact replacements for my beloved 520b. But I'm an electronic tech (although I don't do much to the component level) and I'll admit I'm frugal, so I figured I should take a stab at troubleshooting my equalizer. I've got nothing to lose. I opened up the case. Since this unit is so old, most components used are discrete and likely pretty generic. My first clue to the problem was the hum; likely 60Hz hum, so likely a power supply problem. This equalizer power supply is not too complicated, making it easy to figure out the layout of things. And fortunately, the 40 year old manual I saved had that rarity of, a schematic! And for a bonus, if I used a magnifying glass while reading it, I could see they identified the components by generic part number. In my case, I was given another clue in that the leads to the diodes used in the bridge rectifier looked like they overheated. Now, testing components while in-circuit isn't exactly the right way to test, but it's a good way to triage things and the numbers I was getting with my voltmeter testing the diodes told me at least one of them was shorted. I was also getting weird readings from those two blue capacitors even though they didn't look blown, so I decided to replace all the diodes rather than pull them from the circuit to test individually, and replace both capacitors. Hey; the cost would be much cheaper than buying a new equalizer! Fortunately, there's a "Radio Shack on steroids" electronic store nearby, so obtaining replacement components was a breeze. Looking at the photo now, I see I brain farted and did not buy matching capacitors (they were hanging on the same hook); they both should have had a 25V working voltage rating. No matter; better higher than lower. The only way I noticed was I observed the prices in the photo were different. I can't believe I paid an extra 30 cents! Replacing the components and reassembling things took 30 minutes. I'll admit the component positioning wasn't as professional as factory installed but that's my only disappointment. I'm not sure what that brown smudge is; it doesn't look like a burn mark from overheating or overcurrent flow. I added the marks next to the components to observe correct polarity. Prior to reinstalling the equalizer to see if it would work, I tested all the components out of circuit; one diode was in fact shorted. The capacitors tested fine. I reconnected the unit and it worked! Here's to the NEXT 40 years! 5 Quote Link to comment Share on other sites More sharing options...
absolve2525 Posted June 30, 2019 Share Posted June 30, 2019 I have a couple of those same eqs! Neat units. Good work! 😃 Quote Link to comment Share on other sites More sharing options...
Deang Posted June 30, 2019 Share Posted June 30, 2019 Nice job. I love the old stuff. Discrete sounds so good. 1 Quote Link to comment Share on other sites More sharing options...
pzannucci Posted June 30, 2019 Share Posted June 30, 2019 Considering age, you probably did the right thing getting rid of those caps. Quote Link to comment Share on other sites More sharing options...
JohnKuthe Posted June 30, 2019 Share Posted June 30, 2019 10 hours ago, Deang said: Nice job. I love the old stuff. Discrete sounds so good. Yep! Back in the Daze, everything was in it's own BOX and you had to wire everything together! And I am GOOD at that! Inputs and Outputs!! Phono or line level. No Digital CRAP! All ANALOG! Then the stupid rich Capitalists decided we should all have COMPUTERS in everything and it all went to hell! 😞 John Kuthe... Quote Link to comment Share on other sites More sharing options...
Panelhead Posted June 30, 2019 Share Posted June 30, 2019 It is great you are able to fix equipment. Plus find parts on a Saturday. There used to be multiple stores here that sold both new and NOS parts. Down to a couple now. Support them if you still are lucky enough to have one. Quote Link to comment Share on other sites More sharing options...
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