I scored a non-working AU-555 off Ebay a couple of weeks ago for ~$150. Here's the ebay photo:
Got it on the bench, right channel is weak, noisy and drifts. I plan on doing a restore anyway, so I ordered replacement caps and transistors from digikey. But I wanted to restore a working unit. Hopefully these steps help others and saves a few of these beauties from the dumpster.
1. Deoxed the F out of all pots and switches. This cleared up the noisiness on volume, mute switch and balance control. Mute switch is odd, it's engaged in the off position. Shouldn't mute on be muted?? As expected I still have a bad right channel.
2. Broke out a super old school technique! A super simple DIY audio probe. This is a great way to find problems in a circuit. To build, hack a 1/4" phono plug, solder a wire with alligator clip to the ring, and slide an ohmmeter probe over the tip. Clip to ground on the unit, then use probe to probe audio at various parts of the circuit. I started at the RCA input, and worked my way to the first transistor that signal hits. For those with the repair manual it's TR-602 and TR-604. Replaced them (They're 2SC458LG) with KSC1845. This thing has so much solder!
3. First test, no audio! Hit the joints again with heat, and bent the lead in line with the PCB trace, solder again. Eureka!
The luck
Anyway, this thing sounds AMAZING! Even through my much hated Audioengine P-5's, over rated, overpriced garbage -- unless you like the sound of apple ear-buds.
4. The unit was pretty dirty, 40+ years of grime on face plate, cover and knobs. I used soap and water and 0000 steel wool. Now it looks and sounds amazing. The cover still had plastic tape around the corners! I removed it and the faceplate is near perfect now.
I didn't put the power knob back yet, largely because I will bring this to a friends place who has an oscilloscope, do a recap of the main power amp board, and then bias.
Confession: I wanted a 555 because I really like how it looks, but now am very surprised at how much I like its sound. Very nice low end, subtle graininess in the mids, and nice clear highs. I'll update this post once the restore is finished.
A few questions:
1. The paint on the top metal case is showing signs of age. The paint is spotty, and a couple of narrow scratches have begun to rust. Should I sand and re-paint or leave it is is?
2. If I like how it sounds now, how much improvement can I expect after re-capping? Should I bother doing a full restore?
More to come.
Got it on the bench, right channel is weak, noisy and drifts. I plan on doing a restore anyway, so I ordered replacement caps and transistors from digikey. But I wanted to restore a working unit. Hopefully these steps help others and saves a few of these beauties from the dumpster.
1. Deoxed the F out of all pots and switches. This cleared up the noisiness on volume, mute switch and balance control. Mute switch is odd, it's engaged in the off position. Shouldn't mute on be muted?? As expected I still have a bad right channel.
2. Broke out a super old school technique! A super simple DIY audio probe. This is a great way to find problems in a circuit. To build, hack a 1/4" phono plug, solder a wire with alligator clip to the ring, and slide an ohmmeter probe over the tip. Clip to ground on the unit, then use probe to probe audio at various parts of the circuit. I started at the RCA input, and worked my way to the first transistor that signal hits. For those with the repair manual it's TR-602 and TR-604. Replaced them (They're 2SC458LG) with KSC1845. This thing has so much solder!
3. First test, no audio! Hit the joints again with heat, and bent the lead in line with the PCB trace, solder again. Eureka!
The luck
Anyway, this thing sounds AMAZING! Even through my much hated Audioengine P-5's, over rated, overpriced garbage -- unless you like the sound of apple ear-buds. 4. The unit was pretty dirty, 40+ years of grime on face plate, cover and knobs. I used soap and water and 0000 steel wool. Now it looks and sounds amazing. The cover still had plastic tape around the corners! I removed it and the faceplate is near perfect now.
I didn't put the power knob back yet, largely because I will bring this to a friends place who has an oscilloscope, do a recap of the main power amp board, and then bias.
Confession: I wanted a 555 because I really like how it looks, but now am very surprised at how much I like its sound. Very nice low end, subtle graininess in the mids, and nice clear highs. I'll update this post once the restore is finished.
A few questions:
1. The paint on the top metal case is showing signs of age. The paint is spotty, and a couple of narrow scratches have begun to rust. Should I sand and re-paint or leave it is is?
2. If I like how it sounds now, how much improvement can I expect after re-capping? Should I bother doing a full restore?
More to come.
