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AU-505 Project

David Weaver

David Weaver

AK Subscriber
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I recently acquired an AU-505 as a new project. It was listed as parts/not working and I purchased it a a great price. Not expecting too much, I thought it would be something for me to tinker with. To my surprise, other than the shipping carnage to the case, after cleaning, passing the dim bulb test and making new side panels, the unit looks great and works very well. The DC offset seemed slightly high at 13mv with a load, so I decided to replace the output caps with two 3300uf KG's. After doing so, the DC offset remained the same but the sound went from warm to sharp, with accentuated treble. I checked the original caps and they spec'd fine so I put them back in and the warmth came back. I'm just curious what caused the difference in sound? Could it be the "burn-in" factor or something else? Another issue I hear is a hum through the speakers at no volume. Could this be attributed to the DC offset or could it be a leaky transistor or power supply issue?
Any guidance would be greatly appreciated.

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It is not uncommon to have some sharpness right after new output caps, let alone a full recap. Cap burn in is real and has been experienced by sceptics including me. I would give them more time, possible trying a film bypass, however if the old caps sounds better after a while.....

Transistors can produce hiss or pops, hum is usually more a grounding/loop issue. You can try and reverse the plug 180 degrees, this may reduce hum in rare cases. You can also reroute the sometimes ungodly long power cable and check it's sleeve for any exposures, like dog bites from 1976, including wear and tear where it enters the chassis)

Also consider replacing the large 2200 supply cap - this old chap can be hum prone too, while doing so you can also also resolder it's ground point.

Check inputs like aux, tape, tuner to see if hum is related to any of those specifically.
 
Thanks Stereofun,
I will do as you recommend as well as reflow the grounds and check for other potential cracked solder connections.
The unit must have taken a pretty good jolt in shipping to damage the case like it did.
I have a spare 6800uf 63V 381LL that I was going to use to replace the the 2200uf main power cap.
Do you think I would need to replace the 10DC-1 diode with a beefier pair or do you think it should handle the increased in-rush?
Also, my unit does not have the .01uf ceramic cap across the main cap. I suppose that is a snubber? Is it necessary?
Thanks again!
 
Some have the 0.01 some don't. Two of mine don't and the latest '74 model do, but in turn are missing a stock cap by design....revisions are quite normal on the earlier models for sure.

6800 may be pushing it. I don't think it will hurt right here right now, but I wouldn't leave it in longer term before a beefier cap rectifier is in. We also have to worry about the power switch.

Placing a 1uf film on the backside of feedback cap c809/810 imo improves overall sound, especially the top.
 
I got a -505 recently and have a mild zero-volume ground hum in the left channel which gets worse as I turn the balance AWAY from left and then finally vanishes when I get full right.

Still don't know what to do about that, but please post your hum solution and I'll see if it helps my situation!
 
Thanks all for your responses.
Moe, great technical background! When I was in High School I always had a Stereo Review mag with me to peruse during study hall. I remember it was all about the spec's; slew rate, S/N, dampening factor, THD, etc. Nice to know there was some actual science behind the marketing!
As for my 505, I will post my results with my findings and suggested modifications.
I figure after checking the basics, I will rebuild the F-2001 filter board entirely since it is soldered to the chassis.
Any harm in doubling the two remaining filter caps (C006-C007) while I'm at it?

Dave-
 
archibael said:
I got a -505 recently and have a mild zero-volume ground hum in the left channel which gets worse as I turn the balance AWAY from left and then finally vanishes when I get full right.

Still don't know what to do about that, but please post your hum solution and I'll see if it helps my situation!
Click to expand...

Archibael,
Not sure if the following is a solution to your hum issue but I would think since the issue changes with the position of the potentiometer, I would first try to rule out the pot as the culprit.
I've been watching a lot of xraytonyb's videos on youTube in an effort to learn as much as I can. His videos are very informative and thorough but long so you have to have patience.
One video is about the improper use of DeOxit D-5 and as it turns out, I have been using it improperly from the git-go.
The bottom line is D-5 is for switches, F-5 is for potentiometers and using the minimum amount to do the job is key. I've stopped "embalming" my potentiometers with D-5 and use F-5 sparingly along with working the pot back and forth to condition the contacts.
It may not resolve your issue but I think it is good, "sound" advice for everyone working on their gear.

 
Last edited:
David Weaver said:
Archibael,
Not sure if the following is a solution to your hum issue but I would think since the issue changes with the position of the potentiometer, I would first try to rule out the pot as the culprit.
I've been watching a lot of xraytonyb's videos on youTube in an effort to learn as much as I can. His videos are very informative and thorough but long so you have to have patience.
One video is about the improper use of DeOxit D-5 and as it turns out, I have been using it improperly from the git-go.
The bottom line is D-5 is for switches, F-5 is for potentiometers and using the minimum amount to do the job is key. I've stopped "embalming" my potentiometers with D-5 and use F-5 sparingly along with working the pot back and forth to condition the contacts.
It may not resolve your issue but I think it is good, "sound" advice for everyone working on their gear.

Click to expand...

Thanks. I'll check it out.
 
archibael said:
Thanks. I'll check it out.
Click to expand...

Picking up on this issue i have found hum induced when units are dissasembled and cured when the nuts are nipped up,whether this has anything to do with the grounding or the solder joints i don't know,worthy of investigation as sometimes it can be as simple as the washer has not been replaced!
 
I believe I found the cause of the hum problem on the 505!
While trying to isolate the problem, I connected my 1/8" to RCA cable to Aux port and turned on the unit and the hum was very noticeable.
I then touched my finger to the chassis and the hum reduced dramatically. Investigating further, while the unit was still powered on, I removed the RCA jacks.
Low and behold, the hum went away. I reconnected the jack and the hum came back in both the AUX and REC IN inputs.
When cleaning up the unit, I cleaned the grounds on the RCA jacks but I failed to clean the positive signal connections. With a few Q-tips and some D-5, the hum is gone.
Several lessons learned: 1. Clean all contact points. 2. Don't jump to major parts overhaul until you rule out #1!
I did notice some hiss since the hum was eliminated but nothing major. I would imagine its likely a transistor issue.
At this point I will probably rebuild the power supply since I ordered the parts and perhaps perform a Leestereo overhaul at some point in the future (like my first of two 555a's).
But for now, I will enjoy her native vintage tone.
Thanks to all for your help!
Dave-
 
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