AU777

ToxicTele

ToxicTele

AK Subscriber
Subscriber
Starting to refresh my AU777 and have a couple of questions. First - Stay with carbon comp resistors or move up in the chain? I want to keep the dynamics as original as possible and am leaning towards carbon. I'm starting with the main amp boards (8RB & 8LB). Already have new trimmers. Open to all suggestions. Thank you.
 
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If you like the sound of it now, I recommend that you do not replace the resistors. If you change the original carbon composition resistors for carbon replacements it will sound different.

It just depends on what you are trying to achieve.
 
Thank you Hyperion. The unit is tired, sounds good at low volume but breaks up when upping the volume. Tried adjusting the bias, right side adjusted just fine, left side flaky. I have new trimmers to install but figured if I pull the boards I may as well replace a couple of out of spec resistors. I'll stick with my original plan and replace the trimmers. Re-bias and see how that goes. This unit is all original with the exception of one cap on the bottom side.
 
You can't measure resistors in circuit accurately. However, if you take them out to measure, you might as well replace them while you are in it.
Those carbon resistors do not drift much, BTW.
 
I have spent the last 9 months overhauling my 777. It still has all the carbons in it except for the protection board and part of the power amp. I had recapped it first using a 777a upgrade list. That move set up a world of hurt. I had to reorder original value caps and bring everything back to new original before going forward with it. With that said, some of the caps are upgraded to lees 777d values. Spent may hours trying to replace the 2sd143 as 1 was blown. Like others have said "do not do it". Finally ordered replacements (2sd143) and scope finally confirmed that it worked. If you don't have a scope, get one. I suggest it go on the scope after every single board is done and don't go further until it passes. Typically it will oscillate and my best guess is due poor connections on the output transistors.

Others, much smarter then me have said throw out everything you know about amps to work on this one, I concur! I have not posted this even on my post about my 777, but I think a lot has to do with whether you find the 2sd143/144 soldered in or not. The key to doing this amp correctly is right in this spot.

With all my late nights, frustration and many times thinking just to sell it for parts.......it sounds amazing! I have a updated 555a that I thought could NOT be outdone. When I a/b it, the 777 mopped it up. Tons of power (easily reaches 105db at my listening seat) does not get hot, and has tons of bass. In fact, never a need for the presence or loudness to be selected on. But you must know that owning one.

Be careful on upgrades to your wirewound resistors. Mine scope said it did not like the change. Oh ya, never figured out how to get this amp working part by part. You know.....get power supply working, check it out, NO. The whole thing has to be together to check, or things will not be right that will throw you off the scent. Keep it complete and do one board at a time.

Sorry to be passionate about it, just trying to help......

Cheers
 
Thank you Bigdrive. At this point the only change I have planned is to replace the VR803 & VR804 trimmers. I have 3296P 200ohm trimmers in hand. I'm debating on drilling the 8LB & 8RB to fit the trimmers - unless somebody has a better idea.
 
Trimmers- first I tried not drilling, but it was just too flimsy. I ended up drilling. Are all trimmers responsive and adjustable before you start?
 
ToxicTele said:
I have 3296P 200ohm trimmers in hand.
Click to expand...

Be careful with those trimmers, I'm nearly certain that your adjustment will result in the opposite of the originals.
When you remove the old ones, first measure the resistance at the pins and note, also note which direction lowers/raises the resistance at the pins.
Do this check with the new 3296P, I think you will see its the opposite.
 
Left trimmer is responsive, right is all over the place. Of all the research that I have dug up, it seems almost universally that these trimmers do go bad. I figured them to be my starting place. Original trimmers still have the white paint so I will measure at the original locations and set the new ones the same. Thank you Gentlemen.
 
Replaced both trimmers. I was able to mount by drilling only one hole per side. This resulted in trimmers being slightly tilted but was the least intrusive method. Right side biased at 28ma, left side at 32ma. Could not bias below 32. As it is, the unit is connected to a Fisher FM-100-B, Bozak 300's and simply sounds wonderful. It bothers me that I could not bring the left bias down to 28ma. Hmm?trimmer.jpg
 
Had the same problem. The 3296 will work fine but you must 1)Verify you were in MA on the meter and probes are in the correct spot (if it applies to your meter). 2) Verify NO input to amp and volume at 0. 3) Verify speakers not connected or a scope not connected at all.

Good luck.
 
Thank you Bigdrive - all of the above were verified,,ma on the meter,,no input and no speakers connected. Currently listening to the AU-777 via Fisher FM-100-B and all sounds,,,damn good. Tomorrow it will get coupled to my Thorens and go through some LP paces.
 
Great! Are the temps even at across the outputs? Did you get the bias below 32 and 22?
They do sound wonderful.
 
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