AU-666 Bias issue

[AnalogueBen]

Hey guys,

I've ran into a problem while adjusting the DC offset and Bias. The offset adjustment went fine, however when turning pots on the bias I've somehow lost current, think that I heard a faint click when power dropped out.
Fuses are removed for test lead hookup and everything set to 0 and on AUX.
Hoping someone knows what may have happened?
Thanks

 

[kevzep]

Check the fuse in your meter.
Turn the amp on first, then connect the meter, if you connect the meter first the turn on surge can blow the mA fuse in your meter.

 

[AnalogueBen]

Winner winner chicken dinner
Much appreciated for the fast reply. Although I had known about to power on and connect, I think it had to do with something else as it blew when adjusting..

Cheers!

 

[kevzep]

Its very common, although you might want to investigate why that happened, maybe the bias pot is a bit intermittent or something like that....

 

[ivandezande]

Those amp boards have to be rebuilt, those white disc transistors have to go, as well as the trimmers. They're cards that are so easy to take out, there's no reason not to.

 

[AnalogueBen]

Yeah the layout is pretty good on the amp, I'll be doing an order soon and I want to change out all trimmers on my amps, the multi turn bourns seen to receive great praise for getting values set right. Haven't had much time with this amp aka the "beast"! I have compared it to my stock 555a and I find I like the 555a a little more, I just makes you feel happy when listening to it. The 666 is great and I can't wait until its hooked up again, but although It's less warm, it does make up for it in reproducing clear sonics and good imaging, very grateful to have them both. I'll post some pics soon and probably start up a new thread for it as they're a bit less common as I understand.

 

[avionic]

Check the fuse in your meter.
Turn the amp on first, then connect the meter, if you connect the meter first the turn on surge can blow the mA fuse in your meter.

Hate it !! When that happens.. Fluke fuses are $10 apiece.

 

[AnalogueBen]

Hey Guys,

Update on bias setting.. The Fluke fuse hasn't arrived so I thought I'd set via the emitter resistors as I know many people do.
set speakers to A and on AUX with all controls where they need to be..
I now cant get a reading across these resistors? Whats up???

 

[slimecity]

I now cant get a reading across these resistors? Whats up???

This may be a dumb question - but you are measuring in mV (not mA) across the resistors?

Hopefully this is the case, as the meter set to Amps setting will present a dead short across your probes......

 

[AnalogueBen]

Hey Mate, yes I'm now triple checking my meter setting to make sure I'm on the correct function.
But now I have a definite problem,
I've blown F003 & F005 (left channel) from being impatient and not waiting on the parts to come to do the job right...grrr. I fitted new fuses and they poped as soon as the power button is turned on. What a goose I am

 

[AnalogueBen]

Blown my outputs...just tested

 

[ivandezande]

Build a DBT (dim bulb tester) before you do anything else. I apologize no one including me suggested it so that this wouldn't happen.

 

[AnalogueBen]

I'll do that for sure .. thanks

 

[stereofun]

There are great replacements for the outputs - it sucks, but it's easily fixable ;-)

As for measuring over the emitters, you have to take into account what output config serves your amp as the measuring methods are different. In short, if you set your bias on your 666 (emitters in series) the same way as your 555 you will double you bias mA and possibly run your outputs too hot. That's why I don't recommend the emitter method unless you are aware of the the emitters config for a particular amp.

 

[AnalogueBen]

Hey, I've been over the options for replacement outputs and have decided to use MJ21193G/94G, like you said Stereofun, all's not lost Before I start a new recap thread (whilst building DBT!!) I just have a couple of questions..
Should I order the right amount of outputs or, add a couple extra for matching?? Would it be worth the cost trying to match?
Has anyone ever noticed a difference for better or worse with new output subs?
Oh, last question, should I upgrade the drivers while I'm doing the outputs many other TO-92's across the amp?

I agree with setting as per service manual, makes sense if followed correctly.... We have a saying in Australia called "She'll be right mate" many a many of clusters have been begun from those words

 

[stereofun]

Those outputs are excellent, I've used them in an EIGHT, 999 ....and even a 505 repair, thinking they would be ridiculous overspec"d - but it was all I had on hand, and owner was impatient. Sounded really good. I think they are on par with originals, as long as you don't sub them for 25mhz originals.

I would not worry about matching. Nor would I replace the drivers. So it's really a "plug and play" repair for the output part, just "continuity" test between new outputs and heatsink to make sure that the mica- insulators are doing their job after you torque the bolts. Then fire up the 666 beast and re-check bias.

 

[AnalogueBen]

All understood mate,, now going to begin a new thread for a recap, seeing as thought I'll be wielding an iron in there might as well do the caps too!