Need Help For Adcom Gfa555ii

[VALEMAO]

HI, THIS IS A NEW DYIAUDIO MEMBER!
I AM AN OWNER OF MY SECOND ADCOM GFA555II, THAT I LOVE VERY MUCH,
EVEN IF THE FIRST ONE BROUGHT ME A LOT OF PROBLEMS (LOUDSPEAKERS, AND BJT BLOWN).
NOW I LIKE TO AVOID FUTURE DAMAGES, SO WITH THE HELP OF
A TECHNICAL FRIEND I WOULD LIKE TO UPGRADE ITS WEAKNESS..(DC SERVO,ZENER, RESISTORS

OVERLOADED..Any comments or ideas appreciated) AND INSTALL RELAIS IN THE LOUDSPEAKERS

OUTPUT TO PREVENT SWITCH ON BUMP AND DC VOLTAGE ON THE OUTPUT TO SAVE THE LOUDSPEAKERS.
SO I WHOULD NEED THE SERVICE MANUAL (READBLE) or at least expected voltages at key points,

THAT I AM LOOKING FOR BUT THAT I STILL HAVE NOT FOUND.
I FOUND ONE ON EBAY BUT THE SELLER DIDN'T ANSWER ME...
IF YOU COULD PLEASE SUGGEST TO ME WHERE I CAN FIND IT OR MAIL IT TO ME YOU WOULD MAKE IT

EASIER FOR ME.

I READ IN THE DISCUSSION ABOUT THE CAPACITOR TIPOLOGY AND THE ATTACHED FILE LOOKS LIKE
WHAT I NEED.. SO IF YOU COULD PLEASE MAIL ME.....

THANKS IN ADVANCE

ALESSANDRO

[anatech]

Hi ALESSANDRO,
Please type with the caps lock off. You are yelling in caps.

The stock GFA-555II uses a thermal switch to turn it's amplifiers off when the temperature exceeds a safe limit. You can also install a fuse in series with the speaker. They are generally very reliable, unless serviced poorly or abused.

Your idea to add a DC offset protection network is good (turn on thumps are minor and will not hurt your speaker normally). This will also give you a turn on delay that you wish.

The amp does not have many glaring weaknesses, to tread carefully or you might destroy the amp. There is nothing wrong with it's DC servo.

Is your friend an actual qualified technician with the necessary test equipment? Also with experience?

-Chris

[djk]

I agree with Chris, don't mess with the basic amplifier.

I added an inrush current limiter and a DC sensing speaker relay to the ones I owned. I used the Hafler DH 500 relay board and drove a 24V 3P 13A relay.

[anatech]

Hi djk,
Does the Halfler board implement both functions? Those are the things I would have changed in this design. Just curious how you did it.

I don't know if we helped Alessandro or offended him. No new post.

-Chris

[djk]

Hi Chris.

I used a DH500 relay board (I was a Hafler Dealer).

The stock DH500 used a 5A 48V 2P relay, and no inrush limiting (later they added a 0.5 ohm thermistor). I changed the relay to a 13A 24V 3P type, changed the series resistor for the relay coil (the 24V unit needs more current). I added a 2.5 ohm thermistor across the third pole of the new relay and wired it in series with the primary (removing the 0.5 ohm thermistor on later units).

In an Adcom you must add a high value resistor in parallel with the speaker contacts so the amp has DC feedback with the relay open (it was easier that way, rather than make a gross change in the wiring).

The DH500 relay board is quite nice. You can change the turn on delay time and the DC sense point quite easily. Schematic and board layout are at the Hafler library.

http://www.hafler.com/techsupport/in...D=3#amplifiers

[anatech]

Hi djk,

Thanks!

I see it on PC9. Okay, makes sense. Another way to do the surge is to use a resistor with relay contacts across it. The coil is powered by the secondary DC. So when the supply rises enough, the contacts close. You could also use a timer. Another advantage is that if you have shorted outputs, it will pull the supply down and the relay drops out. The series resistor will then open if the fuse hasn't gone yet.

-Chris

[VALEMAO]

Hi guys, you did not offend me i was on holiday!!
i got the schematics and now we are studing it!
Your help is very appreciated. I'm going to
give a look to this DH500 relay board with the friends of mine.
He works for the electrical hardware department in the factory
where i work.
He quite clude up in his job. I trust him.
I also have another friend that has a service center for all types of
electronic devices.
The first thing i thinking of doing is to substitute all the caps
in particular the first one c101,c151, that one in polyprolilene
(as anatech said good service), but i'am worried to damage the
pc board...i read a previous tread about it..

thanks everybody, i'll keep you posted..

Alessandro

[VALEMAO]

Hi, everybody, and thank to Echowars!!

It was me suggested to cange somthing:
-R109 and R159 that are to close to their rated power
-IC servo amp with an AD820
-R127 and R125 to 2,2M and C107 and C110 to 0.47uf
(and the same ones on the other channel)
i think this last should cange the frequency cut off of the servo

what do you think?

[anatech]

Hi Valemoa,
Replacing R109 and 159 is solid advice. Many makes have similar problems. A higher wattage, metal oxide is what I use.

I don't know about increasing the cap size. Some resistance in series with the cap would prevent the op amp from going into limiting trying to drive that. The lower frequency should end up in the same ballpark. Perhaps lowering the series resistor and also putting an RC on the output of the op amp. That would reduce broadband noise and keep the op amp happy.

-Chris

[VALEMAO]

interesting!!
now i would like even to add some dissipators to
Q108, Q107, Q106, q105.
Infact i gave a look to the pc board and i found close the transistors some overheat spots.
The friends of mine told me the the transistor life decrease
of half every 10c° increased!!
here below .....