Dead Adcom GFA-555II

[K-amps]

Chris... some off topic stuff.

Fixing a 555ii. Got 49v on one channel and 78v on other channel's OP.

OP's are ok, no current draw with no connexions, so no short there...

Input diff seems ok too...

before I sink more time looking (I suck at trouble shooting ) care to share possible things to look at.

By the way it has the Adcom 3A DC servo.. also marked as LT 9021 so any one wanst a sub, just use LT9021.

Thanks!

[anatech]

Hi Arif,
On the speaker outs?

-Chris

[K-amps]

Si Senhor. AC via 100 watt bulb... no current draw , bulb not lighting up...( till I hook dummy load... )

[anatech]

Hi Arif,
Sounds like you've had fun. I started this in a new thread for you.

What are your supplies measuring? The channel with 78 V may have shorted outputs and the negative emitter resistors may have blown. The main supply ground is common to both channels.

Your 49V offset channel may be suffering from leaky cap residue. Of course, check the 2SA1210 and 2SC2912 predrivers for leakage or other failures. This holds true for both channels.

So, what was the history of the failure?

-Chris

[EchoWars]

Check R109 and R159. At 1/4W, they are underrated.

[anatech]

Hi Glenn,
Many of those resistors tend to "exit, stage right" when they blow. Warning, they can look perfectly fine and be open. Same goes for all those resistors.

-Chris

[K-amps]

Originally posted by anatech


So, what was the history of the failure?

-Chris

Can't say, got it from eBay...

If I blow em, the OP's go... not the pesky input stages.




Thanks Glenn I'll Take a look.

[anatech]

Hi Arif,
Can you take some measurements then?

Shorted outputs?

Continuity from the output buss to either rail?

Smoked components? Blown fuses when you got it?

That's how I normally start looking.

-Chris

[K-amps]

R109/159, R104/154 all good. Servo suppies all ok at 19volts, lytics measure 463uF and 455uF for the rated 470uF also fine no signs of leakage. The 1210's on the PCB also fine not shorted.

I will check each OP device and associated ER now....

[K-amps]

All OP's , ER and base R's all ok.

Not checked the driver / predriver yet as I have not opened the sinks from the chassis yet.

Gnd to OP bus, over 8 mega ohm left channel and over 23 mega ohm rt channel... The reading keeps growing so I didnt sit while it charged some cap somewhere...

Now both channels read 79vdc on speaker OP's ???

Anyway here's a huge clue (not seen this before even in blown amps) but both clipping LED's are on full blast... tells the input is not happy...