Adcom GFA-585 Blown 20w 4.7ohm on soft start board

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#122
I M sofa king we todd ed:cannotbe:

I did all that work, and I replaced transistors, diodes, resistors, op-amps, ultrasonic cleaning, and I forgot that one? ground.
Nope
I am so embarrassed. I should LIE. I missed the speaker ground on the "bad" channel. Little bit of DC offset over there, and none! on the other channel🙂 I cannot set the bias properly because there is someone sleeping here, but tomorrow I Will get-r-done.
BTW even at low levels, and crap test speakers it sound Soo sweet🙂 🙂
 
#125
I kinda figured that out, but I am happy that I have progressed this far. The DC offset is nill on one channel, and 24mv on the other. I will hook up to some decent speakers for a real test.

Can I run GFA-565 mono's at 2ohms safely?
What kinda wattage am I realy looking at?
 
#128
the soft start resistor is just that....for a soft start. Instead of hitting the transformer with the full 120 off the wall, and jolting everything to life, they run it through the soft start resistor for a few seconds, till the caps are partially charged and then the relay on the board basically jumpers the resistor out of the circuit, giving you full power to the primary of the transformer.

The resistor should be hot, because it drops a pretty large voltage across it but then it is taken out of the circuit. If the resistor gets hot and stays hot, then the relay isnt doing its job, or the circuitry that controls the relay is kaput.

In the event that the relay ISN'T working, then the 120 is going through the resistor all the time and it will get hot and burn out. Also, your amp will be operating as if in a brownout. This will stress the circuitry on the bias board even more to try and keep the amp stable, because with that resistor in there all the time, you are starving the amp for voltage and current.

I will hunt up my schematic and see what the possible causes might be. laters...Rob
 
#133
musinteg said:
the soft start resistor is just that....for a soft start. Instead of hitting the transformer with the full 120 off the wall, and jolting everything to life, they run it through the soft start resistor for a few seconds, till the caps are partially charged and then the relay on the board basically jumpers the resistor out of the circuit, giving you full power to the primary of the transformer.

The resistor should be hot, because it drops a pretty large voltage across it but then it is taken out of the circuit. If the resistor gets hot and stays hot, then the relay isnt doing its job, or the circuitry that controls the relay is kaput.

In the event that the relay ISN'T working, then the 120 is going through the resistor all the time and it will get hot and burn out. Also, your amp will be operating as if in a brownout. This will stress the circuitry on the bias board even more to try and keep the amp stable, because with that resistor in there all the time, you are starving the amp for voltage and current.

I will hunt up my schematic and see what the possible causes might be. laters...Rob
Click to expand...


Any luck Bob?
Right now its a very pretty limited edition paper weight🙁
Ben
 
#134
Hi musinteg,
Also, your amp will be operating as if in a brownout. This will stress the circuitry on the bias board even more to try and keep the amp stable, because with that resistor in there all the time, you are starving the amp for voltage and current.
Click to expand...
Completely untrue.
The amplifier will be running at reduced voltages, but this will not stress anything except the soft start resistor. That will burn out before too long.

HI Ben,
Can I run GFA-565 mono's at 2ohms safely?
Click to expand...
No.
These amps are designed for 4 ohm minimum, stable if the load drops briefly to 2 ohms.
Why would you want to do this? Sound quality goes down as the load impedance goes down.

The 680 R resistor is a dropping resistor for the soft start circuit. It runs very warm to hot. That is normal. The surge resistor should be on the order of 4.7 to 10 ohms. Something close to that range. It normally may warm up during start, but then runs at ambient temperature. It will only get hot if there is a failure somewhere.

Check the solder connections on the soft start PCB. The relay may have burned contacts. Do you hear it go "Click"?

-Chris
 
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