Adcom GFA-4304 extremely hot - diyAudio
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Old 9th March 2013, 03:41 PM   #1
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Default Adcom GFA-4304 extremely hot

I am working on a GFA-4304, it had a blown up power supply. I replaced the IRFP044 with the IRFP064. The 6.1V Zeners were also shorted, so i replaced those.

When the amp is idle, the power supply gets really hot really fast on the heatsink, around 150 degrees idle with no speakers. Is this normal?

180 degrees idle if on for about 5 minutes. Too hot i think.

All 4 channels do have output.

Last edited by mbates14; 9th March 2013 at 04:03 PM.
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Old 9th March 2013, 04:32 PM   #2
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Drawing 10 amps on idle, So i turned down the 4 BIAS pots, and can only drop it to 7.79Amps with bias turned down on all 4 channels.

Is this normal? I have never worked on one of these amps before.
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Old 9th March 2013, 05:13 PM   #3
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No its definitely not running right. These idle around three amps at the most.

I would have used the original IRFP044 devices unless you changed the gate resistor. As I recall these drive directly from the chip so the added gate capacitance can cause issues since their is no totem pole drivers to handle gate drive. You might also want to scope the signal to the gate leads the PWM chip may have been damaged when the fets failed.
You could also remove the supply fets back to one device per primary winding to see if the fets are the problem. This should drop the capacitance back near its expected range enough to let the amp idle properly.

Its pretty tough to hurt one of these old Adcoms. I have seen power traces blow open like fuse links and once repaired the amp went right back into service with no other issues, so for the fets to be damaged in the supply and no blown open traces it sounds like it may have been revered polarity hooked up. I own several of these personally and while I do have schematic info its paper not PDF file so I don't have it handy to send top you, sorry I just never got around to scanning it.

Also when you get the chance check the DC output on each set of speaker terminals. That is what that other adjustment in each channel is for. Perhaps you lower a bit closer to zero and get lower current draw also if the amp channels have damage in them.

Let me know what you find....
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Old 9th March 2013, 05:40 PM   #4
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The gate drive looks fine, I had checked that already. If i remove the Zener diodes, (basically to drive the FETs harder), its even worse and gets hotter faster.

Also like i said, with the BIAS fully CCW on all channels, current draw is 7.75amps. When first powered up, it starts off at 1.2 amps then ramps streight up to 7.75 amps.

I also recapped it except for the snap-ins.

Last edited by mbates14; 9th March 2013 at 05:42 PM.
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Old 9th March 2013, 06:38 PM   #5
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Are the outputs getting hot ? Please check for DC output on the channels also. If all of that is ok then I tend to think its only in the power supply and either the PWM chip is defective or the feedback loop is defective. Let me know what you find...
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Old 9th March 2013, 06:50 PM   #6
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No, the outputs dont appear to be. only the power supply is getting hot.
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Old 9th March 2013, 06:57 PM   #7
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What are the rail voltages?
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Old 9th March 2013, 09:16 PM   #8
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Compare data sheets on your replacement parts? The 064's gate cap is 3 times that of the 044. What is the value of your gate resistors?
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Old 9th March 2013, 09:46 PM   #9
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The original gate resistors are 220 ohms. Even with the deadtime that's programmed into the drive circuit, that's too high. I'd suggest using 33 ohm gate resistors. With the lower value gate resistors, you'll probably have to remove the 6.2v Zeners to prevent damaging the IC.
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Old 9th March 2013, 09:56 PM   #10
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Adcom is kinda cultish in following so I would have kept it original for them. For as widely available as the 044's are, I would have just ordered a tube at 2 bucks each than subbed forcing you into making more changes.

If you want to continue, Perry is on the money. The original resistors are too high by a mile. If you can wait, lump a tube onto a mouser order or something and keep it OEM. JMHO.
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