Adcom GFA-4304 extremely hot

mbates14

Member
2002-06-02 2:14 am
non
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.
 
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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....:)
 

mbates14

Member
2002-06-02 2:14 am
non
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.
 
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MatthewS

Member
2008-02-22 9:19 pm
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.
 
The 064's are nice devices but the circuitry probably does not like them with the designed gate drive the way it is from the factory. Seen this same type of issue many times before. This is why I try to stay with original part designations most of the time.
The same issue will happen on old PPI amps that use MTP25NO5 fets if you use Z44's etc.. only Z32,s work properly with the gate drive as built. Any upgrade of fet to a supply will likely need some sort or tweeking to make the circuitry operate properly. After all it was designed each and every component to be the way it is, so any changes make need for other alterations to make the changes compatible.

That is why I suggested removing fets till you only had one per primary winding. This would reduce the gate load back near what the circuitry was designed to handle, and should drop off all that excessive current draw you were seeing. Unless I am absolutely sure of myself I only install a single fet per primary winding anyway just to test my work before jumping in with both feet so to speak....Hope it all works out for you. if not come on back and lets look at it again... I will try to dig up my paper files and see about scanning them for you if need be....:)


Oh and Thanks Perry, and MatthewS....;)
 
Well the same thing you said with the gate resistors and zener clamps is what I thought was funny as well, and its something I havent seen either. why clamp the FETs at 6.2V when the gate drive is +/-20? Wouldnt it make sense to push the gates at max voltage to get the peak efficiency?

It just didnt make sense how the circuit is laid out, and thats what i wanted to verify and that question is answered.

When I get back to the shop monday i will fix it.
 

mbates14

Member
2002-06-02 2:14 am
non
I skimmed over the paper, it makes sense now at least. Theres a tradeoff on the gate. Drive it harder in voltage, for reduced high frequency performance. Drive it lower to gain higher performance in high frequencies do to reduced on/off times (not charging it as much). But at the cost of on resistance.

Sorry for the whitepaper stuff, But even though I am a full time repair tech, I also have only a 2 year degree in electronics engineering, and we never got that far and into detail with MOSFETs so i like to know how things tick.
 
I had 47 ohms on hand, the 47ohm and removed zener modification worked perfectly. It now idles around 3 amps or so.


Cool, glad you worked it all out....:)

Now to re-bias the amp, as I recall these do not have resistors to measure voltage drop so you likely going to have to forward bias till you get idle current draw increase to go by....:)
Perry has a tutorial on his web site for this. please check it out if you can...regards