Boss 4800 d

Hi, this amp is the same board as the ssl5000d is so similar the clones of this type 16 pcs irf44 n for the Psu and 12 pcs IrFb31n20d for the outputs, only diference is it has the driver audio circuit integrated in the main board, i have been working with this and found all psu fets blown an some gate resistances too, the sg3525 Was blown too, i changed the sg3525,one psu driver transistor Was blown, so i decided to change all four transistors, it uses 2 c1027 and two a1023, in another post i saw this transistors could be replaced with a1220 and c2690 so i replaced with this kind (i do not have c1027 and a 1023.thinking in a little improvement too in performance), one thing concerns me with doing this replacement, before i changed the Psu driver transistors in one grpup i have gate voltage ~5.4 volts, in the other i read 0 volts( driver transistor in short i think) with the replacement i got 8.4 volts in all Gates of the fets.

Can you tell me please if this 3 volts diference could in some way afect the performance of the Psu or even worse, overheat the fets and blown them prematurelly??

mpsa42 in the driver outputs has very low gain~3 so i replaced with mpsw42, Will install in this a little heatsink to help with the cooling, i Will try to sandwich the board too lubricate the fan, and i Was thinking in install little heatsinks to the New driver psu transistors to

As always i Will apreciate another coments from you, maybe some tip to improve the reliability of this particular model

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Excuse me Perry, aparently theres nothing wrong with voltages, any way i Will post a picture of my cheap scope, i compared the values of this amp versus another planet audio and are similar, i make a mistake taking the 8 volts as a rms voltage and 8 volts is máx, i post both images. The image with the Vrms of 6.44 volts is a ok working amp.


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The amp present some issues, i changed q35a and q36a with New mpsw42 i have~+/-70 volts in rails only one Fb31n20d per side, but even witho no audio signal this Fb31n20d gets hot quickly some a56 and a1023 Was damged too i can not find another bad transistor in the audio drive stage, but the b31n20d are gettin hot very fast
Another problem the amp has is that it turns On with B+ and ground, when i conect remote only the fan turns on
But with only B+ and ground a have rail voltage, some way the sg3525 is getting power oscillate and puts psw to work, without remote i have 70 volt in rails
Thank you so much Perry, i'm afraid this is the moment i must have a better scope, so i can troubleshoot this problem, with out it, i think the only solution Will be desolder the transistors one by one to check it outside the circuit with mi tester, until i find the bad one, because in the circuit i can not find a shorted one.
The mosfets im using are the original installed in the amp, i test all 12 mosfets with a tester the pair im testing aparently are Good, but there is a posibility this are degraded, because other paralleled mosfets in the amp were damaged in the same bank overheating the others, i have New ones, o bougth from ebay seller:( i tested it and measures like the originals but i'm not shure i Will pos a picture.
I can not assure this are genuine but i have fonund with this tester bad quality counterfeit parts, and at this moment when the test result are so close to the originals (this is the case)it have worked pretty decent in another amps, i think if this is not genuine, this could be a Good quality copy.
In the results of a counterfeit the test display show values out of range, for example the Vth diferent the rds(On) high the capacitance low i Will post a pic of a IrFb31n20d i bougth and im completely shure is counterfeit, is weird but the genuine original mosfets IrFb31n20d do not show all parameters like the pictures i show it shows a dide between 2 and 3 leg an a capacitance of about 2.75 nf when i conect to this tester


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The last picture posted is a fake Fb31n20d i bought from aliexpress the parameters are completely diferent, i did not tried to install this mosfet because i Know is going to i not work blown inmediatelly or in the worst case explode.
I Know that this tester results is not going to assure me i only get genuine mosfets, but at least the mosfets that shows the parameter close to the original in Vth, capacitance and RDon, for me have worked nice.
As i commented i think this tester only give values that tells me that the mosfet tested is close or not to the original specs, but i have no 100% certain og a genuine part, no until i boguth from mouser or digikey, that is mi conclusion.

I hope my comment i have been helpful for you, you have help me so many times.
Thank you
I forgot to coment, after the results and get suspicios parametter i break the mosfet and take a look at the cristal texture and size, normally the mosfets out of range has small cristals, that explains the low capacitance results i think.
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The values displayed above seem more what I'd expect to see from a good FET.

Yes at the beginning i Was thinking the same, but i do not know why the tester gives the other results in genuine mosfets(im talking only with Fb31n20d, because the other like 3205, Irfz44n 48n irf640 gives parametercorrect) i have measured various mosfets Fb31n20d i have desoldier from amplifiers that the problem has been the PS so the output mosfets are in Good condition, and looking at the soldier i think are original from factory, so when i get this weird results in especifically this kind of fets i asume they are Good to go, maybe not original but Good enough to work. I have tried at high power and low impedance the amp gets hot but it havent blown.