Pheonix Gold M50 - Protection

[qsilverza]

HI,

Bought this amp for a steal at 50 AUD and it was working ok for about a year but than the protect light comes on whenever power is applied, not even to the remote.

Now I am new at reparing amps and I have lots to repair because I am good at breaking them !

I tested the Rectifiers and the FET's and they look to be fried. So, could somebody point me in the right direction of somewhere that can supply everything I need to get this amp up and running ?

I plan on refurbishing and so will replace caps etc at the same time.

So far I have the following codes off of what I believe to be the broken parts...

But I am having trouble tracking them down and or choosing the correct replacement, can anyone help ?

IRFZ44 IR 9303 1k 9H Mosfets - Varying readinds - 0.00 ohms to 13

DII 31 SF163A - Showing 0.00 OHMS

I will post more information later on !

Thanks

[Jacampb2]

First off, welcome to the phorum! We can probably help you.

You can find the IRFZ44's on ebay. You may try digikey.com as well, I believe they sell and ship international. Mouser.com will likely have them as well. International rectifier lists the IRFZ44 replacement as IRFZ44NL.

Mouser.com has the SF163 in stock.

Make sure you check all of the FET gate resistors. They will likely be bad, and if they are open, then the replacement FETs can get destroyed instantly with the application of power.

If the FET's went in a violent fashion, then check their driver BJTs as well.

Lastly, something made the power supply shit the bed. It is very rare for it to go without some attributable cause. Check the output transistors, you will probably find at least one shorted output BJT.

Good luck,
Jason

[qsilverza]

Thanks for the warm welcome ! I have been looking for an amp specific forum for a while and I was lucky to stumble upon here with what looks to have some valuable contributors from some of the threads I have read !

Could you check attached picture from ampguts and let me know ? I have marked what I know so far !

This amp I believe went because it was in my fathers car who didnt mount it properly thus the cables came loose and shorted for a while until the amp protected is my assumption.

*Meant to put Gate RESISTOR and not transistor on picture

[Jacampb2]

1st off, I gave you the wrong part number. The IRFZ44NL is the wrong case style, you want the IRFZ44e.

Top left hand corner of pic, PS fets. The first lead on the left of each fet is the gate, there should be a resistor in series with that lead vary close to the fet. That is the gate resistor. Follow that trace back further to the PWM IC (SG3525) and you will find a pair of small transistors, they are the gate drivers mentioned. There should be a pair for each set of fets.

If all you have at your disposal is a DMM, then the fets themselves should be tested from Drain to Source, there should be a nearly infinite resistance from drain to source. From source to drain, they will always conduct and show a very low resistance. The fets have an internal diode which always conducts in one direction, but won't in the other. Get it? Point is, that if you aren't checking correctly, you may think they are bad when they are not. Go to alldatasheet.com and look up the data sheets for each par tin question. The IRFZ44 sheet will show you which leads are which. Fet's should really be tested with special equipment, but for a simple check of if they are shorted, you can get by okay with the above. Fet's are extremely static sensitive, so you can screw them up just by messing with them. Just a caution. Be static safe, and if you remove or handle them, keep all leads shorted whenever possible.

Upper Right corner, the part shown, the SF163 and SF163A are the power supply rectifier diodes. One for the positive rail, and one for the negative rail. You cannot check them while they are in the amp. The two leads that are marked on the case with a ~ are each tied to one leg of the transformer. W/o having them removed, they will never test correctly. You remove them, and test with the diode function of your DMM. Again, pull the data sheet, but the SF163 is electrically inverse of the SF163A, so the test leads get reversed. It is probably fairly unlikely that there is a problem with either of them.

Bottom left corner, the large Toshiba transistors are the output transistors. The next two smaller ones in between them are the predriver transistors, and the smallest one is the bias transistor. Most likely one or more of the output transistors in shorted, causing your problems. If you find a shorted output BJT, replace both it and it's compliment, regardless of if both show shorted. If you don't, you will do this again soon. You must use a direct replacement if possible for the output BJTs. Some of these are very hard to find now (obsolete), and if you run into that, then either try to order direct from PG, or you will have to change all of the output BJTs to a part that is similar, and still active.

Bottom right, the three resistors circled are not blackened, they are that color because they are kind of odd ducks. They are fusible resistors with a flame proof coating, thus the odd blueish green color. The last black band signifies that they are fusible. They will be open if they are bad. They won't shift in value (much) or burn like a normal resistor. If you have to replace them, it is difficult to find fusible resistors. Digikey has some vishay parts that are 1/3w fusible. I have used them, but I have also replaced them with non fusible types.

Later,
Jason