What mosfets to go with ??? PPI 2450m

[Samuel Rock]

Well i got a few PPI amps in to work on. one of them has P25N05 switchers and the other IRFZ8N.

the one with the IR switchers has 680 ohm gate and 2 2200uf @ 50v caps

the one with the Moto Switchers has 470 ohm gate and a single 2200uf @50v

I think the Moto fets are the stock ones.. as the solder job looks the same throughout the amp and the amp that has the IR looks like it has been flowed with a iron.

What is running around in my head is what setup to go back with, the MOTO or the IR

both came in for service from the same guy, the amp with the factory Moto's looks like lightning hit it. all the gates are reading open and all the fets are shorted drain to source. and the caps well they plump when you cookem other than this the amp is in good shape i powerd the amp section up with a power supply and she works good.

now the other amp that came in that had the IRs in it customer said the amp was kicking off and on at any given time.. that one was easy it just had a cracked joint on the remote lan.


OOO one thing I did not say is the working amp with the IRFZ48Ns had an ornage sticker on the bottem that said PPI SERVICE DEPT the date is no longer readable.

does any one know why they would bring the gate resistance up with a 48N. i allways thought when you switch to a 48n you had to go with like a 47-50 ohm gate. also the caps are toasted this must be from the ripple current right

[Samuel Rock]

OOOPs the subject should have been a 2150m...

[1moreamp]

right

also the caps are toasted this must be from the ripple current right

Caps this old, just die of old age lol lol... All PPI's this old need new electrolytic caps usually through out the entire amp, by my experience. A repair becomes a restoration piece on amps this old.... these 2200 uf caps were mounted across the mosfets ?? They go south when the fets die usually, if the leads don't just break off the board from vibration.

The amp that uses the 25n06 fets should be able to use a IRFZ-32 or 34 as a replacement.

gate resistors were like 100 ohm < I think >.

IRFZ-48's do not sound right < not at 72 ampere ratings > this amp never handled that much current as I recall. And if my head is correct tonight unless you want to deal with design issues M series amps should go back to 25 to 35 ampere rated mosfets like the 25n06 or Z32 or Z34... Nothing but trouble trying to put the wrong fets in these amps.
Add more power storage caps on the internal rails if you want to improve it, and prevent it from cutting out on over current situations. But this will cause a delayed turn off also from the extra energy stored on the internal rails....hope this helps some..

[Perry Babin]

I think the original gate resistors were 470 ohms in all of these older amps. If you don't lower the value, you have to use an FET with very low gate capacitance (Ciss on the datasheet).

Are you sure that the gate resistors aren't 68 instead of 680 ohms?

[Samuel Rock]

Perry they are 680 i am thinking they put the 680 in when they put the 48n switchers in.

1moreamp yes the caps are across the fets, i have worked on a few amps in this era before but did not remember what fets were insatalled by PPI

I know the caps die with age, but i have never seen them swell up as large as this one did...

I think i am going to change the fets in the working amp too and put the 25n06 or an equal and the right gates into it, i just dont want this amp to go out on this guy he sends me a lot of equipment to work on.

[Perry Babin]

The 680 ohm resistors with the Z48s likely caused/allowed cross conduction between the PS FET banks and that's what caused the caps to overheat and swell.

[Samuel Rock]

when i put the 25n06 back in should i go back with the 470 gate or go lower and let the fets switch faster.

The reason i ask that is i found a post on here someone said they put 47ohm gate when the amps went in for service to PPI.

[Perry Babin]

If you lower the gate resistor value, you can go with slightly better FETs. 68 ohm and IRFZ44s would probably be a good choice. You don't want to go with an FET with too much gate capacitance. The Z44 is a good compromise between high current handling and relatively low gate capacitance.

If you want to use the 25N06 or the Z34 (originals in some of these amps), lowering the resistors to ~100 ohms would help insure that they switch off quickly enough to prevent cross conduction.

[1moreamp]

1moreamp yes the caps are across the fets, i have worked on a few amps in this era before but did not remember what fets were installed by PPI
I know the caps die with age, but i have never seen them swell up as large as this one did...
I think i am going to change the fets in the working amp too and put the 25n06 or an equal and the right gates into it, i just dont want this amp to go out on this guy he sends me a lot of equipment to work on.

As I recall IRFZ-32 and 34's were stock for these and 25no6's were also used . I only owned three of these myself back then < ahh the good old dazes >

The cap most likely saw a reverse polarity type situation that caused the internals to swell and vent rather then explode like they can and do.

I would put her back stock, they ran just fine this way. And the only thing I ever did to make them batter was to install larger rail caps on the secondary side. It a pretty commonly known thing to do and it works usually very well.

I also strive to restore things 100% original. Less headaches down the road, and at least if there are issues it was not due to going over board on replacement parts. < the simple life >

If you want to use the 25N06 or the Z34 (originals in some of these amps), lowering the resistors to ~100 ohms would help insure that they switch off quickly enough to prevent cross conduction.

Thanks Perry......C

[Samuel Rock]

kool i have 44n fets and 68ohm instock.. i would have to order the 32 or 34... i think i am going to run it by the customer see if he wants to wate on the oem fets or install the 44n.