Kenwood KAC-628

[mike49504]

I recieved this amp in for repair. The power suply fets are IRFZ34G's Wondering if i can swap them out for IRF1010N's?

 

[Perry Babin]

It looks like they will work (assuming that the drive circuit is the same as the 728) but you'll need to confirm that they don't run hot at idle. If they do, you'll have to replace the 68 ohm gate resistors with 47 ohm resistors.

 

[mike49504]

Perry,

I stuck the IRF1010N's in this amp and it draws excessive current and the 1010n's get red hot. I checked the drivers they seem to check fine But i cant seem to find the gate resistors for this amp.

 

[mike49504]

Only resistors i see connected to the first leg of the fets are 1.5K ohm (brown,green,red,gold)

 

[brandes.cm]

Don't know how close the KAC-606 or 626 are to one you are working on, but the manual can be found here.
(bottom of page)
R104-108 look to be 100ohm.
https://sites.google.com/site/brandescm/home/projects

Zooming in on your pic shows the board #'s to match the service manual in my link.

 

[mike49504]

Ok only resistors i saw connected to the fets are

LEG1:1.5K ohm

Leg3:10 ohm

With the IRF1010N's in the amp they heat up within a sec and blows the fuse to the amp.

The drivers test fine So wondering what do i check or need to do at this point

 

[brandes.cm]

I was looking at the outputs when I quoted R104-108. The power supply fets look to be driven directly from the 494/594 through R123 and R124. Those tie to 1.5K R126 and R125. R125 and 126 are acting as pull down resistors, keeping the gate low when not on. Are the 10ohm parts OK. Is the xfrmer shorted or anything in the secondary side to the supply?

 

[Perry Babin]

If the only component that's pulling the gate voltage down is the 1.5k ohm resistor, you will need to reduce their value to use the 1010s. A 330 ohm should be sufficient. If it's not, you may have to use a different FET (one that's easier to drive).

 

[mike49504]

Wondering would the 2sc4064's work for the fets?

 

[Perry Babin]

That's not a good sub. Did you try try reducing the value of the gate pulldown resistor?

If all else fails, you could order the original parts.

 

[brandes.cm]

IRF540 or FQP30n06 looks to be a good cross. 30A, 60V and the input cap. is close to the same as the IRFZ34's.

 

[mike49504]

Ok i found some IRFZ34's put a different package style so i put mica insulators under them and used thoose plastic washers. Amp is up and working.

The Irf1010n's with the 330 ohm pull down resistors Will not work in this amp tryed it and blew the fets alomost instantly

 

[brandes.cm]

The input capacitance of the IRF1010's is over 3X that of the IRFZ34, probably had fets conducting at the same time.

 

[Perry Babin]

You should not have blown a single FET when trying substitute parts. Having them clamped tightly to the sink and using an inline current limiting resistor will prevent any damage to the FETs. A $10 current limiting resistor would have paid for itself with this one repair.

In my first post, I stated that, if the drive circuit was the same as the 728, they should work. I would have provided the service manual if asked so he could have compared them. Obviously it was different. The 528, 628 and 728 share the same owner's manual which meant that they were manufactured at the same time. Many times, amps that are manufactured at the same time have the same basic circuits, the only difference being a different number of power semiconductors.

 

[brandes.cm]

Here is a partial pic of the 626 schematic, it listed the same board # as the 628. I don't know how it compares to the 728, I don't have it that manual.

At this point I guess it just doesn't matter.