Zx 350 Amp repair

Fluid76 · Post by **Fluid76** » Sun Mar 11, 2012 9:43 pm

I am interested in repairing my faithful old ZX350 amp, I'm in Australia and don't know of any local repair shops and shipping to the US is not viable. As you can see it has a badly burnt FET and some damage to the PCB. I have removed all 6 of what I believe to be the power supply FET's (P55N06). All 6 tested bad (short between Gate and source or between the Drain and source), I have also ohm the fuse resistors 4 of the 6 read 33ohm the two outer resistors to the right of the burn on the PCB read 26 and 25 ohm. There is also signs of heat on the underside of the resistors nearest the rail cap's but all resistors read 118 ohm. Not clear on how to check the PWM power supply controller chip or the BJT's. When the amp first failed It was still showing the green LED but the internal fan was pulsing. The smell of burnt electronics and no output was the signs of a problem. Any help with components to check, Or contact for someone in Aust that could repair would be very helpful. thanks in advance.

Post by **Eric D** » Mon Mar 12, 2012 10:09 am

Remove the 6 resistors you have shown. Also remove all the leads to the 6 FETs you removed.

Hook the amp to power and remote turn on. Put a 5A fuse or at most 15A in line with it.

Turn it on.

Connect DMM from main negative power lead, to DMM negative, and DMM positive to any one of the PWM controller side of each of the missing 6 resistors. Put DMM in AC mode and turn it on.

You should get around 4 or 5 volts AC, and in DC mode you should get 5 or 6 volts as well. If you get something really different than this, then the PWM controller is probably bad. If you get something close to the numbers I have posted, then...

Replace the 6 resistors and 6 FETs.

Put heatsink back together.

Power on amp with 15A fuse and see what happens.

Fluid76 · Post by **Fluid76** » Mon Mar 12, 2012 3:46 pm

Thanks for your help Eric I will do this today and post my results.

valeks1 · Post by **valeks1** » Mon Mar 12, 2012 6:50 pm

Check Output transistors.

Fluid76 · Post by **Fluid76** » Mon Mar 12, 2012 9:49 pm

Ok Eric all tasks completed (see second pic). Amp powered up well with a 10 amp fuse in line, I got a green LED and the fan was running. When I tested the resistor terminals on the PWM controller side the readings where 10V~ in AC mode and 4.6V in DC mode all of the opposite terminals read zero except R109 lower terminal which reads 1V in DC mode. I also checked the FET terminal's an the centre leg positions(No.2 position on PCB) all read 26V~ AC mode and 12.6 V in DC mode all other positions read zero except the No.1 position on the burned position on the PCB, this reads 1V in DC mode (I suspect a small short on the PCB from the burn)?

valeks1 How would I check the output transistors, are they the ones in the first pic?

Post by **Eric D** » Tue Mar 13, 2012 2:17 am

The output transistors are the large ones in the top photo of your above post. You would test them the same, looking for shorts between their pins.

R109 should not give you 1V on the bottom. Also, you should not see 26V AC on any of the FET pins, since your power supply is only 12.6V. Either your DMM does not work well in AC mode, or your power supply is putting out excessive ripple.

Fluid76 · Post by **Fluid76** » Tue Mar 13, 2012 4:34 pm

I will see if I can get hold of another DMM and retest.

Thanks again Eric D.

Fluid76 · Post by **Fluid76** » Tue Mar 13, 2012 4:36 pm

I will get hold of another DMM and retest.

Thanks again Eric D.

Fluid76 · Post by **Fluid76** » Wed Mar 14, 2012 8:46 pm

Ok so I checked again with another DMM (of same brand) and got the same readings, also performed the same test on my good working ZX450 and got the same readings, so I think my DMM's don't work so well in AC mode. I am still getting a small 2.6V at the lower position of R109 and No.1 leg position of the burnt FET, these are Isolated on the PCB so I am thinking of creating a direct link to the No.1 leg of that FET to the lower lead of the R109 resistor to eliminate the voltage leak?
On closer inspection I found the power supply caps are leaking so I will replace these as well.
Would it be wise to replace the PWM controller (3525) just to eliminate it from possible faults.

Parts list. SG3525ANG (PWM controller)
EEUFC1C822 (Cap 16V,8200uF)
CFR16J33R (33ohm resistor)
RFP70N06 (FET's)

Post by **Eric D** » Thu Mar 15, 2012 2:01 am

The PWM controller is cheap enough, it would be a good idea to replace it. I often mount a socket for the new one when I replace them. This just makes it easier to swap one out in the future. Not a requirement by any means though.

Fluid76 · Post by **Fluid76** » Thu Mar 15, 2012 6:08 pm

Thanks for that idea I will surely be fitting a socket now.
Parts are ordered should be here in a couple of days, I will be sure to post my progress.

Fluid76 · Post by **Fluid76** » Tue Mar 20, 2012 3:09 am

So my parts arrived today AU$74 ( this includes caps and spares for my ZX450) I removed the caps as described in another post on this site (thanks guys) then removed the PWM controller ( Not in a rush to revisit this job) and fitted the 16 pin socket then the new SG3525ANG. Next I replaced the caps and fitted the new RFP70N06 FET's and the 1/2 watt 33 Ohm resistors, you will notice my small modification (red srink wrap) to bypass the PCB trace, this was the best I could come up with. Now for test time, I powered it up with 2 x 3.6 Ohm resistors (from an old 64 chev heater control) in series to limit voltage and a 10amp fuse in line, result- everything working well. Reassembled the amp with all new heat sink compound. Then I removed one of the 3.6 Ohm resistors and tested with a small vol to the subs still all good. Finally I removed the resistors and ran the system under a lite to medium load with the 10 amp fuse still in line just in case, result- My trusty ZX350 is back ( very happy ). I would Like to say a BIG THANKYOU to Eric B and valeks1 for there assistance and also all the people who have posted there repairs and modifications on this site, ( I read a lot before I posted my problem ) and this awsome site you rock. Next job will be to recap my ZX450, I may even attempt rail caps in the future?