Yorkville NX55P

[bsco]

Got this unit and it works great with the exception that when you turn this thing off, it makes an awful loud pop which seems to come from the horn...The woofer section seems fine....Can't find anything gone in the Silent On/Off circuitry....and don't want to start swapping out stuff till I can figure out what is going on.....
cheers,
Bernie

[g1]

Schematic:
http://yorkville.com/downloads/servman/sm_nx55p-r2.pdf

[g1]

Is Q9 Fet going low resistance as soon as power is turned off?

[bsco]

Hi g-one......I was trying to post a schematic but I still have problems trying to figure out how to use some of the features of the forum. I did notice that these systems use different fets for the silent on/off..some of these units use a 2N5638 while others use a J111 Fet.....I guess for all intents and purposes they are the same..but I think the pin-outs might be different.....There is also a cap used for power switch spikes.I am going to check that out tomorrow...as I can't find anything wrong with the Fet circuits.....then again, I could be missing something....I'll let you know how I make out.....
Cheers,
Bernie

[bsco]

actually, I did notice this but I thought this was due to circuit components in the circuit.....I was going to take it out to test it tomorrow.....

[bsco]

Hey g-one....I replaced the fet as the readings didn't look right.....same problem....so I grounded the gate which should put the horn amp into mute all the time....same thing.....So I sprayed the differential circuit area with some freeze spray just to see what would happen......problem went away....heated it up and the problem returned....so i made up an insulated shield so that I could spray individual components....When D7 or Q7 was hit, the problem was gone so I replaced both of them.....same thing...So I started to hit other individual components again and Q5 showed up...replaced that but still no go......You had mentioned about the low resistance on Q9 fet when the power was turned off.........what was on your mind??????
Cheers,
Bernie

[g1]

As far as the Fet, that was just to see if it was doing it's job. But you grounded the gate and still had the pop. So the problem seems to be within the horn amp and the silent circuit can't help with that.
The results from the freeze spray lean toward an imbalance in the diff. amp causing the pop. Did you try replacing D7? For the price of the parts and time involved, I would probably replace D7 and the 4 diff. transistors.

[bsco]

Thanks for the reply g-one.....Yes, I did replace D7 and I also replaced Q5 and Q7 as these were the transistors that were affected with freeze spray....
When I sprayed the other components in that area it did not have any effect....so this is why I replaced just these three components....I'll replace the other two transistors tomorrow as it is late here now.....but for some reason, I do not think that is the problem...but I;ll try it and see what happens....any other suggestions or comments greatly appreciated......
Cheers,
Bernie

[g1]

Forgot to mention this, but to be completely sure about the mute Fet Q9, you should try with the Drain grounded.

[bsco]

OK.....I will check that out...One thing I noticed...different grounds....when I checked them with a meter, the horn ground and the low voltage power supply ground are one and the same....but the schematic shows them as being different....at least they are labeled in such a way as to lead you to believe that they are different.....

[Enzo]

They are different. That doesn;t mean they don;t all connect together somewhere. Different branches of a star ground.

[bsco]

I ended up getting side tracked on this one..but getting back to it now.......I replaced the 4 transistors...(Q5,6,7,8) and 18V zener diode,(D7), in the differental circuit and still have the same problem......The problem must be further downstream towards the output....funny that when I cooled the components down with freeze spray, the problem went away....Will have to tackle this tomorrow....getting late here.....
Cheers

[bsco]

They are different. That doesn;t mean they don;t all connect together somewhere. Different branches of a star ground.

Enzo....if it is a star ground, and they all lead back to one point, then should it make a difference which node of the star you use for ground?? just curious....
Cheers...

[Enzo]

The star ground is just the one node. All grounds connect to it.

The point of the star is that "grounds" have resistance, and where there is resistance, current causes voltage drops. Your meter may have a hard time resolving it, and a half an ohm may not sound like much. But if you connect a filter cap to the same star branch as your input stage ground, you will get 120Hz hum from ripple currents.

[bsco]

Thanks for that tid bit of info Enzo.....I scoped the horn output section of the amp and whenever you turn the amp off, you will see a spike at the horn connector...If I work backwards I see this spike on various mosfet and transistor leads but one side of the circuity seems to have a greater spike than the other...the problem is being caused by something after the mute fet and differential output......anyway, I have to do some more tests first...then I am going to post my findings.....
Cheers.....