Jfet muting

[bsco]

Hi...I have a question. Been working on a Yorkville unit....a 100KW and when you turn the power off you get this nasty click from the speaker........When you turn it on, it is fine....only happens when you turn it off....The only thing in the ckt is a JFET and a tap off the power supply to supply -18V to the gate...The Fet in question, (a 2N5638), has a turn on resistance of approx 30 ohms. The resistance is there in both directions....just like a shorted junction.So I sourced a replacement...a NTE467....Same problem...so I took a couple of new ones and tested them with my meter...I discovered that you can't get the gate to turn on and off...it is always on and therefore shows this low resistance reading between the drain and the source. Is this defective, or is this a characteristic of this particular device?? The store that I purchased these from only sell parts..they don't know very much about the specs and such...Has anybody else ran into a problem like this??
Cheers,
Bernie

[Steve Conner]

You're seeing the 30 ohm reading because JFETs are "On" by default. They need a negative voltage to the gate to turn them off.

I expect the JFET is fine and the problem really lies somewhere else. Maybe the mains switch contacts are arcing, producing a burst of RF?

[bsco]

Thanks Steve....I am going to check out other possibilities today...I will try to apply a small negative voltage to the gate to see if it will turn off....I was using my dmm to check it.....I'll try it with an analog meter and look for a simple test circuit which I can use to test these...Once again, thanks for the reply.....
Cheers,
Bernie

[Jazz P Bass]

Yotkville 100KW Service Manual

I am not convinced that the jfet could cause a click.
The circuit is a delayed unmute.
If you disable the mute circuit (always muted) that may tell you if the circuit is causing the click.
Removing R40 (2.2K) which feeds the circuit will disable it. (page 4, upper right corner)
If you still hear the click then it is something else.

[bsco]

OK. Jazz P.....I will try that and get back to you....
Cheers,
Bernie

[bsco]

Hi Jazz P....tried what you had suggested...and you were right..the loud click is still there.....so it has to be coming from somewhere else....Just for the hell of it, I dis-connected the power switch to see if that wsa it but no difference...I'll have to dig a bit deeper...
Cheers,
Bernie

[g1]

The mute circuit is for turn on, not for turn off. As Jazz stated it is a "delayed unmute" when you power up the amp.
To have this mute circuit affect the turn off would require a little time machine in the amp . It would need to go back and turn the mute Fet on before you had turned off the power switch.
Turn off thumps are often due to an imbalance as the power supplies are collapsing. They are often inherent in the design, or can be a result of tolerances in supply components. If you tried a dozen 100KW's maybe half would do it and half not, it's hard to guess.
Two key questions are: how bad is it?, and did it always do this?

[bsco]

Hi g-gone...Thanks for the reply. The amp only started to do this recently and didn' do it before..It is fairly loud....sometimes worse than other times. I did scope the supply lines and noticed a very sharp pulse when this occurred.....SO to check for a power supply imbalance, I would have to scope the plus and minus outputs and watch them as I turn it off??? I am thinking it might be filter cap related and there are four of them there...due to the hassle of disconnecting all the wiring and removing the board it would be easier to replace all four at once....I am going to try and scope the supply lines to see if they both fall at the same time....that is what you are referring to by an imbalance???
Cheers,
Bernie

[Jazz P Bass]

[QUOTE=g-one;291359]The mute circuit is for turn on, not for turn off. As Jazz stated it is a "delayed unmute" when you power up the amp.
To have this mute circuit affect the turn off would require a little time machine in the amp . It would need to go back and turn the mute Fet on before you had turned off the power switch.
QUOTE]Oohee!
Pretty sharp there G-One.
Hmmm. A time machine.

[mozwell]

Its uneven collapse of the pwoer supplies, which puts a momentary DC onto the speaker.
No problem, unless the DC is really big, can you measure it (connect a DC voltmeter onto the speaker terminals & measure the turn off DC tump voltage).
thsi turn off thump is inherent in the actual amp design, it will vary between amps due to transistor gain differences (& other things)

You cannot rely on the Mute Fet to fix this turn off thump, the mute fet is just for turn on.
If all the power supply caps look ok, then one option is to fit a speaker protection circuit.
These would work fine & uses a relay to disconnect the speaker as soon as the mains is turned off. It will also protect against DC on the speaker
Loudspeaker Protection and Muting
Project 111 - PIC Based Speaker Protection

[g1]

Like Mozwell said it is uneven collapse of the supplies. Yes, scope them both and watch at turn off. It could be in the higher voltage supplies or the lower voltage supplies.
It could be a cap or even bad solder on one of the caps. For that matter there are probably many places where an iffy solder joint could cause this problem.
Not sure the age of the unit but like you say, replacing all 4 caps at once is probably easiest.

[bsco]

Thanks Mozwell......I scoped the supplies and they look pretty normal. There are a bunch of resistors there as well...560 and 680 ohm...5%.....they measure within tolerance....the 2200uf caps measure around 2000 uf...the 470uf ones measure around 430uf.....I could see it if one of the caps were down something like 50% but they measure roughly the same within each set.....however they are a bit lower than the value marked on the case....they usually have a 20% tolerance so they should be fine...I don't have any new ones here to put in....I was thinking of installing a couple with higher voltage and capacitance ratings just for a test...install 3300 instead of the 2200's and probably 820 or 1000 for the 470's....all rated at 100volts.......just to see if the problem goes away or remains...what do you think??

[Enzo]

Even if the two rails both discharge at exactly the same rate, many amps are unstable below certain voltages, and when that discharge gets down that far, the amp can swing rail to rail - your loud noise.

[bsco]

Thanks Enzo...I scoped the power rails again and noticed that on the +and- 16V lines, everytime I turned the amp off I got a very quick burst of noise on the scope display..and this blip on the scope would occur as soon as I switched the power off.....I double checked the +and- 38V rails and they looked good....The owner of this amp said this problem just started to happen....so I guess there must be something there that is doing this..by the way, I temporary installed two 1000uf 100V caps for the 470uf/50V ones( which filter the 16V rails)...same thing....and all the supply rails are dropping at the same time....so either the owner is not telling me everything or I am missing something here...maybe you are right ....maybe the noise was there all the time...I did notice that sometimes the noise is not as loud and there are a few occasions where the noise was gone altogether......this is weird for sure.....

[bsco]

Hi Enzo...and whoever else is out there. The guy who owns this amp said that it was always quiet...this problem has happened just recently......he also mentioned that there was a hissing sound coming out of the speakers as well...but I never heard any hissing sound unless you crank all the controls up.....he said that the hissing sound was there even with the controls turned down...I went through all the connections and touched up any that I thought might need it.....made sure I had good grounds, etc, etc......the popping noise is still there...going to call it a night and give my brain a rest.....