Jfet muting

[Jazz P Bass]

Alright, you want to nail this puppy.
You seem to have the equipment so look for where the impulse to the power amp comes from.
Is the pulse on the EFX out.
If not, that narrowed it to the power amp.
Start at the mosfets. Is the gate getting driven.
Which one.
Or is it random.
Once you can see the pulse you can work back to where it starts.
You may have a blip on the 16 volt supply, but a circuit may not like it.
If you can narrow it down to that circuit you can then see what can be done to alleviate it.
Or not.

[Enzo]

The problem with blips on the power supply is that any loud impulse signal will cause a blip there. A loud pulse puts a transient drain on the main power rails, and the low voltage rails are either derived from the high voltage rails or at least comes from the same transformer, so they can see it too. Take a normal working amp, scope the rails as you put impulse noise through it. See what happens.

[bsco]

Yes Jazz P. I want to nail this as I will learn some very good troubleshooting in the process.. I'll scope it out again today and let you know what I find...there has to be a reason for this to happen as these amps don't have that problem when working properly...by the way, the amp plays fine.....I will let you know what I find....
Cheers,
Bernie

[bsco]

I did as you suggested Jazz P.....the loud high freq click or pop at turn-off is at the effects send jack....so I went and scoped the outputs of IC1 and IC2....same thing....scoped the 38V supplies....clean....scoped the 16V supplies, this pop is visable on the trace to both 16V supplies(+ and -) s soon as you turn it off.....it is immediate...just for the hell of it, I used another power switch....same thing. So, the only thing left is the first op-amp or the supplies that feed it....I have already swapped out the caps in the 16 supplies with 1000uf/63V caps...same problem. I checked all the resistors in that circuit to see if any had changed value and they are all within spec....I was thinking of disconnecting the 16V supplies and running them from external supplies or even a couple of 9V batteries...I only just want to test it for a couple of power cycles..just to see if the pop goes away...At least then I will know for sure if it is power related or not. Any thoughts on this procedure before I attempt it??

[Jazz P Bass]

My only thought is that with the amp muted (Post #3) the amp still made a turnoff noise.

[bsco]

I don't follow you Jazz P....Some of the posts have stated that the circuit is a power on mute..and not a power off mute...therefore, the noise is getting through somehow...There are two different grounds in the amp....a chassis ground which is isolated from the signal ground through a .047uf cap...I scoped the chassis ground and I can see that spike...it appears to be everywhere, on low voltage power rails, input and output pins of ic's....one thing I did notice.....every so often when you turn off the amp it is very quiet.....but nine times out of ten....that loud pop is there....and another thing....sometimes this pop will have a different volume level...I have banged my head off the wall long enough for one day....I am going to relax tonight, have a jam with a few friends and a few beer...and come back to this in a day or so....
Thanks everybody for all your input so far....
Cheers,
Bernie

[52 Bill]

Try plugging in a cord to the FX return jack or better yet a shorted plug. Does that kill the pop? If it does the noise is being carried on the audio signal, if it doesn't go away it is being carried on the power supply rails.

Also do the control settings change the pop level?

[Steve Conner]

The JFET circuit should be powered off a rail that collapses quicker than the others, so it will mute the worst squeals and thumps at turn-off.

But as someone stated above, muting the RF noise from arcing switch contacts would require a time machine, because it appears at the instant the contacts break, before the power supply rails even start to fall. And the description given above is consistent with arcing contacts. Removing the power switch doesn't prove anything, because you still have to use some other switch to control the power, and that one will arc instead.

It is common to use a 0.1uF capacitor across the transformer primary, or across the power switch contacts, to snub the RF. Sometimes with a 10 or 100 ohm resistor in series. Maybe the capacitor died or came loose from the board. Maybe it was never there, in which case the popping may well be "by design", no fault to fix.

If you decide to add a capacitor, look for a "Class X" unit festooned with safety agency approval logos.

[Jazz P Bass]

I don't follow you Jazz P....
Bernie

My point was that you jumped to the preamp.
In post #3 you disconnected the unmute.
Therefore the 'input' to the power amp was basically grounded.
And you still had the pop.
That would steer me to the power supply/ power amp.
You may very well see weird stuff on the preamp.
There very well may be issues with the preamp.
But the preamp cannot make the power amp pop if it 's signal is grounded at the power amp.
Follow?

[bsco]

OK. Yes.....SO when the gate voltage is removed from the Fet,it's resistance is still at 25 ohms or so...so then the pre-amp is always grounded??I will dis-connect that R40 again and retest everything..to be quite honest, this has me totally confused...
Cheers,
Bernie

[loudthud]

Have you checked the bias on the output amp? (See note next to QA4) If it's too high, that would cause the rails to collapse faster than they should. The circuit driving the JFET should mute the amp when the amp is turned off. D8 discharges C31 through R41 although there will be a slight delay. R42 causes a long delay at turn on.

[J M Fahey]

Agree with Steve Conner.
Get a .047uf or .022uF **250VAC** rated.
This is *not* 250V DC by any means but a capacitor which has been *Power Line* rated for power lines up to 250V (so it includes 240/230/220/120/110/100V)
A modern name for that is "X" rated (no, not what it sounds like )
Solder it across the switch contacts.
It should tame the switch off sparks big way.
*Sometimes* it works better with a 10 ohm to 100 ohm resistor in series, and, as they say, YMMV .

In some very critical situations (Yoga Meditation Center / large very silent Catholic Cathedrals / etc.) I have added a "standby switch" which actually cuts the live speaker out and instruct them:
"Turn power on, count to 10, turn standby on.
To turn off, start with the Standby switch, count 10, turn Power off
Between Services / whatever, turn *only* Standby Off/On as needed"
Works like a charm.

Yup, I have such customers.

[bsco]

OK Steve and J M.... I'll try that capacitor...I think I have a couple that came from non-working TV's..I'll check the ac voltage rating on them to make sure they are high enough...The stand-by switch is a good idea as well however, this guy won't go for that as he wants the amp exactly as the way it used to be...I'll also check the bias on the amp "loudthug" and see what it is.....I 'll get back at this hopefully tomorrow afternoon.....Thank you everybody for your suggestions and help....
Cheers,
Bernie

[bsco]

Finally got a chance to get back at this beast. I had a cap from an old scrapped tv so I just temporarily hooked it up to see if it work work(rated for 125V at .047uf)...It worked. Tried it a few times and no loud pops....Just checked the schematic to see what in the hell is going on and the only cap in the primary is a 22nf/400V but it is connected across the transformer primary....there is one other capacitor connected between ground and neutral and that is a 4.7nf at 250V which is also in the transformer primary circuit. I connected my .047 cap directly across the switch...I will pick up one rated for 250V and install it. I'll check this out some more tonight just to make sure. This problem was not there before so maybe one of the original caps in the primary are bad......just a thought......Thanks for all the help everybody......I'll let you know how I make out...
Cheers,
Bernie