Crate BX50 noise

[sunshine_bassman]

Resoldered the noisy intermittent input jack, and upon reassembly there’s a loud swooping distortion that sounds only right when you turn it on. Only happens with the internal speaker, doesn’t happen with a speaker hooked up externally. Not sure where to start with finding the source of the noise. Both 4700uf caps read 22 volts.

Schematic: http://web.archive.org/web/200305231...+/23701H9_.PDF

[nosaj]

Resoldered the noisy intermittent input jack, and upon reassembly there’s a loud swooping distortion that sounds only right when you turn it on. Only happens with the internal speaker, doesn’t happen with a speaker hooked up externally. Not sure where to start with finding the source of the noise. Both 4700uf caps read 22 volts.

Schematic: http://web.archive.org/web/200305231...+/23701H9_.PDF

Ok few suggestions. A bigger screen than a smartphone is most helpful for schematic work. It you don't have one I can get you one( I like to trade stuff so if your interested).

Second a scope would be handy here. One for see the waveform on the speaker an after that you can probe pin 2 on the 12pin ribbon cable to see if noise is present there at turn on. What we are doing here is halving the problem. If it is present here it's further back in the preamp. If it is not here at pin2 it is coming from the output transistor.
Being that problem disappears when an external speaker is connected makes me think load. Check the resistance on the speaker.
Let us know what you need there is more than enough abundant knowledge here.

nosaj

[The Dude]

Have you checked the speaker itself? Try subbing in another speaker to the existing speaker connections (not external speaker). Try hooking up the existing speaker to something else.

[sunshine_bassman]

No way to hook up the speaker to another source, not here in the shop anyway. The internal speaker is an 8 ohm, as is the external speaker I used before.

[sunshine_bassman]

So I was just able to test a different speaker, and it still had the swooping distortion. There’s also noise on pin 2 of the ribbon cable when probed

[nosaj]

So I was just able to test a different speaker, and it still had the swooping distortion. There’s also noise on pin 2 of the ribbon cable when probed

Ok using the scope on TP1 and TP2 See if you see that swoop at those points. After that I'd suggest reattaching the PCB back to chassis and putting it back in the cab and see if it's still there.
nosaj

[g1]

A definition of 'swooping distortion' would also be helpful. I'm completely confused as to what that might mean.

[nosaj]

A definition of 'swooping distortion' would also be helpful. I'm completely confused as to what that might mean.

It's real hard to describe, when I was there it's just about a second of noise. Maybe Sunshine can record it.

Also look back over your soldering on the input jack make sure there's no excess solder that could be bridging something,

nosaj

[sunshine_bassman]

A definition of 'swooping distortion' would also be helpful. I'm completely confused as to what that might mean.

It’s half like a buzzer on a game show, half like a fuzzed-out guitar bending a whole step. It lasts about half a second.

[DrGonz78]

I would measure for any DC offset on the output at the time the amp is turned on. Probably not but just curious if there is any DC offset that relates to the noise. Also curious what that opto OC1 function is in this amp.

[DrGonz78]

I know the optocoupler must be part of the limiter circuit. Curious how it functions.

[Enzo]

The output of one of the power amp ICs is sampled back through D5 D6. It is kinda convoluted, but they light the opto when needed. The resistor side of the opto is simple shunt across the signal going to the PA input.

[J M Fahey]

The output of one of the power amp ICs is sampled back through D5 D6. It is kinda convoluted, but they light the opto when needed. The resistor side of the opto is simple shunt across the signal going to the PA input.

Yes.
The typical Crate/Ampeg (SGM will hate me for this ) optical limiter is a VERY clever passive comparator.
I LOVE elegant minimalistic solutions to problems.

1) you have a 3 resistor string from +V to -V rails, here some 25+25V DC.
We are talking R26/27/29 here.

2) R26 and R29 drop somen 10V each, so they set up "reference voltage" points 10V below and above respecyive rails.

3) optocoupler Led is connected to speaker out and both reference voltages through a diode network, so when speaker out peak is "near one rail or the other" LED turns on and LDR limits drive voltage.

How near?

a) to light a red Led you need 1.9V across it.
Plus 2 series diodes (0.7V each) you now need 3.3V
Add typical power transistor loss of about 4V and we need some 7.5V

Add maybe 0.5 or 1V for various losses (usually emitter resistors ) and we are *almost* at maximum possible undistorted pek voltage.

So this clever circuit reliably stops clipping, only "wasting" about 1 V peak ... nothing.

Best feature: it actually tracks rail voltage, no matter what.

As of my SGM mention: this Crate amp is basically same as diamond blue tolex clad "Ampeg" Rocket 50

[DrGonz78]

Okay got it thanks for the great explanation. I was thinking some sort of comparator with the -/+ voltage rails on either side.

[sunshine_bassman]

Is the optocoupler supposed to prevent noise like what I’m getting? I shorted the resistor side with alligator clips and still got the sound.
Prolly worth noting that it doesn’t happen as soon as the amp turns on, but anout three seconds afterward