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SA7700 protection relay dropping out

It's not - it's drawn badly.

What ABOUT Q21? Have you done the test? Pulled Q21? it's a 2sa720. you could use ksa992.
It's used to get the Q19 & Q20 protect signal back to the pa3004 in a way the pa3004 understands.
Q21 connected to the +48v line and it conducts putting DC on pin 4 of the pa3004 when the base (+48v) is pulled down a volt or so by Q19 or Q20.
USUALLY I see this type of circuit connected to the +13v used to power the pa3004, but nooooo... not this one.
ANY leakage through Q21 will do false triggers, and with that high of a voltage leakage (and shot noise) is much more likely.
shot noise and leakage from Q19 or Q20 is much more likely with this +48v on their collectors too.

so, have you pulled the cursed thing and did it stop the bad behavior?
 
markthefixer said:
It's not - it's drawn badly.

What ABOUT Q21? Have you done the test? Pulled Q21? it's a 2sa720. you could use ksa992.
It's used to get the Q19 & Q20 protect signal back to the pa3004 in a way the pa3004 understands.
Q21 connected to the +48v line and it conducts putting DC on pin 4 of the pa3004 when the base (+48v) is pulled down a volt or so by Q19 or Q20.
USUALLY I see this type of circuit connected to the +13v used to power the pa3004, but nooooo... not this one.
ANY leakage through Q21 will do false triggers, and with that high of a voltage leakage (and shot noise) is much more likely.
shot noise and leakage from Q19 or Q20 is much more likely with this +48v on their collectors too.

so, have you pulled the cursed thing and did it stop the bad behavior?
Click to expand...
I replaced the q21 with a 2sa992. Same issue
I’ll check the bias again. And report back.
 
Don't hook up speakers, use dummy loads, Q21 out disables part of the protection circuit. IF the circuit is false triggering through Q21, removing it will disable the false triggering as well. It is an information gathering step. If you have speakers hooked up with a disabled protection circuit you risk damaging your speakers. See MTF's post, #24.
 
merlynski said:
Don't hook up speakers, use dummy loads, Q21 out disables part of the protection circuit. IF the circuit is false triggering through Q21, removing it will disable the false triggering as well. It is an information gathering step. If you have speakers hooked up with a disabled protection circuit you risk damaging your speakers. See MTF's post, #24.
Click to expand...
How do I find that post? Thank you
 
markthefixer said:
It's not - it's drawn badly.

What ABOUT Q21? Have you done the test? Pulled Q21? it's a 2sa720. you could use ksa992.
It's used to get the Q19 & Q20 protect signal back to the pa3004 in a way the pa3004 understands.
Q21 connected to the +48v line and it conducts putting DC on pin 4 of the pa3004 when the base (+48v) is pulled down a volt or so by Q19 or Q20.
USUALLY I see this type of circuit connected to the +13v used to power the pa3004, but nooooo... not this one.
ANY leakage through Q21 will do false triggers, and with that high of a voltage leakage (and shot noise) is much more likely.
shot noise and leakage from Q19 or Q20 is much more likely with this +48v on their collectors too.

so, have you pulled the cursed thing and did it stop the bad behavior?
Click to expand...
Yes. Problem went away when q21 was pulled.
 
The usual fix for false triggering is to replace the 2 overload sense transistor (Q19, Q20) with KSC1845F, if you have them, and 505-MKS2C032201BKI00 (or similar film) for C319, C320.
 
he's already done new c319,c320,q19,q20.

put in a new Q21 (2sa720 >> ksa992), then if it still does it, pull d17.

merlynski - this circuit is different than usual because the Q21 used to fake a DC in for pin 4 of the pa3004 is
being run off of 48 volts, instead of the usual 13, making q21 much more liable for leakage.
 
markthefixer said:
he's already done new c319,c320,q19,q20.

put in a new Q21 (2sa720 >> ksa992), then if it still does it, pull d17.

merlynski - this circuit is different than usual because the Q21 used to fake a DC in for pin 4 of the pa3004 is
being run off of 48 volts, instead of the usual 13, making q21 much more liable for leakage.
Click to expand...
Do I replace D17? I think I have a replacement. 1N4148. Or just leave it out?
 
IF you needed to continue testing, because Q21 went back in and the problem reoccurred:
THEN you would disconnect D17 - it's disconnection places an air gap between Q21 and the triggers from Q19 & Q20.

A D17 replacement is NOT indicated when D17 is eventually reconnected.

Without Q19 & Q20 triggers coming in we can see if Q21 is causing the mess itself, either by it's leakage or, say as an example, a solder filament or flux blob on the solder side of the board.
Flux blobs become a real possibility when Q21 is run at 48 volts instead of 13 volts. It could be between a speaker trace and a trigger trace - I haven't started looking for that in the documentation - YET- .
 
markthefixer said:
IF you needed to continue testing, because Q21 went back in and the problem reoccurred:
THEN you would disconnect D17 - it's disconnection places an air gap between Q21 and the triggers from Q19 & Q20.

A D17 replacement is NOT indicated when D17 is eventually reconnected.

Without Q19 & Q20 triggers coming in we can see if Q21 is causing the mess itself, either by it's leakage or, say as an example, a solder filament or flux blob on the solder side of the board.
Flux blobs become a real possibility when Q21 is run at 48 volts instead of 13 volts. It could be between a speaker trace and a trigger trace - I haven't started looking for that in the documentation - YET- .
Click to expand...
Thanks for the help. I’ll get back on it tonight
 
Hello. Check
markthefixer said:
IF you needed to continue testing, because Q21 went back in and the problem reoccurred:
THEN you would disconnect D17 - it's disconnection places an air gap between Q21 and the triggers from Q19 & Q20.

A D17 replacement is NOT indicated when D17 is eventually reconnected.

Without Q19 & Q20 triggers coming in we can see if Q21 is causing the mess itself, either by it's leakage or, say as an example, a solder filament or flux blob on the solder side of the board.
Flux blobs become a real possibility when Q21 is run at 48 volts instead of 13 volts. It could be between a speaker trace and a trigger trace - I haven't started looking for that in the documentation - YET- .
Click to expand...
Checked the solder joints and flux blobs and cleaned up any sketchy areas. Re installed Q21 and I can actually turn up the volume a little more past 5 but it still will kick open the relay. Would replacing the diode associated with this circuit benefit me ? The 48v is leaking somewhere and it’s a bitch to find. Thanks again for your help.
 
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