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SA7700 protection relay dropping out

The resistors in the protection circuit are probably fine, it seems to be working. The problem is in the overload detection, which is Q19 and Q20 and their related components. Lifting D17 removed overload detection from the protection circuit, proving that is the path the trigger is coming from. What is still to determine is Left, Right, or both.
The process Mark is trying to get you to do is:
Turn the balance control all the way to the left and leave it there, play music and slowly increase the volume until it trips the protection circuit, or increases past the point where it was tripping the protection circuit. Note where the volume control is if it does trip. Then reduce the volume, turn the balance all the way to the right and leave it there, play music and slowly increase the volume until the protection circuit trips or increases past the point where it was tripping the protection circuit. Note where the volume control is if it does trip. This will determine whether it is the Left channel or the Right channel, or both causing it to trip. At this point it is irrelevant that Q19 and Q20 are new, as replacing them did not change the outcome of testing.
Did you change C319, C320 to 0.22µF film caps?
 
1. after Q19 & Q20 swapped, test again, using balance and speakers connected. which channel ?
2 then use balance with NO speakers, does it trip ? which channel?
 
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markthefixer said:
1. after Q19 & Q20 swapped, test again, using balance and speakers connected. which channel ?
2 then use balance with NO speakers, does it trip ? which channel?
Click to expand...
Wellllllllll......... Upon further investigation Q20
was in wrong. It got swapped at the beginning and I assumed it was right. The silkscreen on the board is super hard to read and I had the emitter and collector backwards. It’s waking the neighbors now. My bad. I’m using readers, a bright light AND magnifiers. My eyes suck. To EVERYONE who helped me THANK YOU. I now know an SA 7700 protection circuit very well . I should have looked at the schematics better. God I feel stupid. Oh well. Live and learn. Thomas Edison once said he never made mistakes. He just found 10,000 ways not to do something.
 
Stereoboy70 said:
Wellllllllll......... Upon further investigation Q20
was in wrong. It got swapped at the beginning and I assumed it was right. The silkscreen on the board is super hard to read and I had the emitter and collector backwards. It’s waking the neighbors now. My bad. I’m using readers, a bright light AND magnifiers. My eyes suck. To EVERYONE who helped me THANK YOU. I now know an SA 7700 protection circuit very well . I should have looked at the schematics better. God I feel stupid. Oh well. Live and learn. Thomas Edison once said he never made mistakes. He just found 10,000 ways not to do something.
Click to expand...
markthefixer said:
1. after Q19 & Q20 swapped, test again, using balance and speakers connected. which channel ?
2 then use balance with NO speakers, does it trip ? which channel?
Click to expand...
it was always the left channel. After putting the Q19 transistor back in it plays a lot better. But if you run it so the peak lights flicker on it will eventually shut down again. I suppose this is normal? I mean you can’t just run an amp full blast all the time.
 
The protection circuit is not proactive, it reacts to current conditions.

IF it pops after "pouring on the coal", it's because something in the amp was disturbed. Find where the trigger is coming from.

I have had badly clipping high power amps run super hard without tripping the same protect circuits you have.

Among other things I don't trust any transistor that was run in an incorrect installation, my favorite quote is"
"it was in a race to destruction and placed second".
 
markthefixer said:
The protection circuit is not proactive, it reacts to current conditions.

IF it pops after "pouring on the coal", it's because something in the amp was disturbed. Find where the trigger is coming from.

I have had badly clipping high power amps run super hard without tripping the same protect circuits you have.

Among other things I don't trust any transistor that was run in an incorrect installation, my favorite quote is"
"it was in a race to destruction and placed second".
Click to expand...
I replaced the transistor with a new one. I had it jamming and it only trips if you keep seeing the peak lights stay on. Not sure why you would want to run a 60 watt amp at 9 for very long anyway.
 
Hi everyone,

I am having the same issues as stereoboy with my sa7700 and have followed this thread to see if I could remedy the issue. Unfortunately no luck so far. I have done the following:

Replace q19 q20 q21 c319 c320

I have the following observations:

Bias has been checked and is good. Measure about 7mv at l/r speaker connections

At Overload IC:

Does not activate relay with Q21 pulled (can turn it way up)
13V to Ic is high 17v
-13.5 v is low -10v

Power supply investigation:
Q26 resoldered (runs pretty hot)
VU Meter voltage is ok (7.6V)

70V across C603 (should be 50V)?
-72V across C602 (should be -50V)?
100VAC across T2

Relay always trips at 1-2 watts of output on the VUs. Trips regardless of channel used or balance. Interestingly it trips both when I use an 8ohm dummy load as well as when no speaker is selected and nothing is connected!

I have a multimeter but no scope.

Thanks guys!
 
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the term bias is a bit too nebulous.

There is DC offset measured at the speaker terminals.
There is Idle Current measured at left channel TP 22 & TP 25, right channel TP 26 & TP 28

examining your post, I see dc offset of 7 mV on each channel. Considering the circuit, A re-reading is warranted. too perfect, too similar.

Does not activate relay with Q21 pulled (can turn it way up)
Click to expand...

Relay always trips at 1-2 watts of output on the VUs. Trips regardless of channel used or balance. Interestingly it trips both when I use an 8ohm dummy load as well as when no speaker is selected and nothing is connected!
Click to expand...

Past history of that symptom (always trips irrespective of load) was that the over current sensing transistors (Q19 & Q20) were installed incorrectly.
if not (prove they are in right!!) then
Put Q21 back in, pull D17, see if trips.
Put D17 back in, pull Q19 & Q20 and see if it trips. looking for shorts on the pattern.


70V across C603 (should be 50V)?
-72V across C602 (should be -50V)?
Click to expand...
Don't sweat it, higher doesn't hurt and it is dependent upon the incoming power line voltage.
 
Sure enough, it was q19 and and q20 that were the problem.
When I replaced the transistors, I noticed they were the older 2sc869 style, so I simply assumed they were original and put the new ksc1845s in with the same orientation.
After scrutinizing the schematic, we realized the old transistors were in backwards! I flipped both transistors around and sure enough, problem solved.

My big lesson: Never make assumptions.

Thanks for all your help, this community is awesome!
 
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