• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

SA-8100 protection circuit issue

if you pull both dc offset resistors and over-current sense d5 and protection trips it has to be protect circuit .
you did this though and protect was stable .
only other common thing is grounding . we checked power supply and nothing noticed there .
i thought it was narrowed to one amp output .
its a teaser for sure .
 
I agree, we checked all of that, and I had it narrowed down to the channel on pins 16/23 on the power amp. However, there still seems to be some randomness to when it will trip or not trip, and I do not feel confident that if I set up a set of conditions and see what happens, that it is repeatable. More than once I have set up the same conditions. One time protection trips, another it does not. I'm also puzzled by the fact that it will only trip once the entire time the amp is on, no matter how long it is on. Also, if the amp has been sitting off for hours, then is turned on, protection will trip sometime in the first 15 minutes. I can turn the amp off and then on again, even waiting half an hour, and protection will not trip. I have concluded this is because the temperature of the components has increased, and have not cooled off yet, which does point to the fault being temperature dependent. I've wondered if I place the amp in a warm room will the circuit trip at all? I happen to have it in a room that never gets above 70 F.

As I mentioned, I can try the scope on the resistors, but if I don't find anything, I'm at a loss and assume I'll have to live with this issue.
 
Used the scope tonight and checked the signal on the transistors I had not replaced on the power amp board. Nothing unusual there. Also checked pin 16 and 23 signals. More variation on 23 as noted when I used a dmm, but not outside the +/-20mV of spec.
 
Decided to recap the power amp board. Replaced all of the electrolytic capacitors using the list found here:

http://www.audiokarma.org/forums/index.php?threads/sa-8100-cap-list.463178/

However, the unit now does not come out of protection. Pins 10 and 16 are 0V, but pin 3 has -34V and pin 23 has -0.2V, so issue seems to be on the channel connected to pin 3. Did several measurements following the circuit diagram for the board. All pins that should have a specific voltage do: pins 5 & 21 = +50V, pins 27, 30, 33, 36 = +38V, pin 13 = +13, pin 12 = -13V.

I think the problem is on the circuit connected to pin 5. R53 has +50V on the side connected to pin 5, but only drops to +49 V on the other side, and is delivering +49V to Q9 collector. In contrast, the pin 21 circuit has +41V at Q10 collector. The circuit diagram calls for +40V at each collector. I lifted one leg of R53 to test resistance, and it was ok (1.5k). However, there is almost no drop in voltage across R53 unlike R54 to bring the voltage down to the +40V spec. I can replace R53, but I won't be able to get one until tomorrow at the earliest. Just wondering if there is anything else I should look for?
 
Have you recapped the amplifier board? Pins 33 and 30 should be -38V no +38V. Is that a typo? Did you mean 0.2V ie 200mV for Pin 23? Just making sure. IF that is the case, one of your amps has almost railed and the other is quite badly ofset. Voltage to the board look good and there's no DC coming in on your inputs (10 & 16) so its definitely within the amplifier...

No voltage drop after R53 means that the circuit after that is not drawing its usual amount of current which would be the case if the amplifier has railed. Have you made sure all ground connections from the amplifier board are good? Test them from the trace side to the chassis with an ohm meter. They should be very very near to 0 ohms.
 
Last edited:
Sorry, yes typo on pins 33 and 30, they are both -38V. Checked pin 23 again, -0.23V or -230mV.

Have not recapped any other boards, just power amp board and protection board.

Not sure what you mean by the "amplifier board"?
 
Sorry, I meant the power amplifier board. The one we're talking about now is the power amplifier board. You have recapped the power amplifier board? and you have changed Q1-4, Q9 and Q10?
 
QSilver said:
Sorry, I meant the power amplifier board. The one we're talking about now is the power amplifier board. You have recapped the power amplifier board? and you have changed Q1-4, Q9 and Q10?
Click to expand...
Yes, replaced all electrolytic caps on the power amp board, and Q1, 2, 3, 4, 9, 10.
 
C23 ground connection is reading 0.43 ohm, all other screws on the board to the chassis are reading less than 0.2 ohm. I'm not seeing any other obvious ground connections on the board - are there others?
 
Ok, found the other ones on the schematic, pins 4, 8, 9, 14, 17, 18, 22 all read 0.2 ohm or less.
 
QSilver said:
Have you tested Q5, Q7, Q11 and Q13?

Check Q9 for 40V at the collector and 1.2V on the emitter.
Click to expand...
Already posted Q9 and Q10 voltages in post 309, but here are all the ones you requested all in Volts:

E, C, B
Q5: -30, -30, -29
Q7: +46, -30, +46
Q9: -32, +48, -31
Q11: -32, +38, -31
Q13: -33, -38, -33
 
The readings on Q9, Q11 and Q13 look like to be okay, as they are dropping some voltage.. Q7 and especially Q5 look like the problem path. Can you check the voltage on the Emitters of Q1 and Q3? You should have around 600mV here. If not, check R13 and check to make sure there are no cold or cracked solder joints. Sometimes a stray piece of solder stuck to the PCB can also connect two paths through the insulated trace enough to cause problems.
 
Far from 600mV:

E, C, B (in V)
Q1: -26, -42, -17
Q3: -26, -38, -17

R13 has -26V on side connected to Q1, 3 emitters, +13V on side connected to pin 13.

Will have to check solder joints tomorrow...
 
Reflowed a few joints that did not look good. Success! Amp comes out of protection and voltages are down where they should be at the transistors. Set idle and offset.

I'm hoping recapping the power amp board will solve the original issue of the amp dropping into protection for a second or two after about 10 minutes of on time. We'll see...

Thanks for the help Qsilver!
 
You must log in or register to reply here.
Back
Top Bottom