Gosh dang it. I just tested one of my 2sc3280 and it did not pass 6 way test. I just testeds these last week. I will order a few 2sc500 replacements from Digi. If there is a better suggestion for a replacement let me know. By the way it was Q327 and while I am at i will replace Q323 even though it passed. Makes me wonder what made it go bad. Sigh its depressing for sure😒
I'm not convinced those transistors are the issue tbh.
The whole output stage, drivers and outputs should just follow the voltage that appears on Q311. The emitter and collector volts of Q311 should always be fairly similar and the main amp output follows that.
Output transistors usually fail short and failure of the 2SC's would just apply positive rail voltage to the output.
With fault finding we have to continue to work through any tests. It stayed negative...... but it should not have done. We need to find out why.
The whole output stage, drivers and outputs should just follow the voltage that appears on Q311. The emitter and collector volts of Q311 should always be fairly similar and the main amp output follows that.
Output transistors usually fail short and failure of the 2SC's would just apply positive rail voltage to the output.
so there is something causing Q311 to turn negative which is causing the outputs to be negative
Yes there is but it is complex because the amplifier has to be looked at as one whole loop.
The high negative voltage at the output should be turning Q303 in the input pair on harder and your readings suggest that is happening. Q303 turned on in turn switches Q305 on harder and that pulls the collector of Q301 toward the negative rail.
Your readings suggest that is happening with Q301 collector more negative than Q301 collector. That is the take away point of all that.
Q301 collector is therefore turning OFF Q308 by pulling the base voltage down. With Q308 OFF the output stage should all swing to a high positive voltage.
Try removing Q308 and see if the output goes positive.
I know it must all seem random and going around in circles but its not, and its really difficult to try and explain simply what is going on. With something like this in front of me it would all make more sense than trying to keep picking it up every few hours 🙂 That is the reality I'm afraid.
The high negative voltage at the output should be turning Q303 in the input pair on harder and your readings suggest that is happening. Q303 turned on in turn switches Q305 on harder and that pulls the collector of Q301 toward the negative rail.
Your readings suggest that is happening with Q301 collector more negative than Q301 collector. That is the take away point of all that.
Q301 collector is therefore turning OFF Q308 by pulling the base voltage down. With Q308 OFF the output stage should all swing to a high positive voltage.
Try removing Q308 and see if the output goes positive.
I know it must all seem random and going around in circles but its not, and its really difficult to try and explain simply what is going on. With something like this in front of me it would all make more sense than trying to keep picking it up every few hours 🙂 That is the reality I'm afraid.
I do apologize for the intermittent communication. I do understand what your saying in regard to having everything in front of you, I was the sameway with computer and network troubleshooting. I eill remove Q308 after i get my boys to school and report back.
That shouldn't be. Lets try and nail this.
Leave Q309 out and now remove Q307 as well. Does the output now go positive?
If it does not we also remove Q341 and we now take a 10k or something close to that and connect it between collector and base of Q315.
That forces the output stage into conduction and the voltage on the 0.22 ohm's must rise to the positive rail value. That should prove if the upper half of the output stage is OK or not.
Leave Q309 out and now remove Q307 as well. Does the output now go positive?
If it does not we also remove Q341 and we now take a 10k or something close to that and connect it between collector and base of Q315.
That forces the output stage into conduction and the voltage on the 0.22 ohm's must rise to the positive rail value. That should prove if the upper half of the output stage is OK or not.
If those cascode transistors aren’t shorted and it’s still pulled negative there almost has to be a short on the board somewhere.
We're just going down a rabbit hole with this one. The output transistors must be in place, the upper npn's are the only route by which positive voltage (with any significant current) can be applied to the main output line.
OK, put the 2sc3280 back in place, I am at the Q307, Q309, Q341 are out and the Q315 has a 10k between collector and base
OK the DBT light stayed very bright and the voltage stayed negative but dropped to around 11vdc. I reattached the 10k just to make sure and the voltage is -6.5vdc at the .22 ohm resistors
First thing of note is the bulb should be dim.
Leave it all as it is now but add a link across Collector and Emitter of Q311 which is the bias generator. That forces zero bias. If the bulb is still bright there has to be an undiscovered issue with the driver and output stage.
So this is what you have now. The output stage should just go to + positive rail value. I haven't added a link here because it should not really need it. Add the link, force zero bias and if the bulb is bright there is an issue in the driver/output stage.
With the link added the circuit reduces to this, a standard emitter follower output stage. The output voltage should follow the voltage applied to the what is now the link. If we applied +9v to the link we should see +9 (approx) at the output, Apply -27 and we should see -27 come out.
Leave it all as it is now but add a link across Collector and Emitter of Q311 which is the bias generator. That forces zero bias. If the bulb is still bright there has to be an undiscovered issue with the driver and output stage.
So this is what you have now. The output stage should just go to + positive rail value. I haven't added a link here because it should not really need it. Add the link, force zero bias and if the bulb is bright there is an issue in the driver/output stage.
With the link added the circuit reduces to this, a standard emitter follower output stage. The output voltage should follow the voltage applied to the what is now the link. If we applied +9v to the link we should see +9 (approx) at the output, Apply -27 and we should see -27 come out.
Unfortunately I have to go to work. I will follow your directions when I get home tonight and wait until I hear from you until I do anything else.
OK, I applied the jumper to the emitter and collector of Q311. I totally understand the theory. I am getting some understanding on the theory of operation and I will continue to study it.
This is what I got. Still bright light. The bias did go to Zero just as you anticipated. The voltage at the .22 ohm resistors is -8.30 vdc. The voltage at the cvollector of Q311 is -6.88vdc.
This is all I can do tonight. I have to be up at 430am to take my boy to catch a bus for a wrestling meet. Hopefully I will be back at 9am my time tomorrow.
So we have an issue with the driver/ouput stage??
Phillip
This is what I got. Still bright light. The bias did go to Zero just as you anticipated. The voltage at the .22 ohm resistors is -8.30 vdc. The voltage at the cvollector of Q311 is -6.88vdc.
This is all I can do tonight. I have to be up at 430am to take my boy to catch a bus for a wrestling meet. Hopefully I will be back at 9am my time tomorrow.
So we have an issue with the driver/ouput stage??
Phillip