Thank you anatech, that helps.
I had Q11/Q12 inverted so it was actually Q11 that was drawing all the current. It seems to be doing what you said, Q11 is trying to fight Q12.
There seems to be alot of discrepencies between the schematic and the board and Q1-Q5 are too tightly packed for me to get a reading.
I had Q11/Q12 inverted so it was actually Q11 that was drawing all the current. It seems to be doing what you said, Q11 is trying to fight Q12.
There seems to be alot of discrepencies between the schematic and the board and Q1-Q5 are too tightly packed for me to get a reading.
Okay, is Q10 on or not (rechecking base-emitter bias)? If not, it may have a fault and Q12 is fine. If it is low or reverse biased, Q12 is probably faulty.
Okay, let's recap to make certain of a few things.
Base- emitter voltages on the drivers and outputs are all biased into conduction, a bit too high I think (that's the entire output stage). Did you measure the voltage across the emitter resistors? If so, what is it? Let us know what amount of current you think it is. This is for both Q11 and Q12, no assumptions. Measure those resistors as well. No shortcuts.
What is the speaker output voltage with respect to common? That's called DC offset. Confirm both sides of C1 are at common potential ("ground"). Confirm one side of C1 has 0 ohms, or very low resistance, to your circuit common, the "0" point of the main power supply.
What is the voltage drop across R13 and across the pair of D1, D2? Also are these two resistors intact, R17, R18? These feed the drivers. If you had a blown driver and it's base resistor opened it might give you funny readings compared to the assumption they are okay.
The very first rule of any troubleshooting is "never assume anything, follow the evidence and prove it". This is followed by, "follow the evidence". If you don't do this, you'll just go around and around without finding the problem. Sometimes the actual issue is totally unexpected.
Base- emitter voltages on the drivers and outputs are all biased into conduction, a bit too high I think (that's the entire output stage). Did you measure the voltage across the emitter resistors? If so, what is it? Let us know what amount of current you think it is. This is for both Q11 and Q12, no assumptions. Measure those resistors as well. No shortcuts.
What is the speaker output voltage with respect to common? That's called DC offset. Confirm both sides of C1 are at common potential ("ground"). Confirm one side of C1 has 0 ohms, or very low resistance, to your circuit common, the "0" point of the main power supply.
What is the voltage drop across R13 and across the pair of D1, D2? Also are these two resistors intact, R17, R18? These feed the drivers. If you had a blown driver and it's base resistor opened it might give you funny readings compared to the assumption they are okay.
The very first rule of any troubleshooting is "never assume anything, follow the evidence and prove it". This is followed by, "follow the evidence". If you don't do this, you'll just go around and around without finding the problem. Sometimes the actual issue is totally unexpected.
I'd like to be more clear but the schematic doesn't match what I'm seeing. Where it shows R13 there are two 100R resistors in series, at the connection point it goes to the cathode of D2. Across the diodes I get .708v.
The speaker output starts at 0v and rises to 5v where it where it starts to slow down.
Just noticed R22 is measuring 0 ohms. Should be .25 ohms, R23 is ok. Need to remove it and test. Will get back later when I have time.
Constant interruptions in this household, hard to concentrate 🙂
The speaker output starts at 0v and rises to 5v where it where it starts to slow down.
Just noticed R22 is measuring 0 ohms. Should be .25 ohms, R23 is ok. Need to remove it and test. Will get back later when I have time.
Constant interruptions in this household, hard to concentrate 🙂
Sorry, R22 is fine.
Across R22 I measured 26.6mV and R23 26.4mV. That equals a normal 110mA bias, of which 120mA is max recommended.
Across R22 I measured 26.6mV and R23 26.4mV. That equals a normal 110mA bias, of which 120mA is max recommended.
You have said that both boards behave in a similar fashion. I also recall that you indicated that the output(s) slowly drifted DOWN, but your recent information is the reverse of that. Have the failure signatures changed? Has your measurement protocol changed?
That's a typo, should read rises to -5v.
Yes, both boards behave the same which is the most perplexing part.
Maybe some current left in the PS caps and I zapped them when I reconnected. Dunno.
Yes, both boards behave the same which is the most perplexing part.
Maybe some current left in the PS caps and I zapped them when I reconnected. Dunno.
A slow drift in offset sounds suspiciously like a floating ground somewhere. One that normally doesn’t carry a lot of DC current, but may have AC currents coupled into it by large-ish caps. That may also trigger oscillation, which could result in high output stage current draw.
It’s not unheard of for the ONLY place the input and feedback grounds connect to be at the RCA jacks where they connect to chassis. Remove the boards to test on the bench, and Look Ma, no ground!
You mentioned that you are measuring ~7M between the output collectors and the heat sink. Was the heat sink floating when you did that? Was that an in-circuit measurement?
The reason I ask is that the collectors are connected to the supply rails, which certainly connect to other parts of the circuit as well. So that 7M reading may not have anything to do with the problem you are seeing.
The reason I ask is that the collectors are connected to the supply rails, which certainly connect to other parts of the circuit as well. So that 7M reading may not have anything to do with the problem you are seeing.
I said there was no continuity, not 7meg.
I first tested it with the heatsink grounded but also briefly unmounted from the heatsink with the input shorted and completely isolated from chassis gnd.
Mark'51 might have a point. Is your amp fully assembled when you do the tests? Reason I ask is I worked on a home theatre amp and to get to the faulty part I removed all screws on the back panel. After the repair I did a quick test and both main speaker outputs rose to 14 volts. Unfortunately I had speakers connected at the time. One screw on the back panel was providing the 0V reference. In retrospect I did notice a warning triangle next to it but the damage was done.
I did specifically ask about this earlier. I asked about everything for a reason. All those things are bits of information needed to troubleshoot efficiently. That's why I have not been commenting.
Holy cow!
I just checked and there is no connectivity between the input and output grounds, even with the board mounted.
Not understanding how it could ever have worked. Everything is isolated from the chassis.
Some designs isolate the grounds with a low value resistor (which may burn open). You also may have a mechanical connection that provides this electrical connection.
You're right, it will never work unconnected. It must have been connected at some point, or if you installed these modules you missed a connection.
You're right, it will never work unconnected. It must have been connected at some point, or if you installed these modules you missed a connection.