Here is the rundown of what voltages/waveforms are on what pins of the LM361N with the probe grounds referenced to negative speaker input. I changed the scale on some to better view the waveform.
Pin 1: +12VDC
Pin 3: One picture AC coupled, one DC
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An externally hosted image should be here but it was not working when we last tested it.
Pin 4: One picture AC coupled, one DC
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An externally hosted image should be here but it was not working when we last tested it.
Pin 6: -12VDC
Pin 8: +3.1VDC
Pin 9: +1.98VDC
Pin 10: -1.98VDC (Should the GND pin be at a different voltage than the negative speaker terminal?)
Pin 11: -1.82VDC
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An externally hosted image should be here but it was not working when we last tested it.
This is approximately the same style of waveform that shows up on the output transistors
Pin 13: +3.1VDC
Pin 14: +3.1VDC
After looking through the datasheet for the LM361N, it looks like everything is correct as far as inputs go... two triangle waves at the input (except maybe the DC offset on pin 3?) and both strobe inputs are held high. The output is obviously low. Is it a fair assumption the LM361N is bad or are there some other components I should look at?
Pin 10 should be at -2 (as it is). The later models had this pin grounded but these are on the -2v reg.
You should not have a triangle waveform on both pins 3 and 4. The comparator (LM361) may be defective. I've seen several that did this.
There is a resistor between pin 3 and an op-amp output (maybe pin 1 of U11). What is the DC voltage on each side of that resistor (black probe on negative speaker terminal)?
Do you have full negative rail on the drains of the output transistors?
You should not have a triangle waveform on both pins 3 and 4. The comparator (LM361) may be defective. I've seen several that did this.
There is a resistor between pin 3 and an op-amp output (maybe pin 1 of U11). What is the DC voltage on each side of that resistor (black probe on negative speaker terminal)?
Do you have full negative rail on the drains of the output transistors?
I do not have full negative rail on the drains of the output transistors. I think I have a picture above that shows it sitting around 0V (IRF640 Drain Picture)
I will check the DC voltage on each side of the resistor you mention later tonight and report back. Would pulling the LM361 and remeasureing the pads for pins 3 and 4 help at all? I am not sure if having an open load there would make the measurement invalid or not.
Now I wish I had waited to place my order with Avnet a couple days so I could have included a LM361... $5 in parts and $8 to ship is crazy. Digikey has gotten quite expensive for parts lately too even though they have cheap shipping
Don't order anything yet.
With -2v on pin 11, the drains of the output transistors should be at the negative rail. There may be a problem with the drive circuit beyond the LM361. That would make the output of the op-amp drive the input of the LM361 too low and could cause the triangle waveform to show up on both inputs of the LM361.
Do you have 11.5-12v across the following Zener diodes?
D107
D108
D120
D121
With -2v on pin 11, the drains of the output transistors should be at the negative rail. There may be a problem with the drive circuit beyond the LM361. That would make the output of the op-amp drive the input of the LM361 too low and could cause the triangle waveform to show up on both inputs of the LM361.
Do you have 11.5-12v across the following Zener diodes?
D107
D108
D120
D121
perry babin said:
R259 (Marked 222 on package - 2.2kOhm)
On opamp side: -10.28V
On LM361 side: -7.47V
d107: +12.45V
D108: +12.45V
d120: +12.37V
d121: +12.37V
They're generally less than 12v but yours are OK. Sometimes they short or leak reducing the driver voltage to a point to where the FETs can't be driven on.
Are all of the output FETs in the board?
What DC voltage do you have on the 4 transistors between the diodes. Black probe on -sp terminal.
Q104
B:
C:
E:
Q113
B:
C:
E:
Q126
B:
C:
E:
Q135
B:
C:
E:
Are all of the output FETs in the board?
What DC voltage do you have on the 4 transistors between the diodes. Black probe on -sp terminal.
Q104
B:
C:
E:
Q113
B:
C:
E:
Q126
B:
C:
E:
Q135
B:
C:
E:
All components are on the board except the Q04 protection circuit transistor, and the preamp daughterboard right now. If necessary, I can provide a signal directly to the preamp card connector for an audio signal on the "IN" pin.
Q104
B: -1.58V
C: +69.1V
E: +5.94V
Q113
B: -1.78V
C: -69.1V
E: -4.48V
Q126
B: -1.65V
C: +69.1V
E: +5.95V
Q135
B: -1.78V
C: -69.1V
E: -4.47V
R183 (33kOhm) removed did not cause excessive current draw. Still have 1.55-1.6A at idle. After removing R183 -->
Q104
B: -1.63V
C: +69.1V
E: -5.95V
Q113
B: +1.78V
C: -69.1V
E: -4.49V
Q126
B: -1.66V
C: +69.1V
E: +5.95V
Q135
B: -1.79V
C: -69.1V
E: -4.47V
Removing that resistor appears to have had little effect.
Dang... I did type different values. I am out of town for the weekend, so I will have to check if they are actually different or if I can't type. I will get the corrected numbers on Monday. Thanks for your help so far.
Here are the correct values. They did not change removing the resistor. I must have just mistyped last time.
Q104
B: -1.66V
C: +69.1V
E: +5.95V
Q113
B: -1.78V
C: -69.1V
E: -4.48V
Q126
B: -1.67V
C: +69.1V
E: +5.95V
Q135
B: -1.77V
C: -69.1V
E: -4.47V
Looks like one of the transistors around R183 might be bad. With R183 still out of the circuit, here are the resistance values between pins. Not sure if they are BJT or FET, so if the terminology is wrong, you know what I meant between gate/base etc
(Pin 1 = Gate Pin 2 = Drain Pin 3=Source)Q121:
Between 1-2: 72 Ohms
Between 1-3: 82 Ohms
Between 2-3: 19 Ohms
Q122:
Between 1-2: 23 kOhms
Between 1-3: 14.63 kOhms
Between 2-3: 18 kOhms
Q123:
Between 1-2: 23 kOhms
Between 1-3: 8.4 kOhms
Between 2-3: 17 kOhms
Looks like Q121 is broke. I pulled it from the circuit, and it still tests bad.
With Q121 pulled, I repowered the amp. It now current limits pulling 2A (@10V) and when pulling 3A (@11.25V). Something in the PS section starts mildly squealing so I only let that go for about 5 seconds before I turned it off. Safe to say that this is my only problem or are there more things I should keep hunting for? Also, what is the replacement part for that SOT-223 transistor (Q121)? Markings show CY 603. The CY is near the gate/base pin and 603 is near the source/emitter pin.
KTC4373 should be the correct replacement for Q121.
Except for R183, Q121 and Q04 being out of the circuit, what other mods/changes remain?
With the limiter set to ~5 amps and the transistors clamped, twist all of the inductors and the power transformer to see if the idle current changes any.
Except for R183, Q121 and Q04 being out of the circuit, what other mods/changes remain?
With the limiter set to ~5 amps and the transistors clamped, twist all of the inductors and the power transformer to see if the idle current changes any.
Those are the only mods/changes. I guess if you say that the preamp daughterboard is disconnected, then that is a mod also.
All transistors are clamped, but I'm a little confused at what you mean by twist all the inductors and the power transformer? How can I twist them while the leads are soldered down and the black silicone holding them down?
Also, should I do this once I replace Q121 or before? I'm a little concerned about the PS squeal that certainly does not seem normal with Q121 removed
Also, what is a suitable replacement for Q04? I do not like unsoldering and resoldering really small parts (SOT-23 in this case) because of the heat involved with the desoldering. Plus, I am assuming it is a standard part which is inexpensive. I have to buy the other transistor anyway, might as well make the order worth it!
Sidenote: Do people do small chip swaps on this board at all? I have a bunch of surplus chips from other repairs and would not mind trading them out for ones I currently need.
Grab and twist. You only need to move it 1 degree. If there is a short, it's likely to change the idle current as you twist.
Q04 is likely a KTA1504S. I'd use an MMBTA56 if the original wasn't readily available.
Leave Q121 out for now. It's part of the over-current circuit. I'm not sure why it failed. It's not doing any significant work.
There may be another problem (like shorted inductors) that is causing the protection circuit to engage.
With the transistors clamped and the current limited, there's very little chance that you'll damage any of the power transistors.
Q04 is likely a KTA1504S. I'd use an MMBTA56 if the original wasn't readily available.
Leave Q121 out for now. It's part of the over-current circuit. I'm not sure why it failed. It's not doing any significant work.
There may be another problem (like shorted inductors) that is causing the protection circuit to engage.
With the transistors clamped and the current limited, there's very little chance that you'll damage any of the power transistors.
OK, will check tomorrow. Something about twisting an active high power component kind of worries me
Guess I'll find some insulating gloves before I do this!Digikey stocks the MMBTA56 for cheap, so that seems like a better solution.
Is the power supply squeal nothing to be concerned with then?
Guess we'll find out when I start twisting!
Last thing... the KTC4373 seems to be hard to find here in the US. Fairchild shows a direct replacement in their cross reference. Have you had any experience with the KSC2881? They say it is a direct replacement and easier to find in the states.
http://www.fairchildsemi.com/ds/KS/KSC2881.pdf
The Fairchild part seems to be a good sub but I've never used it.
I wouldn't be concerned about the squeal at this point. You're starving it for voltage/current so it may not behave perfectly.
You have 4 50+ amp FETs in parallel in the power supply. It's unlikely that you'll damage them with 5 amps of current unless they overheat (which they won't do if they're clamped).
If nothing (twisting the various components) makes a difference in the idle current, remove the limiter and insert a 10 amp fuse in the B+ line to see if it will power up. If it blows the fuse, pull the rectifiers to see if it will power up without blowing the fuse.
Does it look like either of the output inductors has overheated?
I wouldn't be concerned about the squeal at this point. You're starving it for voltage/current so it may not behave perfectly.
You have 4 50+ amp FETs in parallel in the power supply. It's unlikely that you'll damage them with 5 amps of current unless they overheat (which they won't do if they're clamped).
If nothing (twisting the various components) makes a difference in the idle current, remove the limiter and insert a 10 amp fuse in the B+ line to see if it will power up. If it blows the fuse, pull the rectifiers to see if it will power up without blowing the fuse.
Does it look like either of the output inductors has overheated?
I ended up going with a Zetex FCX493 (instead of the fairchild or KEC4373 part) because that's what digikey had in stock. It was a direct match on their cross reference.
I am not worried about the power or output FET's blowing up, I'm worried about starting to arc +/- 70V while I'm holding it and twisting
DC burns hurt...not as much as RF, but still no fun!The output inductors did give off some heat previously, but nothing that seemed excessive. Visually they look fine.
On Wednesday I'll try the suggestions and report back!
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