Thanks. I'll definately check those when i get back on it this evening. I'm holding off on ordering a thermister for now.
I did try and compare mine to service manual. Bias resistors seem to be only thing different. They took 1 resistor out put the pot in and slightly decreased other resistor. I think if I had the pot at 250 ohm it would be exact same as the service manual.One of the 'problem' here is that we are not looking at the right service manual after all. His model has a trimpot for adjusting offset. The worst thing is that I can't find a service manual with this upgrade. In other words, we are looking at the wrong picture. Probably not a major upgrade, couple of resistors and a trimpot. Guys, be my guest ! 😊
@gto127, we have no weekends here. We work 24/7.
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I found solution to zener diode problem but still not fixed. R111 which appeared to be OK was162K out of circuit but I noticed changed value widely after heating up with soldering arm 30K so I figured it was changing value under a load. I changed that and now zener is 3.3 volts. Bias is stuck at 2.2 volts now instead of 2.1. and DC offset will not go below 20mv now. Q108 and 109 are both 80,79.3 and 77.2. If these resistors can fail in this way I'm good mind to replace all the small ones with really good ones before I go further to eliminate possibility other ones bad. Anyone have suggestions for good replacements for 1/4 watt ones?
Good work on finding the flaky R111 ! I'm surprised--- resistors are usually reliable, especially if conservatively sized re power dissipation. My inclination would be to be alert for more; if you find a second defective resistor, maybe suspicion about bad choice of resistor manufacturer is warranted. But I'd wait for more evidence rather wholesale replacement.
I'm also curious about the output offset trimmer. I presume there's a pot spanning the positive and negative supplies, and a large value resistor between the pot's wiper and R121 (either end). Would you confirm the offset adjustment is something of that nature?
I discovered from the schematic that there's no global negative feedback from the amp output! Rather, feedback is taken from R120 at the junction of R118, R119--- essentially "center-tap" between Q115 and Q116 emitters. From Q115 and Q116 emitters onward, the entire Q116-Q133 transistor array has unity gain, with only local feedback (eg. Q118, Q120 and paralleled brethren).
I believe this suggests a method to test the front and back ends of the power amp, quasi-independently.
Start from the properly adjusted working channel:
1. Record output offset voltage.
2. Record voltage at junction of R118, R119.
3. Record voltage across R136 (i.e. bias current).
Record voltage across R124.
Record voltage across R128.
Measure voltage at base Q118 and Q119 re ground.
Note: voltages across analogous resistors should be similar.
4. Set bias current to minimum via VR11 and repeat measurements in steps 1,2,3.
5. Measure range of output offset trim at junction of R118, R119.
Armed with this data describing nominal behavior, test the bad channel:
Set bias current in the bad channel to minimum via VR11 and perform step 5.
With success, rererse test procedure with the saved data as test goals.
Let us know if this at all useful.
Good luck!
I'm guessing these details refer to evolution of the bias current adjustment?
I'm also curious about the output offset trimmer. I presume there's a pot spanning the positive and negative supplies, and a large value resistor between the pot's wiper and R121 (either end). Would you confirm the offset adjustment is something of that nature?
I discovered from the schematic that there's no global negative feedback from the amp output! Rather, feedback is taken from R120 at the junction of R118, R119--- essentially "center-tap" between Q115 and Q116 emitters. From Q115 and Q116 emitters onward, the entire Q116-Q133 transistor array has unity gain, with only local feedback (eg. Q118, Q120 and paralleled brethren).
I believe this suggests a method to test the front and back ends of the power amp, quasi-independently.
Start from the properly adjusted working channel:
1. Record output offset voltage.
2. Record voltage at junction of R118, R119.
3. Record voltage across R136 (i.e. bias current).
Record voltage across R124.
Record voltage across R128.
Measure voltage at base Q118 and Q119 re ground.
Note: voltages across analogous resistors should be similar.
4. Set bias current to minimum via VR11 and repeat measurements in steps 1,2,3.
5. Measure range of output offset trim at junction of R118, R119.
Armed with this data describing nominal behavior, test the bad channel:
Set bias current in the bad channel to minimum via VR11 and perform step 5.
With success, rererse test procedure with the saved data as test goals.
Let us know if this at all useful.
Good luck!
Thanks. I was suprised to find bad resistor as well. Nak used close tolerance parts but mabye they just didn't hold up well over time. Regarding the changes I meant DC offset not bias. The service manual has no dc offset trimpot but in that circuit where there is a couple resistors one has been removed and a trimpot in its place. I dont have enough adapters and clips to have both sides open at once but may try this by writing down all valuse of the good side If I cant figure it out. Regarding the amp design its using something they call stasis which if Im grasping the concept correct they are running the beginning stages in class A and its driving the output stages somehow to make it sound better. Thanks for all the tips
Without casting aspersions on their design philosophy, IMHO it's marketing spin to imply that operating beginning stages in class A is in any way unusual. I believe all beginning stages operate in class A and only output stages dynamically move between Class AB and Class B. But not having the output stages enclosed within global feedback is unusual.
Taking a wild guess, is the offset pot introduced near the junction of R118 and R119? Irrespective of how output offset is implemented, I recommend characterizing stop-to-stop adjustment range for potentially useful data.
Good to have bias on the Zener sorted out. Seems there's still something not right about bias across Q139 and the entire power transistor array. Maybe more data from the good channel will provide clues.
One side of the pot goes to same trace as D105/Q112 the other side of pot goes to R113. The center of pot is shorted to adjacent pin so its basically just 2 sides to pot instead of 3.
R113 on this one is 1k instead of 1.33 so pot at .333 would be same as service manual. I think I have it adjusted near 0 to get lowest DC
That's a very peculiar way of adjusting amplifier offset.
It's possible you've gotten the early stages into working order. I suggest a few voltage checks before returning to bias current problems in the output stages.
Check voltage across R101. For this test you want the input jack open because you want to detect any abnormal gate bias current at Q101. Use the working channel for comparison, but I would expect voltage across R101 to be less than 0.1mV. The voltage across R120 should similarly low, although leakage through C104 might raise the observed voltage. If these voltages are similar to the working channel, odds are good that the circuitry from input though Q114 and Q115 is OK, but it wold be a good precaution to poke around for significant discrepancies.
If the above goes well, that brings us back to the output devices. I suggest comparing channels as bias current is varied. Voltage across R124 and R125 may be a significant clue, particularly since you describe voltage across R136 as being stuck at 2.1mV. How do these voltages change as you raise bias current drive? Re the 2.1 mA bias while in the "stuck" state: can you sense with your DVM where this current flowing to? What is drop across nearby emitter resistors and on the opposite pairs?
I also note there's a bunch of jumpers that appear to allow opening the emitter resistor path to each power transistor, but the details are too faint to be understood. Could you provide some details or pictures?
It's possible you've gotten the early stages into working order. I suggest a few voltage checks before returning to bias current problems in the output stages.
Check voltage across R101. For this test you want the input jack open because you want to detect any abnormal gate bias current at Q101. Use the working channel for comparison, but I would expect voltage across R101 to be less than 0.1mV. The voltage across R120 should similarly low, although leakage through C104 might raise the observed voltage. If these voltages are similar to the working channel, odds are good that the circuitry from input though Q114 and Q115 is OK, but it wold be a good precaution to poke around for significant discrepancies.
If the above goes well, that brings us back to the output devices. I suggest comparing channels as bias current is varied. Voltage across R124 and R125 may be a significant clue, particularly since you describe voltage across R136 as being stuck at 2.1mV. How do these voltages change as you raise bias current drive? Re the 2.1 mA bias while in the "stuck" state: can you sense with your DVM where this current flowing to? What is drop across nearby emitter resistors and on the opposite pairs?
I also note there's a bunch of jumpers that appear to allow opening the emitter resistor path to each power transistor, but the details are too faint to be understood. Could you provide some details or pictures?
Well I had a setback. ZD14 I think went out. 6.4 volts across. It was previously ok when I first tested everything. Not sure at what point it went out. Its strange because it measures 6.4 across but if i measure each side individually from ground I get 80v on one side and 68 on other a difference of 12v. This doesn't make sense. It supposed to measure 12 across as well. Im going to order another 12v zd. Its a 1/4 watt. Would it hurt to do 1/2 watt for better durability or is it also meant to be a protection device with smaller wattage?
Are you sure it's failed? You wouldn't see 12V across ZD14 unless conditions around Q134 caused the transistor to present more than 12V, inducing it to clamp at 12V. I think when you used a volt meter from the rai, the meter's resistance provided the necessary current to bias the Zener into conduction.
Is the amp in protect mode? I haven't studied the protection circuit.
Is the amp in protect mode? I haven't studied the protection circuit.
No its not in protect mode. I had assumed its failed but may be wrong. Every Zener in this has measured right at Zener voltage. I will try and test other side to be sure.
You are right about the zener diode. Same on both sides. R101 is 0 volt and R120 is near 0. The only difference I see so far is bad side main supply is about a half volt more and zd12 is 3.3 volt even when i put back the original 3.2 zener. Not sure if significant enough to matter.
I suspect the problem is the the power transistor array; the stages from input all the way through Q115 are probably working. I only suggest some of the tests in post #29 to avoid overlooking a remaining problem.
I suggest getting a feel for proper behavior by gradually bringing bias current on the working channel from minimum to nominal. As you do this, notice how the voltage drop across R124 and R125 grows with rising bias current. Then repeat this process on the problem channel; I suspect the behavior and voltage drops will be markedly different. If this is so, additional clues may be found by comparing voltages across the emitter resistors, again referring to the good channel for working perspective.
Let me know what you observe. If you confirm peculiar differences but still can't localize the defect, I will offer more suggestions.
Good luck!
I suggest getting a feel for proper behavior by gradually bringing bias current on the working channel from minimum to nominal. As you do this, notice how the voltage drop across R124 and R125 grows with rising bias current. Then repeat this process on the problem channel; I suspect the behavior and voltage drops will be markedly different. If this is so, additional clues may be found by comparing voltages across the emitter resistors, again referring to the good channel for working perspective.
Let me know what you observe. If you confirm peculiar differences but still can't localize the defect, I will offer more suggestions.
Good luck!
Just found 2 more resistors not like the service manual. R163 and R122 both 1k instead of the 750 ohms in service manual. These are both directly in bias circuit. Starting to wonder if I may have a engineering sample before they got the kinks worked out. I've ordered a few resistors that are slightly off before I go much further just in case they may be breaking down further under load. When these come in I will go through and compare the 2 sides as you mentioned. I reaalyy hope the thermisiter is not the problem as it is not available anywhere that I can find.
Just got back to it. The off tolerance resistors mad no difference. I still havent taken good side all the way down but was able to compare most of 1 watt resistors between both sides with it put together. most 1 watt resistors on good side are .04 volts across. Bad side are .036. This does not count R124 and 125 those are both .6 volts on bad side with bias at normal and .4 with bias at low point. I cant access these points on good side due to location. Also just a question about the varistor. According to tech at digikey the varister is a 1k that changes with temperature. Could I test this by temporarily putting a 1k resistor in its place since the problem occurs even when cold.
You mean thermistor , NTC ? Personally I have never encountered a failed thermistor in such applications. Well, mechanically damaged yes. Check the resistance of that NTC when cold and report here. Btw, you could place a fixed resistor for the sake of quick testing but beware, without thermal tracking, the output transistors might go bang due to the thermal runaway. Q139 is there for the same purpose. Keep it mounted on the heat sink all the time while testing.
Above post remains my favorite conjecture about possible defect. But I would add Q120 through Q133 as additional suspects; I believe open collector junctions could cause similar symptoms. And please reconfirm Q116-119 are the devices that are inordinately hot.
Re the emphasized print above, are any of the 14 resistor voltages dramatically different than 0.036V? (i.e. R130 though R143). Any such associated transistors may be suspect re wg_ski quote above. What are voltages across R126 through R129?
Thanks!
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That could definitely clear up a lot of things. Now I'm not sure anymore which ones are getting too hot, Q114/Q115 or Q116-Q119 !?
@gto127, are you trolling us ? 😆
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