Got a tougher one. I have a Nikko Alpha III that I cant set the idle current on. I just installed some used Exicon mosfets. A bit of history. I bought this off ebay along with a Nikko Beta 30. The beta 30 was completely recapped along with some new transistors in the power supply and installed some LM4562 opamps and it sounds great. The Alpha III was doing very well and had bought all the hardware needed for a complete recap. Bad storm hits and pulled my power lines loose enough I had a lost neutral condition and my power protector got a 160v surge, uh yeah. Alpha III smoked and the J49 mosfets took a dive. The 134's survived. I started working on it with another fellow tech and I replaced the trannys on the complimentary differential board which were 2sa872 and 2sc1775. I did hfe matching of the transistors on the left channel only. Once completed it came out of protection but I can not adjust the offset or idling. Following the manual when adjusting the offset and using a shorting pin on the direct input jacks the lowest I can get is 1.62 vdc, left channel and the right side I can adjust right on the nose. Idling adjust ment is stuck on .2mv and no adjusment no matter how much I turn the pot both sides. Funny thing is if I connect the DMM to the pins it reads .2mv with no power.
I know its alot but it helps give you some insight. I have attached the manual. Any suggestions would be greatly appreciated
Phil
I know its alot but it helps give you some insight. I have attached the manual. Any suggestions would be greatly appreciated
Phil
I've worked on enough stuff over the years to know that the damage caused by a floating neutral on a three phase supply can cause major issues. Everything will have been over volt-ed such as caps in the power supply etc etc.
Have you checked ALL the rails are correct? There are stabilised supplies to the front end in that amp all fed from stabilised supplies.
Have you checked ALL the rails are correct? There are stabilised supplies to the front end in that amp all fed from stabilised supplies.
Ill be in my tech room this afternoon and will start taking measurements. Yes that floating neutral took out microwave (replaced main circuit board that was actually fried) Keurig (trash) wife's bullet juicer (trash) transformer in central air (replaced along with motor run cap) everything else I got unplugged and salvaged. The other outlets was the lower voltage half of the lost neutral and they wouldn't run at all like computer, tv.
Ill check to make proper voltages are present coming off the transformer. I replaced all the caps in the main power supply.
Ill check to make proper voltages are present coming off the transformer. I replaced all the caps in the main power supply.
OK, those are interesting results and a bit unexpected...
We have to start somewhere and assuming the manual is correct it shows 60 volts AC on the upper winding of the transformer. The lower terminal will ground. 60 volts AC would give approx 60*1.41 which is approx plus 84 volts DC on D805 and minus 84 volts DC on D806. Those figures tie in with what I see on the diagram but not with your readings. You should see that same -/+84 volts DC on the collectors of the regulators.
Q816 seems like it might be working OK with -55 volts DC at the output even though the input is low, Q 815 output seems high and the input low of course.
Are there two issues here? or even more. This is typical of equipment damaged by severe over voltage.
Is the -/+84 volts DC low because of a an over current situation on Q815 output pulling the raw voltage down. The supply is a low current one and the transformer winding will be limited in current delivery. Symptoms suggest this may be what is happening. The positive regulator also seems as if it might be faulty because the output is higher that +56 volts.
No easy or quick fixes... you have to work through all the scenarios.
Check the current (by measuring volt drops across resistors) in the amplifier driver stages to see if one of these is pulling the voltage down.
Just picking one channel as an example try isolating both points 4 and 17 to remove the -/+56 volt rails and see if the voltage come up.
I think you will find the + regulator is faulty and its output might rise dramatically when you reduce any excess current.
If you remove fuse 801 you can check the raw AC off load.
We have to start somewhere and assuming the manual is correct it shows 60 volts AC on the upper winding of the transformer. The lower terminal will ground. 60 volts AC would give approx 60*1.41 which is approx plus 84 volts DC on D805 and minus 84 volts DC on D806. Those figures tie in with what I see on the diagram but not with your readings. You should see that same -/+84 volts DC on the collectors of the regulators.
Q816 seems like it might be working OK with -55 volts DC at the output even though the input is low, Q 815 output seems high and the input low of course.
Are there two issues here? or even more. This is typical of equipment damaged by severe over voltage.
Is the -/+84 volts DC low because of a an over current situation on Q815 output pulling the raw voltage down. The supply is a low current one and the transformer winding will be limited in current delivery. Symptoms suggest this may be what is happening. The positive regulator also seems as if it might be faulty because the output is higher that +56 volts.
No easy or quick fixes... you have to work through all the scenarios.
Check the current (by measuring volt drops across resistors) in the amplifier driver stages to see if one of these is pulling the voltage down.
Just picking one channel as an example try isolating both points 4 and 17 to remove the -/+56 volt rails and see if the voltage come up.
I think you will find the + regulator is faulty and its output might rise dramatically when you reduce any excess current.
If you remove fuse 801 you can check the raw AC off load.
OK, remember were plugged into a DBT if that makes any difference. Also I fu*ked up big time and apologize. When I installed the replacement exicons (TO3) I inserted the screws but didn't screw them in, sigh, found out by moving the amp on the workbench and a couple fell out. I tightened the screws down and now have a little different results. Makes a big difference when you add the collector to the transistors.
I now can dial in the idle current and the offset, but can not dial in the offset with the direct input pins shorted out. The lowest I can get is 40mv and that's maxed out on the VR. It also doesn't come out of protection and the dbt bulb is a little brighter than before which would indicated to me that we still have a current issue. I again really apologize for that **** up, feel bad but at least I caught it before anymore in-depth troubleshooting had occurred in the wrong direction.
I will again measure the transistors and see what we get but I'm leaving it plugged into the DBT
I now can dial in the idle current and the offset, but can not dial in the offset with the direct input pins shorted out. The lowest I can get is 40mv and that's maxed out on the VR. It also doesn't come out of protection and the dbt bulb is a little brighter than before which would indicated to me that we still have a current issue. I again really apologize for that **** up, feel bad but at least I caught it before anymore in-depth troubleshooting had occurred in the wrong direction.
I will again measure the transistors and see what we get but I'm leaving it plugged into the DBT
These things happen 🙂
What I would do is this. First turn the bias current as low as it will go on both channels. Still using the DBT is the output of that suspect regulator correct now or not?
That is the first thing to check. Make sure you have the correct -/+56 volts present.
What I would do is this. First turn the bias current as low as it will go on both channels. Still using the DBT is the output of that suspect regulator correct now or not?
That is the first thing to check. Make sure you have the correct -/+56 volts present.
very substantial drop. Is that because of the draw the output transistors?
Q821
e 12.47vdc
b 8.17vdc
c 7.51vdc
Q821
e 12.47vdc
b 8.17vdc
c 7.51vdc
Most likely yes.
Turn the bias down and see if the voltage rises. If the output of the regulator goes over -/+56 v you have a problem in the reg. If the bulb is still lit you can force zero bias by adding a link across the bias resistor chain. Also of course check that the bias really is now zero by measuring across the 0.22 ohm source resistors.
That is because the unregulated input to the regs is to low... because of the higher current draw. Force zero bias as above and look and work on the regulator first.
Turn the bias down and see if the voltage rises. If the output of the regulator goes over -/+56 v you have a problem in the reg. If the bulb is still lit you can force zero bias by adding a link across the bias resistor chain. Also of course check that the bias really is now zero by measuring across the 0.22 ohm source resistors.
That is because the unregulated input to the regs is to low... because of the higher current draw. Force zero bias as above and look and work on the regulator first.
The lowest I could get the bias by turning R721 on both boards is 12mv but the voltages at Q815 and Q816
Q815 Q816
E -53.5 vdc E 52.3 vdc
B -54.5 vdc B 53.4 vdc
C -54.7vdc C 54 vdc
The bulb is very dim. Turning the bias down made a difference. The relay actually click on
Q815 Q816
E -53.5 vdc E 52.3 vdc
B -54.5 vdc B 53.4 vdc
C -54.7vdc C 54 vdc
The bulb is very dim. Turning the bias down made a difference. The relay actually click on
The collector volts on Q815 and 816 tell us that the raw supplies are to low for the regulators to function.
12mv bias (so this is the voltage you are measuring between test points 9 and 10) is telling us 55ma is flowing, if both channels are similar that is 110ma and that is probably enpough (depending on wattage of the bulb) to pull the supplies down. Apart from concerns over the regulator all else seems OK at this point but it is not conclusive because we know you measured a high voltage from one of the regulators.
You have two choices 🙂
Either risk full mains and check the output of the regulators or keep the bulb and do as I suggested earlier and force zero bias which should see the supplies rise enough to check the regulators.
12mv bias (so this is the voltage you are measuring between test points 9 and 10) is telling us 55ma is flowing, if both channels are similar that is 110ma and that is probably enpough (depending on wattage of the bulb) to pull the supplies down. Apart from concerns over the regulator all else seems OK at this point but it is not conclusive because we know you measured a high voltage from one of the regulators.
You have two choices 🙂
Either risk full mains and check the output of the regulators or keep the bulb and do as I suggested earlier and force zero bias which should see the supplies rise enough to check the regulators.
I dont like risks. Ive burnt up enough stuff knowing that could be catastrophic. I will force zero bias as you suggested
Installed jumper to force zero bias. I will send you the new voltage measurements as they have substantially increased.
Q815
E 60.3 vdc
C 67.5 vdc
B 61.08 vdc
Q816
E -55.3 vdc
C -70.8 vdc
B -55.75
Q821
E 11.5 vdc
C 11.6 vdc
B 12.23 vdc
Q821 didnt change much but you really wouldnt expect to. The regulators increased quite a bit. Is that because there is no current to the output mosfets?
Q815
E 60.3 vdc
C 67.5 vdc
B 61.08 vdc
Q816
E -55.3 vdc
C -70.8 vdc
B -55.75
Q821
E 11.5 vdc
C 11.6 vdc
B 12.23 vdc
Q821 didnt change much but you really wouldnt expect to. The regulators increased quite a bit. Is that because there is no current to the output mosfets?
Last edited:
That seems to show a problem.
The emitter of the two regulators is the output and should be at approx -/+56 volts
You will have to check around Q815 and all its components. Check the the Zener has 15 volts across it. Any/some/all of the three transistors could be damaged by the initial overvolts condition as could C821 on the reg output.
There is no quick fix I'm afraid, you will have to test all those transistors out of circuit but you need to be sure the tests and interpretation of the results is good 🙂
Q821 although just an auxiliary supply is also showing a problem because the collector voltage should be much higher. That could be a problem before or after the regulator or both. I would disconnect the output (point 22 on that board) and see if you get the correct 12 volt output and with nearer 20 to 25 volts on the collector. If you do the circuit that rail feeds is likely zapped which are the LED driver chips.
If the voltages don't come up you have problem before the reg which would be D803 and the diode and cap either side of it.
Such is the nature of these things (severe overvolts) that both issues can be present together.
The emitter of the two regulators is the output and should be at approx -/+56 volts
You will have to check around Q815 and all its components. Check the the Zener has 15 volts across it. Any/some/all of the three transistors could be damaged by the initial overvolts condition as could C821 on the reg output.
There is no quick fix I'm afraid, you will have to test all those transistors out of circuit but you need to be sure the tests and interpretation of the results is good 🙂
Q821 although just an auxiliary supply is also showing a problem because the collector voltage should be much higher. That could be a problem before or after the regulator or both. I would disconnect the output (point 22 on that board) and see if you get the correct 12 volt output and with nearer 20 to 25 volts on the collector. If you do the circuit that rail feeds is likely zapped which are the LED driver chips.
If the voltages don't come up you have problem before the reg which would be D803 and the diode and cap either side of it.
Such is the nature of these things (severe overvolts) that both issues can be present together.