Nikko Alpha 220 Keeps blowing output transistors

Hi All,

I'm working on a Nikko Alpha 2220 power amp. When I got it the Rch had shorted output transistors as well as some other dead transistors and resistors. I replaced everything I found that was dead as well replacing the polarized electrolytic caps and silicon diodes.

With the output transistors disconnected everything seems fine. With them hooked up it will light my dim bulb tester (40W) and has even shorted out a number of output transistors (while on the dim bulb!). I thought the whole point of the dim bulb was to prevent things from blowing up.

I could use some help troubleshooting.

The original output transistors are 2SA1095/2SC2566 which I was unable to source. I am using 2SA1943/2SC5200. They have adequate  Amp and Voltage Ratings but lower fT then the originals. Service manual is too big to attach in entirety. I've attached the schematic. Let me know what other pages you would like, or you can download it here: https://www.hifiengine.com/manual_library/nikko/alpha-220.shtml

shorted output devices is a common amplifier failure mode and can be from voltage spikes or overheating. Overheating is most common.

I can't be sure of the values of the emitter degeneration resistor (in series with each emitter. R781, R785, R783, and R787, but for trouble shooting you can replace the very low resistance connection with a modest value in series with each... say 10 ohms...

This will not provide enough output current to drive a speaker but will connect the feedback loop and allow the amplifier to operate without a load.

Measure the sundry voltages and look for an outlier.  If you have one working channel that should reveal what the voltages should be... With it working properly with 10 ohms , replace the original low Z emitter degeneration paths.

JR

JohnRoberts said:

shorted output devices is a common amplifier failure mode and can be from voltage spikes or overheating. Overheating is most common.

I can't be sure of the values of the emitter degeneration resistor (in series with each emitter. R781, R785, R783, and R787, but for trouble shooting you can replace the very low resistance connection with a modest value in series with each... say 10 ohms...

This will not provide enough output current to drive a speaker but will connect the feedback loop and allow the amplifier to operate without a load.

Measure the sundry voltages and look for an outlier.  If you have one working channel that should reveal what the voltages should be... With it working properly with 10 ohms , replace the original low Z emitter degeneration paths.

JR

Click to expand...

Thanks. I will try that. The emitter resistors are two 0.1ohm resistors in a single package in series.

samgraysound said:

...With the output transistors disconnected everything seems fine. With them hooked up it will light my dim bulb tester (40W) and has even shorted out a number of output transistors (while on the dim bulb!)....

Click to expand...

You should check also  output transistors bias circuit,  Q728,730, R742,744,746 etc. The DC voltage between bases of Q 724 and 726 should be about 4Vbe.

JohnRoberts said:

shorted output devices is a common amplifier failure mode and can be from voltage spikes or overheating. Overheating is most common.

I can't be sure of the values of the emitter degeneration resistor (in series with each emitter. R781, R785, R783, and R787, but for trouble shooting you can replace the very low resistance connection with a modest value in series with each... say 10 ohms...

This will not provide enough output current to drive a speaker but will connect the feedback loop and allow the amplifier to operate without a load.

Measure the sundry voltages and look for an outlier.  If you have one working channel that should reveal what the voltages should be... With it working properly with 10 ohms , replace the original low Z emitter degeneration paths.

JR

Click to expand...

Ok. I've got 10ohm emitter resistors on the Rch. Took a bunch of Vbe readings. This is all with the idle current trimmer all the way down. Outliers look like Q720, 722, 724, 726.

Q715: 0.529V - Q716: 0.542V
Q717: 0.520V - Q718: 0.544V
Q719: 0.106V - Q720: 0.230V
Q721: 0.117V - Q722: 0.211V
Q723: 0.581V - Q724: 0.451V
Q725: 0.585V - Q726: 0.341V
Q727: 0.653V - Q728: 0.637V
Q729: 0.637V - Q730: 0.627V
Q731: 0.346V - Q732: 0.433V
Q733: 0.389V - Q734: 0.362V

I've replaced or checked every components in that section except D702 and D704.  I'll pull those and check em.

Alright! Found a bad germanium diode - 0A90.

Do I need a germanium replacement? Found two datasheets one says 30v 500ma other says 10v 300ma. The still working one has a voltage drop of .643.

Seems like a 1n4148 would be fine...but then again why did they use a germanium to begin with?

samgraysound said:

Alright! Found a bad germanium diode - 0A90.

Do I need a germanium replacement? Found two datasheets one says 30v 500ma other says 10v 300ma. The still working one has a voltage drop of .643.

Seems like a 1n4148 would be fine...but then again why did they use a germanium to begin with?

Click to expand...

Those are in the class A bias network so the smaller Vbe of germanium is probably on purpose.

The drinking lamp has already been lit so I'll look at this again tomorrow.

JR 

It is now coffee o'clock so lets see what you have there

samgraysound said:

Ok. I've got 10ohm emitter resistors on the Rch. Took a bunch of Vbe readings. This is all with the idle current trimmer all the way down. Outliers look like Q720, 722, 724, 726.

Q715: 0.529V - Q716: 0.542V
Q717: 0.520V - Q718: 0.544V

Click to expand...

OK

Q719: 0.106V - Q720: 0.230V
Q721: 0.117V - Q722: 0.211V

Click to expand...

OK.. all cut off at low current. Looks like current limiting. Instead of sensing voltage drop across emitter resistors, it detects voltage including Vbe of output devices.

Q723: 0.581V - Q724: 0.451V
Q725: 0.585V - Q726: 0.341V

Click to expand...

Driver transistors for darlington output stage.

724/726 not biased up as hard...as 723/725

Q727: 0.653V - Q728: 0.637V
Q729: 0.637V - Q730: 0.627V

Click to expand...

This is unusual configuration.. those devices are probably mounted on heat sink to track thermally with output stage devices when they heat up. Drops look OK (same is good).

Q731: 0.346V - Q732: 0.433V
Q733: 0.389V - Q734: 0.362V

Click to expand...

output stage definitely idling along at very low current.

I've replaced or checked every components in that section except D702 and D704.  I'll pull those and check em.

Click to expand...

Can we assume the final output voltage is roughly 0V?

JR

samgraysound said:

Alright! Found a bad germanium diode - 0A90.

Do I need a germanium replacement? Found two datasheets one says 30v 500ma other says 10v 300ma. The still working one has a voltage drop of .643.

Seems like a 1n4148 would be fine...but then again why did they use a germanium to begin with?

Click to expand...

I have never used germanium diodes in a design but AFAIK they are generally a smaller forward drop that silicon diodes. BUT the data sheets I find characterize them at 1V forward, which is much higher than typical silicon diodes. So I don't really know.  :-\

I would be apprehensive about substituting different technology parts in that particular section of the amp that is setting the output stage bias/class A current.

Note: too much voltage across the output stage bias can easily release smoke from the power devices.

JR

If you do your testing while powered up through a variac set at a low, safe voltage, you can probably troubleshoot the over-current condition without blowing any more transistors

JohnRoberts said:

I have never used germanium diodes in a design but AFAIK they are generally a smaller forward drop that silicon diodes. BUT the data sheets I find characterize them at 1V forward, which is much higher than typical silicon diodes. So I don't really know.  :-\

I would be apprehensive about substituting different technology parts in that particular section of the amp that is setting the output stage bias/class A current.

Note: too much voltage across the output stage bias can easily release smoke from the power devices.

JR

Click to expand...

Ok. I'll try to source the germaniums. The NOS are going to be hard, but a lot of places are listing the NTE109 as equivalent.

https://www.newark.com/nte-electronics/nte109/switching-diode-80v-60ma-do-7/dp/21M5148