Sansui AU 517 with hum in both channels...

[jonah1979]

About the amp...

I cleaned, recapped, and did all of the traditional upgrades to my AU517 (fusible resistors, diodes, etc.) and the amp was running fine and sounded great for about five years.

Recently, it refused to come out of protection so I opened it up and worked through the protection circuit. I must admit I took a shotgun approach and replaced the transistors (TR601-TR607), the relay, and a couple of diodes (D607, D608) on the protection board. It now comes out of protection and behaves as normal; however, when powered on there is a low hum which I believe to be 60hz that wasn't there before I worked on the protection circuit.

​

About the hum...

It is in both speakers and can be heard at the headphone jack.​

The hum is constant regardless of volume position.​

Front panel controls have no effect.​

Hum exists when there are no cables at the inputs.​

​

On a side note, the amp will come out of protection with disconnected driver boards. As far as I know, this shouldn't happen, and curious to know what could be causing this. Perhaps it's not an issue. It also seems to take longer to come out of protection than before...

Any help in diagnosing the hum would be great and MUCH appreciated. Thanks in advance!

 

[spark1]

It is normal for protection relay to close with driver boards out.

Have you checked your work carefully for correct orientation of new parts, solder bridges, etc?

 

[Ray Gianelli]

About the amp...

I cleaned, recapped, and did all of the traditional upgrades to my AU517 (fusible resistors, diodes, etc.) and the amp was running fine and sounded great for about five years.

Recently, it refused to come out of protection so I opened it up and worked through the protection circuit. I must admit I took a shotgun approach and replaced the transistors (TR601-TR607), the relay, and a couple of diodes (D607, D608) on the protection board. It now comes out of protection and behaves as normal; however, when powered on there is a low hum which I believe to be 60hz that wasn't there before I worked on the protection circuit.

​

About the hum...

It is in both speakers and can be heard at the headphone jack.​

The hum is constant regardless of volume position.​

Front panel controls have no effect.​

Hum exists when there are no cables at the inputs.​

​

On a side note, the amp will come out of protection with disconnected driver boards. As far as I know, this shouldn't happen, and curious to know what could be causing this. Perhaps it's not an issue. It also seems to take longer to come out of protection than before...

Any help in diagnosing the hum would be great and MUCH appreciated. Thanks in advance!

I had a similar issue with an AU-719. You can see that here:

 

[jonah1979]

I had a similar issue with an AU-719. You can see that here:

Thanks for passing this along. Super interested to hear about your experience. I'll watch it and sit down with the amp this afternoon.

 

[AlTinkster92]

60 Hz or 120 Hz?

 

[spark1]

The OP stated that he thinks his hum is 60Hz.

 

[jonah1979]

The OP stated that he thinks his hum is 60Hz.

Sooo... I just got home and used the frequency analyzer on my phone and what I heard this morning as 60Hz hum is actually 120Hz. My apologies for any confusion.

I had a similar issue with an AU-719. You can see that here:

Just watched your very thorough and excellent video and will start down that path. I took the bottom cover off to access the boards and did as you described but want to make sure I'm understanding the first step in isolating the hum. With the amp powered on and with an audible hum, I disconnected the cable to each driver board and it seemed to behave in the same manner as your amp as soon as the cables were removed the hum disappeared in each channel. I sadly don't have access to a scope, so I'm relying on my ears. Is this enough to discern that it is not coming from the output stages but from the regulated power supply?

 

[jonah1979]

And voltages at test points on the power supply board. Obviously, looks like there is something a little funny on the right channel. The lower channel in the attached image. I have to admit I'm relatively new to electrical engineering so troubleshooting is a struggle. Any thoughts and pointers would be much appreciated.

 

[Ray Gianelli]

Sooo... I just got home and used the frequency analyzer on my phone and what I heard this morning as 60Hz hum is actually 120Hz. My apologies for any confusion.


Just watched your very thorough and excellent video and will start down that path. I took the bottom cover off to access the boards and did as you described but want to make sure I'm understanding the first step in isolating the hum. With the amp powered on and with an audible hum, I disconnected the cable to each driver board and it seemed to behave in the same manner as your amp as soon as the cables were removed the hum disappeared in each channel. I sadly don't have access to a scope, so I'm relying on my ears. Is this enough to discern that it is not coming from the output stages but from the regulated power supply?

Thanks for the kind words, Jonah!

2 things: the cables I pulled were the signal input cables into the output modules. It sounds like that's what you did, but I just wanted to make sure. I needed to use headphones, but my work area is quite noisy

Other thing that jumps out at me is the voltage you measured at Point e, which is higher than designated. Measure the AC voltage between Points between Points f and d ( to see what it should be), then f and e, where it looks like you have a problem. There should be virtually no AC voltage at the output of a regulated DC supply.

Let us know what you find.

 

[jonah1979]

2 things: the cables I pulled were the signal input cables into the output modules. It sounds like that's what you did, but I just wanted to make sure. I needed to use headphones, but my work area is quite noisy

Yes, this is what I did. The two conductor quick disconnects at the end of the boards.

Other thing that jumps out at me is the voltage you measured at Point e, which is higher than designated. Measure the AC voltage between Points between Points f and d ( to see what it should be), then f and e, where it looks like you have a problem. There should be virtually no AC voltage at the output of a regulated DC supply.

So there's definitely some AC voltage at the output of the right channel. There was some fluctuation but both eventually settled down. Here's what I found:

Points f & e (Pins 18 & 20): 14mV AC
Points f & d (Pins 18 & 16): 1.1mV AC

Thoughts?

 

[Ray Gianelli]

Yes, this is what I did. The two conductor quick disconnects at the end of the boards.


So there's definitely some AC voltage at the output of the right channel. There was some fluctuation but both eventually settled down. Here's what I found:

Points f & e (Pins 18 & 20): 14mV AC
Points f & d (Pins 18 & 16): 1.1mV AC

Thoughts?

Well, 14 mV is not much. Just how loud is the hum?

I would check ZD04's anode voltage, and with power off check TR04. Not much else there.

Curious about the lower voltages going into the regulator as well. Might be a good idea to check the rectifier diodes D10, D12, D14 and D16.

 

[jonah1979]

Well, 14 mV is not much. Just how loud is the hum?

It's audible and noticeable but not obnoxious. I know... that's an impossible description.

I would check ZD04's anode voltage, and with power off check TR04. Not much else there.

Curious about the lower voltages going into the regulator as well. Might be a good idea to check the rectifier diodes D10, D12, D14 and D16.

Will check the ZD04 and TR04. With TR04 am I looking for a short? I assume I have to remove TR04 to test... I wish this board was easier to access.

At what point should I measure the voltages going into the regulator? For the rectifier diodes, I will pull and check to see if they're in spec.

 

[Ray Gianelli]

It's audible and noticeable but not obnoxious. I know... that's an impossible description.


Will check the ZD04 and TR04. With TR04 am I looking for a short? I assume I have to remove TR04 to test... I wish this board was easier to access.

At what point should I measure the voltages going into the regulator? For the rectifier diodes, I will pull and check to see if they're in spec.

Give TR04 a quick in circuit diode check, all junctions and emitter to collector as well.

You notated the schematic already with the voltages going into the regulator. They are feeding into the collectors of TR02/04. Both are low compared to the other channel. That's why I became curious about the rectifier diodes.

 

[jonah1979]

Have the board out and poking around...

Lifted a leg of ZD04 and D10, D12, D14, and D16 and all seem fine.

ZD04: OL and .645V
D10, D12, D14, and D16: OL and .57V

Removed TR04 and here's what I found... Not super familiar with diagnosing transistors...

Base to Emitter: .045V
Base to Collector: .001V
Emitter to Collector: .045V

 

[Ray Gianelli]

Have the board out and poking around...

Lifted a leg of ZD04 and D10, D12, D14, and D16 and all seem fine.

ZD04: OL and .645V
D10, D12, D14, and D16: OL and .57V

Removed TR04 and here's what I found... Not super familiar with diagnosing transistors...

Base to Emitter: .045V
Base to Collector: .001V
Emitter to Collector: .045V

You have a base-collector short.

 

[Ray Gianelli]

BTW, you just needed to check the zener's voltage. They can check OK as a diode, but not have the correct reference voltage.

 

[jonah1979]

You have a base-collector short.

Thanks for your eyes on this! Moving forward, should the Base to Collector also read .045V or should there be no reading? I'll order equivalent replacements for TR04.

While I'm at it, would you recommend replacing TR03 in the left channel as well? And with that, replacing its complimentary transistor in TR01 and TR02?

TR01 and TR02: 2SD438 (Replacement: KSC2383)
TR03 and TR04: 2SB560 (Replacement: KSA1013)

Honestly, I just never want to take this board out again... already have a couple of lifted traces.

BTW, you just needed to check the zener's voltage. They can check OK as a diode, but not have the correct reference voltage.

Noted. Is that just checking the anode when powered up?

 

[jonah1979]

And for my own understanding... would the shorted transistor in the right channel be solely responsible for the hum in both outputs? Isn't this something a dual mono PS would relegate to the offending channel? I've probably got it wrong but hoping for clarification.

 

[Ray Gianelli]

Thanks for your eyes on this! Moving forward, should the Base to Collector also read .045V or should there be no reading? I'll order equivalent replacements for TR04.

While I'm at it, would you recommend replacing TR03 in the left channel as well? And with that, replacing its complimentary transistor in TR01 and TR02?

TR01 and TR02: 2SD438 (Replacement: KSC2383)
TR03 and TR04: 2SB560 (Replacement: KSA1013)

Honestly, I just never want to take this board out again... already have a couple of lifted traces.


Noted. Is that just checking the anode when powered up?

A silicon junction drops around .5 to .6 volts. I don't know why yours show .045, I'm guessing it's .45 volts, which is close.

You can replace them if you want, that's up to you. But yeah, those boards are a PITA to pull, so maybe that's not a bad idea.

And yes, just checking the voltage at the anode when it's on. For the positive regulator you would check the cathode.

 

[Ray Gianelli]

And for my own understanding... would the shorted transistor in the right channel be solely responsible for the hum in both outputs? Isn't this something a dual mono PS would relegate to the offending channel? I've probably got it wrong but hoping for clarification.

You know, that's a really good point. Unlike the 719, your 517 has 2 separate regulated supplies. Frankly, I wasn't paying close enough attention, having just come away from the issue with the 719.

Let me think about this.