9090DB bass tone switch issue. Please read.

Robisme

Robisme

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I am working on a 9090DB for a fellow AK'er. I have resolved three issues but have one more.

There is a slight hum when first powered up that goes away in a few minutes.
You can only hear it if you are near the speakers and the volume is at minimum.

When the bass tone switch is flipped, the hum gets louder and there is a small pop with woofer movement.

I am thinking if I recap the tone board it may cure these issues.

Has anyone had this problem?

Thanks.

Rob
 
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It sounds like you have a leaky cap on the tone board causing the pop and woofer movement, and also a bad cap in the power supply, causing the hum.
 
There are several ground wires on the tone board, you need to make sure they are all attached at both ends, if not you can get some really big pops in the output. The hum is probably a power supply cap or a bad regulator on the power board. The voltages for the driver board come from the regulators. There are a couple of 100uf caps in the bases of the regulators that could cause the hum. Of course the main power caps could cause it also.

I am assuming that the pop you get is rather loud. All the 9090DBs that I have worked on have a little noise in switch but the only time you can hear it is with no input and you really have to listen for it. (this is before and after a recap)
 
CDFixer said:
I am assuming that the pop you get is rather loud.
Click to expand...

Actually you can hardly hear it but it is there. The woofer only moves about 1/16 or 1/8 of an inch.

I know the main caps could cause this, but I am hoping it's not them.

The owner did not mention the hum, but now that I know it is there, I must do something about it.

I will start with a tone board recap, then the afore mentioned regulators and their caps, and if that doesn't work, the main caps.

Thanks for the replies.

Rob
 
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Robisme said:
...The owner did not mention the hum, but now that I know it is there, I must do something about it.

I will start with a tone board recap, then the afore mentioned regulators and their caps, and if that doesn't work, the main caps.

Thanks for the replies.

Rob
Click to expand...


That's because I never heard it! Thanks Rob and all you guys!

I dropped my 9090DB off today at Rob's and had a great time hanging out and listening to couple of his wonderful Sansui systems. I'm really looking forward to hearing his magic!

Thanks again!
Carl
 
I'd have my scope out sniffung around to see if I could find both the hum and click/noise source. PS caps for hum. do you have a big uF cap you could use to parallel one of the PS ones? Just some thoughts.
 
oldhifiguy said:
PS caps for hum. do you have a big uF cap you could use to parallel one of the PS ones? Just some thoughts.
Click to expand...

Not that big. Except in my 9090DB.

I will separate the main and pre. If I do not get the hum when I use the 9090DB as an amp, I would think it would not be the main caps.


I will still recap the tone board. I will also pull the scope out.

Thanks

Rob
 
To test any filter cap, use a scope. If the peak to peak ripple is 10% of the DC voltage or less, the cap is ok.
 
I pulled my 9090DB out and fired it up. It does the same thing with the bass tone switch. Can someone else try this. With the volume all the way down, flip the bass turn over/defeat switch up and down.

Do you hear any noise?

I was using FM. Shouldn't matter though.

Funny thing is the hum is gone now. Maybe I had the power cord wrapped up with the speaker cables. Or maybe my shop lights were causing it.

Rob
 
Shop lights

Robisme said:
I pulled my 9090DB out and fired it up. It does the same thing with the bass tone switch. Can someone else try this. With the volume all the way down, flip the bass turn over/defeat switch up and down.

Do you hear any noise?

I was using FM. Shouldn't matter though.

Funny thing is the hum is gone now. Maybe I had the power cord wrapped up with the speaker cables. Or maybe my shop lights were causing it.

Rob
Click to expand...

Got fluorescent tube shop lights?
Hum inducing items those....
Casey
 
Robisme said:
I pulled my 9090DB out and fired it up. It does the same thing with the bass tone switch. Can someone else try this. With the volume all the way down, flip the bass turn over/defeat switch up and down.

Do you hear any noise?
Click to expand...

In Phono I hear a very faint hum when I'm not in defeat on my 9090db.

I had that same popping problem, when using the 9090DB as a preamp I had a pop when playing around with controls, i thought for sure it was from dirty pots or switches, but it was the cluster of grounds on the back of the board.

All I had to do was suck the old solder out, and re-solder the grounds. I also ran another ground wire to another part of the frame to make sure I didn't have to worry about it.
 
I've seen some hum when the bottom cover off and the unit tipped up on it's side. Lay it flat and the hum is gone. It's probably interference from the fluorescent lights. It doesn't happen when all the covers are on and it lays flat.

- Pete
 
LBPete said:
I've seen some hum when the bottom cover off and the unit tipped up on it's side. Lay it flat and the hum is gone. It's probably interference from the fluorescent lights. It doesn't happen when all the covers are on and it lays flat.

- Pete
Click to expand...

Funny you say that. I had the bottom cover off and the unit on its side.

And.........it doesn't happen with all the covers on and it lays flat.

Thanks for that tid bit.

Rob
 
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