Sansui RZ3000 Receiver problem

[Quirt]

I inherited an RZ3000 stereo receiver with a control amp problem. It
*appears* that the balance control is bad as the left channel never pots
down all the way, driving the amp to about 3 watts when the volume is all
the way down.

At the balance pot, the left channel signal pin never drops to ground
potential. This would generally indicate that the pot is bad, which it
probably is. However... the main volume board follows the output of the
control amp and the main volume pot has an extra pin on each gang which
appears to be a center tap of the resistive element. As far as I can tell
from tracing the circuit, the connections are thus:

The first pin is GND.
The second pin is the WIPER OUT.
The third is the SIGNAL IN.
The fourth pin is the CENTER TAP.

The CT loops back to the "Variable Loudness" control circuit, which I
assume is using the loop to modify the loudness control's preemphasis
curve in relation to the volume level. I might be totally off-base here,
but that's the best I can decipher of this setup without a schematic.

I'm not sure what, if any, interaction this extra loop would have on the
balance circuit. Since the balance pot is just a standard 3 lead pot, I'll
change it out first since I have a replacement handy. I've noticed that if
you wiggle the shaft rather violently, the channel intermittently goes
quiet. I took it apart and it looks like the composition is wearing off
that side of the pot's PWB. If the problem persists after changing the
balance pot, then I'll know the main volume pot is bad. Makes sense to me
anyway, unless anyone has any other suggestions.

Quirt Evans

 

[Jerry G.]

It seems that you will need to change the defective pot. It is normal that
the internal resistive material can wear out.

--

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Jerry Greenberg GLG Technologies GLG
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I inherited an RZ3000 stereo receiver with a control amp problem. It
*appears* that the balance control is bad as the left channel never pots
down all the way, driving the amp to about 3 watts when the volume is all
the way down.

At the balance pot, the left channel signal pin never drops to ground
potential. This would generally indicate that the pot is bad, which it
probably is. However... the main volume board follows the output of the
control amp and the main volume pot has an extra pin on each gang which
appears to be a center tap of the resistive element. As far as I can tell
from tracing the circuit, the connections are thus:

The first pin is GND.
The second pin is the WIPER OUT.
The third is the SIGNAL IN.
The fourth pin is the CENTER TAP.

The CT loops back to the "Variable Loudness" control circuit, which I
assume is using the loop to modify the loudness control's preemphasis
curve in relation to the volume level. I might be totally off-base here,
but that's the best I can decipher of this setup without a schematic.

I'm not sure what, if any, interaction this extra loop would have on the
balance circuit. Since the balance pot is just a standard 3 lead pot, I'll
change it out first since I have a replacement handy. I've noticed that if
you wiggle the shaft rather violently, the channel intermittently goes
quiet. I took it apart and it looks like the composition is wearing off
that side of the pot's PWB. If the problem persists after changing the
balance pot, then I'll know the main volume pot is bad. Makes sense to me
anyway, unless anyone has any other suggestions.

Quirt Evans

 

[Quirt]

It seems that you will need to change the defective pot. It is normal
that the internal resistive material can wear out.

Yes, I changed the balance pot and the problem improved, but still
persisted. I then swapped out the 8-pin main volume pot with a standard
6-pin stereo pot, which corrected the problem at the expense of defeating
the "variable loudness" circuit. I now have fully-working Bass, Treble,
Balance and Main Volume controls, but a temporarily inoperative Loudness
control.

Since I don't have the remote control for the unit, I just removed the
motor for the pot and controller IC altogether. It's no big loss to me
really. I can use the motor and good side of the pot in another project
I'm building anyway.

It's the best I could do with no schematic or replacement pot. Seeing that
it's going to be for my own use, I don't really mind if the circuit is
modified as long as it meets my needs. It was basically on it's way to the
dumpster when I intervened, so I figure if I can get it working at all, it
gets new lease on life and I get a better sound system for my gaming
computer. It works for me!

I'm still curious about this loudness control though, mainly because I've
never seen such an odd configuration in my 20 years of repairing broadcast
gear in radio. The center tap on the main pot is evidently where the
loudness circuit gets it's audio input from. There are also two traces on
the PWB for the main volume that follow along with the rest of the audio
traces but end abruptly near the pot. There are no connections to the far
end of these traces, which do have audio on them. There's no solder pads
at the end of the trace and it's a single layer, one-sided board.
Capacitance, perhaps?

What I think I'll do until I can run down a replacement for the 8-pin pot
is simply leave the standard one in there and add a pair of trimpots to
supply the audio feed to the loudness circuit and tweak them until it
works properly. Then in the event that I *can't* find a replacement for
the main pot, at least I have a fully-functioning receiver.

Anyone have an idea where to look online for these 8-pin center-tapped
pots? I tried my usual parts places and had no luck at all... nothing even
close, at least in PCB-mount! I found a few CT'ed stereo pots with solder
loops, but that's it.

Quirt

 

[Tony Marsillo]

Try this link. They come in 10K to 500K range.

http://www.dalbani.com/CGI-BIN/LANSAWEB?WEBEVENT+LD71F9C4F4859C3BE87F1C6O+M3
6+ENG

 

[Tony Marsillo]

Looks like the link does not work. Go to www.dalbani.com and search for item
number 08-1380. That is a 50K dual pot with tap off.

--
Tony Marsillo
Nutmeg Repair
-----------------------------------------------------------------

Try this link. They come in 10K to 500K range.

http://www.dalbani.com/CGI-BIN/LANSAWEB?WEBEVENT+LD71F9C4F4859C3BE87F1C6O+M3
6+ENG

--
Tony Marsillo
Nutmeg Repair
-----------------------------------------------------------------