Panasonic SL 721 cd4 turntable

[johnd393]

It was just a $20 turntable from a Facebook local seller. Didn't know what it was still I got it home and started fixing it. Some cleaning, gluing, & lubrication and it has responded well to my effort. It spins fairly accurately, changes records like it should. Has the original EPS 451qd strain gauge cartridge & stylus and built in cd4 decoder. It works an stereo. I had no quad records to try on it. A Fisher test record has arrived from a ebay seller. I'm testing with a Radio Shack mini amp and headphones. Front & back channels are on selectable amp inputs so I can a/b the front & back. Playing the test record, the Quad LED lights when a cd4 record is played. If I play the front or rear test I don't hear much difference when listening to front or back. What's supposed to be front only or back only is heard both front and back. My land line and internet is out and I'm on my tiny Tracfone so I hope this is not nonsense.

 

[Doug G.]

Does the Panasonic have separation controls? If so, play the record and adjust the back channels for minimum output when the correct track on the Fisher record is played.

Doug

 

[johnd393]

There are two holes on the side of the turntable base that give access to 2 unlabeled adjustments. I have no instructions or schematics for the control board

 

[Doug G.]

Most likely, those are the separation adjustments, particularly if there are no similar paired adjustments on the unit.

There is currently a service manual on eBay for the SL-721.

Doug

 

[johnd393]

When I get my land line and DSL back I can post some pictures of the circuit board. Took a closer look. There is clear tape over the 2 adjustments. It's hard to see but it is labeled. Separation, Left & right.
This has a BSR C141R1 changer. At least it's not one with a plastic platter. There is a 2 pole motor that has a extra coil on it that acts as a transformer secondary to power the decoder.

 

[Doug G.]

Hmm, I was thinking maybe somebody messed with the separation controls and ruined the separation but, if the tape is still over the controls, it seems less likely although not impossible.

It still wouldn't hurt anything to adjust them, anyway. Just make note of where they are now so you can return them to their current positions. If you are getting identical output from front and rear now, something isn't right, anyway.

Doug

 

[johnd393]

Just like volume & tone controls get scratchy and make intermittent contact with age, no reason to think that wouldn't happen to a cheap little trimmer pot. The stereo/CD4/Aux switch was noisy until exercisd a few times. There are inputs on the back to pass another 4 channel source through this thing

 

[johnd393]

Do I have this right? Playing a front only left or right warble sound, adjust the separation for minimum rear channel output.
I haven't got up the nerve to adjust anything yet. Still trying to get by testing with headphones, switching back & forth front and back.

 

[Doug G.]

Yes, correct. Set the balances so you only hear the left back channel and play the appropriate track and adjust for minimum output from the left back channel. Then, repeat for the right back channel.

Doug

 

[johnd393]

I tried to adjust. There is no separation front to back. Even if I switch to aux and introduce hum by sticking. a wire into the front or back aux input jacks there is no separation. I touch either front or back input and I get output from both front and back. I Wish had a schematic.
I downloaded a schematic for a SE-405 free standing CD-4 decoder to try to get some idea how they do stuff.
If the switch to aux input is like on the SE-405, the aux signal just passes through a switch to the output cables. There should not be front back mixing. I gotta re do this test to make sure I'm seeing what I think I saw.

 

[quadsearcher]

I'd like to see a picture of the back or where the aux 4ch input is located. If an input to aux LR (for example) comes out of both LF and LR, either there is a 2/4 channel switch we're overlooking, or maybe someone rewired for 2 channel?

 

[MidiMagic]

Is that a 2-pole motor?

 

[johnd393]

Yep it's a 2 pole motor. The motor has a second winding that acts as a transformer secondary to power the decoder. A strobe disk shows the speed is close.

Electronics has ways of tricking me down the wrong path. If the rear aux input is terminated to something, no output comes from the rear output if hum is introduced to the aux front. I tested it 2 ways. (1)I plugged a phono amp in, (2) I stuck a resistor from the input to ground. The wire from the aux jacks to the board is a unshielded 5 conductor ribbon cable. If a input is floating it can pickup from adjacent conductors. So, there is not crosstalk from front to back with the aux inputs.

Here's what I think. with the sep control at minimum and a CD-4 source, I should be hearing a difference signal on the rear. Someone discribed it as a hollow sound. I don't think I'm hearing it. I don't think the difference signal is being decoded.
There are 2 more unlabeled trimmers on the board. The LED that indicates a quad track is recognized is on solid when a quad track is played, so it's seeing the 30k signal.

 

[Owen Smith]

Is that a 2-pole motor?

Looks to me like it has a secondary winding on it so it can act as a transformer to supply power to one of the boards, as well as being a motor.

 

[Soundfield]

This has a BSR C141R1 changer. At least it's not one with a plastic platter. There is a 2 pole motor that has a extra coil on it that acts as a transformer secondary to power the decoder.

Yes, the BSR-C141R1 auto-changer was fitted a ‘BSR power unit’. This was a shaded pole induction motor with a secondary winding on the stator to supply other parts of the assembly with low voltage ac.

 

[MidiMagic]

I had to replace one of those motors once. It was easier and cheaper to buy a new motor than to get the bearings. The problem was that this was installed in a compact stereo, and the motor ran whenever the radio was on (to supply the power). It was used mostly for the radio, and when they tried to use the turntable, the bearings were worn out.

 

[johnd393]

Seems like it would be easy to add a transformer to eliminate using the motor as a power supply.

I think I'm stuck until I find a schematic. It works for stereo. I don't want to put wear on that special CD-4 stylus playing stereo records. I have several other turntables & cartridges that track at lighter tracking force.
I'm not willing to spend $25 for the service manual on eBay.