CD player strange behaviour - Aiwa CX-N500Z (NSX-500)

[Jeroni Paul]

I am looking at the CD player part of an Aiwa CX-N500Z (NSX-500), many times does not focus when a disk is loaded, but retrying sometimes starts and plays the disk perfectly all the way.

I have swapped the entire optical pickup assy from an identical working unit and the problem is in the drive electronics, not the pickup. Both pickups work fine one one unit and both fail the same way in the other.
Mostly Sony parts, pickup: KSS-210A, ICs: CXA1081A, CXA1082BQ, CXD1167Q, BA6296FP.

I have scoped some signals, when it its trying to focus the FOCUS-OK signal from CXA1081A activates momentarily but nothing happens, this should trigger the servos and start of spindle motor. Sometimes the focus locks and disk starts to spin and suddently loses focus for no obvious reason. However while playing it does not lose focus easily, buttons FF or REW behave fine, but jumping tracks sometimes cause it to lose focus.

A secondary sympthom (maybe related) is when playback is stopped through the stop button, the sled returns home and at the end the sled gears slip some teeth because the inertia, the motor does not brake in time. BUT if playback is stopped with the stand-by button instead, it does the same without teeth slippage, because it brakes. I ran some tests shorting the switch contacts and with the Stop button the sled moves some extra millimeters due to inertia. I can't explain how the BA6296FP can stop the motor without brake as its MUTE signal is wired to Vcc.

[Jeroni Paul]

Jeroni Paul wrote:
> A secondary sympthom (maybe related) is when playback is stopped through the stop button, the sled returns home and at the end the sled gears slip some teeth because the inertia, the motor does not brake in time. BUT if playback is stopped with the stand-by button instead, it does the same without teeth slippage, because it brakes. I ran some tests shorting the switch contacts and with the Stop button the sled moves some extra millimeters due to inertia. I can't explain how the BA6296FP can stop the motor without brake as its MUTE signal is wired to Vcc.

I found this happens because when the CD stops and the sled returns home, the controller cuts power to it so fast that does not allow the driver to brake the sled motor. I have jumped the power-on signal to always active and it brakes properly. Design defect?

But the initial disk reading problem remains, maybe they are not related. I tried to readjust the focus bias pot and it was already right. I'm doing some tests heating the board and that seems to have some effect but will require more tests to narrow down.

Service manual: https://elektrotanya.com/aiwa_nsx-500_xg-500_cx-3500.pdf/download.html

[Jeroni Paul]

Jeroni Paul wrote:
> Service manual: https://elektrotanya.com/aiwa_nsx-500_xg-500_cx-3500.pdf/download.html

I've somewhat fixed it but maybe not a proper fix.

I analyzed various signals: FocusOK, FocusError (CXA1081) and DATA, CLK and SENSE (CXA1082).
The controller sends these commands to CXA1082 over the DATA/CLK bus:
$47, $40, $47, $40 - two attempts to focus with the CXA1082 autosequencer
$22, $20 - sled forward jump
$47, $40 - third attempt to focus
It waits 920ms between $47 and $40 during which the SENSE output (¬BUSY signal) remains low - the CXA1082 is not succeding at focusing. When occasionally focuses OK it does so in around 300ms and the ¬BUSY signal goes high for a very short time.

Now I carefully scoped FOK and FE while it tries to focus and they look as expected, FOK goes high and within that time FE does an up and down sine oscillation. At this point I would assume the CXA1082 is defective, however something called my attention:

When it succeds in focusing, FE signal changes offset about 1V higher. This offset jump is (apparently) caused by Q2 transistor, when "on" it changes the FOCUS BIAS point by connecting one resistor in the focus bias divider circuit to Vref. This transistor is "on" when it tries to focus initially and sometimes when jumping tracks, and is "off" while playing (MON signal).

So, what's the purpose of that transistor? I was tempted to disable it but it's all SMD, I would have to cut a track.

Instead, I readjusted the BIAS OFFSET pot R8 to somewhat compensate or reduce that offset jump and now it focuses properly first try almost always, but at the cost of a lower and noisier eye signal while playing. I think this is probably not how it is supposed to work. While it seems to play mastered CD fine I think it has more difficulties playing CD-R.

Previously I had already adjusted the BIAS OFFSET according to the manual, and apparently was set right from the beginning. Still not sure I am doing it right, because the manual refers to a test disc TCD-782 (YEDS-18) composition 2 and I'm using a standard mastered music CD, maybe this invalidates this procedure. However I suspect that noisy eye pattern is not right.

Any idea what Q2 is for? Should I go ahead and try to disable it?

[Jeroni Paul]

Jeroni Paul wrote:
> Any idea what Q2 is for? Should I go ahead and try to disable it?

Fixed!!! I disabled that transistor, restored FOCUS BIAS to original setting and now works perfectly. Focuses first try and never loses it, went through several disks in random mode, focuses and seeks quickly and reads all perfectly including scratched CD-R. I don't get what was that transistor supposed to do.

I decided to do that because, while misadjusting FOCUS BIAS made it work somewhat better, it still kept losing focus while jumping tracks and sometimes would not focus back and give up. Also, other schematics using the same ICs for example Aiwa NSX-800, do not have anything altering F-BIAS, it is simply fixed by a pot.

[Peter X1]

Well done en thx for sharing this story.
--
grtz. Peter