Reference material on CD/DVD control systems and waveforms - any suggestions? - Page 1
Forum section: electronics / repair

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: October 27, 2016, 05:01:35 am
content: I posted on this Aiwa mini-hifi repair a while ago (https://www.eevblog.com/forum/repair/aiwa-mini-hifi-audio-amp-protection-circuit-how-to-diagnose-cause-of-tripping/) and have fixed the tripping and several other bad connections/joint problems. Unit is now working fine on all functions but it won't read CDs or DVDs. Spends 60secs reading disk then reports "Can't play disk" error. Have cleaned the lens, confirmed the tray and sled motors are being driven and responding OK, confirmed visually that laser red light comes on and laser carriage is going through a focus attempt. Also confirmed the tray open/close and inner sled limit switches are working. With a disk inserted, the rotation speed starts up slowish and ends up pretty high and constant, cycling up and down a few times on the way - so I figured the spindle drive was at least functional. With everything on the DVD mechanism and the DVD drive chip (LA6560) appearing to work, I figured next step was to check the focus/tracking error amp (an LA9703).I've searched a lot for a useful reference to the mysteries of DVD control mechanisms that would help with interpreting the signals on the MPEG/DVD board. I found some useful bits and pieces but little on the detail of CD/DVD control systems and the sorts of waveforms I should be seeing and how to interpret them. I wonder if anyone can point me to something quite comprehensive on CD/DVD control systems that could help. Something with a practical rather than an engineering/maths focus would be best - but I'd be happy to find anything worthwhile at present.I've had a look at signals related to tracking and focus error on the LA9703 RF amp and focus/tracking error amp - see 1st two images. The flags in the FE image are (in sequence) - with tray open, with disk inserted, when it displays "Reading disk", and when it displays "Can't play disk". The TE image is missing the 1st flag. The focus error signal in the first 20sec or so after startup looks much like the ones I've found in online material but the tracking error signal looks nothing like them - I was expecting to see some sort of sine or triangular waveform emerge. The unit's also not distinguishing CDs from DVDs. The DVD/CD detect pin (27) (see last two images) on the LA9703 switches Lo/Hi several times immediately after start but ends up low (indicating a CD) in the last 40secs regardless of whether there's a CD or a DVD inserted. So that's a fail. The FE and TE signals both go high for the last 40s of the disk reading window and revert to the on-spec 1.66V when the "Can't play disk" message pops up. Figured if I could get some good reference material I might be able to figure out what this is telling me and where to look next. I'm treating this as a learning exercise on DVD/audio systems - the device itself is nothing special.I've attached a schematic for the board in case it's useful - I think it's readable at postable size.

author: singapol
author_rank: Frequent Contributor
timestamp: October 27, 2016, 06:09:06 am
content: I'm really surprised but optical disk technology has matured and pretty standard with regards to mecahnical control. Don't blame you as you are not experienced about such things. For us we virgins started with the original CD audio players, video is just an extension. As the race to reduce costs one very important component nay most important one you will soon find out is the laser lens.Mass produced lasers are or can be or most commonly a source of failure...nothing to do with conrtol, it simply can't read because of weak lasers or associated components on the laser lens or complete mechanisms it. A whole industry/business has been built up just to replace the laser lens module. Just seach youtube or audio cd sites to learn how to change or replace such a unit. Look at the label on laser lens module at the back it will normally have the part number or in house part.no. eg. KSS213C , a sony part but now made by oem companies in china in fact most of the japanese brands and philips have "abandoned" this business. There are NOS (new old stock) but majority are modern clones. They do work of course.Do note that some laser modules require you to remove the solder short for antistatic protection before installation. If you forget you have to take it out again.Good luck..be caredful of the filmsy flex cablemaybe difficult if you have big hands.