EEVblog #289-1 - VCR Teardown & Handifax Followup

[station240]

I've actually got one of these DVD/VCR combo machines, mine's a LG V782W but looks almost the same from the outside. I struggle to see any difference at all in the internals, exact same PCBs, not fitted components, and everything else. It was even a dumpster dive like Dave's one.

Anyway the none of it worked, I've found any fixed two problems:
1. VHS section didn't want to take tapes, was simply misaligned somehow, seemed to almost fix itself.
2. The DVD section would make horrible noises when trying to play discs, and the open/close button only worked for opening the drawer. Tracked to to a funny microswitch with a lever arm in the middle of the mechanism in plain sight. Switch was OC and not very springy, fixed by unsoldering it, which magically fixed the contract problem . Prying up the two ends released the top cover of the switch, and letting the contract out and then sticking it back in fixed the springyness.

Only thing now is it still wont play DVDs, it scans the disc, then gives up and spits it out. I think the laser is operating, which just leaves the read mechanism. Anyone have ideas on how to troubleshoot that part ?

[jahonen]

I used to fix VCR's in my previous life, and I still remember some points:

The drum spins 25/30 times per second (half turn takes same amount of time than it takes to send a field in video) so in 50 Hz PAL land, the drum spins 1500 RPM. Video heads are arranged so that each head records a single field as it passes the tape. The head switching is done during vertical blanking interval, so discontinuities won't be visible. 6 heads means that there are 2 heads for standard speed (SP), 2 heads for long play (LP) and 2 heads for hifi sound.

The fixed sound head actually contains two heads, one is for mono audio (not that hifi-one) and another is for sync track. It is important that tape is in correct phase against the drum phase, so that each head flies (those grooves in the drum act as an air guides to form an air cushion between the tape and heads, otherwise it wouldn't work!) just along with the recorded video tracks. Tracking head is used to read out a reference which is then used to adjust the capstan motor phase to maintain correct tracking. Hifi sound is actually recorded on top of the video tracks with frequency modulation using separate pairs of heads having very different azimuth angle, so that hifi sound won't spoil the video. Hifi sound was actually quite good, and some VCRs even contained a mode where it could be used as an audio recorder. Only problem with the hifi sound was actually during the head switching, which could be heard as a buzzing sound in background. To suppress that, signal processing circuitry contained a compander (compressor/expander) which flattened out the dynamics of the signal to be recorded and expanded it back during playback so that during silent periods, the buzzing noise could be made less distracting.

Regards,
Janne