68kMLA Classic Interface

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LC 575 refurb options?

Posted by: Arbee on 2023-09-16 14:06:47 I can fix the lack of schematics, although if the headphone jack works your problem may not be on the logic board.

Posted by: ClassicMacintosh on 2023-09-18 14:40:07 Appreciate it. Time to take it apart down to the bare plastic and find out where the speakers are connected.

Posted by: ClassicMacintosh on 2023-09-19 04:53:21 Currently at this stage of the disassembly - everything is apart down to the front speakers & shield. Can anyone help make sense of where to start to troubleshoot this? Don't want to break anything, even though perhaps the connection on the inside is already frayed.

Posted by: pizzigri on 2023-09-19 08:41:24 @Arbee, you don’t happen to have the schematics of the analog board? That’s where the audio amplifiers are located….

Posted by: Arbee on 2023-09-19 09:05:25 I have no analog boards, sorry.

Posted by: ClassicMacintosh on 2023-09-19 15:20:55 @pizzigri @Arbee Appreciate the support! Will have to do it without schematics, and hope we stumble across the fault. Have taken it down to the bare plastics - Nothing immediately obvious below the bottom shield, apart from being a fair bit less dusty than the rest. The only obvious thing are these two posts where the speaker wires connect to the AB. Zooming in, it looks like there's some green material on one of the metal "prongs". Here's where they connect to on the AB. Guessing a clean with iso and then a quick sanding down would be the way to go?

Posted by: pizzigri on 2023-09-19 22:29:15 The two TDA7052A chips are the audio amplifiers… surely cleaning the contact posts can’t hurt

Posted by: joshc on 2023-09-20 00:28:37 Recapping your AB wouldn't hurt as well, those capacitors are probably way beyond the end of their design life and at least a few of the ones that are near high temperature areas are probably out of spec.

Posted by: ClassicMacintosh on 2023-09-26 16:01:16 OK, so thanks again to all - cleaning up the posts on the AB and removing the gunk from the connectors did the trick. @joshc Everything appears to work, however now on power-on there appears to be a hissing/whining, almost sounds like a kettle. Sounds like it's coming from the back, near the flyback, or even where the AB connects to the back of the CRT. It only appears to be producing this sound when the crt powers up (after the sound of the initial static). Couldn't isolate it to the speakers - could it be related to the caps, as you mentioned? Have checked all connections.

Posted by: Arbee on 2023-09-27 16:41:55 @ClassicMacintosh That's a classic symptom of "corona discharge", the high voltage finding a path to ground somewhere. It's often due to dirt/dust or cracked insulation on wires. Worst case, it can be due to the insulation inside the flyback transformer breaking down, in which case you're unfortunately on borrowed time. Try powering the machine up in a dark room with the screen brightness turned down and look for kind of a blue/purple glow to track down where it is.

Posted by: ClassicMacintosh on 2023-10-01 00:28:54 That doesn't sound ideal. Alright, will power it on in a dark room - could it be due to the shielding below the AB (which was pretty rusty, & bent out of shape a little when reinstalling it)? Alternatively, can a 580CD AB + CRT fit in the same spot? Got one that the case exploded into showers of brittle plastic, but the insides survived.

Posted by: pizzigri on 2023-10-01 09:38:16 I really do not think so, the 580 uses a shadow grille crt instead of a Trinitron, it’s totally different

Posted by: pizzigri on 2023-10-01 09:42:15 to be more precise, even if the connections from the AB to the LB were compatible (they are not) the 575 bezel has straight sides, while the 580 has curved sides (cylindrical vs spherical crt).

Posted by: cheesestraws on 2023-10-01 09:55:37 Yeah, if you look at it and go 'oh, that's a really pretty purple', it's probably corona discharge which isn't a good sign unfortunately...

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