Fixing an old Micro Hi-Fi System - Bodge job? - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: June 14, 2014, 04:55:55 pm
content: Hey,So I'm troubleshooting thisPhilips Micro Hi-Fi System MCM390, going over some burnt caps and other possible damage I might find. The unit let go some magic blue smoke.When I opened the unit up, I noticed this odd "bodge" job which I wanted to share and get some input. While I'm not sure it is a bodge job, it doesn't seem to have been "properly" designed if it isn't.In picture A, you can see a rather large piece of paper (kind of like construction paper / cardboard) that has a copper foil glued to it. To one end, is connected to a Capacitor, and, at the other end, what looks to me to be connected to the GND plain.In picture B, I've located the capacitor in question, green, labeled 244K. I take this to mean 0.22uF, and it looks like a Polyester cap with 10% tolerance (K), but maybe I'm wrong.Anyway, the question is, why the huge copper foil to connect between the cap and the GND plain? Surely a wire would have been sufficient? Did they try to create some kind of capacitance / inductance between the cap and the GND Plain? Or some other kind of effect? I don't see why they would have done this, perhaps you guys have some idea.

author: SeanB
author_rank: Super Contributor
timestamp: June 14, 2014, 05:19:59 pm
content: The copper on paper board is there for shielding, added during manufacture to reduce either noise emitted or received by the audio circuitry underneath. The wires are there to ground it, the wires are both connected to a ground trace on the board, it just so happens that the one chosen on the left side is also attached to one leg ( ground as well) of the 220n capacitor there. They probably used this to avoid having to add another soldermask hole to solder the extra wire to the back of the main board like on the right where they left the soldermask slot for it. Second wire probably was a production line addition when they found that the single ground wire was not effective at removing most likely GSM phone noise effectively with a phone in use close by the unit, and the second wire was added to lower the impedance of the ground plane some more to improve the noise rejection.

author: made2hack
author_rank: Frequent Contributor
timestamp: June 14, 2014, 05:37:27 pm
content: Ah ok, interesting.But would it have been a cost cutting measure not to have say a large piece of shielding to cover the entire pcb? I mean, it seems rather flimsy to me, whereas if you would have covered the entire PCB I could see it being more effective?

author: SeanB
author_rank: Super Contributor
timestamp: June 14, 2014, 05:42:56 pm
content: Yes, it is a cheap method to save having a shield over the whole board, which might have needed a redesign of the casings ( new dies at $50k each, possibly quite a few as well) and is added to only cover the most critical areas. Often the shield is designed in using another PCB which is etched, soldermasked and silkscreened and cut to shape complete with cutouts for assembly screws and adjustment points. That then is soldered to pins inserted specifically for the shield. I have seen some with lovely patterning to shield certain areas only, and multiple shields over sections each having a grounding pin.