martoon69 Posted May 29, 2014 Posted May 29, 2014 Hi Guys, Perhaps someone can light the way here.. I've shouted myself a couple of gainclone amp kits off ebay, and, (I really should know this), I've forgotten which way round polarised caps go on a pcb by convention. To start with I googled pcb's and capacitor layout to get a fix on convention. There doesn't seem to be any, and what's really confusing me is, as you'll see in the attached pictures, that I expected to see the hatched half of the pcb cap circle to be positive, the other side being ground. I would have kept on thinking this but taking a look at the populated board, it seems that the caps are mounted wrong way round. I can't help feeling I'm having a dribbly moment and not seeing the obvious, so if anyone feels inclined to stare at the two pix and compare them, then tell me if the main polarised caps are mounted correctly or not. It would have helped if they'd been marked +/-, but sadly not... Not a promising start to a simple kit build, but I do get better... Cheers, Martin
Guest Muon Posted May 29, 2014 Posted May 29, 2014 (edited) The band down the side of each cap denotes the negative, as I think you know. If you want to make sure what side on the PCB is negative just use a DMM to easily check for continuity on the ground plane, but I think the crossed section is indeed the negative? Edit: Ok, more I look at that PCB, the more I'm confused, need confirmation from someone else Although the assembled pic should be correct. Edited May 29, 2014 by ortofun
Sub Sonic Posted May 29, 2014 Posted May 29, 2014 (edited) Can you follow the +/- rails from the rectifier to establish which should be which? Regards, SS Edit: does the ebay site have a photo of the board? You should be able to see which way round they are. Cheers! Edited May 29, 2014 by Sub Sonic
martoon69 Posted May 30, 2014 Author Posted May 30, 2014 Ok, thanks guys, I'll know more when I get the boards to hand and I can examine them in detail. It's a bit pathetic to be thrown by this basic requirement, but when I started to look closely at the pix something seemed amiss, hence my question. I was attracted to this kit because it offered a board with the power supply built in for a small footprint. I intend to go dual-mono with separate iron, probably 300va with 22+22v from what I've researched. They didn't cost a lot and there's not a lot lost if the caps go pop and poke holes in the ceiling ( unless I get in the way, and/or the war department finds out) Re. Sub Sonic. Those pictures came from Ebay. There's 4 taken from different angles of the populated board, and a couple with the bare bones. On another DIY subject, I'm currently building a pair of mini Hedlund horns, 1200mm tall. I drew one out full size, found a cnc router guy who transferred my dimensions to DXF, and using 25mm MDF, made 2 pairs of side pieces. It's up to me now to laminate the fronts and backs with 6 layers of 3mm MDF for a total, including glue layers, of around 19mm. The sides are rebated 20mm leaving me to clean off around a mill with a router. Speakers are Fostex FE166en. I've been taking hi-res pictures, so if anyone is interested I can post them as construction proceeds. They're fiddly, require lots of clamps, wedges, chocks, head scratching, cups of tea, frequent breaks and cursing and puffing on my electronic pipe, but it's all great fun. I'm using 5 to 1 epoxy resin, absolutely essential in my view for laminating thin MDF under a lot of bending stress, and thickened with talc as required. How will they sound when completed? Might sound crap for all I know, but currently that's less important than the actual build, which at my current state of progress, plus all the other jobs around the house the war department keeps lumping on me, is going to take several weeks or more. Don't care, saves me going loopy watching soaps with a mug of cocoa... Let me know anyone, if you'd like to see progress pix... Cheers, Martin 1
Sub Sonic Posted May 30, 2014 Posted May 30, 2014 Please!!!!! We all love to see build pics :-) Regards, SS
Addicted to music Posted May 30, 2014 Posted May 30, 2014 Assuming the picture of the diagram is right and of a working module, the part that is shaded with white painted lines is negative - and the unshaded area is positive + Thats looking at the caps that are in place on the pcb of that board. Alternatively to back what I have said:
martoon69 Posted May 31, 2014 Author Posted May 31, 2014 Thanks pchan, I appreciate you taking the time... I was drawn to the same conclusion, or was getting there. I'd hoped that the pic of the completed board which I borrowed off the ebay supplier site was indeed correctly populated. As it happens this supplier (Ivy textiles) also has another gainclone-type amp module for a stereo board. I bought one of those too $20 inc post, but minus the LM3875's, which I thought was pretty cheap and worth a fiddle with. It too has a number of pics on the site and the convention is the same as the board in this discussion, the hatched half of the cap circle being negative. I wasn't keen on buying LM3875tf et al chips off ebay because I understand there are fakes around, so I got mine from Element14, an Aussie/UK Farnell site, not expensive, cheaper than JayTronics and with at least a good chance they'll be the real thing, not that I would know the difference:) I await the postie... Cheers, Martin
A J Posted May 31, 2014 Posted May 31, 2014 The 3875 is a stunning chip - I'm using my amp to drive a set of Kef LS50's and the result us quite stunning for the $$$ Sent from my iPad using Tapatalk
Addicted to music Posted May 31, 2014 Posted May 31, 2014 Not sure whether you have built before, but if this was my project I would solder the SMD 1st. Start populating the smallest to the largest. For the SMD, i did post what I did on another thread is that with a normal iron, I sticky tape the SMD component and position it where I want it. Hit one a pin with solder 1st, take the tape off, accurately correct the position if Im not happy with it and then finish solding the other point.
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