Hi, Sorry to be flooding the forums with bawws this weekend, but it's my 'project' weekend My amp (Beomaster 4000, c.1970) started humming recently, and it really gets loud when it's warmed up. I did an amptopsy and determined: - The transformer and wiring is all held in rectangular thin sheet steel box (~10cm x 7cm x 5cm). - This box is filled with solidified plasticy resin which locks everything together. - The long edges of the box have pulled away from the resin, and are now vibrating at mains frequency. - Pushing very hard on the sides of the box, squashing it against the resin, reduce the hum massively. I tried to manufacture a method of holding it using cable ties, but it's very cramped and I couldn't get it to work suitably. I cannot get the box out as it is clearly the first thing that was assembled into the amp, but I can move it slightly if unscrewed. Any ideas? Possible thoughts: #1 Replace transformer with modern one, but as far as I can tell it needs to output 22VAC and 47VAC, 250W total (not sure how it's split). #2 Somehow inject resin down the sides on the box to rehold the box sides. I prefer #2 as it would keep the amp more 'original', and be much less hassle. Can anyone recommend a suitable resin, and most importantly, how do I get it in there: the gap is around 2mm wide in the centre, taping to nowt at the edges, and about 5cm deep. I've got an RS order to put through as soon as it's back online, so preferably something they have in stock - but the choice is huuuuuge! Thanks guys! Will post pics when I open it up again (uber engineering pr0n ). tl;dr Need some glue with a long injector to fix a vibrator in place.
Its bottom, transformer on the right. Size 120 x 100 x 70 mm. Close up of transformer, from the top and its lid off (un pictured metal lid screws on top) Rectifier - White + red carry 54VAC, rectifies to 69VDC in red + black which feeds to main board. Grey + black pair carry 22VAC which feeds straight into the other end of the main board. The metal case can be pushed in a couple of mm with a strong thumb. The gap is pretty inaccessible, I couldn't even get my knife into it, but it's definitely there. Not sure if you can see it, but there's a tiniest crack between the green and the metal.
I'd be tempted to try and fill the crack up with some silicone to rebond it all back together. Something nice and strong for car use would probably be best (I'm thinking black loctite from Halfords) because it's easy to work with and reasonably cheap.
This, and also replace then center capacitor. That thing looks like it is losing it's electrolytic compound.
Something along the lines of this http://cpc.farnell.com/jsp/displayProduct.jsp?sku=SA01707&CMP=e-2072-00001000 I like the idea of loctite, it'd go into the thin crack more easily than trying to force silicon into it. Leaky capacitors are on the list, but it'll be a shipbuilders syndrome problem I suspect It doesn't seem to affect the sound quality though; I cleaned up all the switches and sliders properly and there's now no crackling
