RandomBass Posted September 20, 2013 Share Posted September 20, 2013 (edited) Back again with another Trace Elliot repair - this time a GP7SM250 which I recently acquired. When I received the amp I noticed the fan was louder than normal, and whining a fair bit. Some more close listening revealed the bearing was pretty noisy - something to think about changing soon. Well, as it happens, sooner than I thought! After a practice session the fan sounded extremely poorly - then suddenly stopped. This was followed a few seconds later with a 'pop' from inside the amp, closely followed by that classic blown electrolytic cap smell. Oh bugger. Got the thing apart and removed the fan - absolutely no resistance across its terminals: open circuit failure. The blades wouldn't rotate by finger, the whole assembly was welded together. These fans have a sleeve bearing, where basically two surfaces rotate against each other, lubricated by a type of grease. They are generally very quiet at the expense of robustness, and once the lubricant eventually expires you are left with two surfaces rubbing against each other - getting hotter and hotter until they basically weld together. Once this happens there is a spike in the current demanded by the fan which can cause them to burn open or closed circuit. Clearly this one went open. The question was though, why did a capacitor blow too? A quick look at the PCB track, and then the circuit diagram revealed all. Once the fan had gone electrically open circuit, the fan resistor used to drop the voltage from 70v rail to 12v had no current through it - meaning that 70v was suddenly presented to the cap in parallel with the fan: and this cap is rated at only 16v. Goodbye cap. For info, it looks like the cap is there to reduce eddy currents in the fan motor, and reduce resultant RF interference. I was lucky to source an exact replacement 12v fan online, at a bargain price. They are basically 80mm PC cooling fans, but this particular one has specific side mounting lugs that TE use to mount it in place. The replacement cap is a standard 1000uF 16v item. New fan now in place and much much quieter, and cap replaced. So if your fan does the same, you may well need a cap replacement too. Its the cap situated directly behind the fan screw terminals on the main power PCB, and luckily it is easy to get to and replace without having to completely dis-assemble the power board. Next week, we will be trying to work out why TE specified 63v smoothing caps for a +/-70v supply. Cheers Geoff Edited September 20, 2013 by RandomBass Quote Link to comment Share on other sites More sharing options...
icastle Posted September 20, 2013 Share Posted September 20, 2013 [quote name='RandomBass' timestamp='1379688244' post='2215766'] Next week, we will be trying to work out why TE specified 63v smoothing caps for a +/-70v supply. [/quote] In an ideal world, an electrolytic should only be run at about 80% of it's voltage rating. Are the two caps in series? If not, my best guess would have to be that 63v caps were used because they couldn't fit 100V ones in the space and they were relying on the fan to keep the caps cool and raise their resistance to allow for the underrated component that had been selected. Quote Link to comment Share on other sites More sharing options...
RandomBass Posted September 21, 2013 Author Share Posted September 21, 2013 This is common to quite a few of their amp modules, they run a 70-0-70 supply with a 4700uF 63v cap across each of the 70v rails, to ground. So yes the caps are in series, and each has 70v across it. I've replaced these before with 100v versions which are identical in size and lead pitch - I've just done this to my AH200 head, which doesn't have a cooling fan so the inside of that head must get quite sweaty at times Quote Link to comment Share on other sites More sharing options...
rmorris Posted September 26, 2013 Share Posted September 26, 2013 [quote name='icastle' timestamp='1379718748' post='2216263'] If not, my best guess would have to be that 63v caps were used because they couldn't fit 100V ones in the space and they were relying on the fan to keep the caps cool and raise their resistance to allow for the underrated component that had been selected. [/quote] I don't get that idea - increasing the resistance of a capacitor by cooling ? Quote Link to comment Share on other sites More sharing options...
RandomBass Posted September 27, 2013 Author Share Posted September 27, 2013 [quote name='rmorris' timestamp='1380207953' post='2222342'] I don't get that idea - increasing the resistance of a capacitor by cooling ? [/quote] Me neither. Hopefully just a little confusion there. Quote Link to comment Share on other sites More sharing options...
RandomBass Posted September 28, 2013 Author Share Posted September 28, 2013 Quick update. Used the recently fixed SM250 last night for it's first gig. A large social club, with sizeable dance floor between stage and the seating area, and then the bar in the far distance lol. Into my two uprated Harley Benton cheapo-cabs (replaced the 10" drivers with some Celestions) this thing was superb. Plenty of volume for such a large location, even with the master vol on 3 (gain set to just under clipping). Of course additional coneage helps there. Sounded really good tonally out front, I must say. Cracking piece of kit. And it got a little warm, nothing more. Very pleased. Cheers Geoff Quote Link to comment Share on other sites More sharing options...
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