PaulDF Posted May 12, 2011 Share Posted May 12, 2011 Hi, I have been asked to repair an amplifier board by one of my mates, think it is made by Kool.I have taken out the faulty board and am just really after a second opinion as to what the white rectangular components makred 5WR15J are in the photo: http://i215.photobucket.com/albums/cc91/Spark-123/DSCF0276-1.jpg I am thinking they are just 5w 15ohm ceramic resistors. I have removed them from the board and measured the resistance across each one and am getting <1ohm each which is why I am scratching my head as to wether they are resistor or something else, or is it possible that there has been a domino effect causing them all to go at once?I have also changed the burned up looking 82R and 10R resistor in the pic along with a few other componentsCheers Paul Link to comment Share on other sites More sharing options...
abbotsmike Posted May 12, 2011 Share Posted May 12, 2011 I'm being a dope, despite sitting here revising devices and circuits all day. My brain is fried!!! Time for dinner me thinks. Link to comment Share on other sites More sharing options...
KevinE Posted May 12, 2011 Share Posted May 12, 2011 They are actually 0.15 ohm, 5-watt ceramic cased wirewound resistors, (5WR15) these are called emitter resistors and are always a low value. Your meter, unless very accurate, will show them as more or less short circuit. I think they're ok! The R in the figure denotes the decimal point. So 15 ohms would be 5W15RJ 1,5 ohm would be 5W1R5J 0.15 ohm is 5WR15J etc I'd be more inclined to investigate that 10R resistor at the bottom of the pic by the middle lower output transistor. It looks like there's a flash mark by it. If so, it suggests the output bridge is probably blown with possible driver problems too. Link to comment Share on other sites More sharing options...
PaulDF Posted May 12, 2011 Author Share Posted May 12, 2011 Thanks for that,They connect to the large transistors which are soldered on the opposite side of the board. I have already checked all the transistors and found 5 of the 6 u/s hence have replaced all of them, one had blown completely and had a big crack in it and tell tale smoke trail, the other 4 didn't bell out with a fluke on diode test and showed short ckt.At the moment am just going through likely causes of the large transistors failing (bar the human element). They are actually 0.15 ohm, 5-watt ceramic cased wirewound resistors, (5WR15) these are called emitter resistors and are always a low value. Your meter, unless very accurate, will show them as more or less short circuit. I think they're ok! The R in the figure denotes the decimal point. So 15 ohms would be 5W15RJ 1,5 ohm would be 5W1R5J 0.15 ohm is 5WR15J etc I'd be more inclined to investigate that 10R resistor at the bottom of the pic by the middle lower output transistor. It looks like there's a flash mark by it. If so, it suggests the output bridge is probably blown with possible driver problems too. Excellent, thanks - at that I'll put them back in.Just waiting for another transistor to arrive in the post and then I can put it back together again Thanks again Link to comment Share on other sites More sharing options...
KevinE Posted May 12, 2011 Share Posted May 12, 2011 Are the base resistors ok? That one I mentioned looks physically blown.If that isn't cleared up, it may work on the bench but will overheat under load after a while. Link to comment Share on other sites More sharing options...
boatman Posted May 12, 2011 Share Posted May 12, 2011 Oh dear, oh dear. First of all I'm going to give you the standard BR response: "If you have to ask that question you shouldn't be doing this". 5WR15J is a 5 Watt 0.15 Ohm ceramic resistor used as a matching resistor for the power transistors. A 5 Watt 150 Ohm resistor would be marked 5W150RJ and would quite likely catch fire when passing the sort of currents found in the output transistor of a power amp. They might be hard to find in the UK but US imports can be had from Farnell14 Link to comment Share on other sites More sharing options...
PaulDF Posted May 13, 2011 Author Share Posted May 13, 2011 No problem, thankfully I asked for a second opinion as I wasn't 100% sure, was mainly to do with resistor sizing as I didn't believe 6 resistors would fail like that. My job is an instrument technician and deal with TTL day to day but don't normally deal with ceramic resistors that small, tbh was considering that it could have been some sort of inductor as it has the suffix J. Was just a brain fart on my behalf that the position of the R was relevant to the size, had the R been mid number I'd have realised!! Tbh was thinking that 15R would cause a lot of power to be dissipated across the resistors, hence I thought I'd ask here.I don't consider myself not competent to do the job however I do realise when I need to ask others when I am not 100% sure which, to me, is part of actually being competent if u know what I mean.Thanks everyone for the replies Paul Are the base resistors ok? That one I mentioned looks physically blown.If that isn't cleared up, it may work on the bench but will overheat under load after a while.The base resistor in the pic was open circuit as was another one, another 10R one was reading about 500R so have replaced them all with new 10R ones. I have also replaced all 6 large transistors and am awaiting delivery of a small transistor which was also showing bad readings with a fluke, 0,0,0v between b,c&e Link to comment Share on other sites More sharing options...
Dmills Posted May 13, 2011 Share Posted May 13, 2011 Replace both the driver transistors on general principle and also check the diodes and the transistor or diode that is (probably) in thermal contact with one of the output transistors, a total blowup like that will usually take at least some of the driver components out and unless the drive is fixed replacing the power devices will likely just result in more blown power transistors as soon as DC is applied. Low value ultra fast fuses or a severely current limited supply are a good thing when first powering up after an output stage brew up. Also be careful when you re fit the board to the heatsink to ensure that everything is clean and that any isolating pads are fitted correctly, been plenty of transistors killed by a little swarf puncturing the mica and plenty more by poor thermal contact. Regards, Dan. Link to comment Share on other sites More sharing options...
PaulDF Posted May 13, 2011 Author Share Posted May 13, 2011 Yeah, I have changed all 6 large transistors on the other side of the board which drive the output and checked out the other 5 transistors (4 under the rectangular boxes to the right of the board and one between the two lower ones). I have also checked the two transistors on the top of the board and found one blown.I have some heat sink compound and the insulators on standby for when I put it all back together, I have a current limited PSU on my bench so will power it up slowly.ThanksPaul Link to comment Share on other sites More sharing options...
PaulDF Posted May 19, 2011 Author Share Posted May 19, 2011 Hi,Just to say thanks to everyone for their replies, the amp is now up and working. :) Pauldf Link to comment Share on other sites More sharing options...
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