tesla13BMW Posted August 28, 2012 Posted August 28, 2012 Having looked for someone to service my Plinius MA102's in Two Cannel Discussion, Clay at Speaker Specialists on the Gold Coast was recommended. Unfortunately he was not keen on working on big amps like these. Issue is that a couple of the electrolytics are showing swollen end caps. I have purchased replacement caps (ROE 47 uF/100V) and will replace all of them. However, my question: do I simply replace the caps (observing polarity) or is there adjustment that generally needs to be made. I see on the top side there is one variable resistor but have not removed the boards to see if any others are underneath. Due to the age of these amps Plinius have no documentation (or wont look for it!) I've attached a couple of photos (hope they attach) which you can see the swollen cap in. Any advice or DIY person around Brisbane that would be interested would be greatly appreciated.
A J Posted August 28, 2012 Posted August 28, 2012 Is that Adrian Edmunson in the background? Hey well, it should just be a simple case of replacing the caps if you have identical values - I'm a little confused though, you mean 4700uf caps not 47uf? I can't imagine those are 47's. My guess is that the variable resistor is the output bias adjuster - just check when you recap that when powered up and settled down without any speakers connected, that you see a very low DC output across the speaker terminals - it should be close to zero, but anything -/+ a few millivolts is fine.
tesla13BMW Posted August 28, 2012 Author Posted August 28, 2012 Hi AJ. Guilty as charged. Got to love the young ones.... my kids don't seem to get it though. But, they don't think Monty Python's funny either. They are ROE 47/100V and my purchase of the identical ones measure between 47 and 50 uF. These aren't the DC supply caps. Those are the 6 (3 on each side). The one on its own is 22000 uf and the two coupled are 15000 uF. I am only looking to replace the 6 "smallish" gold electrolytics on the board. I will replace all of them as a number on both blocs are showing signs of swelling. Getting the board off the heat sinks will be interesting as I can't see how they are connected to the transistors mounted on the heat sink tunnel beneath the board. The variable resistor mentioned is on the top right corner of the second photo. The PCB has printed on it "Standby". There is a stand by toggle switch on the front of the amp which switches it from Class A to Class A/B. So am not too sure of this variable resistor.
A J Posted August 28, 2012 Posted August 28, 2012 If that's the case - just replace them that should be no problem. Someone else could chime in with a view as to what the resistor might do, I could be a resistor that adjusts the class A bias, that is the idle current when in class A/B - but I'm not enough of a guru to be certain.
Guest Muon Posted August 28, 2012 Posted August 28, 2012 (edited) Personally, as there are failing/failed caps already in places, I would replace all the electrolytic caps. Edited August 28, 2012 by datafone
tesla13BMW Posted August 28, 2012 Author Posted August 28, 2012 Hi Datafone Are you suggesting all the electrolytic caps on the board only or on the power supply aswell?
Guest Muon Posted August 28, 2012 Posted August 28, 2012 (edited) Yes, all, every electrolytic including the big filter caps. Only to safe guard against failures... that could possibly lead to the failure of some other components. Edit: If I am correct, the MA102's were introduced in 1987, that would mean some units are 25 years old, that's more than the life span of electrolytic caps. Might be worth checking the values of any carbon resistors, especially in high heat areas too, as they can drift out of spec. Would be great to get hold of the service manual, but like yourself, I haven't been able to find any on the net. Edited August 28, 2012 by datafone
ngquan Posted August 28, 2012 Posted August 28, 2012 Yes, if you can find the manual then it is the best thing to do. But i agree otherwise recapping should be perform thouroughly to prevent further failures.
tesla13BMW Posted August 30, 2012 Author Posted August 30, 2012 Yes these are old girls. In fact they were Peter Thomsons personal amps. I have been in touch with Plinius and as helpful as they've tried to be it seems they do not have any documentation. Seems a bit strange to me though. I have a strong affection for these amps (maybe its the kiwi connection or that I managed to go to their "scoungy" factory in Palmerston North a couple of times while at uni). Perhaps when I have them apart I will draw up a circuit diagram for them (assuming they haven't removed parts id's like some manufacturers do).
ngquan Posted August 31, 2012 Posted August 31, 2012 Yes these are old girls. In fact they were Peter Thomsons personal amps. I have been in touch with Plinius and as helpful as they've tried to be it seems they do not have any documentation. Seems a bit strange to me though. I have a strong affection for these amps (maybe its the kiwi connection or that I managed to go to their "scoungy" factory in Palmerston North a couple of times while at uni). Perhaps when I have them apart I will draw up a circuit diagram for them (assuming they haven't removed parts id's like some manufacturers do). Got to love that. Please draw a schematic for them.
skippy124 Posted August 31, 2012 Posted August 31, 2012 If you are replacing the caps on the PCB, be careful not to damage the plated through holes as this is a double sided PCB. Cheers John
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