RonJ Posted June 3, 2011 Share Posted June 3, 2011 I have a Pioneer A400 Amplifier which suffers from, what seems to be a typical problem in A400. My problem is on cold start(read fully discharged) after the delayed power relay click (which comes almost 20-30secs afterwards); the amp first start with a loud continuous popping/motor-boating sound(what seems to be DC) on the speakers. This Amp motor-boating has killed one of my design acoustics tweeters once. Once switched off from this state, the very second time amp starts with a loud hum on the Right channel along with normal operation on the Left channel. And usually on one more power cycling, the amp starts to work normally. One way to circumvent this problem is, to start the amp with a headphone plugged in at a decent volume level followed by waiting for few minutes. Later I can unplug the headphone still the amp continues to work normally. After searching various forums, I got few leads, but none of the solutions worked for me... Trying to rectify the problem, I have replaced the 10000MFD/50V reservoir caps (C505 and C506) with Panasonic FC caps of the same value. I also replaced the other power supply related capacitors (esp to the OP-AMP) C501, C502 (10MFD/50V) and C503 and C504 (47MFD/25V). I used NICHICON Aluminium Electrolytic of same value for C501, C502 (10MFD/50V). While I found that on the circuit daiagram C503 is shown as 10MFD/50V while C504 shown as (47MFD/25V). Anyway I replaced both with 100MFD/35V capacitors. I also replaced the 2.2MFD/50V Caps C201,C202 and C301,C302 with EVOX RIFA PHE series polypropylene capacitor of 2.2MFD/400V. Anyway despite all these replacements, my problem persists… And I need to cycle power using the front panel switch for 2-3 time before the Amp starts working after a loud motor-boating session. By the way I did check the rail voltages and they are normal. I really appreciate your inputs…. Circuit Diagram can be found here http://www.hifiengine.com/manuals/pioneer/a400.shtml Or Here as JPEG Link to comment Share on other sites More sharing options...
synthesis1624705793 Posted June 3, 2011 Share Posted June 3, 2011 Problem with the relay? Link to comment Share on other sites More sharing options...
RonJ Posted June 3, 2011 Author Share Posted June 3, 2011 I don't think relay could be a culprit, I doubt it.... Relay problem, shouldn't cause the hum on Right Channel on the second power cycling. Totally it takes 2-3 power cycling before the amp stabilizes... And once its on... switching of power for couple of minutes don't have any effect... it starts right back on normally.... Link to comment Share on other sites More sharing options...
RonJ Posted June 3, 2011 Author Share Posted June 3, 2011 Both the channels are affected initially. Second powercycling, only Right Channel is affected though... But this time no motor-boating, but just severe hum on the right channel with no audio. Left channel works normally this time. Speaker relay continuously clicks during the motor -boating. Volume control level doesn't have any effect on Initial Motor-boating or subsequent severe hum on Right channel after second power cycle. With headphone plugged in, the amp works well even on cold start... And if I unplug the headphone after few minutes I could hear the normal relay click and the amp functions normally with the speaker load.... Once settled subsequent power cycles don't have any effect up to couple of minutes and Amp continues to work normally after each power on... :Longer power-downs repeats the problem... Link to comment Share on other sites More sharing options...
wck Posted June 3, 2011 Share Posted June 3, 2011 My 5cents worth. If the 'motor boat' appears after the click and the volume is zero, should be a transistor. If the 'motor boat' appears after the click and only when you turn up the volume, then should be a capacitor. Link to comment Share on other sites More sharing options...
synthesis1624705793 Posted June 4, 2011 Share Posted June 4, 2011 I had my old subwoofer that developed farting problem. It's fine when playing but made noises when switched off. It's probably similar with your problem and it was due to aging capacitors. If you have LCR meter, check capacitors that drift the value so much and replace them. It's not necessary the power supply caps. Link to comment Share on other sites More sharing options...
DIYer Straits Posted June 4, 2011 Share Posted June 4, 2011 If you can, open the amp's top cover and look for any caps that have popped open or have a split top. Link to comment Share on other sites More sharing options...
RonJ Posted June 4, 2011 Author Share Posted June 4, 2011 Well I have gone beyond opening the top cover... I think You havent read my post :) I even replaced many caps already including the PSU caps. DC Offeset on the RIght Channel is an issue. Righ Channel reads up to -1.5to -2V DC offset on Cold start in alternate fashion. Which is causing the loud pops along with relay cuts. subsequent power cycling it starts reducing eventually to settle around -75mV to -90mV. :Left channel stays at +20mV through out. What could be the reason? failing transistor? I re-soldered all power transistors, but the problem persists.... Link to comment Share on other sites More sharing options...
DIYer Straits Posted June 4, 2011 Share Posted June 4, 2011 My bad. ;D Have you checked for cold solder joints then? I'm clutching at straws here since cold joints normally cause crackling. Link to comment Share on other sites More sharing options...
wizardofoz Posted June 4, 2011 Share Posted June 4, 2011 My bad. ;D Have you checked for cold solder joints then? I'm clutching at straws here since cold joints normally cause crackling. More often than not in a high current area like power supplies ... Could also perhaps be something like a diode breaking down - did you replace those? Link to comment Share on other sites More sharing options...
mHiFi Posted June 4, 2011 Share Posted June 4, 2011 Old electronic "popping/motor-boating sound" usually cause by dry joint as someone mentioned above. A bad filtering CAP will cause continuing humming. You have to remove the PCB and check all the solder point. Resolder those suspected joint. Also may be the inter state coupling cap, transistor leaking as you mentioned the DC offset is out of spec. Link to comment Share on other sites More sharing options...
synthesis1624705793 Posted June 4, 2011 Share Posted June 4, 2011 Keep the chassis and build new amp!!!! ;D Link to comment Share on other sites More sharing options...
RonJ Posted June 4, 2011 Author Share Posted June 4, 2011 Yeah, considering that I have seen used A400s from $80 up, I think its not worth changing the power transistors or something that drastic... I have already spent a whole day yesterday, so not sure, whether its worth bang my head on more... Or may be start looking for a better used A400... hmmmm Link to comment Share on other sites More sharing options...
Spunky Posted June 6, 2011 Share Posted June 6, 2011 My bad. ;D Have you checked for cold solder joints then? I'm clutching at straws here since cold joints normally cause crackling. Cold joints...probably a point on the ground. With no proper ref. the signal can become very huge...but FB will bring it down...and then the cycle repeats...giving u the motorboating at the o/p. just my guess...it happened to me once when the gnd got detached at the i/p due to intensive i/c changing ;) Link to comment Share on other sites More sharing options...
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