aleph 2-thermistor coat cracked

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Hi guy`s
Maybe some one can help me on this one. After a few weeks of listening of my monoblock`s it seem`s that I have a problem and no idea what caused the problem. Firs day of listening I was using CD player directly conected to the input of the amps. No hum no buz, no problem turning the amps on. After a few days I conected CD on the amps via preamp using single ended conection. Audiable bot not very loud toroid freq. buz (did not use shorting plugs) from the speakers. When I made my xlr conections and pluged them in there where almost silence. About three weeks there where no problems except one bad ground conection which was taken care of. Mesuring during days, showed all basic values on the boards in limits. Few days ago I noticed the litle pops and like bad ground conection noises when amps was turned on. After a few minutes of warming amps on, those interuptions stoped. Yesterday I thurned on first chanal went to the second and thurned it on and after 5 sec. a had the room fol of sound like somebody disconected main ground wire. After few second`s ewerything stoped. Boath my monoblocks where death-switched off, without the power???. I thurned the switches off, checked out all conections and parts in turn off position.... Found blowen fuse on one chanal and that is all. Than I thurned on unconected(cd, speakers) amps one by one, meshured basic parameters on boards, off set on output and everything was normal. Conected CD directly to the amps turned them on and they where playing. I thought that I coud have a problem with balanced output on preamp so today I conected preamp with SE conect. And ther was a buz audiable 5m away from the speakers (much louder then in the first days of listening sesions). Tryed directly CD player, no buz. I opened amps and foud what I missed yesterday - blown thermistor TH1 (main plug to ground). Coating of thermistor was cracked but amp was still playing. I was using NTC 10A/5ohm (buerklin), TH2 and TH3 same walue paralel on mains. (BTW - guy`s in Europ - what typ of thermistors are you using)
PSup. - 1,5kW toroid 33+33 per ch, 136.000uF. (Wireing in ZIP file)
All on board parameters are still normal, but something must be wery wrong. I know that it is hard to give any sugestions based on this kind of report but I hope that someone will have some idea. What to check, what to do?
thank`s on advance
regards - Marijan


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I`ll try preamp with another amp tody, but still; what are the possible reasons or what should hapend in the PS or input circuit(teoreticly speaking) it can couse the reaction like this-blown fuse and NTC. I`ll replace thermistor easely, but I`d like to prevent this in the future. Theoreticly is it possible that I had a kind of short cut some there betwin ground point and 230VAC mains?
I`m not asking for soulution, just for any kind of sugestion.
rgds Marijan
I hate answering on my own questions although it is usualy most eficient and useful, but it could helpfull to someone who didn`t know this like I did not. It could save some trouble.
1. NTC need only a few sec. to reach the op. temperature and decrise it`s own resistence in circuit to the level that is almost insignificant regarding V drop in the PS circuit. By the time the resistence drop down during the NTC heating up, the curent flowing through it won`t harm your components any more.
2. But NTC need those 10 ohm (like in A2) every time you turn your amp on. One of the operating caracteristic`s of every NTC on the position of the surge limiter is a cool down or recovery time of thermistor( that is what I did not know) . Depending of varius factors (enviroment temp. , how it is mounted, model and quality...) that cool down time is usualy terminated in area of 1 to 3 minutes. Cool down time is a time, needed for NTC to recover it`s zero point resistance (10 ohm in A2). If you try to thurn on your amp immediately after short power break for example, thermistor won`t act like inrush current suppressor, becouse it`s resistence will be to low. So if you turn your amp off, it is not adviseble to thurn it on for at least three minutes, if you are using NTC certanly. It can do harm to your fuses, switches, thermistor`s... I did that few times, silly me, and that coul`d be one of I supouse many reasons why my NTC was blown away.
Hi Marijan.

Just idea that you may want to test and have checked all the connections around the power supply capacitors, they are usually well known to reproduce weird hums and current spikes which can damage your fuse, bridge and as well NTC. Either bad connection (corrosive contacts caused by hi current) or bad capacitor. Check also temperature on power-capacitors.

Hope this help.

Thank`s for your sugestion. I`ll rewire almost all conections you mentioned when replacing NTC. On the first fast check I could confirm only that I do not have a conection, center tap betwin caps through NTC to main earth point so I know that NTC is completly death in one chanal. BTW, when you are talking about caps temp. - what should be normal aproks. operating temp. of filter caps in A class design?
rgds Marijan
Hi Marijan.

In any case lower that for what they were intended to work in. Every BigCan Caps have max. temperature indicated on them were the capacitance value is. For me it would be more important to check them with digimeter, haw they charge and discharge, it may show some anomalies, unless you have special capacitor meter to check if some of them failed. This is only my thought which may not necessary be the case.

For instance I would use those caps with 105C. One more time just my oppinon.

As well I would order original NTC , CL-60 as Mr.Pass has indicated just to be sure. Here is the link of digi-key, I think they do ship worldwide.CL-60

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