Overheating Class A

[cln.white]

I have a Musical Fidelity P270-2, It has been much modified over the years with Black Gates, schottky diodes, star earth etc. I have had no problems for a long time and was quite proud of it considering I am no electronics expert! the amp itself sounds superb! I used it with Musical Fidelity P173, kit valve and lightspeed pre-amps with no problem whatever, having had the amp returned from Portugal by freight recently, it now overheats on the right channel only, in fact the heatsink gets so hot it would cause a burn if touched.The amp is working but I use a gainclone kit instead but just not the same quality. Has anyone had similar problem with large Class A amp? I would be interested and any help would be appreciated.🙁

 

[nigelwright7557]

Check the standing current.

Check if output transistor has become loose.

 

[cln.white]

Fault found

I now know that a resistor is burning out, I have marked this on the drg. I downloaded from this site which was kindly posted for another problem.
Does anybody have any idea why this resistor would keep blowing? The amp still works but I guess because the other two FETs are having to work harder the heatsink on this side gets so hot I feel it could cause further damage over a period of time!😱

 

[nigelwright7557]

Could be a gate to source short on the MOSFET.
I had exactly the same problem with a new amp I was testing.

 

[Woody1911a1]

bias too high . do you have more of that schematic to show us ? or a service manual that explains checking and adjusting ?

cheers Woody

 

[Tekko]

The fet with the affected resistor have likely failed, replace it and see if that fixes the problem.

 

[cln.white]

Input section of drg.

Hi, Thanks for the quick replies, I have attached the input side of the amp and I will try and source a replacement FET.

 

[noSmoking]

check the current

The fet with the affected resistor have likely failed, replace it and see if that fixes the problem.

hi
Put a dvm in line and check the current draw on that fet,or do the math across the resistor for current,If the resistors are good ,not much left,
good luck

 

[cln.white]

Hitachi J162 Mosfet

Hi, Have found that Aura Note Premier uses these devices. I cannot find any Hitachi devices on the www I can only find Renesas, would these be drop-in replacements for the faulty one or would I be better to replace all to keep everything ballanced? I am a bit wary of buying on the www from China source due to the stories of fake items being produced there.🙁

 

[sregor]

Just a suggestion - if the dc currents seem ok, use or find someone with a scope and check for oscillation. High frequency oscillation will cause an amp to run hot, also. Good luck.

 

[davidsrsb]

Renesas is spawned out of Hitachi and therefore is the same part

 

[cln.white]

Renesas is spawned out of Hitachi and therefore is the same part

OK Thanks, will buy and try!

 

[analog_sa]

OK Thanks, will buy and try!

As for testing, you can just remove the suspected device by lifting the source and gate resistors and see if the rest are ok.

 

[AndrewT]

the three PmosFETs are very likely selected at the factory for matching of operational currents.
You may have to buy a matched set of three devices.

 

[cln.white]

WOW, How lucky is that, I just ordered a matched set of four. Which I intended to keep as spares so will now replace all, thank you.

 

[cln.white]

Now singing sweetly

Thanks for all the advice guys, on removing said Mosfet small hole in back (not visible from front) hole in insulator and also heatsink! have now replaced and amp is once again singing sweetly! Thanks again to all, most helpfull.😀

 

[cln.white]

I now have another problem with the same unit, I shorted the output on one channel and now have no output on that side. I noticed that one set of fet's were not heating up so I have changed these but still no output. There is no sign of any component failure etc. any suggestions please?

 

[cln.white]

No output on one channel

I have shorted out the output of one channel and it is now dead! I noticed that a pair of the j162/k1058 were not heating up so I changed them but still have no sound. I can see no damage to any components so I am stuck, I will try and post a picture. Any suggestions please?

 

[AndrewT]

Test the devices you removed.

 

[kasey197]

Also while at it check if there is any voltage across the drain and source of the output fets in the dead channel. If its close to zero and the fets aren't warm then you need to check the supply for that channel.