Newbie help w/MOSFET transistors

[RossInDenver]

Hello all,

I'm a hobbyist with audio equipment. A friend has an amplifier that appears to have blown a resistor, maybe a diode, and at least one transistor that is a K952 or 2SK952. 

I'm trying to understand how to figure out what is an acceptable substitute. 

I found something similar with the STM STP3N80K5 where most specs seem the same except the STM has Rds(on) at 3.5 ohms while the original K952 spec is 5 ohms typical and 7 ohms max. Can you help me understand how to think about that difference and whether it is generally acceptable to make this kind of substitution (maybe in one direction but not the other, i.e. going toward a lower Rds(on) is OK but going higher isn’t, or vice-versa?)

Also, if you have a thought on a good substitute for the K952 I’d sure love to hear it!

Thanks in advance for any help and I apologize for the rookie question to you experts.

Best,

Ross in Denver

[AScha.3]

Hi,

if the 2SK952 is dead (you are sure?) , the STP3N80K5 might replace it , its also 800V/2,5A N-Mos.

But really need a 800V mosfet ? Whats it used for ? Which amp?

You have any circuit of the amp?

[RossInDenver]

 

It's called AudioPrism Debut II. I don't have any schematics for it… But I can't say that I have looked very hard!

https://www.raptorelectronics.com/subpage2.html

 

[Peter BENSCH]

The picture and the marking help a little further. As this is obviously a tube amp with EL34, the MOSFET is probably used as linear regulator to create the anode voltage. Such linear regulators usually have a relatively simple design, so that the schematics should be easy to reconstruct/redrawn.

If it is such a linear regulator, a MOSFET in planar technology was almost certainly still used there, which was the typical technology for MOSFETs in the last millennium. However, the STP3N80K5 is already manufactured in MDMesh technology with vertical structures, which tends to form hotspots in the drain-source channel under linear load, as is the case with such linear regulators, which ultimately lead to destruction. For this reason, linear operation is no longer recommended with MOSFET based on that technology.

The problem is then to find a MOSFET in planar technology that has at least the required reverse voltage (greater than the input voltage) and approximately the Rdson. I'm afraid that you'll only find old types from the last millennium.

Regards
/Peter

[RossInDenver]

Thanks, Peter, that's fascinating. So my last question (maybe) is whether you think I could trust what appear to be current Chinese production of the K952. 

[RossInDenver]

BTW, I'm not SURE it's dead (cue Monty Python dead parrot scene or Holy Grail)...BUT it's missing a chunk, roughly the lower right quarter of the body of the transistor so it's at least rather wounded.

[Peter BENSCH]

If the lower right quarter has been blown away, the MOSFET is definitely dead. Under no circumstances would I just use any MOSFET, but I would definitely determine the schematics of the environment and try to understand it. The explosion of the MOSFET also had a reason, which must be found and eliminated before replacing components. In such old amps, the capacitors have often dried out and could, for example, have become low-resistance, which could have overloaded the MOSFET.

Only then can you decide which candidate is suitable and can be tested.

[AScha.3]

This parrot is dead, my friend. Not just sleeping...

+

you cannot "trust" the actual K952 - but just try it. And buy more than one...you might need it...

or try any other cheap 800V mosfet , like IRF840 ;

can still get it, ie ebay:  5 pcs at about 1$ each...

https://www.ebay.de/itm/332450970778

on digikey 2.60 $

https://www.digikey.be/de/products/detail/vishay-siliconix/IRF840PBF/812044