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Mos-Fets replacement
Mos-Fets replacement
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Old 16th February 2013, 08:23 AM   #1
Riddles is offline Riddles
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Default Mos-Fets replacement

Hi,

I'm currently repairing a Yamaha YST-SW800 subwoofer and have found some of the Mos-Fets to be dead but am unable to locate the ones that i require.
This is what the service manual says and the parts that i need-

Q16- V5877200- FET- 2SK2378
Q18- V5877200- FET- 2SK2378
Q19- V5877000- FET- 2SJ406

Are there alternatives to these?
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Old 16th February 2013, 09:25 AM   #2
pergo is offline pergo  Italy
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2SK2378 ---> IRF640
2SJ406 --> IRF9640

for the IRF640, choose the non-N version or the -A version (IRF640 or IRF640A, not the IRF640N)
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Old 16th February 2013, 09:40 AM   #3
Riddles is offline Riddles
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HI,

Brillaint thank you, have found the IRF9640 but all the IRF640 seem to be N-Channel although have found a IRF640A so think might be in luck.

Thanks
Jake
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Old 16th February 2013, 09:43 AM   #4
pergo is offline pergo  Italy
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you can also use FQP19N20 & FQP12P20
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Old 1st October 2017, 06:13 PM   #5
jlithen is offline jlithen  Finland
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Are you guys sure IRF 640/9640 work in these amps?
I just soldered in a parallel pair of IRF9640 in the same subwoofer and the amp now seems to operate fully in class-B I think.
Not switching. Of course something else may still be blown in it.
From the datasheets it can be seen that IRF has much higher VGS than 2SJ/2SK at least...

And yes all IRF640 are N-channel, but an IRF640N is a bit different from IRF640. That is what was meant earlier in the thread
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Old 2nd November 2017, 08:59 PM   #6
jlithen is offline jlithen  Finland
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For the record I bought some from aliexpress for 0.60€ each, fake, could be noticed from VGSth being high.
Bought some from UK for 10.20€ each, fake, RDSon was skyhigh at VGS= -4V
Sooo, back to trying with IRF.
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Old 1st June 2018, 01:18 AM   #7
digitallake is offline digitallake  United Kingdom
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Hi all
I'm currently repairing a YST-SW800 and have run into the same issue(s) as above.

I was wondering if anyone found any suitable equivalents in the end for the switching stages, because the 2SJ406/2SK3278 are "low voltage drive" MOS-FETs, and I can't seem to find any that are similar i.e. VGS(th) = 1.5 > 2.5V

I tried IRF640/IRF9640 but ran into the same trouble as above with no switching occurring and am assuming the difference in threshold voltages is the cause (will do some more measurements to verify this)

Did anyone manage to get this amp running in the end?
Cheers
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Old 3rd June 2018, 05:41 AM   #8
Eva is offline Eva  Spain
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I have checked the schematic against the description of the problems you report.

There are 2 cases to analyze:

- Q16-19: Class AB amplifier. Upon inspection of the schematic I can't find a problem with higher Vgs-th MOSFET. There is a bias adjustment. If there is no way to bias the amplifier out of class B: R14 can be increased for more range (increase in small steps, turn bias pot to minimum before starting each test, check with multimeter for no mistake before power up). If there is no switching or no output I think the problem is not the Vgs-th of the class AB FETs but some other part damaged.

- Q22-25: Buck regulator. Current hysteresis driven. Upon inspection of the schematic I can't find a problem with higher Vgs-th MOSFET. Gate drive is suboptimal. Vgs is also Vds when switch is ON. Higher Vgs-th just reduces efficiency. But it wasn't high-efficiency to start with. If there is no switching or no output I think the problem is not the Vgs-th of the switching FETs but some other part damaged.

I assume you have checked the PSU is working. Bear in mind the SMPS has no current limiting, so it is likely to fail in case of shorted power devices in amplifier section.

EDIT: Due to the current hysteresis design of the switching regulators, these won't start switching with no signal unless the output stage is biased into class AB, above a certain threshold of bias like 100mA (2*50mA). It may not switch at all at lower bias levels like 50mA (2*25mA). A signal and a load is required to see the regulators switching.

btw: For perfectionists: To search for MOSFET able to operate at Vgs=5V search for "logic level MOSFET". However, from the point of view of semiconductor usage/performance the circuit is not perfectionist to start with.
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Last edited by Eva; 3rd June 2018 at 05:47 AM.
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Old 3rd June 2018, 12:00 PM   #9
digitallake is offline digitallake  United Kingdom
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Quote:
Originally Posted by Eva View Post
EDIT: Due to the current hysteresis design of the switching regulators, these won't start switching with no signal unless the output stage is biased into class AB, above a certain threshold of bias like 100mA (2*50mA). It may not switch at all at lower bias levels like 50mA (2*25mA). A signal and a load is required to see the regulators switching.
Hi, many thanks for all your comments and analysis of the circuit, I will double-check the above to make sure there is enough current to cause switching. As far as I can tell, all other sections of the circuit are operating correctly. Thanks again
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Old 3rd June 2018, 06:04 PM   #10
digitallake is offline digitallake  United Kingdom
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Hi Eva,
I've completed some more testing today and the switching stages seem to now be working once their is enough current as you say, as shown on the trace below. The upper and lower traces are the switching rails B+ and B- on the schematic (x1) and the centre trace is the output (x50)

The behaviour of the bias adjustment however doesn't seem to be matching what is written in the service manual. The voltage measured between TP1 and TP2 is as follows under these various conditions:

Load (Ohms) | Output (VRMS) | Max possible adjustment range TP1-TP2 (mVDC)
Open | 0 | 0 > 0
Open | 25 | 1 > 6
Open | 35 | 8 > 10

16 | 0 | 0 > 0
16 | 1 | 56 > 57
16 | 10 | 68 > 68
16 | 20 | 68.5 > 68.5

8 | 0 | 0 > 0
8 | 1 | 62.0 > 62.5
8 | 10 | 70 > 70
8 | 20 | 70 > 70

This seems a little odd... I'm concerned that the bias cannot be adjusted between 50mV and 250mV when there is no signal, as is obviously the usual procedure, but the amp seems otherwise to be working. I will continue testing but thought I would let you know how far I had got because you kindly took the time to look into the circuit. Thanks

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