MTX Thunder 6500D Resistor value questions

[Steve]

I have an MTX Thunder 6500D which had blown power supply & output
stage fets. I've got everything replaced except for the two SM
resistors that connect to the gates on the power supply fets (R628 &
R629). From what I can tell, the unit uses a TL494C to control the
supply. It drives two SM fets/transistors labeled
D228
A5G
These feed into what looks like a balun (not sure, the signal is DC
coupled going into the coil & ac coupled coming out), which feeds the
blown resistors, which each feed a pair of fets P/N P55NF06. The
resistors are in series between the balun & power supply fets.
Unfortunately, the resistors cracked in the middle and are illegible,
and MTX doesn't give out info. I know MTX repairs in house for a flat
rate, but I've already put the money & time into it, I'd hate to pay
$95.00 or whatever it was just to replace two 8 cent resistors.
I've posted a picture of the resistors in a.b.s.e.

Also, there aren't any three digit part numbers on any of the other
resistors, and the only letters in the part numbers are for the low
values(10R0,3301 etc...)

All that I can read off the blown ones is 4 5. If there were two
nubmers in the middle, this would put it in the 40 meg ohm range. I'm
not an expert, but that seems too high to drive the gates on the fets.
I would expect a 40 something ohm value to be more realistic. Am I
wrong in this assumption? Therefore, what standard resistor values
are there in the 4x.5 ohm range?

If I'm off base on this, please give me some guidance. FWIW the other
class D amps I have use something in the range of 10ohms - 100ohms for
the gate resistors.

Thanks,
Steve Kamego

 

[Arfa Daily]

I have an MTX Thunder 6500D which had blown power supply & output
stage fets. I've got everything replaced except for the two SM
resistors that connect to the gates on the power supply fets (R628 &
R629). From what I can tell, the unit uses a TL494C to control the
supply. It drives two SM fets/transistors labeled
D228
A5G
These feed into what looks like a balun (not sure, the signal is DC
coupled going into the coil & ac coupled coming out), which feeds the
blown resistors, which each feed a pair of fets P/N P55NF06. The
resistors are in series between the balun & power supply fets.
Unfortunately, the resistors cracked in the middle and are illegible,
and MTX doesn't give out info. I know MTX repairs in house for a flat
rate, but I've already put the money & time into it, I'd hate to pay
$95.00 or whatever it was just to replace two 8 cent resistors.
I've posted a picture of the resistors in a.b.s.e.

Also, there aren't any three digit part numbers on any of the other
resistors, and the only letters in the part numbers are for the low
values(10R0,3301 etc...)

All that I can read off the blown ones is 4 5. If there were two
nubmers in the middle, this would put it in the 40 meg ohm range. I'm
not an expert, but that seems too high to drive the gates on the fets.
I would expect a 40 something ohm value to be more realistic. Am I
wrong in this assumption? Therefore, what standard resistor values
are there in the 4x.5 ohm range?

If I'm off base on this, please give me some guidance. FWIW the other
class D amps I have use something in the range of 10ohms - 100ohms for
the gate resistors.

Thanks,
Steve Kamego

The values of the series resistors to the gates of switching FETs in PSUs,
are usually of little consequence, and are typically in the range of 10 to
100 ohms, as you suggest. Although the input impedance of the FETs is
typically very high, the input capacitance can also be quite high, so you
don't want to be going too high in value with the series R, otherwise you
will knock the corners off the drive waveform, which could lead to sluggish
switching, and an increase in the power dissipation in the devices. I can't
think of any 'standard' values that start with a 4 and end in 5, but I think
that you would be just fine with say 47 ohms. I've just taken a look at a
schematic for the power supply used in a popular series of LCD TVs. It uses
a very similar circuit configuration to that you have described, and the
series gate resistors on that one, are 10 ohms each.

Arfa

 

[N_Cook]

I have an MTX Thunder 6500D which had blown power supply & output
stage fets. I've got everything replaced except for the two SM
resistors that connect to the gates on the power supply fets (R628 &
R629). From what I can tell, the unit uses a TL494C to control the
supply. It drives two SM fets/transistors labeled
D228
A5G
These feed into what looks like a balun (not sure, the signal is DC
coupled going into the coil & ac coupled coming out), which feeds the
blown resistors, which each feed a pair of fets P/N P55NF06. The
resistors are in series between the balun & power supply fets.
Unfortunately, the resistors cracked in the middle and are illegible,
and MTX doesn't give out info. I know MTX repairs in house for a flat
rate, but I've already put the money & time into it, I'd hate to pay
$95.00 or whatever it was just to replace two 8 cent resistors.
I've posted a picture of the resistors in a.b.s.e.

Also, there aren't any three digit part numbers on any of the other
resistors, and the only letters in the part numbers are for the low
values(10R0,3301 etc...)

All that I can read off the blown ones is 4 5. If there were two
nubmers in the middle, this would put it in the 40 meg ohm range. I'm
not an expert, but that seems too high to drive the gates on the fets.
I would expect a 40 something ohm value to be more realistic. Am I
wrong in this assumption? Therefore, what standard resistor values
are there in the 4x.5 ohm range?

If I'm off base on this, please give me some guidance. FWIW the other
class D amps I have use something in the range of 10ohms - 100ohms for
the gate resistors.

Thanks,
Steve Kamego

Wire a connection to each land of a cracked R , so you can connect to a DVM.
Sandwich a bit of kitchen foil between the two parts and squash together to
get the minimum reading, maybe. Could give some idea of value.

 

[Steve]

The values of the series resistors to the gates of switching FETs in PSUs,
are usually of little consequence, and are typically in the range of 10 to
100 ohms, as you suggest. Although the input impedance of the FETs is
typically very high, the input capacitance can also be quite high, so you
don't want to be going too high in value with the series R, otherwise you
will knock the corners off the drive waveform, which could lead to sluggish
switching, and an increase in the power dissipation in the devices. I can't
think of any 'standard' values that start with a 4 and end in 5, but I think
that you would be just fine with say 47 ohms. I've just taken a look at a
schematic for the power supply used in a popular series of LCD TVs. It uses
a very similar circuit configuration to that you have described, and the
series gate resistors on that one, are 10 ohms each.

Arfa

Looks like the only 4x.5 value that is stocked is 47.5 ohms, I'd say
I'm safe with that value.

Thanks for the reply,
Steve

 

[Steve]

Wire a connection to each land of a cracked R , so you can connect to a DVM.
Sandwich a bit of kitchen foil between the two parts and squash together to
get the minimum reading, maybe. Could give some idea of value.

Just to follow up, I got an email from MTX & they are 47.5 ohms.

Thanks everyone!
Steve