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MTX Thunder 6500D Resistor value questions

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

Reply to
Steve
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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

Reply to
Arfa Daily

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

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

Reply to
Steve

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

Thanks everyone! Steve

Reply to
Steve

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