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Old 27th April 2010, 02:05 AM   #51
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Location: Louisiana
The IRF540 are marked ST, is this STMicroelectronics?

**** Yes.

I have noticed that different manufacturers have different specs for the gate voltage etc. Can anyone confirm the ST is the original spec?

**** It's possible that they were used originally. Did the solder joints appear to be flow soldered or had they been replaced before?

The M7 5N06HD

**** Likely MTP75N06HD.

are marked with a tiny motorola sign, and the numbers 716. Does this 716 signify anything specific?

**** Nothing important. It's either a date or production code.

Can't wait to get these parts in and fire her up (with a ten amp fuse

**** and all semiconductors clamped tightly to the heatsink.
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Old 27th April 2010, 09:59 PM   #52
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Quote:
Originally Posted by spooney View Post
You should replace the power supply fets with IRF3205. They are more rugged than the originals and should help increase the amps reliability.
Will do Spooney, is there a particular brand I should purchase?

Thanks Perry, all will be clamped. I will also clip a heatsink to them when soldering back in as I've been told that the heat from prolonged soldering is dangerous as well. They looked original, and the amp was still sealed when I bought it.

Here is a shot of the increased surface area mod I have completed. I have done the corresponding tracks on the other side. Thoughts?
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Old 28th April 2010, 01:57 AM   #53
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Although it won't cause problems, attaching a heatsink when soldering is unnecessary. I was concerned about this at one time but after working with Rockford's MEHSA insulators where you have to tin the backs of the transistors (and the transistors aren't harmed), I am no longer concerned about damaging a transistor when soldering it's leads.

Arrow appears to have the lowest prices on 3205s right now. Click HERE.
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Old 14th May 2010, 01:14 AM   #54
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I'm fixing a Thunder 2300, and I am having an issue with the bias. Both pots are turned all the way counter-clockwise, and the output FETs still heat up fast. All FETS, diodes and transistors have been checked or replaced. I am looking at four of the 3W power resistors R150, R151, R164 and R173. Does anyone know what the values are supposed to be? The colors are faded from heat. R150 & R151 are next to each other by the small toroid transformer, while R164 & R173 are across from each other on the sides of the big capacitor bank (SMPS output bank). Any help would be very much appreciated.
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Old 14th May 2010, 04:01 AM   #55
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The resistors connected to the outputs are 0.1 ohm. The ones near the small driver transformer are 680 ohms. R173 is a 680 ohm. This is from a Rev D amp. I can't find R164.


You state that the outputs heat up quickly. Do they heat up if clamped to the sink?

If so, how much current does the amp draw with them clamped to the heatsink?

Are all insulators in place and in good condition?
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Old 14th May 2010, 03:14 PM   #56
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Thanks a bunch Perry. My resistors are fine then.

I found out what it was. Someone replaced the output FETs with 100V, 50A rated ones, but they forgot to check Vth (Vgs threshold voltage). The ones they put in have a 2V typical, while the original spec IRF540 has a 3V typical. I just replaced them with IRF540. Now, with the bias pot all the way down, I'm seeing a ~3V bias for Vgs on the high-side FETs (what I expect), but only 1.3V for all of low-side FETs on both channels. This of course is giving me some bad cross-over distortion, even when I crank up the bias pot to the point where the FETs get way too hot. I'll be working on this issue today.

The insulators are in great shape. I haven't checked the current draw in idle yet, but I will.
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Old 14th May 2010, 06:00 PM   #57
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You can't adjust the bias current unless the transistors are tightly clamped to the heatsink. Clamp them down and readjust the bias current. Does that eliminate the distortion?
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Old 10th August 2010, 06:11 PM   #58
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Well, I'm stuck, Mouser are out of stock of IRF540's for the channel outputs, and they only have 9 IRF540A's left which Mr Spooney reccomends as a replacement. Do I dare buy crappy ones off Ebay??

10PCS IRF540N IRF540 TO-220 N-CHANNEL POWER MOSFETS on eBay (end time 06-Sep-10 01:47:12 BST)

These look favourite as they actually have something printed on them. I don't know anything about this topic. Can someone educate me a little on sourcing these components?

I do need to source some of those small square block capacitors, 153J100, Mouser list then as 100volts 0.015uF 5% LS 5mm, so I've ordered those today. Hope thats right...Spooney?

Oh, And I've ordered those two powersupply transistors you mentioned Q52/Q53.

And I've bought a E300 TD Mercedes Estate to put it all in. Shame its a diesel but these are well built and rattle free...nice to get some peace, I've got a Fiat X19 at the moment.
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Old 10th August 2010, 08:27 PM   #59
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Quote:
Originally Posted by spooney View Post
You should replace the power supply fets with IRF3205. They are more rugged than the originals and should help increase the amps reliability.
I did that Spooney, took your advice, they are in the post, but the channel fets are IRF540's, 16 of them. Any ideas? Hong Kong ones on Ebay...??
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Old 11th August 2010, 12:52 AM   #60
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I'd suggest against buying any semiconductors from any seller on ebay. I'm sure that some are OK but FAR too many have proven to be counterfeit or otherwise defective.

Digikey has the IRF540PBF in stock.
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