F-5 output device failure mode

[Russellc]

Recently posted concerning bias, junction temp and longevity of the mosfet output devices.

Checking the 4-40 bolts that hold my IRF devices to the heatsink, I found they indeed needed just a slight twist to snug up again....not too tight though. Wondering if they were loose enough that any damage was done to the mosfets, because of not transferring enough heat.

Frankly, they were just a smidge looser, not much at all, but it got me to wondering about how these devices fail. The only ones I've read about failing
on F-5 have been from miswires, usually switching the wrong device in the wrong place on the board.

Do these devices tend to either "work" or "not work" when they fail, or is it a gradual breakdown with worse and worse sound quality or what?

Thanks to any that know,

Russellc

[frags]

Use spring washers to keep bolts tight.
PS. And do not use hand held cordless drill for drilling heat sinks. The holes need to be ideally vertical.

[Russellc]

Originally Posted by frags

Use spring washers to keep bolts tight.
PS. And do not use hand held cordless drill for drilling heat sinks. The holes need to be ideally vertical.

I'm not familiar with "spring washers" Frags, do you have a link or pic?

As to drilling the sinks, I found the 10-32 holes for the angle aluminum were the most difficult to keep square, but mattered the least. The 4-40 holes with careful eyeballing were square as can be....I did the first as a trial and it was true as I could measure, of course there are only 4 of those for my F-5 build. The larger 10-32 were more difficult to keep straight and true....but since they just went through holes in the angle material (whose hole can be hogged out to allow fitting and trueing) so it really wasnt as important as clamping a fet square to the sink surface with the 4-40.

russellc

[westend]

I haven't seen many reports of failure of the outputs other than catastrophic failure due to a miscue. If they do fail, it probably wouldn't be gradual and if your device-to-sink connection was fairly diligent, you should be OK.

I know our two builds are pretty close, same heatsinks, same chassis, different outputs but current handling is the same. I've measured output device temps, case temps, sink dissipation, etc. and was startled to see how low the actual values actually were.

One thing I did to insure a good mating surface was to mirror polish the heatsinks, especially in the area where the devices are positioned. FWIW, I positioned them as close to 1/3 up from the bottom as I could, that was mentioned as a good placement. I don't think it is absoluetly critical, the M&M heatsinks dissipate the heat very well.

[frags]

Originally Posted by Russellc

I'm not familiar with "spring washers" Frags, do you have a link or pic?
russellc

Spring washer:

You can find them in any home hardware store.

To mount FETS I'm using regular washer (10mm outside diameter) plus a spring washer.

[prooptiki]

This is the correct option! Belleville Washer
the above option is just for shake proof

[evsentry3]

Originally Posted by prooptiki

...the above option is just for shake proof

So....if you often shake your amp to get more out of it...like shakin' a printer cartridge, then those would be a good option???

Sorry....couldn't resiste!!

[Nelson Pass]

Originally Posted by prooptiki

This is the correct option! Belleville Washer
the above option is just for shake proof

That's the best. You can buy them at various spring values.

[Russellc]

Originally Posted by frags

Spring washer:

You can find them in any home hardware store.

To mount FETS I'm using regular washer (10mm outside diameter) plus a spring washer.

Oh, I see now, I've just never heard "spring" in reference to them. I'll get some, thanks for the tip!

Russellc

[Russellc]

Originally Posted by Nelson Pass

That's the best. You can buy them at various spring values.

Is there a prefered spring rate for this application? I've never seen this kind, very interesting.

Russellc