Crate BFX 100 - WTF were they thinking??

[DrGonz78]

So anyhow I am repairing a BFX100 right now and could not believe how the output board is mounted to the chassis. I already pulled the board out and doing the repair, no problems. However this board looked upside down(solder side up), but it is not. My rant about this is in regards to where they placed the Bias pot, facing down towards the chassis on the upside down board.

Now my question is about when I go to bias the amp and some advice. Now when I bias the amp it will be done with the transistors not mounted to a heat sink. In fact, this amp has no "true" heat sink and has mosfets mounted directly to the chassis. So I always let amps warm up to set the bias correctly and that has me a bit concerned. Since the mosfets will be unmounted during the bias procedure then how long do I have before the chips overheat? Do you think they will be fine for say 5-10 minutes? Just need some opinions here to help ease my mind a bit once I get to the bias procedure.

I attached a pic that someone uploaded of this amp from another MEF thread on here... I copied the pic from google images as I think the thread is missing the picture. Clearly the mounting of these mosfets and board layout was a total after thought. What a dumb layout

[The Dude]

I can't tell much from the pic, but is it possible to drill a hole in the chassis to access the pot?

[Enzo]

A couple thoughts...

Hmmm, just occurred to me I have a very similar BX100 here in the shop. The output transistor board appears the same. on mine, there is a hole through the circuit board for a small screwdriver to make the adjustment. I am surprised yours lacks the hole that was on the model 6 years older. I have no trouble at all accessing the adjustment with the board mounted.

But lacking that hole, you can easily remove the trim pot, drill an adjustment hole through the board there, re-install the trimmer, and there you are.

Alternatively, remove the trimmer, and mount it on the reverse side of the board on the solder side, and there it will be. I think either of those alternatives will be far better choices that trying to adjust things without mounting the transistors to the heatsink/chassis. I would NEVER do that to a transistor output circuit.

I find it surprising, but I can believe there is no access hole under the trimmer on your board, but to say the engineers never even thought about it is overly harsh, as they clearly had, at least on the BX100. On that small board at least, the boards for the two models appear identical, down to the part numbers. I didn;t compare the rest of the schematic.

[DrGonz78]

Too funny!! I am an idiot! So after reading your comments guys I go out to give the damn thing another look. Of course there is a little tiny hole right there to adjust the bias. Looks like I jumped the gun and totally overlooked it. Naturally when I have done this in the past I have only adjusted bias from the top side of the pot. I guess I did not think of way to get at it from the bottom side. You live and you learn, sometimes looking quite foolish. Thanks you guys always steer me clear of my short sightedness. lol

[52 Bill]

You live and you learn, sometimes looking quite foolish. Thanks you guys always steer me clear of my short sightedness. lol

I give you a lot of credit for admitting your mistake. We all have done dumb things in our lives, but so many people are afraid to admit it.

[soundguruman]

Here is the crate bias adjustment tool. Fits perfect in that little hole.

Handy Tip: Always wear eye protection when using the tool.

[The Dude]

I am reminded of this.


“He has never learned from a mistake…because he has never made a mistake. Mistakes are afraid of him ”

[Enzo]

I don't need to admit to mistakes, my wife always points them out before I can say anything.

[DrGonz78]

I give you a lot of credit for admitting your mistake. We all have done dumb things in our lives, but so many people are afraid to admit it.

Thanks. Yeah I always feel better about something when I own up to it and the same thing goes for telling a lie. I am the worst liar in the world, because I am use to telling the truth. Enzo, if I had a wife she would just say I am just full of it!! lol

[DrGonz78]

Well got it all fixed and biased to go! Looks like this Crate won't make it to being in SGM's potted plant collection, for a while anyhow. Thanks again for the help ya'll.