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Millet MiniMax: Fuse blew after LEDs lit for fraction of a second

post #1 of 3
Thread Starter 

Hello fellow DIY-ers.

 

I'm a newbie to DIY and would be very appreciative of any assistance you could offer. My only prior build was a CMoyBB v2.03R (very pleased with it). Based on that successful build, I've decided to try my hand at the MiniMax.

 

I purchased the great kit from beezar.com and really enjoyed the build (except for drilling out those tube sockets which was a real hassle). As far as I can tell, the workmanship is acceptable given my level of experience. Parts are well-aligned. No solder bridges I could identify. Instructions were followed carefully. Pretty certain all parts are correctly installed/aligned and in the right places.

 

In any event, the reason I'm posting is because when I fired this up earlier the LEDs under the tubes turned on for a fraction of a second, at which point the fuse blew. Wondering whether anyone could point me in the direction of what might be causing my issue. Thus far my searches have proven fruitless.

 

Needless to say, I'll have to pick-up a few more fuses before I can test further, but it seems that I won't have much luck diagnosing the root cause if the fuse blows immediately after I turn the unit on. Hopefully, though, that will make the issue easier to identify. One other thing worth noting is that I didn't install the middle LED...it didn't seem to be in the middle of a path so I'm assuming (gulp) that's not the cause.

 

By the way, you'll notice that the right tube bias connector broke. This is the reason I didn't install the PCB in the case yet. I'll get a replacement part this week, but this is reportedly not going to be the problem.

 

For what it's worth, I'm better equipped than informed (learning more every weekend via YouTube and such) so let me know if I need to take any measurements (ideally which setting to use on Fluke DMM and which points to measure).

 

Please let me know if you need the larger versions of the images below. If so, hopefully you can recommend how to upload larger images.

 

Thank you very much in advance for any assistance you're willing to offer.

 

 

 

 

 

post #2 of 3

Unlike some I've seen lately wink.gif, it's a very nice build.  The photos are very good, too.  I'm at a loss to tell from the excellent photos what might be wrong.  Wiring looks great, no parts are out of position or soldered in a strange orientation.  There are some resistors that don't have their ratings shown on top, but there are enough others to allow some deduction to identify them all and they appear to all be correct.

 

The only thing I would suggest at this stage is to check and re-verify the transistors.  Make certain the TO-92 transistors are in their proper positions - odd-numbered pads are used by the 2N5088's and even-numbered pads are used by the 2N5087's.  Also make certain that the output transistors - QB8-L/R and QB9-L/R are in their proper positions.  If you have the BD137 and BD138 output transistors, the BD137's go at QB8-L/R and the BD138 transistors go at QB9-L/R.  Actually, blowing a fuse requires a significant short or some parts not in their proper position - like the output transistors QB8 and QB9. 

 

Check all of that first and let us know.

 

P.S. So, it was the red tip jack that got broke. wink.gif


Edited by tomb - 2/2/13 at 7:16pm
post #3 of 3
Thread Starter 

Thank you, Thomas, for the compliment and the guidance.

 

Appears as if I've screwed the pooch, so to speak. I ended up with both BD138-16 on QB8R and QB9R and both BD137-16 on QB8L and QB9L. I'd constructed the heat-sink assemblies last week and made the mistake yesterday of assuming that they were all the same so I just threw them in there without paying attention to the associated transistors. If only there was a pithy saying about the dangers of assuming which could've played in my head beforehand...

 

If there's a silver lining, only two of the four need to be disassembled and desoldered.

 

Thanks again. I'll update the post once I've gotten the issue resolved.

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