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Millett Minimax Troubleshoot NEED HELP PLEASE

post #1 of 10
Thread Starter 

Hey guys, first time posting here.

 

After reading bunch of posts from here for months, I have decided to purchase Minimax and today I have finally built it.

 

The problem is, I got way too excited and started "setup and biasing" part without the tube and the tube socket installed (I had a problem with the socket leg so I couldn't put it in),

 

Then after around 10-15 seconds with power on, there was a sudden hiss-ish sound with smoke coming out of PCB, and I was like fck, great.

 

I was panicking and was busy turning off the switch so couldn't locate the exact source of the smoke, but I'm positive it was from the right side of the board, and to the front where pot is.

 

Here is the current situation:

 

1. Everything in place, tubes and tube sockets are installed as well

 

2. Sound not checked yet, I don't want to blow up the cans, but willing to test it on my 20$ earphones if you think it is necessary

 

3. The multimeter reads:

 

27v for P.S. (I have adjusted this before it burnt out) 

 

26.5 for TA2L to gnd, 0.8 for TA2R to gnd (***as far as I know these should be 13.5 each but turning trimmers works on neither side, and numbers are consistent)

 

4. Both readings between TA1<->TA2 (L/R) shows 0, but few hours ago I remember adjusting trimmer for the right one to 70 mv (since the range was 66-110 mv and I just wanted to meet the minimum for now). I guess that means something on the right must have shorted

 

5. SOMEHOW LEDs ARE ALL FINE

 

 

 

So the question is, does anyone have any idea what might have gone wrong? Also, does smoking mean that it needs replacement? If so, how do I know which one is burnt? (no sign of burnt parts, although I kinda smell some from the QB8/9 on the Right side. 

 

*btw I am a complete noob

post #2 of 10

yeah, sounds like the DBs are no longer with us...


Edited by vixr - 12/22/13 at 8:14am
post #3 of 10
Thread Starter 

Thanks for replying, do you think I should change all 4 or is there any way I can find out which one is burnt?

 

Given the measurements, can you tell if some other parts also died?

post #4 of 10
Quote:
Originally Posted by hiperterran View Post
 

Thanks for replying, do you think I should change all 4 or is there any way I can find out which one is burnt?

 

Given the measurements, can you tell if some other parts also died?


At this point, your best course of action would be to replace all four output transistors.  If you can't get it to work after that, then you probably should replace all of the TO-92 transistors in the buffer, too.  PM me with your address* and I'll send you an extra set.  Are there any resistors burned?  If not, this would be a great opportunity to verify that all of them are correct.  I'm concerned that you have something else wrong besides having the buffer turned up too high.  You may want to take a few pics so that some of us can check to see if anything is wildly out of place.  (You can PM me with those, too, if you're embarrassed to post them in public.

 

Also - don't ever connect a set of headphones to the amp until you are positive that all of the measurements are correct. 

 

 

* I've shipped a few MiniMAX kits lately, but I can't tell which one you are from your Head-Fi name. ;)

post #5 of 10
Thread Starter 
Quote:
Originally Posted by tomb View Post
 


At this point, your best course of action would be to replace all four output transistors.  If you can't get it to work after that, then you probably should replace all of the TO-92 transistors in the buffer, too.  PM me with your address* and I'll send you an extra set.  Are there any resistors burned?  If not, this would be a great opportunity to verify that all of them are correct.  I'm concerned that you have something else wrong besides having the buffer turned up too high.  You may want to take a few pics so that some of us can check to see if anything is wildly out of place.  (You can PM me with those, too, if you're embarrassed to post them in public.

 

Also - don't ever connect a set of headphones to the amp until you are positive that all of the measurements are correct. 

 

 

* I've shipped a few MiniMAX kits lately, but I can't tell which one you are from your Head-Fi name. ;)

 

OMG. I can't believe I got a reply from THE TomB

 

You are the greatest man in the world T_T

 

Yea, I bought the kit from you from Beezar with GrubDAC (which I TOTALLY failed and basically stopped at U1, it was just impossible to get that smd mounted - way too small)

 

It shows on my email that the order number is 2356 and the date is Oct 4th, to the 202 Lester place.

 

I am trying to insert image on this post, but when I click on it it says my account is not permitted so I will send them to your email at tomb@beezar.com if you are okay with it.

 

Just few updates:

 

1. While I was removing the heat sinks to see the transistors, I have noticed that I ****ed up yet again and have QB8L and QB9R switched (so BD138 in QB8L  and BD137 in QB9R right now). 

 

2. Similar thing happened way earlier when I was populating resistors, where I used RA4L (1k) for RB4R (100) so in the picture it shows tha RA4L in relay section is actually a resistor that I had (luckily) at home. It is as far as I know a 1k ohm resistor so I hope it works okay.

 

3. Regarding (2), I was going through hell when I tried to recover the soldered RA4L from the RB4R, and finally failed to do so and broke one of the legs of the resistor (thats why I used mine at the end). I can't even imagine going through that same hell with all the transistors in the buffer. Can you give me any tips or methods to use?? (I have about 0 experience with PCB soldering.

 

4. How do I know which resistors and transistors have gone bad? I have the multimeter you have sent me but don't know which mode to use and where to test.

 

Thank you so much in advance, you are the only hope I got right now. Quitting GrubDAC is enough T_T

post #6 of 10
Quote:
Originally Posted by hiperterran View Post
 
Quote:
Originally Posted by tomb View Post
 


At this point, your best course of action would be to replace all four output transistors.  If you can't get it to work after that, then you probably should replace all of the TO-92 transistors in the buffer, too.  PM me with your address* and I'll send you an extra set.  Are there any resistors burned?  If not, this would be a great opportunity to verify that all of them are correct.  I'm concerned that you have something else wrong besides having the buffer turned up too high.  You may want to take a few pics so that some of us can check to see if anything is wildly out of place.  (You can PM me with those, too, if you're embarrassed to post them in public.

 

Also - don't ever connect a set of headphones to the amp until you are positive that all of the measurements are correct. 

 

 

* I've shipped a few MiniMAX kits lately, but I can't tell which one you are from your Head-Fi name. ;)

 

OMG. I can't believe I got a reply from THE TomB

 

You are the greatest man in the world T_T

 

Yea, I bought the kit from you from Beezar with GrubDAC (which I TOTALLY failed and basically stopped at U1, it was just impossible to get that smd mounted - way too small)

 

It shows on my email that the order number is 2356 and the date is Oct 4th, to the 202 Lester place.

 

I am trying to insert image on this post, but when I click on it it says my account is not permitted so I will send them to your email at tomb@beezar.com if you are okay with it.

 

Just few updates:

 

1. While I was removing the heat sinks to see the transistors, I have noticed that I ****ed up yet again and have QB8L and QB9R switched (so BD138 in QB8L  and BD137 in QB9R right now). 

 

2. Similar thing happened way earlier when I was populating resistors, where I used RA4L (1k) for RB4R (100) so in the picture it shows tha RA4L in relay section is actually a resistor that I had (luckily) at home. It is as far as I know a 1k ohm resistor so I hope it works okay.

 

3. Regarding (2), I was going through hell when I tried to recover the soldered RA4L from the RB4R, and finally failed to do so and broke one of the legs of the resistor (thats why I used mine at the end). I can't even imagine going through that same hell with all the transistors in the buffer. Can you give me any tips or methods to use?? (I have about 0 experience with PCB soldering.

 

4. How do I know which resistors and transistors have gone bad? I have the multimeter you have sent me but don't know which mode to use and where to test.

 

Thank you so much in advance, you are the only hope I got right now. Quitting GrubDAC is enough T_T


Thanks for the kind comments!

 

Yeah, sending pics to my e-mail is fine.

 

Responses:

1. Yep - I sort of suspected as much.  You really can't get a "poof" so quickly if the buffer is just turned up too high.  It had to be shorted somewhere.  If the output transistors were swapped, that would do it.  Most of the time, once power transistors like that have been hard-soldered in the wrong place, they're fried.  The best thing would be to replace them all and I'll send them to you.  Be sure you get them in the right place when you get them.  There is ample documentation on the MiniMAX website and also in the typed notes that come with the kit - it's right there on the front page.

 

2. Why not measure it with your meter to be sure?  Set your meter for the 20K or 2000 measurement on the Ω scale and touch the probes (plugged into the COM - black and VΩmA - red probe on the meter) on the leads of the resistor and see if it measures at 1000.  A way to check if it's measuring resistance OK is to simply touch the two probes together and see if it reads zero resistance.  Don't leave your meter on with the measurement set for resistance, because a small voltage is applied through the probes to measure resistance and the battery will run down quicker.

 

3. Removing those transistors are pretty easy:

  1. First, make certain they are unbolted from the heat sinks.
  2. Tie a string, twist tie, or something similar through the mounting hole of the transistor.
  3. On the bottom of the PCB, add enough solder with your soldering iron to cover all three pins at once.  There has to be just enough of a solder blob that your iron can keep it all melted at the same time.
  4. If you're lucky, the transistor will simply fall out.  If not, tug on the string while the solder blob is melted.
  5. Clean up the solder and make certain the PCB pad holes are open with de-soldering braid.
  6. Install the new transistors (mounted on the heat sinks, first, helps).  I like make mine just finger tight.  Then I solder the transistors and heat sinks in.  Finally, I go back and torque down the screw and nut so that they're snug.

 

4. A resistor is not going to go bad unless it's burned.  You can, however, verify that it's the proper value in the proper position.  The transistors are much more difficult to troubleshoot.  It's why I'm suggesting to replace them.  Start with the output transistors, first - QB8's and QB9's.  If you can't get the amp to the proper measurements at that point, then go back and replace all of the smaller ones.  It's really simpler and less trouble than trying to figure out which transistor may be bad and replacing them one at a time through trial and error.  You've already blown the buffer - no reason to assume anything is good, but we can limit your effort by replacing only the output transistors, first.  If the amp works at that, then count yourself lucky.  If not, then replace all the small transistors.

 

The same technique applies for removing the small transistors as described in #3 above.  For resistors, you're better off using some sort of dental pick to push the lead out of the hole while the solder is melted.  Repeat for the other lead and the resistor should be out.

post #7 of 10

Man... I need to quit answering stuff from memory... I knew I wanted to say output transistors, but I said DBs. Jeez...

post #8 of 10
Thread Starter 
Quote:
Originally Posted by vixr View Post
 

Man... I need to quit answering stuff from memory... I knew I wanted to say output transistors, but I said DBs. Jeez...

idek what DB is for (which I was wondering when I was building, even googled it and got nothing) so it is no problem lol

 

I guess it's....Database? lols

 

p.s. TomB I have sent you another email :) please check it out

post #9 of 10
Quote:
Originally Posted by hiperterran View Post
 
Quote:
Originally Posted by vixr View Post
 

Man... I need to quit answering stuff from memory... I knew I wanted to say output transistors, but I said DBs. Jeez...

idek what DB is for (which I was wondering when I was building, even googled it and got nothing) so it is no problem lol

 

I guess it's....Database? lols

 

p.s. TomB I have sent you another email :) please check it out


DB is shorthand notation for a Diamond Buffer. ;) 

post #10 of 10
Thread Starter 
Quote:
Originally Posted by tomb View Post
 


DB is shorthand notation for a Diamond Buffer. ;) 

I say it was a close call :P

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