[tomb]

Quote:

Wow, I must be having a REALLY strong bout of bad luck. I bought the Enercell 18/24 AC adapter and.... it's as dead as my TRIAD!!    Yeah, I suspected my VOM might be the problem but I checked with a second one  with the same result:  0V out of both AC acapters  
 
I know this is only marginally related to the thread but I had to vent somewhere.
 
cheers!

I haven't had any trouble with the Triads in the states and have shipped a few hundred over the years.

 
That said, the fuse on the MiniMAX PCB will protect the secondary side of the adapter, but it won't do much for the primaries.  If there's an issue with the line voltage, shorting at the outlets, etc., it may burn up whatever is used as a fuse inside the walwart.  Unfortunately, many 24VAC adapters will have protection, but it's often non-resettable/non-replaceable.

 

[scottie4442]

Ok, trying to source out the parts to build my miniMax.  I got everything ready to order except QM2, Fairchild discontinued the mpsa14 and the replacements only have .5a continuous collector current, the fairchild has 1.2a continuous collector current.  Any ideas for a replacement for the mpsa14 that would work.  Thanks
 

 

[tomb]

Quote:

Ok, trying to source out the parts to build my miniMax.  I got everything ready to order except QM2, Fairchild discontinued the mpsa14 and the replacements only have .5a continuous collector current, the fairchild has 1.2a continuous collector current.  Any ideas for a replacement for the mpsa14 that would work.  Thanks
 

Allied Electronics has 181 in stock right now.  Or, shoot me a note if you're ordering stuff from Beezar.  I'll drop one in your order.

 

[scottie4442]

thanks, I will do that. I really appreciate the help.  I need to order several parts that you carry for the miniMax, much easier than trying to find all these special parts where ever.  Thanks for having beezar, it helps those of us that are tinkerers but want good quality equipment.  One of my friends compared you to the old Heathkit. (oh sorry about the PM to you and cetoole, it was before I read this post).

 

[the_equalizer]

I haven't had any trouble with the Triads in the states and have shipped a few hundred over the years.

That said, the fuse on the MiniMAX PCB will protect the secondary side of the adapter, but it won't do much for the primaries.  If there's an issue with the line voltage, shorting at the outlets, etc., it may burn up whatever is used as a fuse inside the walwart.  Unfortunately, many 24VAC adapters will have protection, but it's often non-resettable/non-replaceable.

Well the TRIAD worked perfectly for almost three years. I'm a bit mistified as to how it died so suddenly. Anyway for those in need of using one of these RadioShack Enercell 18/24 VAC adapters, a couple of recommendations:

1.- The correct tip for the MiniMax is size 'M'
2.- Take a VOM to the store and test the adapter you buy... You might find it hard to believe but two of these bought in different RadioShack stores in towns 35 miles apart were both as dead as my TRIAD. What are the odds of that happening!? Finally the third adapter I got, after going back to the last RadioShack store and claiming my warranty, worked fine and has been working without a hitch..

 

[scottie4442]

Another MiniMax lives. Just got mine finished today and it sound AMAZING.

 

[tomb]

Wow - those must be white LED's under those tubes!

 
Nice work and congratulations!!

 
P.S. Hey - are those Black Gate NX's at CA2?

 

[scottie4442]

no, nichicons, the red is an Elna (think I spelled that right), I tired and Elna for ca2 but it was about 3mm too tall. I did use a Russian k42 .01 ohm for a bypass instead of a vitamin q, I had a few already. I wanted to try the white and they look really cool, I thought they might be good with the color changing led in the center position, but they are so bright that the drown out the color changer. I am colecting part for a SOHA II and a Bijou as my next two projects.

 

[lordearl]

An odd question given this is a headphone forum, but does the minimax have enough grunt to power a pair of small desktop 8 ohm speakers (90dB efficiency)? Probably only need a few watts at most, and I thought that using the high gain tube might be a perfect option.

 

[tomb]

Quote:

An odd question given this is a headphone forum, but does the minimax have enough grunt to power a pair of small desktop 8 ohm speakers (90dB efficiency)? Probably only need a few watts at most, and I thought that using the high gain tube might be a perfect option.

The MOSFET-MAX would be a better choice, but if you can turn the bias up on the MiniMAX, it may do OK.  It can probably stand 70-80ma without a big issue (we normally recommend 50ma).  Keep in mind that the -3dB bass cutoff with the normal MiniMAX 470uf output caps may be up to ~42 Hz. 

 

[zwack]

I finished my build and decided to fire it up. I measured the DB bias first and these where almost 0 mV, which seemed a bit odd to me. When I measured the power supply voltage I noticed that it measures 1.4 Volts, no matter in which direction I turn the potentiometer (RR3). I verified that the input pin of the LM317 sees the rectified 34 Vdc. I verified that resistors RR1 (121R) and RR2 (2K) are not defective and have the correct values.Checked if the potentiometer is working. It does. All diodes in the voltage supply section are working too and placed in the right direction according to the silk screen.  
 
Some more informations about my build: 
In the diamond buffer section I'n using two pairs of 2SC2238/2SA968. As these have a reverse pinout I soldered them to the MOSFET locations according to the image here: http://diyforums.org/MiniMAX/MiniMAXbjt.php
RB8L/R, RB9L/R are jumpered out, hope that I understood that well
 
Double checked all other transistors and they're all in the right place. For QB1L/R I used J310's instead of PN4392's 
 
I checked the polarity of all caps and they are all positioned the right way. 
 
I'm using a way oversized torroid 24Vac (225VA) transformer. So surely not running out of power here. 
 
Any ideas what might be wrong here?? I'm thinking that it could be a faulty LM317, but maybe anybody has experienced problem this before and knows how to solve it
 
 
 
I have made to little posts here, so I'm not yet allowed to attach any images. Here's a link to a picture of the almost finished board. Please notice that the LM317, tubes and RB14L/R where not yet installed or jumered when I took this picture.
http://www.flickr.com/photos/mandarijnen/9306836728/sizes/o/in/photostream/

 

[tomb]

Quote:

I finished my build and decided to fire it up. I measured the DB bias first and these where almost 0 mV, which seemed a bit odd to me. When I measured the power supply voltage I noticed that it measures 1.4 Volts, no matter in which direction I turn the potentiometer (RR3). I verified that the input pin of the LM317 sees the rectified 34 Vdc. I verified that resistors RR1 (121R) and RR2 (2K) are not defective and have the correct values.Checked if the potentiometer is working. It does. All diodes in the voltage supply section are working too and placed in the right direction according to the silk screen.  
 
Some more informations about my build: 
In the diamond buffer section I'n using two pairs of 2SC2238/2SA968. As these have a reverse pinout I soldered them to the MOSFET locations according to the image here: http://diyforums.org/MiniMAX/MiniMAXbjt.php
RB8L/R, RB9L/R are jumpered out, hope that I understood that well
 
Double checked all other transistors and they're all in the right place. For QB1L/R I used J310's instead of PN4392's 
 
I checked the polarity of all caps and they are all positioned the right way. 
 
I'm using a way oversized torroid 24Vac (225VA) transformer. So surely not running out of power here. 
 
Any ideas what might be wrong here?? I'm thinking that it could be a faulty LM317, but maybe anybody has experienced problem this before and knows how to solve it
 
 
 
I have made to little posts here, so I'm not yet allowed to attach any images. Here's a link to a picture of the almost finished board. Please notice that the LM317, tubes and RB14L/R where not yet installed or jumered when I took this picture.
http://www.flickr.com/photos/mandarijnen/9306836728/sizes/o/in/photostream/

I've seen it before, but it's usually a reversed part in the power supply.  I don't see that in your photograph (a damn good photo, btw!).
 
I'm curious about something though, can you describe how you made the power connections to the power supply terminal block?  Did you use the terminals that would put the fuse in the circuit?  The other times I've seen this has been when there is a major short in the buffer, but the fuse usually blows.  Let us know how you have things connected.
 
Also, it looks like you have the transistor mounting screws touching in the buffer's center heat sinks.  The shoulder washers should be isolating the screws from the transistors, but it's sort of like daring the thing to short when the screws are touching like that.
 
Another time I've seen this has been when the resistor ratings are wrong.  Mouser will often show resistor selections for 120R, 12K, 120K, so forth from a single search for "120R".  So, it's easy for people to order the wrong thing sometimes.  If that RR1 resistor is off by a few magnitudes, you'll get very little to no voltage out of the power supply.  I can't see it the way you have it soldered in and from the angle of the photo.  I know you say you checked it, but it's sometimes easy to get the scales mixed up on a DMM, too.
 
Just some guesses - best I can do with the info so far.

 

[gspence2000]

An odd question given this is a headphone forum, but does the minimax have enough grunt to power a pair of small desktop 8 ohm speakers (90dB efficiency)? Probably only need a few watts at most, and I thought that using the high gain tube might be a perfect option.

I use my MiniMAX with a pair of uFonken speakers featuring Fostex FF85WK drivers, which are 86 dB efficiency. This works will for me in a near-field setup (sitting at my computer). I had asked TomB about this before I tried it, and he advised me to watch that it didn't get too hot. I have not had any heat problems though. I use the high-gain tubes anyways because my main headphones are Sennheiser 580's.

 

[zwack]

Originally Posted by tomb /img/forum/go_quote.gif
 
I've seen it before, but it's usually a reversed part in the power supply.  I don't see that in your photograph (a damn good photo, btw!).

Thanks, it's shot with a 5 year old "Canon IXUS 860 IS". That little photo camera still amazes me. 
 
 
 

I'm curious about something though, can you describe how you made the power connections to the power supply terminal block?  Did you use the terminals that would put the fuse in the circuit?  The other times I've seen this has been when there is a major short in the buffer, but the fuse usually blows.  Let us know how you have things connected.

Here's the connection. It's putting the fuse in circuit.

 
 
 

Also, it looks like you have the transistor mounting screws touching in the buffer's center heat sinks.  The shoulder washers should be isolating the screws from the transistors, but it's sort of like daring the thing to short when the screws are touching like that.

Fixed that!

 
 

Another time I've seen this has been when the resistor ratings are wrong.  Mouser will often show resistor selections for 120R, 12K, 120K, so forth from a single search for "120R".  So, it's easy for people to order the wrong thing sometimes.  If that RR1 resistor is off by a few magnitudes, you'll get very little to no voltage out of the power supply.  I can't see it the way you have it soldered in and from the angle of the photo.  I know you say you checked it, but it's sometimes easy to get the scales mixed up on a DMM, too.

 
I had checked it, but guess what! I overlooked the letter "k" before the Ohm 

.  I had ordered some 121K resistors instead of 121R's. I could confirm it by checking the order details from my latest Mouser order. While I was at it I checked all other components I ordered as well. They are all the same (values) as per the BOM.  
This is the little basterd. 

 
 
 

Just some guesses - best I can do with the info so far.

You did great! Thanks for that! Now to order some new 121R resistors before I can proceed to start testing the amp. 

 

[tomb]

Quote:

Thanks, it's shot with a 5 year old "Canon IXUS 860 IS". That little photo camera still amazes me. 
 
 
 
Here's the connection. It's putting the fuse in circuit.

 
 
 
Fixed that!

 
 
 
I had checked it, but guess what! I overlooked the letter "k" before the Ohm 

.  I had ordered some 121K resistors instead of 121R's. I could confirm it by checking the order details from my latest Mouser order. While I was at it I checked all other components I ordered as well. They are all the same (values) as per the BOM.  
This is the little basterd. 

 
 
 
You did great! Thanks for that! Now to order some new 121R resistors before I can proceed to start testing the amp. 

Ha!  I thought that might be the case about the resistor!  I wish Mouser would fix their searches from doing that.  I have a couple of hundred 221K resistors instead of 221R resistors because of the same thing.

  Anyway, good news and look forward to you getting it working!!

 
P.S. Yes - great camera, but I suspect the operator had a lot more to do with it.