Help with Minimax

Well, the first thing wrong is that you have the output transistors reversed on the right channel.  The green ones are the 2SA1359's.  They go in the middle, not on the outside.  So, you'll have two green ones in the middle and two black ones on the outside.  Refer to this webpage for a refresher:

http://www.diyforums.org/MiniMAX/MiniMAXbjt.php

A couple of other comments:

1. You need a heat sink for that 4th transistor.  You'll never get the biasing done without burning it up without a heat sink.

2. You should work on trying to fit the part leads completely through the holes.  Resistors should be sitting flat on the PCB, with the leads tight and the rating designation visible.

3. You should also work to get those rectifiers down flat on the PCB in the back (the black sausage-like things with the silver stripe on each one).  I don't think you will be able to get the tip jacks on the back plate with them up high like that.

Good luck and let us know how it works out when you swap those transistors!

Quote:

Originally Posted by Simmin 

Well, that didn't work, though the fuse did last twice as long without burning up ( A whole 4-ish seconds...).

There was a piece that wasn't actually going all the way through the hole (RB12 R) but I managed to destroy the leads so I'm going to try to find another one. I was also using .5 amp slow-blow fuses for some reason which probably doesn't help anything. Hopefully those are the only problems... Any other ideas since I'm off to buy parts as it is?

We recommend a 1A, slo-blo fuse.  I'm not sure if a 0.5A slo-blo fuse will blow all the time, but a 0.5A quick fuse will blow - every time.

Try to make sure the trimmers are turned down more.  Turn them clockwise about 10 turns.  That should be safe, regardless of the starting position of the trimmer screws.  If that doesn't help, it's possible that you have some resistors mixed up, too.  Go over the PCB carefully.  Reference every resistor and its value with the BOM rating and the silkscreen position.  Use your DMM to verify the values if you've soldered them so that you can't see the rating.  As it turns out, there are very few resistors on the PCB that are in parallel.  So, you should be able to get some reasonably close readings to the BOM.  Obviously, if a reading is way off, that would indicate the wrong part in the wrong position and it should be swapped out.
 

Quote:

Originally Posted by Simmin 

I managed to get it to stop blowing the fuse, but I'm still having a problem and I've verified that the resistors are all in the right place. I'm not sure what exactly fixed it, but I cleaned up some of the soldering which probably helped.

 

It turns on (I have one led at RA5C so I can tell when it's on) and then turns off after a variable amount of time, between immediately  and about 10 seconds. I connected jumper cables from RB14L/R directly to the headphone jack and get a buzzing noise that continues as long as the led is still on (It either pops instantly, or starts out really quiet and gets louder and louder 'till it pops and turns off).

 

The thing is, the parts are still receiving power since I can still measure the voltage (If I measure it using the ac settings, it says about 27V at any point on the board. If I set it to dc, it says about 13.5V. I hate to ask, but which should I be using?)

 

Does this give you any indication of what it might be?

 

 

1. You never use AC to measure the voltage anywhere on this amp (unless you want to measure the walwart directly, which is kind of pointless).

2. What is the voltage at V+ and Gnd?

3. What is the tube bias at TA2L and Gnd (left tube) and TA2R and Gnd (right tube)?

4. What is the buffer bias at TA2L and TB1L (or TB2L) for the left channel?

5. What is the buffer bias at TA2R and TB1R (or TB2R) for the right channel?

6. Do the tubes light? (You should see an orange glow at the very top of the tube or along the bottom.)

Try to answer these questions, one by one, and then maybe we'll have an idea where we stand.
 

Quote:

Originally Posted by Simmin 

I managed to get it to stop blowing the fuse, but I'm still having a problem and I've verified that the resistors are all in the right place. I'm not sure what exactly fixed it, but I cleaned up some of the soldering which probably helped.

 

It turns on (I have one led at RA5C so I can tell when it's on) and then turns off after a variable amount of time, between immediately  and about 10 seconds. I connected jumper cables from RB14L/R directly to the headphone jack and get a buzzing noise that continues as long as the led is still on (It either pops instantly, or starts out really quiet and gets louder and louder 'till it pops and turns off).

 

The thing is, the parts are still receiving power since I can still measure the voltage (If I measure it using the ac settings, it says about 27V at any point on the board. If I set it to dc, it says about 13.5V. I hate to ask, but which should I be using?)

 

Does this give you any indication of what it might be?

 

 

Put simply you should only have AC coming in on the terminals, it is then rectified via the diodes so is pulsating DC, it then goes through the smoothing caps and is fairly steady DC. Then it is regulated, the voltage will drop slightly after the regulator to a steady constant value, linear regulators need a bit of overhead in voltage, so a little bit higher coming in than out, depends on the type as to how much that is.

There should be a little bit of AC on your DC such as ripple voltage, but not that high, there may be a serious failure...

Most likely though I suspect you have a cheap meter, apparently some cheaper meters will measure the DC and roughly double it and come up with an AC reading (if set to AC)*. I suspect that may be the case here. More expensive True RMS meters should measure correctly just the AC component.

* Not sure exactly why, are they normally just averaging one side of the AC and then doubling it to get their AC readings ? If so that would explain it.

Quote:

Originally Posted by Simmin 

Okay, so here's what I found:

From power input to gnd, 13.5V. From power to V+ is 12.1V.

tb2l -> ta2l is 0V and tb2r -> ta2r is also 0V.

ta2l -> gnd is about .7V

ta2r -> gnd is 1.1V

 

The tubes do not light up.

The heatsink at the power supply heats up a little, and the rest do not.

 

Oh, and by the way, thanks for helping me! I appreciate it alot!

OK, no offense, but can you please quote my post and reply exactly as asked from the quote?

For instance, what do you mean by "power input?"  What do you mean by "power" to V+?  That's only your interpretation and no one else understands exactly what that means.  I might guess that you're referring to one of the terminal block screws on the power input, or perhaps one of the contacts on the power socket.  Unfortunately, referencing a DC voltage to either one of those points is meaningless.

The points I specified in those questions have definite labels on the PCB.  They are VERY SPECIFIC: V+ and GND.

If we are to make any sense out of what you've replied to with so far, we still have an issue with the power supply.
 

A properly built MiniMAX should register about 45uVAC.  Not even the best Fluke will measure that.  You'd need Tangent's LNMP to even see that amount. 

Well I was more just trying to say there will be some AC... measuring it is indeed another story.

Just that obviously something would be seriously wrong if he was really measuring 27v of AC after the regulator, which makes you question the test equipment and methods first, circuit second.

Hard to say where he's measuring as well as he was a bit vague initially, if he had a better meter he might be able to measure AC ripple after the rectifier or so on....

Yes, he has a cheap meter.  The one supplied with the kit is a Harbor Freight special - not intended to measure anything but DC voltage and for that, it is more than adequate.

I have a cheap meter in my tool bag, they're fine for measuring Extra Low Voltage AC generally to check it's not excessively low or high...   not particularly good or accurate though I'll admit, with many caveats.

Simmin, I think brushing up on theory will definitely help with trouble shooting.

All About Circuits is one free resource you can look at.

Oh and 1.5v between +V and GND definitely sounds wrong...   however tomb is far more knowledgeable than me in general and especially with the MiniMax so perhaps he can best advise where to check next. Just to be clear that is between the +V and GND pads ?

Quote:

Originally Posted by Simmin 

Sorry about that... I was measuring from the power terminal to gnd, then power terminal to V+. I'm not sure why I understood it that way, since I didn't do that with the rest...

 

So, from V+ -> GND is 1.5V

 

I'll try not to make up my own measurements from now on, I promise :(

Well, seeing as how you should be registering between 20-30VDC there regardless of how the trimmer is adjusted, I think we have some bigger issues at fault than simply the adjustments.

Now that you've done some more work on it, take us another top-down, high-resolution photo of the PCB - like you did in the first pic up there.  It needs to be something large and clear enough that we can read the writing on the resistors.  Then let's see if one of us can spot something out-of-place.
 

Quote:

Originally Posted by splaz 

 

A properly built MiniMAX should register about 45uVAC.  Not even the best Fluke will measure that.  You'd need Tangent's LNMP to even see that amount. 

 

Well I was more just trying to say there will be some AC... measuring it is indeed another story.

 

Just that obviously something would be seriously wrong if he was really measuring 27v of AC after the regulator, which makes you question the test equipment and methods first, circuit second.

 

Hard to say where he's measuring as well as he was a bit vague initially, if he had a better meter he might be able to measure AC ripple after the rectifier or so on....

 

Yes, he has a cheap meter.  The one supplied with the kit is a Harbor Freight special - not intended to measure anything but DC voltage and for that, it is more than adequate.

 

I have a cheap meter in my tool bag, they're fine for measuring Extra Low Voltage AC generally to check it's not excessively low or high...   not particularly good or accurate though I'll admit, with many caveats.

 

 

Simmin, I think brushing up on theory will definitely help with trouble shooting.

 

All About Circuits is one free resource you can look at.

 

Oh and 1.5v between +V and GND definitely sounds wrong...   however tomb is far more knowledgeable than me in general and especially with the MiniMax so perhaps he can best advise where to check next. Just to be clear that is between the +V and GND pads ?

Yes - all correct and thank you for the help.  Please continue - hopefully he'll have another pic posted shortly.