gar2520 build thread

BD13916  =  BD139-16  In a pinch BD139-10 is OK
Same as above with BD14016

Its hard to get in circuit readings that are meaningful.

GARY

gar381 said:

BD13916  =  BD139-16  In a pinch BD139-10 is OK
Same as above with BD14016

Its hard to get in circuit readings that are meaningful.

GARY

Click to expand...

Thank you very much, Gary.

It turned out the diode in D1A was shorted, as well as the q7 and q8 transistors which were not producing current between the emitter and base, but were functional between the collector and the base. Hopefully this is helpful for those that run into the same issues.

EXCELLENT Allecstatic !!

Glad you found the problem!

GARY

Hello

Is having an HFE Of 400+ for the matched input pair important if using the opamp in a unity gain (balanced line receiver) configuration? I have some low (100-150 HFE) matched pairs laying around with no real use!

From my understanding...the input gain of the differential pair of BC550C's will set the open loop gain. A low open loop gain results in not being able to use the opamp in high gain configurations with lots of feedback because their will not be enough gain at higher frequencies, causing a skewed frequency response. Is this correct?

I know my explanation is oversimplified....just trying to grasp the concepts first, before getting lost in all the minutiae  ;D

Use a matched pair of BC550C and all will be right with 2520 world.

I have matched many thousands of 550C pairs.  I almost never  find "C"s
under 400.  Most are 500 and above on my Peak DCA75.

Best

GARY

gar381 said:

Use a matched pair of BC550C and all will be right with 2520 world.

I have matched many thousands of 550C pairs.  I almost never  find "C"s
under 400.  Most are 500 and above on my Peak DCA75.

Best

GARY

Click to expand...

In the opamps I care about, I do. But I am curious in learning more about differential pairs, and I havent been able to find the answer to my question in my searching unfortunatly!

I just soldered all the resistors of four kits mixed up when it suddenly hit me:
Are the resistors in a kit matched?  ???

As long as all the right values go in all the right holes all is well.

GARY

gar381 said:

As long as all the right values go in all the right holes all is well.

GARY

Click to expand...

Perfect! Thanks!  ;D

My VP28 builds went well except of one thing... I accidentally swapped the BD139 and BD140 in one of my GAR2520.  Naturally, when I put power to the VP28 module, I burned out the "fuse" 10 ohm resistors corresponding to that opamp.  It took me some time to locate the transistor error, and I thought that the problem was a loos pin, so I again applied voltage, but this time, I didn't burn up the 10 ohm resistors, but noticed that the opamp was getting physically hot.

Long story short:  when I figured out the error, I removed and re-soldered them in the proper location, but I think that the damage was already done.  I reinstalled the preamp into my lunchbox, then started to smoke the 5.6ohm resistors on the actual opamp PCB.

My question is this:  Is my theory plausible?  If I killed these  transistors, would this start the 5.6ohm resistors to smoke or should I be looking for something else in my trouble shooting?  I should point out that the voltages at the socket read correctly... I tested this before inserting the opamps.

The fully functional VP28 sounds awesome.  Can't believe I screwed up the opamp--such a silly mistake on my part.

Thanks for your advice,

--Jesse

First check if the diodes in the right place.
I burn the 5,6ohm resistors, as I placed the diodes wrong.
Check the diodes you must twist together. I burned my 5,6ohm resistor, when I had the wrong direction.
The transistors getting hot, but they get much hotter, when you solder the diodes the wrong way.

I am using the 4.1 board, so no twisting of diodes is necessary.

http://www.capi-gear.com/catalog/images/gallery/opamps/gar2520/2520v4.1_manual.pdf

I have checked to make sure that the cathodes are aligned properly (with my magnifying glass).  Of course, I will triple check.  I think that I failed to recognize the fact that there are two different power transformers because I was rushing to finish.  I had built one module that fired right up... I was obsessed with measuring all the resistors and caps, orient the diodes and transisors properly, yet somehow I acted as the each of these BD transistors were the same component.  Whoops. 

Thanks

Don't feel to badly.  This is the most common mistake that people make.
but thats why there all the references in the build manual regarding the orintation
of the BD139/140.  Time to get the Hakko 808 out.  If you haven't fired it up yet
you can fix it.  If you have fired it up and realized  something was wrong quickly enough
you may not have damaged parts yet.  Best of luck !!
Hope you get it working

GARY

thanks.  I Carefully oriented three of 'em.  Just jumped the gun on the 4th.  I have no way of testing the gar 2520 out of the circuit...  Thinking about starting with a new one, taking my time like I did with the previous (successful) three

Well I've met a PITA 2520 that's being a stubborn little bi*T*$...I've built over 40+  of these & 1731's over the past few years.  I've  flubbed a few times in the beginning with the typical mistakes but it's been a good couple years since I've built a 2520 that's giving me issues that I can't remedy. I built this in a bunch of 4 and this is the only one that's not passing audio- I can smack my sm57 testing it in a known working Capi 312 and get some inital audio breakthrough but then  nothing.
even fully cranked input & output just a little hiss noise.

Things I've TRIPLE checked  & confirmed thus far

1- All resistors are correctly stuffed
2- All the capacitors are correctly stuffed
3-Q7 & Q8 are facing the proper direction and in their right positions
4- Absolutely NO solder bridges ON TOP & ON BOTTOM of the PCB or arching resistor tops
5- Touched up EVERY solder point to remedy any potential cold sold joints...fired up same problem.
6- Touched up all the PINS
7-Confirmed no bridges at the no go spots for bridges

My fluke can't test transistors so I wonder if this is a transistor based problem? Only thing I can't troubleshoot and I've never encountered this symptom before. I build these starting fromCd1 & Cd2 caps and slowly stuffing and soldering downwards until I'm complete so I can't imagine mixing up the matched transitors as they are the 2nd to last things I stuff and solder so I'd have to be distracted hardcore/on another planet for that to be a possibility, though anythings possible I suppose! lol

Anyway, care to help me fix this little guy anyone? Can happily link to dropbox pictures if that'll help. Scratching my head wondering....

geardude said:

My fluke can't test transistors so I wonder if this is a transistor based problem?

Click to expand...

You mean it can't test for hfe? It surely must have a diode function??

geardude

99 times out of 100 no output is a result of a small bridge at one
of the input pins.  Triple check with your fluke and a
magnifying glass.  Also check for continuity between the input
pins and the destination of the trace hooked up to the pins.
I saw one instance of a hairline crack at the pin pad about
5 years ago on one PCB.  this is very rare but worth a quick
check.  The crack was actually hidden under the solder at the
base of the pin and not visible by eye.

Just for knowledge purposes,  Q7 and Q8 mixups always bring
"magic smoke" 

While its always possible you got a bad part, its not
very likely as these are all very high quality fresh top of
the line parts from major vendors. 

Best of luck and I hope you fine the problem.

Best regards

GARY

Hey Guys,

Just thought I'd share a timeline of my sanity....  Here are 50 gar2520s I did a couple weeks ago. This is the second batch of 50 that I've done and 3 more batches on the way!

EB

TubeMonkey

That is wonderfully insane ;D ;D

Love it !!

GARY

BTW... Nice looking work !!

Hi all, I built a couple of the gar2520 s. At least one is shorting out. Everything looks good but in the build three of the millmax pins wouldn't fit into their sockets on the circuit board. I filed the short ends of the pins down so they would fit I'm wondering if that's an issue. Other than that there's just what I thought were okay Bridges from the pins to other points. Any thoughts would be appreciated.  Thanks Patrick