Gar2520 overheating

[Buffstuffbuddha]

It's been a while since I've built anything but I've got the itch lately so I ordered a VP28 with 2 Gar2520 (v 4.1) op amps. I'm no expert but definitely not a beginner. I built a 51x rack/power supply, vp 25, vp 26 as well as a bunch of other projects all without a problem.  For some reason this time I was bested. Both of the DOA I just built get super hot very quickly and will actually smoke. Transistors Q7 and Q8 get hot enough to reflow the solder.  I followed the directions dead on step by step and can't find any bridges, shorts or anything. All the diodes are the correct orientation. I tried both DOA in my VP 25 and they still overheated. Then I tried an already working DOA in my newly built VP28 and it ran fine. This way I at least know the something is wrong with op amps I just built but I am at a loss of where to go from here.

 

[Rob Flinn]

Don't take this the wrong way.  Step back & think logically.

You said you have done everything right in the build.    Unless you have a faulty or wrong component, something must be wrong with the build.    I've been in this position a lot of times myself, & undestand how frustrating it can be.  However, it is almost always a mistake I've made, & hardly ever a faulty component.    You have a working Op Amp to compare it to.

 

[Buffstuffbuddha]

I may have worded that incorrectly out of frustration. The fact that both of the op amps are overheating would most likely point to something wrong on my part. I was moreso asking where  would be the best place to start looking with that being the problem. Unfortunately, the working op amps I do have are not the same version as the new ones so the components are slightly different.

 

[Humner]

i would triple check the diode and transistor orientation and then check it again.

 

[Buffstuffbuddha]

Everything is definitely the correct orientation 8) I just find it very odd that I would have made the same mistake on both of these builds.

 

[Buffstuffbuddha]

1 more

 

[Buffstuffbuddha]

Here's the diodes

 

[Rob Flinn]

Got the right types transistors in the right places & the correct orientation ?  It is possible to orientate the right transistor so you have the legs in the wrong place.

Since this is a kit that thousands of people have made successfully, as I said in my first response unless you have a faulty component, then there is a mistake somewhere.      It's very difficult to check stuff out on here from photos.    You could diode check the transistors to see if they're OK, but if one has blown it is probably likely that a fault elsewhere has caused the failure.

 

[Buffstuffbuddha]

The transistors are definitely in the correct spot/orientation. Pins are all in the correct holes. I did just notice that the 2 half watt resistors (r11,r12) seem to have burned. They  both have black spots...is it possible that some Flux on the bottom could have caused anything to short?

 

[Buffstuffbuddha]

I pulled one of those resistors out and it's still reading 6 ohms. Supposed to be 5.6 so that should be fine.

 

[JohnRoberts]

Sounds like an opportunity to learn troubleshooting.

Power up the op amp from a reduced power supply and start probing around the circuit with a VOM...

You will need to learn a little about how the circuit works, but such knowledge is a good thing.  If you have one that works properly you compare voltages at various circuit nodes between them to find the fault.

What doesn't kill us makes us stronger.

JR

 

[Buffstuffbuddha]

Ok, REALLY stupid question and I don't know why I didn't think of this earlier. When I originally installed Q7 and Q8 they were swapped...after I initially turned on the unit and I realized it was wrong I took it out and swapped the transistors to their correct location. I did this on both op amps. Evidently that's what happens when I build these late into the night haha  :-[. Did that fry those transistors? I thought they were a matched pair but I think I was confusing those with Q1 & Q2...Could that be my entire issue? Is there a chance it fried the whole op amp or can I just replace those transistors and hopefully be good?

 

[Buffstuffbuddha]

What should I be using for a reduced power supply?

 

[Buffstuffbuddha]

Of course after reviewing the documentation with a much clearer mind they are definitely different parts. I can get them from Mouser for very cheap but I just want to make sure that it didn't smoke the rest of the op amp. I'm hoping it just fried the transistors and they can just be swapped.

 

[JohnRoberts]

Ok, REALLY stupid question and I don't know why I didn't think of this earlier. When I originally installed Q7 and Q8 they were swapped...after I initially turned on the unit and I realized it was wrong I took it out and swapped the transistors to their correct location. I did this on both op amps. Evidently that's what happens when I build these late into the night haha  :-[. Did that fry those transistors? I thought they were a matched pair but I think I was confusing those with Q1 & Q2...Could that be my entire issue? Is there a chance it fried the whole op amp or can I just replace those transistors and hopefully be good?

I have no idea what Q1 or Q7 is, but there are basic troubleshooting measurements you can make of transistors to confirm if they are good or have been injured.

Many cheap modern VOM will have a diode scale.  A simple bipolar transistor will show a diode drop from base to emitter, and similarly from base to collector, while showing open circuit from collector to emitter, and backwards into the base.  The polarity is reversed  between NPN transistors (plus to base) and PNP (negative to base).

A common failure is shorted collector emitter.

JR

WRT reduce supply, low enough that it doesn't get hot and catch fire... You could accomplish this with dropping resistors in series with + and - rails, but they may get hot too.

 

[moamps]

Ok, REALLY stupid question and I don't know why I didn't think of this earlier. When I originally installed Q7 and Q8 they were swapped...after I initially turned on the unit and I realized it was wrong I took it out and swapped the transistors to their correct location. I did this on both op amps....

Most likely you fried bias diodes D1a  and D1b. Use DVM and test them.
http://en-us.fluke.com/training/training-library/test-tools/digital-multimeters/how-to-test-diodes-using-a-digital-multimeter.html
Also check output transistors as diodes.

 

[Buffstuffbuddha]

Ok I've got some new transistors coming. They're all wide open haha.  Is it imperative to have the diodes out of the circuit to test? At this point I know it's kind of silly to be buying components for a kit that only cost like 12 bucks but I hate feeling bested. 

 

[Buffstuffbuddha]

I was getting some wacky readings from the diodes so I just ordered those too. What a huge pain this turned out to be all because I didn't pay attention haha. Moral of the story is stop working on stuff when you're tired.

 

[PRR]

> Moral of the story is stop working on stuff when you're tired.

That is a very valuable lesson.

And I think you got it quite inexpensively. (Shipping may be more than the parts.)

 

[Buffstuffbuddha]

Ya, parts were like 2 bucks and shipping was $4.99. There's a pretty big supply house like an hour from me but I figured that would cost me more than $5 in gas. I think the bigger pain was just trying to get the components out of the board and clearing the holes since everything is packed so tightly on this board. Let's hope this does the trick because I've got 2 more pres to build and restoring my confidence would be a huge plus haha 8) :-[