Transistor substitution in GAR 2520...

[ben_allison]

I got the GAR2520 kit, and accidentally mixed up my BD139 and BD140. Desoldering from a through-hold PCB like this one is almost impossible! So, I had to cut them out.

I went to a local store to try and get replacements, but they could only offer substitutions:

BD139 > NTE375
BD140 > NTE374

Are these viable subs? Here are the datasheets:

BD139
BD140

NTE 374
NTE 375

Thanks guys!

 

[shabtek]

where are you?
these are like $.05 parts
I did not bother to cross check data sheets.

 

[ben_allison]

Toronto. I picked up the substitutes because finding what I want locally is almost impossible, and I figured I'd snag them in case they do work as it would save me having to order online and waiting 2-3 weeks for the parts to arrive.

 

[dbeeson]

Hey, there's a place near College and Spadina that should have it.  I don't remember the name of it.  Also try active surplus, they should have them also.  I asked this same question in the classic api support thread but then found the parts at sayal electronics.  Which is also in your area.  Did you swap the two by accident and fry them? That's what I did  :

 

[ben_allison]

I went to Sayal, and they didn't have the Fairchilds, just the NTE "equivalents."

The place downtown is Creatron. They'll probably have them. We'll see!

What happened was I soldered them into the board, then realized the mistake. But a three legged device is almost impossible to remove from a through-hole plated PCB!

 

[okgb]

Electrosonic is close to you no ?
if you cut the lead as close as possible to the board
either try a solder wick / braid
or add some solder then suck it out while the iron is still
on it , need to work fast of course

 

[QUEEF BAG]

1    i'm struggling against the urge to dis NTE and say if there is a "real" part
     available, use it instead.

2    3 legged device?   try an unsocketed large scale chip - yeow!
     
     get a good desoldering tool  - a full on station would be great, but
     a nice manual solder sucker will do.  don't even bother with that stuff
     from radio shack and the like,  two of the ones i like are the Paladin 1700
     or 1701 (both the same model, the 1701  has an antistatic tip),
     and the Solda-pullit SS-350.  
     both are a little harder to find than some, and 20 dollars instead of 7 but
     well worth it.
     trying to use a cheap solder sucker pissed me off...and damages circuit boards.
     
     or don't suck out the solder and try to heat up all three pins at once and  pull
     thru from the other side.  the wet solder will lubricate the holes, and also it will
     melt through to the other side quickly. sometimes if you only partially clear the
     holes and there is only a little solder on the other side still, ya got to hold the pencil
     on stuff to long before the heat works it's way over there. but by then pads are ready
     to lift and the component is well done.
     



     sorry if my english is not so good...i think in pictures

 

[bobine]

"Did you swap the two by accident and fry them?"

Me too!  If you stand your Lunchbox on end and switch it on, they'll fall out by themselves!  ;D

 

[ppa]

You can not use these NTE because they have a too low ft.
With this NTE the 2520 could have instability.

 

[Walrus]

You can not use these NTE because they have a too low ft.
With this NTE the 2520 could have instability.

I'm a repairer not a designer, so I don't understand why the NTE with an ft of 140Mhz would cause instability when the BD has an ft of 190Mhz?
Surely if a device has a higher operating frequency range it would be more likely to be susceptible oscillate?

 

[ppa]

the NTE 375 datasheet in the link posted here says that the ft = 8Mhz not 140 Mhz, even if the ft value in the datasheet is at 500mA (so at 10-15mA it can be 2-3 times more high) it so much below of 140Mhz.
The datasheet is original from NTE.

But the problem is NOT only the Ft:

1- the Cbe could be much higher in the NTE (as likely it is), and this is very important value for the stability of the 2520.

2- the polariztion system is designed for the BD139 and BD140 could give a different quiescent current for the NTE's (as likely it is).

3- the hfe in the NTE could be lower than the BD's (as likely it is).

The 2520 is a 4 stages opamp so it required much care to compensate it than many other opamps so change only a transistor type in it change often its stability margin.

I have designed a version of 2520 using different transistors and I have had recompensate it so i know how it is sensitive.





 

 

[Walrus]

Sorry, I just looked at the NTE374 which is 140MHz.
I can understand the other factors you mention, it's just I couldn't see why a transistor with a lower operating frequency range would be more likely to be unstable, which is all you said.

 

[ppa]

yes, the NTE374 has 140Mhz of ft, but the NTE375 not, because it has only 8Mhz of ft.
Infact, the problem isn't the NTE374 but the NTE375, like I've written in my, I was referring to NTE375  
Moreover, if NTE374 works with the NTE375 there are problems of stability the same.
The matter is that the NTE374 is the complementary of NTE373 not of the NT375.
We are not speaking of the change with the NTE373 and NTE374 but with NTE374 and NTE375

 

[ppa]

it's just I couldn't see why a transistor with a lower operating frequency range would be more likely to be unstable, which is all you said.

I've only written that the 2520 is likely instable if it uses the NTE375 in its final stage.

 

[ppa]

Since I'm here for helping Ben I think that, with the couple NTE373 and NTE374, likely the 2520 runs.
I precise that I'm NOT referring to the couple NTE374 and NTE375.
Moreover, the quiescent current of the couple  NTE373 and NTE374 could be likely only 1 or 2 mA max different from the original version with the BD139 and BD140.  
I obviously can not guarantee these allegations because should be done simulations and tests to be sure.


However, I think that the gar 2520 is one of the best API2520 and, in my opinion, sound better than the original.
At this point I council to Ben to use the BD140 and BD139

 

[Jarno]

On this site they also mention NTE184 and NTE185 as equivalents. Personally, I wouldn't go for equivalents and buy the BD parts, like someone else mentioned they are cheap as chips. Does it really take 2-3 weeks for parts to get to you? Is it customs that's causing the delay? Small parts from the US routinely take about 5-6 days to europe.
I think I ordered from this store, but not these BD's, I think I got J201's from them. I do see now they don't have the BD140, that's a shame.

Did see a listing from someone offering 5ea of the BD139 and BD140 for 1,40Euro but he's in Germany and only ships to europe (maybe if you ask nicely) BD pairs on eBay.DE. I've ordered from him and it is a good seller.

 

[dbeeson]

Hey Ben,
Sayal does have the transistors but they come in a kit along with a few other numbers.  It's like $25 for the kit (like 100 transistors) but considering you're going to pay $4-$5 for EACH (that's right folks) NTE equivalent, it's not that bad.  And as some are stating, NTE is not an equivalent in this case.  Sayal should have the kit in stock.  Are you downtown? 

 

[ppa]

I have a lot of BD140/139 because use them in my opamps I could give out one or two couple of these BD for free only Shipping at 1.5 euros with priority mail.   

 

[ben_allison]

Wow, great info guys!!! Thanks for the thought/feedback.

I'm going to get the necessary transistors from Gary, so all's well!

Thanks again all!