transistor substitutes>?

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kepeb

Well-known member
Joined
Sep 4, 2010
Messages
581
Location
Norwich, UK
does anyone know of a site or program that can identify suitable substitute/ replacement transistors?
i have all the specs of an out of production transistor (1024y) but i'm not sure where to begin trying to find an appropriate match.
:(
 
Hello,

NTE has a cross reference webpage.....

http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/
 
not bad, result is very limited tho.
is there one like that that looks at other brands beside nte?
 
yeah, it's only for NTE replacement parts. There was a result for your part tho...I'd give it a go.
 
yea  :( that ones to66
I'm looking for a regular little plastic one to fit in the old space.

1024y.jpg


im just not familiar enough with transistors to recall similar properties when i see them.
In the past i've sought help on forums by just asking 'what transistor can i use instead?' etc.

the answer would be nice :) but i was also curious about how people go about this... i've read what i can find and trying to match the gain, package max current , similar breakdown voltages, power dissipation.
there are simply so many, and the numbers/manu codes are seemingly random and nothing to go by. i just dont know where to start.
i must be going about this the wrong way

 
Ideally you would also look at the demands of the circuit where the transistor is used. No sense busting your butt, to find a 70v transistor if the circuit only needs 40v. No need using an expensive metal package, if power dissipation is modest. Of course it takes some understanding of the circuit to know what parameters matter and which don't.

It is always easier to use the same part for repair, but not necessary as long as the substitute is equal or better for the actual parameters that matter. 



JR

 
The outstanding specs in your table are 150V and 1000mW (1 Watt).

That's not a 9-cent part. The fact they used a $1 part suggests it "needs" to be this beefy. The fact that it failed(?) says it maybe should be beefier.

This is a video output stage, isn't it?

If it is a video-amp part in a 48V Phantom regulator, that's different.

TIP47-TIP50 is a beefier part in TO220 shape, but has lower gain and is slower.
 
hey,
cheers for the replys.
yea i thought about subbing the nte part but was a little concerned i'd get too far of track... i.e. the nte is within 5% difference on some specs of the original, then the sub for the nte is within 5% of that. pretty soon i'd have a 100% different part :)

the part is for a power amp 'alesis ra 500' i'm over my head with it really, i have replaced most everything in the offending section even though they checked good. as per the advice on another forum months ago. (to ensure any damaged or overworked components were replaced just in case) the power transistors, diodes and 5watt resistors have been changed out for new ones.

th schem is available electronics-diy.com/pdf/Alesis_RA500_Power_Amplifier.pdf easy but this is the section im looking at...
ra500-1.jpg

the part in question q115 and q116 (c4793) were the only parts i didnt replace.

i powered it up with a lamp in series because that and a dmm is pretty much my equipment.
still nothing on this channel.

from what i understand the jack was removed while it was on and shorted the output.
this section had burnt components. i thought this would have sorted it tho.
i would normally only want to replace the bad parts only but it seems like others could have been stressed at the time of failure and being a stereo amp i would have like to keep the channels closely matched.
maybe i could replace the right side channels transistor at this section too so they are both still the same.

so what are the qualities i should be focusing on for replacements?:

more current gain than original at least
equal c - e voltage
equal c - b voltage
equal or higher power dissipation

i cant figure out what the important numbers are for it in this position.
 
> a dmm is pretty much my equipment.

A BRAIN is the most important piece of gear.

Use the DMM as an input to the brain.

With lamp-limit, what are the DC voltages at +Vcc, -Vcc, output, and S+ S- ?

Ideally the rails are well apart, S+ S- are a hair above/below zero V, and output is zero V.

If the S inputs are way negative and the output isn't, Q115 is one of several possible culprits.

BRAIN! Learn the polarity and magnitude of happy transistor voltages. Figure out which transistors are not working as they should.

> jack was removed while it was on and shorted the output.

The amp is nominally short-protected. It appears to be a recent-enough design that it really SHOULD be be short-proof in all normal stage abuse.

> what i understand the jack was removed while it was on

You mean the plug? Or the JACK was "removed"? If it has been physically traumatized (assume it has!), inspect for phsyical damage. May be almost invisible: bad contact in a pin-header.

The replacement must be over 120, use the 150V value.

The replacement MUST have a similar fT. This is a high feedback amp. Phase must be controlled past 1MHz. The massive output devices will barely do 1MHz, so all other stages must be much faster than 1MHz. The TIP parts I cited will be too slow.

I strongly endorse power amp repair as brain training. BUT this amp is too complicated for a beginner. Call Alesis support. See if they will cross-trade you the entire output module (they usually come out with a few screws and plugs). IMHO, this will be a lot cheaper than your real cost to repair.
 
ok,
well i sent them a mail.
very much appreciate the time and thought in your answer, any suggestions for the best place to start on this 
BRAIN! Learn the polarity and magnitude of happy transistor voltages. Figure out which transistors are not working as they should.

your right, this is a bit complicated for me, i will check the bits you suggested asap tho, didnt think to check the socket.
the rails well apart? and signal 0v... one should be + and one - tho right, all these referenced to ground? just to be sure.
 
These guys say replace with a 2SA1124:
http://www.bdent.com/KTA1024-p/kta1024.htm

And a 2SC2362K:
http://www.bdent.com/2SC3206-p/2sc3206.htm

 
Actually that looks like a pretty simple amp, as they go... straight class AB, no funny multi rails or tricks.

I second what PRR suggested, if you can bring this up on a variac, or it isn't releasing smoke. carefully measure the voltages to see where it goes astray.

The whole amp output stage is a glorified emitter follower so look for the forward voltage drops, roughly .5V for each device so roughly 3V across the whole output stage base to base..  The stuff on the far left side is first a Vbe multiplier to make the 6 diode drops, and just after that some short circuit current limiting.

Don't be intimidated by the number of parts, the main thing that will make it hot and unhappy is if the 6 diode drops are getting too much voltage and running at too much class A current. The safe-coolest setting will be with VR2 set for max resistance, which will be lowest base drive. (That trimpot should be adjusted after everything is working by measuring the voltage drop across some output device .22 ohm emitter resistors (r150- r155). Use ohms law to figure the voltage drop for say 15-20 mA class A current after everything is working.

Looks like a good amp to get started on. 

JR


 
> a pretty simple amp, as they go...

My first real payday was for fixing the Dynaco ST-80s, six transistors.

There's nothing exotic here, true. Just a really big 741. Rail-2-rail gain stage offscreen left, layers of complementary emitter followers.

But too many parts.

And large too-many-stage power amps have gnarly frequency compensation problems. A beefy-enuff part which is too slow OR too fast can turn it into an AM transmitter (which may trash the whole output stage).
 
2SA1024Y is available here for 0.85 euros:
http://www.indipc.fr/connectique,2sa1024y-transistor,2sa,page,zoom,mod,boutique,gps,4934607-2156,recherche,267966.fr.html

They also have the 2SC4793...

Axel
 
PRR said:
> a pretty simple amp, as they go...

My first real payday was for fixing the Dynaco ST-80s, six transistors.

There's nothing exotic here, true. Just a really big 741. Rail-2-rail gain stage offscreen left, layers of complementary emitter followers.

But too many parts.

And large too-many-stage power amps have gnarly frequency compensation problems. A beefy-enuff part which is too slow OR too fast can turn it into an AM transmitter (which may trash the whole output stage).

Perhaps, but the part of the amp shown in his schematic is the simpler compound emitter followers... They don't even use a quasi-comp output stage so popular in slightly older amps.

The GBW of an emitter follower could lead to higher HF distortion, but it would have to be seriously off the page to cause instability. The only devices with voltage gain in that output stage are the current limiting, and they have compensation caps across them. 

In my judgement the larger concern about substituting devices is that Vbe is in the the right ball park (so it will bias up for proper class A current) and you meet voltage and power requirements. Of course you want to use similar parts in positive and negative swings for signal symmetry.

I suspect he still has a bad part somewhere or bad solder connection, if he has already swapped out so many parts. Time to drag out the VOM and see where the puppies are getting out of the pen... Of course be careful when probing around that you don't inadvertently short something out, but that's why they put small points on the ends of probes.

Go for it, what's the worst that could happen?

JR
 
many thanks, good point, dr pepper would agree :D

so, while i'm waiting to turn down the extortionate Alesis price I fear is coming...

vr2 to max resistance
check the voltages across each diode, ok.
and look for a 3v drop across the output stage.. could you clarify where i am to probe to do that?  :-[
also I only have my series lamp btw no variac  ::)

Axel,
Thats awesome, strange it didn't show up in any of my searches. however, I am sadly not very french :(
is anyone reading making an order there soon?
 
@ Kepeb :
I had a look through their website to see if I would need something from them, in order to help you, but no luck...

Looks like they ship internationally though, and they take Paypal, so I guess your best bet would be to order directly from them.
I'll give them a call on Monday to see how we can sort this out, minimum order, shipping,... and to ask if they can communicate in English...

I'll help you with French translation... How many transistors would you need?

Axel
 
Thats very kind of you thanks. :) maybe just see what the min order/postage is.

I guess I only need 1 or 2 of each for this repair.
i'll see if they have anything else i need
 
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