Transistor Substitution Question

[Chris F.]

Is it possible to substitute a Darlington transistor with a non-Darlington
one? I'm working on a Kenwood amp, it had blown the output transistors
2SD2222 and 2SB1470. I ordered some new replacements and installed them, one
channel worked but the other blew right out again. Having spent $45 on this
amp so far including a service manual, I don't want to gamble any more money
on it. So I was wondering if I can replace those transistors with some
non-Darlington ones I already have. The packages and electrical
characteristics are very similar, but of course this sounds just a bit too
easy.
Thanks for any advice.

 

[Arfa Daily]

Is it possible to substitute a Darlington transistor with a non-Darlington
one? I'm working on a Kenwood amp, it had blown the output transistors
2SD2222 and 2SB1470. I ordered some new replacements and installed them,
one channel worked but the other blew right out again. Having spent $45 on
this amp so far including a service manual, I don't want to gamble any
more money on it. So I was wondering if I can replace those transistors
with some non-Darlington ones I already have. The packages and electrical
characteristics are very similar, but of course this sounds just a bit too
easy.
Thanks for any advice.

--

You're right ! It *is* too easy ... Darlingtons cannot be replaced with
non-Darlingtons, because of the hugely different gain characteristics, which
leads to the requirement for an utterly different bias regime. Also,
substituting almost *any* component in a DC coupled amp, is a recipe for
disaster, and in particular, with a Kenwood ... Sorry ...

Arfa

 

[Meat Plow]

Is it possible to substitute a Darlington transistor with a non-Darlington
one?

No.

 

[Eeyore]

Is it possible to substitute a Darlington transistor with a non-Darlington
one? I'm working on a Kenwood amp, it had blown the output transistors
2SD2222 and 2SB1470. I ordered some new replacements and installed them, one
channel worked but the other blew right out again. Having spent $45 on this
amp so far including a service manual, I don't want to gamble any more money
on it. So I was wondering if I can replace those transistors with some
non-Darlington ones I already have. The packages and electrical
characteristics are very similar, but of course this sounds just a bit too
easy.

You can't.

The idea is to fix the underlying fault before replacing the output devices.
Admittedly you can get away with it sometimes but not in this instance. Those
output devices shouldn't be much more than a couple of dollars each though.

Graham

 

[Michael Black]

Is it possible to substitute a Darlington transistor with a non-Darlington
one? I'm working on a Kenwood amp, it had blown the output transistors
2SD2222 and 2SB1470. I ordered some new replacements and installed them, one
channel worked but the other blew right out again. Having spent $45 on this
amp so far including a service manual, I don't want to gamble any more money
on it. So I was wondering if I can replace those transistors with some
non-Darlington ones I already have. The packages and electrical
characteristics are very similar, but of course this sounds just a bit too
easy.
Thanks for any advice.

Have you considered that it may not be a mere transistor failure? IN other
words, something else went bad, and took the transistors with them. And if
that's the case, the problem won't go away no matter how many transistors
you put in there. You need to figure out if there is something else
wrong. At least you have the manual.

Michael

 

[Charles Schuler]

Is it possible to substitute a Darlington transistor with a non-Darlington
one? I'm working on a Kenwood amp, it had blown the output transistors
2SD2222 and 2SB1470. I ordered some new replacements and installed them,
one channel worked but the other blew right out again. Having spent $45 on
this amp so far including a service manual, I don't want to gamble any
more money on it. So I was wondering if I can replace those transistors
with some non-Darlington ones I already have. The packages and electrical
characteristics are very similar, but of course this sounds just a bit too
easy.

You can sometimes come up with a two transistor combination that will serve
as well. Tie the collectors together and the emitter of the driver goes to
the base of the output. However, some Darlington power transistors have
other items inside the case ... such as resistors and a diode. Those can be
replaced with the appropriate discrete components if you know what you are
doing.

 

[Chris F.]

Have you considered that it may not be a mere transistor failure? IN
other
words, something else went bad, and took the transistors with them. And
if
that's the case, the problem won't go away no matter how many transistors
you put in there. You need to figure out if there is something else
wrong. At least you have the manual.

I went through the output circuits checking for shorts with a the diode
function on a DMM, prior to replacing the transistors. There must be some
kind of fault that won't show up this way, I guess the only solution is to
replace the pre-drivers as well.
I paid about $8 each for the transistors here in Canada, but have since
located a US supplier that sells them for $2 ea. They have a lot of got
prices on semis so may order a bunch of stuff from them. Of course, they
might not be of great quality....
I should have realized the difference with Darlingtons. Why couldn't all
manufacturers just use the same parts, and save us poor techs the trouble
and expense of finding so many different types?

 

[Homer J Simpson]

I went through the output circuits checking for shorts with a the diode
function on a DMM, prior to replacing the transistors. There must be some
kind of fault that won't show up this way, I guess the only solution is to
replace the pre-drivers as well.
I paid about $8 each for the transistors here in Canada, but have since
located a US supplier that sells them for $2 ea. They have a lot of got
prices on semis so may order a bunch of stuff from them. Of course, they
might not be of great quality....
I should have realized the difference with Darlingtons. Why couldn't all
manufacturers just use the same parts, and save us poor techs the trouble
and expense of finding so many different types?

How much is a part for your car?

 

[Jerry G.]

Because of the characteristics of the Darlington, it cannot be replaced
with regular transistors. You must use the proper devices with the
proper specs.

The problem is that you must find the cause of the failure in the first
place, and replace the failed parts. Servicing these amplifiers is a
challange even for the experienced technician.



Jerry G.
======

 

[Jerry G.]

Because of the characteristics of the Darlington, it cannot be replaced
with regular transistors. You must use the proper devices with the
proper specs.

The problem is that you must find the cause of the failure in the first
place, and replace the failed parts. Servicing these amplifiers is a
challange even for the experienced technician.



Jerry G.
======

 

[Arfa Daily]

I went through the output circuits checking for shorts with a the diode
function on a DMM, prior to replacing the transistors. There must be some
kind of fault that won't show up this way, I guess the only solution is to
replace the pre-drivers as well.
I paid about $8 each for the transistors here in Canada, but have since
located a US supplier that sells them for $2 ea. They have a lot of got
prices on semis so may order a bunch of stuff from them. Of course, they
might not be of great quality....
I should have realized the difference with Darlingtons. Why couldn't all
manufacturers just use the same parts, and save us poor techs the trouble
and expense of finding so many different types?

Now there's your problem with a DC coupled amp ... I have had leaky
capacitors in PREAMP stages cause output stage failure. Almost ANY component
from input socket to driver stages, can, and regularly does, cause output
stage failure. If you can bypass the line power switching, you can sometimes
get to the bottom of the fault, without destroying handfuls of transistors,
by using a variac and comparitive voltage checks, if one channel is working.
If you don't have a variac, two alternative methods are to use comparitive
resistance checks between channels, or remove the output transistors from
both channels, and go back to voltage checks. If you do it by one of these
methods, chase down EVERY difference that you find - no matter how small it
seems. If I were in your shoes, I would be checking the driver stages,
including all reistors in them, and between them and the outputs, VERY
carefully. Repairing these can sometimes be a real labour of love ...

Arfa

 

[Eeyore]

I went through the output circuits checking for shorts with a the diode
function on a DMM, prior to replacing the transistors. There must be some
kind of fault that won't show up this way, I guess the only solution is to
replace the pre-drivers as well.

Look for 'burnt out' resistors too. The diode function check is hoplessly
inadequate btw. To check a resistor in parallel with a diode junction, ensure
that the meter polarity reverse biases the junction.

I paid about $8 each for the transistors here in Canada, but have since
located a US supplier that sells them for $2 ea. They have a lot of got
prices on semis so may order a bunch of stuff from them. Of course, they
might not be of great quality....

Why not ? Short of being counterfeits ?

I should have realized the difference with Darlingtons. Why couldn't all
manufacturers just use the same parts, and save us poor techs the trouble
and expense of finding so many different types?

What voltage, current, gain, power dissipation, transition frequency etc would
you have them 'standardise' on ?

You can often substitute with a higher rated part anyway !

Graham

 

[Michael Black]

Why not ? Short of being counterfeits ?

Because he's trying to understand why there's such a big difference in price.
An immediate reason would be the exchange rate between the two countries.
That's less of a factor now than a few years ago because the two dollars
are much closer together, but my take is that we don't see prices here
in Canada fluctuate to match the exchange rate.

There's also a smaller market, so they'd want increased profit to
compensate for having a big stock just lying around.

And since it's a smaller market, they may not buy in as large quantities
as some company in the US, which means they don't get as good quantity
discount from further up the chain.

Finally, a lot of places that might have cheaper prices aren't so friendly
to the hobbyist, or at least to those who only need a few parts. So the
higher price may reflect a company that is willing to go to the fuss
of selling in small quantities, and they want a higher profit to compensate.
Someone buying enough of them, or a large order in total of different parts,
might get a better price because they could deal with a place that isn't
interested in small orders.

It can be a drag to have to buy parts that you don't need at the moment in
order to get good prices. On the other hand, it's often better pricewise
to buy those thousand resistors (or whatever) than pay the single price for
a few resistors. You end up with more stock in the latter case.


Michael

 

[Dave]

Now there's your problem with a DC coupled amp ...

why do you think it's a DC coupled amp? All I see in the description is
"Kenwood".

 

[Eeyore]

why do you think it's a DC coupled amp? All I see in the description is
"Kenwood".

It would have to be pretty ancient to be AC coupled.

Graham

 

[Dave]

It would have to be pretty ancient to be AC coupled.

Graham

Are pretty much all modern amps DC coupled, then, as opposed to capacitance
coupled? My solid state amps are from the golden era high-powered SS audio,
mid-70's to early 80's and both amps and my preamp have capacitor-coupled
inputs.

 

[Michael Black]

Are pretty much all modern amps DC coupled, then, as opposed to capacitance
coupled? My solid state amps are from the golden era high-powered SS audio,
mid-70's to early 80's and both amps and my preamp have capacitor-coupled
inputs.

You are misreading the terms.

IN the days of tubes, pretty much all the stages were coupled either by
capacitor or transformer. A shift in DC in one would not affect the bias
of the next stage, unless the coupling shorted out.

When transistors came along, amplifiers followed that scheme. But it
didn't take long before they were going to a different system, a multiple
active element amplifier where the elements were directly coupled to the
next and the capacitors were only at the input and output. When they
started running them off a split supply (ie plus and minus some voltage)
the output capacitor wasn't even needed, since the idling point was zero
volts.

But in those, if a transistor went bad, or the bias on one transistor
shifted, it would affect all the rest. You were no longer dealing with
a single stage (it's easy to figure out what's wrong when there's only
one active element, and even easy to figure out which active element
is the problem) at a time, you were looking at a much bigger picture.

Michael

 

[Arfa Daily]

why do you think it's a DC coupled amp? All I see in the description is
"Kenwood".

So when did you last work on a Kenwood that wasn't DC coupled ...?

Arfa

 

[GregS]

Is it possible to substitute a Darlington transistor with a non-Darlington
one? I'm working on a Kenwood amp, it had blown the output transistors
2SD2222 and 2SB1470. I ordered some new replacements and installed them, one
channel worked but the other blew right out again. Having spent $45 on this
amp so far including a service manual, I don't want to gamble any more money
on it. So I was wondering if I can replace those transistors with some
non-Darlington ones I already have. The packages and electrical
characteristics are very similar, but of course this sounds just a bit too
easy.
Thanks for any advice.

Any replacement, you have to deal with possible instabilities. You can make a darlington
with two transistors and a resistor. You basically multiply the two gains
to get one.

greg

 

[Arfa Daily]

Are pretty much all modern amps DC coupled, then, as opposed to
capacitance coupled? My solid state amps are from the golden era
high-powered SS audio, mid-70's to early 80's and both amps and my preamp
have capacitor-coupled inputs.

Well, that pretty much says it. From the late 80s on, I think that just
about everythingof half way decent quality that's been made, has been DC
coupled throughout. It produces a far flatter overall frequency response.
Anyway, we're not talking just the inputs here, which may indeed still be AC
coupled even on a modern amp. We're talking all interstage coupling, and
indeed inter-element within stages here. The reason that this gives a big
headache to service engineers, is because of the huge gains involved. A
signal of a few mV on the input, will finish up as potentially several tens
of volts at the output. Thus, a small offset at the front end of such an
amp, which may not even have caused any noticeable problem with AC coupling,
will result in the outputs slamming hard over one way, followed rapidly by
their demise ...

Arfa