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.
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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 ...
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.
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.
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.
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?
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.
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.
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 ...
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.
Why not ? Short of being counterfeits ?
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 !
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.
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.
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.
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.
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 ...
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