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/diy/ - Do It Yourself

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906838 No.906838 [Reply] [Original]

What advice might you offer me on finding replacement transistors? Tell me if I'm wrong on any of the following and post your choice databases.

I've heard that Ueb, Uec, Ucb, collector current, and transition frequency specs can usually be exceeded.
I have also heard both that hFE can be met approximately in either direction and that hFE should be exceeded. What should I do for that and why?
Outside of the obvious polarity, power dissipation, and outline, is there anything else imperative to pay attention to?

Specifically I'm looking to use this in repairing the power amplifier of a stereo, if that's relevant.

>> No.906848

>>906838

It all sounds good. You may want to consider replacing AB amplifier stage transistors in pairs.

>> No.906859

That's a really nice looking receiver, was it a garage sale find? How exactly did you come to the power amplifier stage as the problem? Im just really curious because i found a Yamaha a few months ago on a side walk that was only out putting one channel, and it turned out the solder joints on the balance pot had been broken due to a drop or something of the sort. Reworked the joints and the thing sounds amazing. Have you cleaned out all of the pots via lubricating them, and do all the switches work?

>> No.906862

>>906859
Not op, but transistors are easy to test. I imagine that is how he came to the conclusion.

>> No.906868

>>906848
You refer to AB as in the class of amplifier and pairs being on the left and right channel, yes?
Apologies for my new, just want to make sure I'm getting my terminology right.
>>906859
Flaky power transistor connections caused one of the channels on the power amp to kill itself. Most of the damage was in a single transistor, but there's still some strange voltages showing up, so I was going to replace everything suspect and hope.

>> No.906874
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906874

>>906859
Also not OP, but audio gear is some of the simplest electronics out there to fix. It is an input, and an output. It is literally a matter of injecting a signal into the amp via an input, and probing around to see where the signal fails. It is literally a chain, and you are working out at which link the signal drops. Knowing the order in which to test helps as well.

A typical example (for me at least)

> Powers on? Yes? next step. No? check fuses, power switch, power supply.
> Are all inputs (CD/AUX/TAPE) affected or just one (CD)? Just one? Problem is going to be in the switching matrix, the switch itself, or the preamp for that given channel. Have at it. All inputs are out? Something in the power amp stage.
> Power up, connect speakers, sweep volume from 0 to 10 and back a few times. Hear crackling or hiss as you hear 10? Neat, your main amp stage is likely good, time to check the preamps and EQ stages.
> No whoosh? Something in your main amp stage is likely broken. Again, inject a signal and probe around to find where the signal disappears.

All electronics can be broken into blocks, and all we are doing here is testing each block. I can't find one for a consumer hifi but this TV block demonstrates the same. Assume no audio but perfect picture. Work from your output backwards, testing all blocks until you hit the dead one. In this example you'd be testing all blocks between the antenna and the loudspeaker (inclusive) to see at which point the audio signal drops. There you will find your fault. You would not be checking the sync seperator or colour decoder as these are not in the audio signal chain, and are thus unlikely to be janking out your audio. Again, learn and follow the signal chain and you're not going to get lost testing unrelated circuits.