Fake transistors?

[spooney]

I've repaired the same alpine MRV-1000 amplifier twice now.The first time I replaced the 2sa1265's in the bad channel and all was well for about a month.After the amp died this first time I replaced the 2sa1265's as well as the 2sc3182's in the bad channel.The amp worked great... for about a day. I'm starting to wonder if these transistors that I purchased are up to spec.The place I purchased them from doesn't specify what brand they are anywhere even on the packing slip.All of the transistors that have failed were purchased from the same place at the same time.Not surprisingly these parts were cheaper than any other ones I could find.Does anyone else here have any experience or comments about www.electronix.com ?Both times when the amp failed it was not even at full tilt or warm at all for that matter.I don't see anything else wrong with this amp except that now that the parts have failed again the 10 ohm resistors connected to the first leg of the 2sa1265's appear to be out of tolerance.I'm kinda just scratching my head on this one at this point. I'd like to buy parts from a more reliable supplier and give it another shot but at the same time i'd rather not throw 15 to 20 dollars in parts down the toilet again.

[spooney]

I have some FJA4210 and FJA4310 transistors here and the ratings are similiar to the 2sa1265 and 2sc3182. Does anyone know if these would make for a suitable replacement?

[Perry Babin]

Those look like a viable substitute. If you try them, you need to set the biasing when cold. Then heat the amp up (by playing it) checking the biasing every few minutes to make sure it's not creeping up. You need to do this until it reaches thermal shutdown.

Pop open the transistors and take a detailed photo of the internals. If you don't have a good camera, a flatbead scanner should do a good job. Post the photo here. It will allow others to compare them in the future.

If there's any demand for it, I could post them on my site. I'd be willing to photograph the ones that I remove from various amps to allow other to compare to suspected fakes.

[dave_gerecke]

Hello Spooney, Mr. Elliot doesn't directly address the transistors you have, at least I didn't see them, but he does give a good idea of how to check out the transistors you have. Here's the weblink:

http://www.sound.westhost.com/counterfeit.htm


Peace,

Dave

[spooney]

Perry I've never had to set the bias on an amplifier.How do I go about doing so?Can it be done with a multimeter alone?

[richie00boy]

Did you not replace the driver transistors?

[Perry Babin]

Looking at the schematic, it appears that it doesn't have bias pots.

To determine if the bias current is remaining constant, you can measure the DC voltage across the emitter resistors. From the ampguts page, it appears that the legs of the resistors are not accessible. Measuring the voltage from the emitter of a 2SA1265 to the emitter of a 2SC3182 (both from the same channel) is the same as measuring across 2 emitter resistors.

Compare the good channel to the channel you repaired. If they're approximately the same, begin warming up the amp and monitor the voltage on the channel you repaired. You'll need to measure the voltage from emitter to emitter with no signal.

You'll need to set your meter to the most sensitive range. If there is any voltage across the emitters, it will be well below 0.01v.

[spooney]

did some more poking around and I see that the 10 ohm base resistors are fine,they were just thrown off because of the bad transistors but I see that the IRFZ44N's in the power supply have died again.The amp was pretty much in this same condition when I received it originally. the power supply fets survived the last blowout but did not make it this time. If the driver transistors are the small ones mounted to the heatsink near the outputs then yes I did replace them. I'll have to order more IRFZ44N's before I give this thing another shot as I only have two on hand at the moment.I'll try to get some pics later today of the inside of those transistors.

[Eva]

Use a 15A fuse (or the smallest size that does not blow during normal use) and you will get less IRFZ44 fried (if any)

[sdoom]

I recommend a 2 Ohm resistor (something around 20W or bigger ) in line with the +12V power line , a 15A fuse needs too long to fry and the IRFZ´s may be damaged until the fuse blows.

If you want to use a fuse dont use bigger fuses than 5A , that will do the job until you fixed the amp to that state it can be really driven into higher power .

For a lot of my repair jobs I used BD249 / BD250 (TIP35 &TIP36 in USA) for analog amplifiers. usually i got those from Texas instruments (cost about 2 USD each overhere). Once I got cheap BD249 (50cents a piece) and they turned out to fail at higher currents so I used those for replacements in smaller korean amplifiers (most of the times the cheap amps already came with cheap transsitors.... )