My Transistors, original or copy?

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wg_ski

Member
Joined 2007
Most end up simply giving up and using MJ15024/5 as driver and output. Unless you can find NOS or used 2SB600 or 2SA1117 (and the NPNs). How much do you trust ISC? The innards of their stuff doesn’t inspire confidence…..Finding working pulls of the Jap types is more likely than the 15020 family.

The other option is to retrofit TO-3P’s, which only works in some circumstances. Sometimes there is just no clean way to hook up the collector lead. Damn, I wish they’d put an NJW0281 in a TO-3 can and so does everybody else who repairs vintage amps. Even if they sold them for $25 a pop.
 
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ivoa

Member
Joined 2008
I have temporarilly gave up as I am using MJ15024/25 for drivers until I find originals. Outputs are MJ15003/04. As fT is something that I would like to achieve, this is why I am looking for those type of transistors.

I have opened one of each and here is result:

IMG_20230307_185536.jpg
IMG_20230307_185413.jpg
 
wrenchone

wrenchone

Member
Joined 2004
The die in the picture at the top looks like the proper size for the job - whether it's the actual part in question or a re-marked part is quite another matter. The die inside the part in the bottom picture does not look anywhere near large enough to do its intended job, and would probably blow up wen presented with a substantial load. At least the die weren't coated with that nasty white silicone goo that all the fakers love to use.
 
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wg_ski

Member
Joined 2007
The top one is definitely NOT Motorola - internally it’s simply not their package. Whatever it is, it is probably pretty good. MJ15024 size (200 to 250W) but the fT is anybody’s guess. I’d guess low - Layout looks like a vintage epi-base type. Bottom looks like Motorola 2N3055, or possibly any of their old 100W epi-base types. Probably a 4 MHz fT. You‘re better off leaving those 15024’s in there.
 
wrenchone

wrenchone

Member
Joined 2004
wg_ski -
I haven't made a practice of ripping open my varied TO-3 stock to check construction details, so I will bow to your experience. As I pointed out, at least the innards weren't slathered in that horrible white silicone glue. The really phony device with the logo printed sideways (Motorola never did that) looks distinctly on the puny side. I wouldn't trust either of those transistors in any of my projects. Having said that, though, I rarely use anything but mosfets of vertical or lateral flavor as output devices any more. That's a whole 'nother realm for fakers...
 
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ivoa

Member
Joined 2008
Thanks all for your answers. I agree with all that you sad. I will definitelly not use them and leave 15024/25 for now, unitill I found (if ever) known original MJ150018/19/20/21 or MJ4237/38/47/48.

As fT is of importance, would TO3 replacement be 2SA1117/2SC2608? Or something else? What would be TO3-P replacement since there is room on heatsink to mount it?

Finally, if anybody have MJs for sale, please let me know.

Ivo
 
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wg_ski

Member
Joined 2007
If you could FIND real A1117/C2608 they would be ideal. You might luck out and find some in a dead Jap receiver (or one with pots so bad it’s owner junked it). The D555/B600 has sort of a medium fT (12 MHz) and you might find those as pulls too. If you were going to use flatpacks you could probably use ANY modern output pair. I’d use C5200/A1943, but that’s just because I have them.

That sideways logo isn’t Motorola’s printing, that’s for sure. But if you look at the scrub pattern, the original logo went the other way. It DOES look like Motorola inside. But a 100 watt die, and only 10A bond wires. Probably some old discontinued low value device, like 2N5878.
 
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ivoa

Member
Joined 2008
Well, to summarise, I have very limited options:
-to find original MJ15018/19/20/21 pairs which is close to impossible
-to find 2SA1117/2SC2608, also next to impossible
-to find proper replacement (high fT) in either TO3 or TO218 case.

Here is the picture of heatsink where currently is MJ15025 which should be replaced with better replacement. I am affraid that even TO218 will not fit. I though that it will but when I look at picture I am nor sure anymore:

IMG_20181219_175600.jpg


Square (not rounded) case of 2SC5200/2SA1943 will definitely not fit as hole of transistor goes in hole closer to the edge of heatsink.

So do you haver suggestions for TO3 or TO218 replacement for MJ series with hi(ghier) fT?

Thanks all,

Ivo
 
wrenchone

wrenchone

Member
Joined 2004
If you use TO-247 or TO-3P packages, there may be more room on the heat sink for them than you think. I would try removing one TO-3 device, then laying down the alternate package so that the mounting hole on the plastic package lines up with one of the TO-3 mounting holes. From the top view of the heat sink provided, it looks like you might have a little more room on the top side of the TO3s than the bottom, as viewed from above - correct me if I'm wrong... Connections could be made via jumpers from the plastic package leads down through the holes in the heat sink. If you are lucky, the leads on the plastic package might even align with the proper holes in the heat sink for base and emitter pins.
 
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ivoa

Member
Joined 2008
Unfortunatelly, changing heatsink is not an option since heatsinks are part of the housing. You can see on picture position of heatsinks:

Fotografija0047.jpg


And this is from the inside, or underneath:

IMG_20181220_205926.jpg


So, TO3 or maybe housing with rounded top like old BD249/250 for example, is my only choice:

bd249.JPG


I am not 100% sure that even transistor from picrure would fit.

I hope that you got the picture of problem with space. I forgot to mention and I apologize for my language as english is not my native language:)

My idea was, if unable to find TO3 substitute with high fT, to make this conversion:

to3.JPG


So I am affraid that TO3 or flat rounded package is the only choice.

Ivo
 
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