Obsolete Motorola Transistor Replacement

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have a Tapco CP500 power amplifier with bad output transistors. The transistors are Motorola SJ9017 and SJ9018. I've Googled them and found no spec's. NTE offers replacements but they are expensive and I'm not thrilled with using NTE anyway. I contacted ON Semiconductors and they said these devices were special and no data sheets are available.

I have no problem replacing the SJ's with another device but without spec's, I'm not sure where to start.

Can anyone help me out?


Hi Kirk

There are several transistors which might replace these. Most of the likely candidates (MJ15003/4 etc) are likley to be an improvement.

I have no idea what your originals were; but Motorola made many devices, so the starting point would be what voltage is your amp. operating at; and what output power?

Many older transistors might have fT's of 4 Mhz, and a pretty safe bet replacement might be MJ21193/4 as these also have 4 MHz fT.

As a guess I'd say your devices might have been 150W, 80V rating, in which case the MJ21193/4 would be ideal. Most of the older transistors had fairly signficant second breakdown de-ratings, anyway - such as starting at half the max. Vceo but the MJ21193/4 will sustain 80V at full power....

Hi Kirk

MJ21193/MJ21194's should be a good sub.

They have fT's of 4 MHz and good gain linearity. I presume your amp. ratings were for 4 ohms - +/- 60V would be suitable only for 100-150 W into 8 ohms.

You may not need to parallel four devices - each MJ21193/4 is rated at 250W. MOtorola built some devices at 200W but they would not have had the second breakdown, I think, of the MJ21193/4's.

Two MJ21193's and 4's per side would be a minimum for 4 ohms, but you could use three or replace all four transistors with a fair degree of overkill power to hand. Adding too many transistors increases the junction capacitance without much additional benefit.

Hi Kirk,

Real substitutes are 2SD555 for SJ9017 and 2SD600 for SJ9018.
Bad news: these 2SD devices are obsolete too.
The good news: specifications for these 2SDs are available!

Icmax=15A (10A in some datasheets)
Ft~7MHz (14MHz in some datasheets)

The MJ21193/MJ21194 pair should work perfectly. Be careful to pick up genuine ones (Onsemi or NOS Motorola), there are a lot of fake semiconductor in the markets!

Best regards,
The SJ numbers are house numbers Moto put on them for Tapco. They were not likely custom, they were more likely off the shelf Moto parts, just renumbered.

2SD numbers would not have been Moto, but would easily have been alternative parts. You would either find all 2S parts or all SJ Moto parts in one amp. Just as older QSC amps went from 2S parts to MJ parts without otherwise changing the amp.

Those MJ current production parts might have a 250W rating, but please watch the SOA chart for each piece.

If you replace some, do yourself a favor and replace them all, at least within a channel. DOn;t mix the older SJ parts with newer MJ parts. They may not share current so well, and may not track thermally together as well either.
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