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Matching transistors




 
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K6JEK
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RF in the shack


« on: August 12, 2023, 07:48:18 PM »

Fixing a transistor amp, an SGC SG-500 with a blown pair of output transistors, Toshiba 2SC2290s. Internet wisdom is replace with matched pair. But holy moly $265 at RF Parts including shipping and tax. Meanwhile, genuine Toshiba parts on eBay -- $80 for an unmatched pair.

Since I have access to a curve tracer and someone who knows how to use it, I'm temped to buy two or even three pairs to find a match among them. I'd still be ahead if I could find two out of six.

But what's a match? What's good enough? Anyone with experience doing this? Would I likely need a lot more than six to find a good enough match?

Jon

PS: The amp uses 4 pairs of these, push-pull, input splitter, output combiner. It does not have individual bias adjustments. It does have per pair current sensors which shut the whole thing down for over-current or unequal currents with individual adjustments of the sensor circuits but no info on how to do it.
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n8fvj
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« Reply #1 on: August 14, 2023, 02:43:08 PM »

When building linear power supplies with multiple transistors they had to be closely matched. The HFE could be over double of the lower gain transistors. Unfortunately, you need matched pairs.
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KD6VXI
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« Reply #2 on: August 14, 2023, 03:24:40 PM »

First.

Nobody is selling any Toshiba bipolar devices, that are real, for 80 dollars a pair.   Those haven't been made since Fukushima, the dies where washed out to sea.  All rf devices of a bipolar nature Toshiba was making are gone.

HG has a close device, but they are NOT exact.  They use different bias take a lot less than the magical point seven volts.  They have been known to take off with simple diode clamp bias schemes.  When set up correctly, they do work, and work well!

If it was me, trash all the transistors and move to mrf454s.
  Still made.  Still available.  Who knows for how long.  SGC even used the 454 in that amp before.

CBers are killing the market for Toshiba bipolar devices.  A couple years ago I bought a couple junk radios with good transistors....  The 2879s where worth more to CBers than the radios where!

As to matching.  I've built hundreds of bipolar amps.  Biased and grounded base.  Grounded base battle box cb amps I didn't worry abouy matching.  At all.

On biased amps I made sure all transistors where from the same beta group.  IE,  single dot red, double dot black, etc.  I never had an issue doing it this way.  Motorola said it is good enough to match this way.  When they said this I was buying so many mrf455 that the sales rep had a MR 455 as a license plate!  He was as Mr 455.  Between us, Palomar and Texas star we where buying thousands of bipolars a month.

Good luck.  Those are great amplifiers!

--Shane
WP2ASS / ex KD6VXI
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WB6NVH
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« Reply #3 on: August 15, 2023, 09:43:36 AM »

I have a 100W SEA HF SSB boat radio that someone applied reverse polarity to and blew up the RF PA transistors.

NOS examples of the bipolars are available at far more than the radio is worth,  Short of parting them out from a junk radio I was pondering just using one of the 100W+ RF PA boards on eBay and grafting it inside the boat radio PA compartment, using the existing filters. A lot cheaper than the original bipolars. 
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Geoff Fors
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K6JEK
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RF in the shack


« Reply #4 on: August 15, 2023, 06:38:39 PM »

First.

Nobody is selling any Toshiba bipolar devices, that are real, for 80 dollars a pair.   Those haven't been made since Fukushima, the dies where washed out to sea.  All rf devices of a bipolar nature Toshiba was making are gone.

HG has a close device, but they are NOT exact.  They use different bias take a lot less than the magical point seven volts.  They have been known to take off with simple diode clamp bias schemes.  When set up correctly, they do work, and work well!

If it was me, trash all the transistors and move to mrf454s.
  Still made.  Still available.  Who knows for how long.  SGC even used the 454 in that amp before.

CBers are killing the market for Toshiba bipolar devices.  A couple years ago I bought a couple junk radios with good transistors....  The 2879s where worth more to CBers than the radios where!

As to matching.  I've built hundreds of bipolar amps.  Biased and grounded base.  Grounded base battle box cb amps I didn't worry abouy matching.  At all.

On biased amps I made sure all transistors where from the same beta group.  IE,  single dot red, double dot black, etc.  I never had an issue doing it this way.  Motorola said it is good enough to match this way.  When they said this I was buying so many mrf455 that the sales rep had a MR 455 as a license plate!  He was as Mr 455.  Between us, Palomar and Texas star we where buying thousands of bipolars a month.

Good luck.  Those are great amplifiers!

--Shane
WP2ASS / ex KD6VXI
Thank you Shane. I was going to fall for the fake Toshiba transistors. Yes. The Moto parts are available now. Sounds like a good idea.
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K6JEK
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RF in the shack


« Reply #5 on: August 28, 2023, 11:59:49 PM »

...
If it was me, trash all the transistors and move to mrf454s.
  Still made.  Still available.  Who knows for how long.  SGC even used the 454 in that amp before.

...
--Shane
WP2ASS / ex KD6VXI
RF Parts and a ham with experience fixing SG-500s (trying to remember his name) suggest an even better idea is switch to Moto SRF 3795, said to be more rugged and "a premium grade replacement for the MRF 454" according to RF Parts. Ever heard of this one? Have an opinion? Cost is about the same as the MRF 454s from MACOM.
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KD6VXI
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« Reply #6 on: August 30, 2023, 05:59:04 AM »

SRF devices are a marketing gimmick.  RF Parts got them to relabel them and made a killing selling them to CBers throughout the 80s to today, as upgrades for Motorola transistors.

Same transistor as the MRF454.  Just from the 'double dot black' hFE group.   Same exact xister.

But they are right.  Saturated power out is higher with the double dot black / SRF devices.

Only real downside is they do have more gain.  Possibly have to lower the feedback resistors from collector to base (in series with a cap) if any instabilities are found.

--Shane
WP2ASS / ex KD6VXI

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