BLY87A Amp' question

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MiXiN
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BLY87A Amp' question

Post by MiXiN » Wed Apr 26, 2017 12:51 am

I've just got hold of a BLY87A Amp', and I was told it would do 10W with 1W input, however, I'm only seeing just over 6W with 1W in at 13.8V.

I've had a brief look at the Philips datasheet for this Transistor, and from what I can gather, it should be doing around 8 > 10W with around 0.5W input.

Is there anything I can alter in the circuit to see this figure, or am I seeing "realistic" figures here for this Transistor?

Image


Last edited by MiXiN on Wed Apr 26, 2017 12:51 am, edited 1 time in total.

Albert H
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Re: BLY87A Amp' question

Post by Albert H » Wed Apr 26, 2017 12:59 am

I always found that these needed quite a lot of base - emitter capacitance. The base to deck choke looks OK, but I'd damp it a bit with (say) 1k or 470Ω (non-inductive!) base - ground. My collector to supply choke would normally be 5 - 6 turns. I hope that you've attached it to a heatsink, too!
Is it meant to smoke like that? :shock:

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Re: BLY87A Amp' question

Post by MiXiN » Wed Apr 26, 2017 1:19 am

Hi Albert,

Thanks for the info' mate.

Yeah, it's connected to an heatsink, it's just not visible in the image. The soldering of the Coax at the input/output is also just a quick lash up for testing purposes - not permanent.

The ceramic disc that's currently between the base & emitter pins is 47pF.

So, as well as change the Collector to supply choke to 5 turns & add, say, a 680 ohm carbon resistor in parallel with the base to ground choke, would you recommend altering the value of the current 47pF base/emitter Cap' or shall I leave that be?

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Re: BLY87A Amp' question

Post by Albert H » Wed Apr 26, 2017 1:40 am

Make your other changes, then fire it up, peak it up and see what you get out. Add another 47p base to emitter, and see if the gain goes up a bit.... Experiment a bit!
Is it meant to smoke like that? :shock:

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Re: BLY87A Amp' question

Post by Analyser » Wed Apr 26, 2017 1:04 pm

I was going to say the same as Albert before scrolling down. You might need about 200pF (or more) base capacitance which should help the input tuning. Also increase the collector supply coil to 6 turns.

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Re: BLY87A Amp' question

Post by Analyser » Wed Apr 26, 2017 1:06 pm

Apart from that, the thing looks very old so the transistor could be half knackered. I'd replace those dirty-looking trimmers as well, with a board that old they can tarnish/ collect dust/ short out and that's the end of your BLY87.

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Re: BLY87A Amp' question

Post by MiXiN » Wed Apr 26, 2017 2:36 pm

Top input, cheers guys.

Just ordered some new trimmers, and already have the other components, & coil wire, so will report back when I get time to work on it.

I didn't notice before, but checking the Philips datasheet for the BLY87A, the circuit I have here uses the exact same components & coil windings as the 175Mhz test circuit.

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Re: BLY87A Amp' question

Post by Analyser » Wed Apr 26, 2017 4:06 pm

Just noticed your ouput coax is solder in the wrong place. Take it from the junction of the two output trimmers, not after the coil.

Also try changing output coil to 4t. dia 6.5mm and collector coil to 6t, 6.5mm

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Re: BLY87A Amp' question

Post by Analyser » Wed Apr 26, 2017 4:12 pm


MiXiN
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Re: BLY87A Amp' question

Post by MiXiN » Wed Apr 26, 2017 6:18 pm

Top guy, Analyser, cheers.

The Amp' came with a solder blob at the point where I connected the coax inner, so I automatically thought that's where the output should be - so well spotted, as I failed to see the correct point.

Looks like we're all sorted now, so as soon as I get things fixed up I'll post again. :tup

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Re: BLY87A Amp' question

Post by radium98 » Sat Apr 29, 2017 7:35 am

Just noticed your ouput coax is solder in the wrong place. Take it from the junction of the two output trimmers, not after the coil.
Analyser is right man

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Re: BLY87A Amp' question

Post by MiXiN » Thu Aug 10, 2017 4:09 pm

Only just got around to seeing to this.

I've swapped out all the trimmers for new ones, and now connected the output coax to the pad that's shared by the trimmers and I'm seeing just shy of 10W with 1W in.

There was also a 0.5W 10R Resistor connected to a Ceramic Disc Capacitor that was blackened, so I put a 10R 1W Resistor in its place.

For the extra 1 or 2 Watts I guess it's not worth fettling with the Base Emitter Capacitance or altering the Collector inductor?

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Re: BLY87A Amp' question

Post by Albert H » Thu Aug 10, 2017 8:13 pm

You might be able to get a shade more gain by messing with base to ground capacitors, but it's unlikely to be worth the effort - 10 out for 1 in is good for an old bipolar transistor as long as it's not getting too hot.
Is it meant to smoke like that? :shock:

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Re: BLY87A Amp' question

Post by MiXiN » Thu Aug 10, 2017 11:43 pm

The Transistor is staying nice and cool, Albert, and the heatsink is just toasty - so no worries touching it.

It's drawing 1.89A at 12.75v.

Just one more thing; What's the maximum input power that the BLY87A can take?

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Re: BLY87A Amp' question

Post by Albert H » Fri Aug 11, 2017 3:07 am

You're running it slightly beyond its official rating - maximum dissipation is supposed to be 17.5 Watts, with Pout=8 Watts. Collector current is meant to be 1.25A maximum. I've used them at around 10 Watts to drive a BLY90 to a little bit over 50 Watts, back in the bad old days of 12.5V bipolar transistors.

I built a series of rigs with BSX20 oscillator, BSX20 buffer, BSX20 doubler, a second BSX20 doubler, a BSX20 amplifier to about 100mW, 2N4427 for a Watt, BLY87A and a BLY90 final. The oscillator ran at ¼F to be well within the range of cheap logic ICs for the PLL. In those days, prescaler ICs that would work in Band II were expensive. Each doubler had two critically coupled tuned circuits and the output was surprisingly clean when correctly aligned. The PA had a HUGE heatsink since the amount of heat generated by the driver and output stage was enormous. The supply leads had to be pretty heavy duty, too!

I used diode programmed 74HC CMOS logic - a 74HC4060 for the crystal oscillator and divider, a 74HC4040 for the preset divider, and a phase comparator made from a 74HC7474 and a 74HC7400. There was a passive loop filter, and the phase comparator gave solid locked / unlocked output which was used to enable the PA power switch - the link receiver would switch the PA on when the link was established.

This style of 50 - 60 Watt rig was used a lot in the late '70s and early '80s.
Is it meant to smoke like that? :shock:

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Re: BLY87A Amp' question

Post by MiXiN » Sun Aug 13, 2017 1:00 am

Nice one, Albert.

I bought a few BLY87A off eBay for peanuts a few Months ago, so if this one fries I'll just replace it. It runs cool enough though, so it should be alright.

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Re: BLY87A Amp' question

Post by thewoodstarr » Sun Aug 13, 2017 5:25 am

MiXiN wrote:
Wed Apr 26, 2017 12:51 am
I've just got hold of a BLY87A Amp', and I was told it would do 10W with 1W input, however, I'm only seeing just over 6W with 1W in at 13.8V.

I've had a brief look at the Philips datasheet for this Transistor, and from what I can gather, it should be doing around 8 > 10W with around 0.5W input.

Is there anything I can alter in the circuit to see this figure, or am I seeing "realistic" figures here for this Transistor?
That is a very old 8 Watt device. I notice you have no filtering. No your be alright!

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