Phase Linear 300 (series2) output transistors.

[DigitalJunkie]

Hey guys,

So I have a Phase Linear 300 Series 2 here on the bench,symptoms were no output,and no output relay 'click'.

I opened it up and quickly noticed some burned resistors in both channels,a zener shorted in one channel,and a pair of the outputs shorted in the other channel.

So,long story short,I have one channel working with the original outputs,etc.
These output transistors are 2SB706A and 2SD746A,which are no longer made. I did source some on the 'net,but at $30 each,I think we'd be better off to just replace them all with something new (better/cheaper).

Which brings me to my main question, What would you suggest as replacements?
Quiescent rail voltage is +/-64V,or so,and there are two pairs of outputs per channel.

So far on my post-it note I've jotted down,
MJL4281A/4302A
MJW21193/21194
MJW21195/21196

Does anybody have a favorite,or a part they'd like to add to the list?


Second question: What should these amps be biased at?
At first check,there was no bias. I've got the working channel idling at about (7.5mv/0.33R) ~20ma/device,or so?


I have a schematic that I've tweaked to match what I have in front of me.
http://i199.photobucket.com/albums/a...eLinear300.png

I'll attach the original PL300 (non-series 2) PDF to this post.

[Tajzmaj]

Hi
I would go with MJ15024/25
I installed 15024 at my 700b and it is ok. Don't go with some ultra modern and ultra fast devices because of oscillations.....Maybe you will be forced to install some b-c caps to the outputs.....check with scope.
Bias...hm Probably something from 30-50mA should work but again I would check it by scope.....
Regards, Taj

[jaycee]

Look at Jan Dupont's Lynx power amp, which is a modernised version of this circuit.

[DigitalJunkie]

Originally Posted by Tajzmaj

Hi
I would go with MJ15024/25
I installed 15024 at my 700b and it is ok. Don't go with some ultra modern and ultra fast devices because of oscillations.....Maybe you will be forced to install some b-c caps to the outputs.....check with scope.
Bias...hm Probably something from 30-50mA should work but again I would check it by scope.....
Regards, Taj

Are those TO-3 transistors? Do they come in TO-3P?
I need TO-3P(TO-263),TO-247,or similar.

Also,I will go dig up the Lynx. Thanks for the input guys,keep it coming!

[Tajzmaj]

Ah sorry I checked some strange old daata sheet for original units and it seems to me that there you have to3 devices...uh
So MJL21193/4
And check what Jaycee suggested. I'm not familiar with that amp but every information is worth to check...
Regards, Taj

[DigitalJunkie]

Hey Taj,
Here's a pic of the original (shorted) transistors.
I'm not quite sure what this package style is,curiously these have 2 Collector leads. With a bit of lead-bending I think I can get some TO-263/TO-247 transistors to fit.

http://i199.photobucket.com/albums/a...0/PICT0001.jpg

[Tajzmaj]

Oh my God....vintage....uh
There you will have a lot of mechanical work to repair it.....
Those MJL21193/94 should work fine. They are not too fast and will handle at least so much of current, power,voltage.......
Maybe there you can stick in some big plastic units from San Ken too but it is probably not worth.
You can find MJL ON Semi units cheap there in the US.....and those are excellent devices.
Of course you must replace all transistors at both channels to get same performance...and re adjust idle current....
Maybe than you can make some sot of metal plates with 2 holes to mount new transistors if you want to avoid making new holes.....
Best regards, Taj

[sakis]

stay close to relatively slow transistors or as close o the original ....too fast transistors will take you to oscillations... opposite might do the same ....replace the op amp with something too fast with slow outputs ....

point is that beyond speed considerations be sure that construction pcb and layout of the amp will not support F1 speeds

at this age of course you need to take a look at all small electrolytics... you need to verify that local stabilization of the low voltage of the IC is working properly and efficient enough .

Finally depending on the year of make and model there is a chance that the legend used resistors that drift from original value if exposed to excessive heat or moisture ... check carefully that also its critical ...

Kind regards

sakis

[Tajzmaj]

Exactly my point....from Sakis as well. Don't go with something fast....you will create nice transmitter....or it will looks like that it is everything ok but you will be surprised what will happen at clipping.....so something nice and slow....hehe
Regards, Taj

[DigitalJunkie]

Indeed. I read a similar post here that mentioned the MJW2119x's as a good candidate for these older amps,because of the speed issue.
Maybe I'll go with those? I'll have to look at the datasheets.

Any thoughts on using those?