HELP ALS-600 NO OUTPUT

[KC8VWM]

MRF150 datasheet:

http://www.datasheetcatalog.org/datasheet/motorola/MRF150.pdf

[WB2WIK]

>RE: HELP ALS-600 NO OUTPUT  Reply  
by W5DWH on May 31, 2009  Mail this to a friend!  
Ameritron specs call for up to 100W of drive.<

::Not really.  The Ameritron manual is very clear about the absolute requirement to connect the ALC output of the amplifier to the ALC input of the connected transmitter.  There isn't anyplace where they say it's okay to run without ALC connected.

Under those conditions, the amplifier can accept "up to 100W PEP," because the ALC will limit the transmitter's output to something much less than this, anyway.

The ALS-600 I have here on the bench runs 600W output power with about 35W drive.  If I connect it to my TS-850S with the ALC cable connected (as I have it set up right now), I can just run the Kenwood "normally" without turning its power output level down, because the ALC in the 600 does that for me, and the rig itself never runs more than about 40W output.

The amp is pretty well self-protecting with a few different input signals that can shut it down, including excessive power, excessive reflected power, excessive temperature.  However the input attenuator inside can still be damaged by overdrive if you don't use the ALC judiciously and somehow manage to keep the output, reflected and temperature low enough to not shut the amp off.  I've already seen that happen at least once, which is why the amp is now on my bench, after repairing it for a local who overdrove it and cooked the resistors.

WB2WIK/6

[AC0FA]

Fb, Steve.
The resistors I could trace and easily get to along the RF input path were within spec.

The amp was a used purchase from a local shop.
The shop is currently checking things out under their labor warranty.

They have not completed their inspection yet but their first impression was the finals may be finalized.

I didn't notice any flash bulbs going off and the alc kept the drive below 40w.

If the finals do go out. Typically is it just 1
MRF 150 or do we need to replace all 4.

Is this MRF-150 transistor something a ham can just solder on or does it require special equiptment and alignment?
Erik

[K5DVW]

Usually the MRF150s are sold as matched quads. How much they're really matched or need to be, I dont know. It's anyone's guess if all 4 blew or just one shorted.

Can you replace them yourself? Yes, but it's not easy unless you have a hot air reflow setup. The transistor tabs and PCB traces are very wide and will sink the heat from a small soldering iron. YOu'd be there all day trying to get the solder to flow unless you have a large iron. I've done this before with a Weller iron and it just takes patience.

But... once you replace the transistor, you will  have to reset the resting current on it. All four if you replace all of them.

This is a neat site where a guy built one himself and he documents what he did. It's good reading if you want to understand how that amp works. I think he might talk about setting the bias too.

http://www.k0gkd.com/ssamp.html

[WB2WIK]

MRF-150s are usually about $50 each.  A "quad" is about $200.

There's a 10-pack (new) on eBay right now for $360, which is $36/ea:

http://cgi.ebay.com/10X-Motorola-MRF150-150-Watt-50-VDC-150-MHz-FET---New-_W0QQitemZ220411044356QQcmdZViewItemQQimsxZ20090509?IMSfp=TL090509123002r28276

I find them pretty easy to unsolder or solder.  You have to unscrew the two machine screws that hold the transistor against the heatsink *first* before even trying to unsolder one, or you'll damage the transistor and probably the circuit board trying to unsolder it.

But once it's "unscrewed," I find a regular 60-65W soldering station with a 1/4" blade tip will do the job okay.  Same process when you go to "install" a new one: You can align the screws and turn them in a few twists to hold the transistor in the right location, but never tighten the screws before soldering all four of the tabs.

WB2WIK/6

[AC0FA]

Ok,
Finals were finalized. Appeared to have gone out in pairs. The first pair the bias was real high. The other pair had a short from the gate to the drain.
Or some other from of kaboom.

The ALS-600 appears to have a seperate bias setting for each MRF-150.

The shop was un-clear if ordering they would get a matched quad. Two matched pairs. Or if they just took the transistors out of a big bin at the factory and set the bias individually.

And what if they were to send a randomly selected transistor that does not want to align with the others
to get full output. Be linear.

What if the amp had other problems that caused the transistor to short.

Shop decided to send the amp to ameritron they most likely have a wide range of MRF-150s to choose from and if they make a mistake we are not out $50.00.

They might have more sucess with alignment and output.  

There may be some minor changes that a factory tech could make to cure known problems with the original design.

The power supply seems to be ok but shipped it along just in case.

Ameritron will waranty the repair.

I am just required to cover the 200-250 in parts.  

Made sense to me.
Erik