Harris Z FM FET replacement

FM does it with frequency!
Post Reply
User avatar
OnAir
Posts: 2
Joined: Wed Feb 06, 2008 1:27 pm
Location: Vancouver, BC

Harris Z FM FET replacement

Post by OnAir » Fri Jul 12, 2019 9:08 am

It there anyone in the group that has experience replacing Z series PA pallet FETs? Harris recommends that you let them do it as it requires specialized test gear to align, etc, etc... Surely it can't be that complicated. The physical replacement appears straightforward. What I'm curious about is the tuning / alignment procedure. Can anyone offer any advice? Thanks.

grich
Posts: 451
Joined: Fri Sep 02, 2011 11:19 am
Location: MP89.5, Mason City Subdivision

Re: Harris Z FM FET replacement

Post by grich » Fri Jul 12, 2019 10:03 am

I've only had one Z PA failure, and I didn't fix it myself because I didn't have the time, but Paul Thurst's broadcast engineering blog has details on swapping Z PA FETs. I don't believe he had to align anything.

One thing to be mindful of is the age of the transmitter. Harris changed the FET supplier early in the life of the Z series. Modules (-001) with the older FETs (I think Motorola) cannot live in the same Z-plane as the newer modules (-002, -902) because of phase differences. You'll know it too, ISO temps will skyrocket. That was my case when the factory-exchange module was installed in this particular Z5. Walter Freeman at Gatesair told me to just put the new module in the IPA slot where it could live in isolation. I put the old IPA into the open PA slot, and all was well.

The big issue is sourcing replacement FET's that aren't cheap Chinese knockoffs.

User avatar
Dale H. Cook
Posts: 903
Joined: Thu Dec 20, 2007 9:08 am
Location: Roanoke/Lynchburg, VA
Contact:

Re: Harris Z FM FET replacement

Post by Dale H. Cook » Fri Jul 12, 2019 10:08 am

OnAir wrote:
Fri Jul 12, 2019 9:08 am
The physical replacement appears straightforward.
The one thing that may be critical in the physical replacement is the torque. If the manual doesn't specify replacement torque I like to measure it. I use an analog dial inch-pound torque wrench with a follower needle. The follower needle shows how much torque it took to break the fastener so that it can be correctly re-torqued when installing the replacement MOSFET.

Another thing that I like to do when replacing power MOSFETs in transmitters is to use a high-quality heat sink compound with high thermal conductivity. Because the solid state transmitters that I work most on are Nautel I use Aavid Thermalcote 249G, which is what the Nautel factory uses. A major goal of mine in any repair is to return the transmitter, as far as feasible, to like-new condition.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

DaveSt
Posts: 49
Joined: Mon Jul 23, 2018 2:21 pm
Location: UK

Re: Harris Z FM FET replacement

Post by DaveSt » Sat Jul 13, 2019 2:39 am

There is some calibration needed, not least of which is the bias. It is better to send it back rather than risk killing expensive MOSFETs.

radio_guru
Posts: 127
Joined: Sat Oct 02, 2010 11:23 pm
Location: Illinoid

Re: Harris Z FM FET replacement

Post by radio_guru » Sun Jul 14, 2019 8:47 pm

As much as I hate to write (or have written) an oversized/inflated check to Harris/Gates/GatesAir, we have always returned the pallets in pairs on a heat sink to the OEM for reasons previously outlined as well as scale of economy (cheaper by the pair). Particularly the bias matter.

In another part of my life, I repaired MSF5000 PA's and I spoke to the designers of those amps. They advised each FET needs compensation at construction to minimize oscillations and work in their hybrid splitters and combiners. And a standard pallet only gets them in the close range where precision and automated compensation occurs using laser etching of certain traces (precision changes in capacitance). I can't say whether the same is done for the Z pallets, but I can imagine something close given the sheer number of pallets out there and something of a similar design...just 50 or 350Mhz lower...

Then there is the matter of torque. Nautel advises 6 INCH pounds of torque on their pallets. Yes---INCH/Pounds. There are 16 inch pounds to a foot pound. You would be surprised at how little 6 inch pounds are. There is no torque spec on the Gates/Harris pallet hex screws. I bought an inch/pound torque tool on E-bay. Even if it's off by a couple inch/pounds, I can at least say it's close. Anyone servicing SS TX's should carry one in their tool kit.

I need to look at a couple of my PA's for the change in FET matter as their reject loads are running hotter than the others by some amount. If that's the case, then they need to be marked as "IPA only". And these are older Z series TX"s ( 3 sided back).

RG

kcbooboo
Posts: 565
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: Harris Z FM FET replacement

Post by kcbooboo » Mon Jul 15, 2019 4:04 am

"Nautel advises 6 INCH pounds of torque on their pallets. Yes---INCH/Pounds. There are 16 inch pounds to a foot pound."

There are 16 ounces per pound but only 12 inches per foot.

Perhaps you mean there are 12 (twelve) inch pounds to a foot pound, since there are 12 inches in a foot.

Bob M.

User avatar
Dale H. Cook
Posts: 903
Joined: Thu Dec 20, 2007 9:08 am
Location: Roanoke/Lynchburg, VA
Contact:

Re: Harris Z FM FET replacement

Post by Dale H. Cook » Mon Jul 15, 2019 6:57 am

radio_guru wrote:
Sun Jul 14, 2019 8:47 pm
I bought an inch/pound torque tool on E-bay.
I use an American-made low-range torque wrench which cost about $80.00 and came with a calibration certificate.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

radio_guru
Posts: 127
Joined: Sat Oct 02, 2010 11:23 pm
Location: Illinoid

Re: Harris Z FM FET replacement

Post by radio_guru » Mon Jul 15, 2019 3:21 pm

Correct....12 inch pounds per foot pound. Wrong conversion....

:shock: :shock: :oops: :oops:

Post Reply