MASTR II Power Amp Repair
By Ray Vaughan KD4BBM
Here are some pictures from inside the Power Amp before and after a recent PA repair. The symptoms were weak signal. It tested at about a 1/10th a watt. But the power supply voltage dipped from 15 to 13 volts when the PTT was applied. So the PA seemed to be drawing current. Normal current draw with weak output means the transistors haven't blown yet, but the RF isn't getting out. This is a very common failure mode for this PA.
Inside the 100 Watt PA. To make things more visible while on the bench, the PA is upside down from what you're used to.
The problem is very common to the MASTR II repeater PAs. A solid jumper breaks between the PA board and the harmonic filter (box in the lower left in the picture above). Look for a tiny crack in the solder between the two screws.
With the cover of the harmonic filter removed:
The crack is more visible here. I found a small strip of metal under the solder that had been used to jump between the boards. It worked for a while, but it eventually failed at the right end of that metal strip. There was pitting between the bottom of the strip and the PCB land on the board on the right.
Here's a close up of the same jumper after the repair. It's hard to see from above, but there's a small jumper of solder wick in a little hump, like a small upside-down U. The hump is still flexible since the solder is on both ends, but didn't suck into the middle. You can use needle nose pliers on the middle to act as a heat sink to keep the center too cold to draw in solder. Now when the boards expand and contract, the flexible solder-free gap will flex without breaking. The upside-down U needs to be small or it could act like an inductor. Don't go any wider than the existing circuit board traces or that could also effect the impedance. After this picture, I cleaned up the excess flux.
Here's a view from the side to give you some idea of the height of the jumper. It's more of a ripple than a hump.... just enough to allow expansion and contraction.
After this modification, the Amp could make up to 140 watts. It was set to 90 watts. My thanks to Chris Jensen for his help with the troubleshooting, removal and installation of the Power Amp.
Additional Suggestions from the mailing lists:
The problem you and many others have experienced is expansion and contraction of the PC boards due to the warmup / cool down cycle. It eventually causes the crack you documented. One additional modification that I have found quite helpful is to add washers under the two adjacent screws and then solder the washers down to both the PA and filter boards while the screws are backed off (loosely left in place to 'locate' the washers). This modification holds the distance between the PA and filter boards constant over temperature thus relieving the mechanical stress placed on the jumper.
Ed Yoho WA6RQD
You mentioned using needle nose pliers on the middle of the braid to act as a heat sink to keep the center too cold to draw in solder... you can just put a drop of light clear oil (like "Three-in-One" brand oil) in the center of the hump while you solder it. And lastly, depending on whom you talk to this mod is sometimes called the "omega" mod (from the shape of the greek letter omega) or the "inchworm" mod, and is applicable to low band, high band and UHF amplifiers.
Mike Morris WA6ILQ
Copyright 2003 Ray J. Vaughan, KD4BBM, All Rights Reserved.