Replacing the PA module on a GTX mobile

[kcbooboo]

I have a GTX mobile with a sick PA module. It puts out about 6 watts at best; the radio draws 3-4 amps, which is about right for that amount of power.

I have a brand new PA module - the exact same part number and made by Motorola. Problem is, the PA module's heatsink is soldered to the underside of the GTX circuit board. I could probably melt the solder and get it off with a small butane torch, but that's not the right way to do it.

Does anyone have the proper surface-mount rework equipment to remove the old one and install a new one? The alignment isn't critical - the holes have to line up and it should be centered in the opening.

Is there another procedure I could use to do it myself (cut the solder with a thin knife blade, heat the circuit board with a gun, etc)?

Contact me via PM if you can do this.

Bob M.

[jackhackett]

Ugh.. low power GTXs are junk. Even when the PA is working 6 watts might be all you'll get. I've seen them anywhere from there up to 12 watts. I rarely see them do the rated 12 though, 8 would probably be average.

When the modules go bad you'll get milliwatts, and they seem to go pretty often even though the module is rated at 20 watts... you'd think they'd last forever running at half power.

As for replacing them... what I've done is unsolder the old one (I use huge soldering iron meant more for repairing your car radiator), clean off the solder pads where the heat sink was soldered to the board pretty good, and put a very thin layer of silver solder on them, it should be just tinned, and smooth.
Then I put in the new module and just solder the pins, put the board back in the chassis and just let the pressure between the module and board make the contact without soldering the heatsink.
That seems to work fine, I've done a bunch that way with no problems.

The high power GTXs seem to be much better by the way, believe they use a transistor output stage.

[wavetar]

Then I put in the new module and just solder the pins, put the board back in the chassis and just let the pressure between the module and board make the contact without soldering the heatsink.
That seems to work fine, I've done a bunch that way with no problems.

As have I...many, many times. Never had a return yet. I think the heat required to actually resolder them to the board does more harm than good.

I will say I've never had a problem getting them to put out rated power once the PA was replaced.

Although generally when they go bad it's in the milli-watts, I have also seen where they will put out a few watts that can't be adjusted, and replacing the PA fixes the issue.

Todd

[kcbooboo]

Well, this one isn't completely dead. I can get the power up to maybe 7 watts at the maximum softpot setting (after RSS complains), and it slowly drops to about 6 watts from there. I can set it to 6 or lower and it's completely stable. It draws appropriately less current as well. I measured the voltages on the brick - they're all where they should be. The control voltage (pin 2) varies with power setting and gets as high as 11 volts, so the module is being told to make lots of power.

I suppose if I change the module and get the same thing, my problem must be elsewhere. I don't have any easy way of measuring the input RF power, nor do I know what it should be, but I suspect the module has about 18dB of gain.

Bob M.

[kcbooboo]

Update: I bought a new RF power amp. After buying an inexpensive heat gun, I tried getting the old PA off the circuit board. My first attempt did nothing.

I then played with the heat gun on a junk power supply board to see how much heat, and how close, it took to melt the solder enough to pull a wire out. I found that I needed close to maximum heat but I had to be within 1/2 inch of the joint I wanted to melt.

So I went back to the GTX, used a small putty knife to divert the hot air away from some other components, and gave it a try. After 5 seconds, one end of the PA module unstuck itself from the board. I then attacked the other end, but didn't have the putty knife in the right spot. After 4 seconds, a small ferrite bead / inductor went flying off the board, because the solder holding it to the board got hot enough and the moving air blew it away. A second later, the PA module was completely free.

I melted some fresh solder on the circuit board pads, then removed as much as I could with a solder-sucker. I scraped the high spots with a knife and cleaned the area with some flux remover. I then carefully positioned the new PA in the opening, soldered the 5 leads, added some heatsink compound to the PA and audio chip, and reinserted the board into the chassis. Tightened all the screws, plugged the front panel back in, and closed it up. Oh yes, I did put that ferrite bead back where it belonged.

I applied power and keyed the mike. 5 watts. Then I remembered that I had turned it down that low because it was stable there. I started the programming software and pushed the output power pot from 40 to 140, and now had 12 watts. I went through the PA replacement procedure and had 12 watts on the 896-902 band and 10 watts on the 935-941 band, and it's all very stable now.

So for about $40, I fixed the radio. I paid $60 for it, but I think I'm still ahead of the game because it could have cost a lot more.

I learned some valuable lessons here. The amount of air coming out of most heat guns is way too much. The hot-air rework stations seem to use a very low volume and a temp around 800-1000F. Various tips on the tool attempt to aim the hot air just around the component you're trying to work on, and with insufficient force to blow it off the board. You need a rather small hot air opening, about the size of most SMT components, or the soldered junction itself.

Thanks to all those who gave me suggestions on how to remove the PA.

Bob M.

[jackhackett]

Good deal, glad it worked out for you :)

A little tip for putting on heat sink compound (a little late for this job, but might help someone else)... I use the razor blade from a utility knife to spread the compound out, since it has a good straight edge it makes a nice thin even coat.

I've been wondering how the stuff for CPUs, like Arctic Silver, would work on a radio...

[kcbooboo]

I did clean the chassis really well after removing the board. Obviously the new PA didn't have any compound on it. I add a dab and smear it around with my finger. As long as there's a coating everywhere on the module's heatsink, it'll work fine. As you said, using a razor blade will leave just about as much as my finger does; in fact if you can see the metallic pattern through the compound, you've got enough there.

If you put on too much, it'll just squeeze out anyway.

The only thing I didn't have was some kind of conductive material to put between the bottom of the circuit board and the module's heatsink, to make a real good electrical connection. Right now, it's just pressure put on by the mounting screws. The board was cleaned by me before installation, and I did consider trying to solder one edge of the heatsink to the pcb, but decided against it.

Bob M.

[wavetar]

Good job, and I doubt you'll have any problems with just using the screws to hold the PA in place. We haven't had a return yet, with about 120 units done so far over the years. As I said, I think the heat required to solder the ground does more harm than good.

Todd

[Tony Soprano]

I can't afford to fix those things in-shop, the module retails at $182, by the time I add labor I'm right at flat-rate. I can fix them, but not competitively.

[wavetar]

The Canadian 'end-user' flat-rate is $421...we can fix them up for under $300...not great, but good enough to make it feasible.

Todd

[Tony Soprano]

The Canadian 'end-user' flat-rate is $421...we can fix them up for under $300...not great, but good enough to make it feasible.

Todd

Christ! That's rough! Our end-user is $287, and I can fix them for just under that. Makes sense now.

[wavetar]

The Canadian 'end-user' flat-rate is $421...we can fix them up for under $300...not great, but good enough to make it feasible.

Todd

Christ! That's rough! Our end-user is $287, and I can fix them for just under that. Makes sense now.

Yeah...Motorola Canada's last flat-rate price change was done in 2001, when the Canadian dollar was worth about a nickel U.S.

Todd

[kcbooboo]

Since you can buy a GTX mobile on eBay for under $100, it's almost not worth sending one out for repair. In my case, I took my chances and got one that was less than perfect. In fact, there were other things that I had to replace on it (volume knob components) too, so someone figured they'd dump it cheap.

Several months ago Motorola told me the PA module would cost about $250, and I was not able to find a suitable replacement until recently. Still a nice learning experience.

Glad I didn't ever try to send one back for depot repair - in either country.

Bob M.

[transistor747]

Bpb. care to share where you got the replacement module?

Sam

[kcbooboo]

I just happened to search for it on Google, and it gave me a hit on eBay. I then searched on eBay and a guy was selling one for $20 (buy-it-now), so I did. Brand new, exact same MHW2821-2 that was in the radio to start with.

As a matter of fact, right now on eBay the same guy is selling 25 for $19 each, and you can get a pack of 10 for $149. Search for mhw2821. Make sure you get the -2 parts for 900 MHz GTXs.

Bob M.

[transistor747]

thanks Bob!