CDM Tuner Problem

[ricp]

I was tuning up a VHF 25 watt CDM, doing what I have done on other radios before.
Went top to bottom then got to the auto tune for PA Limits. Set it off and completed as it should
BUT, then I had two problems: 1) it now puts out only 7 watts in either high or low power ( set to 5/25), it did get 19 or 20 before, and 2) when doing the PA Bias the Voltage setup does bring up supply current, BUT the Bias 2 doesn't raise it any. Sounds like a hardware problem, BUT it was fine until I did the AutoTune, so I suspect pot setting, but nothing affects it.
Any ideas ? Thanks
Ric

[escomm]

Whose autotuner are you using for your radio?

[ricp]

I was using the motorola Global Tuner for Pro Radios R02.15.00. Under TxAlign there is PA Voltage limits and the button is labeled AutoTuner. Looks like it is a final automatic step in the tune up process, BUT......
Ric

[Mfire39]

I was tuning up a VHF 25 watt CDM, doing what I have done on other radios before.

I have to ask, do you use a service monitor on any of the radios that you "tune up"?


-Marc

[ricp]

Yes Marc,

The service monitor is THE test equipment to align and verify the radios Rx & Tx performance.

Ric

[Mfire39]

Yes Marc,

The service monitor is THE test equipment to align and verify the radios Rx & Tx performance.

Ric

I'm fully aware of what a service monitor is.. I'm just wondering if you were using one with the tuner software, or just making tuner adjustments blindly without a monitor...


-Marc

[Al]

To the OP, do you know whether you've got the bipolar RF PA or the LDMOS PA? Sure sounds like something went crazy when you told it to autotune, and the tuner caused too much forward bias to be applied to the PA gate. I'd suspect you've got a defective PA if it's the LDMOS board.

[ricp]

Marc,
I must appologize for my phrasing, I didn't mean to offend, I should have written "...is THE test equipment I use to align ...". I use the combination, the "Tuner" I see as sort of the slug and cap tuning tool. Yes I am that old. And the SM to make the adjustment. But the PA Voltage Limit part is automatic and jsut does it's thing, sort of like the final sleanup after you make all the other adjustments, or at least that is what it seems. I AM NOT an expert in it's theory and use, for sure.

Al,
The 25W has only the MOS option, and it is claimed to be a 12 db gain, and putting out 7 watts, my thought at present is more like the bias if off as 7 watts is a lot for a totally bad device, and it's drawing 2.7 amps. So, I am presently trouble shooting that path. CDMs aren't a lot of fun as you can't take them apart and trouble shoot the PA as there is no heat sink.

Thanks for all the inputs, it's appreciated. Will keep you all updated as this will likely help someone else later.
Ric

[ricp]

To Al et al,

The mystery is solved. It WAS the LDMOS final.
It was surprising it was still an RF amplifier even with damage. It had 160 ohms resistance, gate to ground, so my assumption is that it was still an RF amp, but couldn't get biased properly with the "punch thru" near short. Live and learn with these devices.
Also, they aren't a lot of fun to change without the SMT tools, BUT it got done and done well.

The occurrance with the tuner software was either a coincidence or an element already partly damaged and the stress did it in. I hadn't had it apart prior to the failure, but it was purchased used, so who knows.

So, on to the next challange.

Ric

[Al]

Glad you got it, Ric. I've seen almost identical results after someone inadvertantly applied too much bias to the LDMOS PA manually with the tuner, and it still amplified but gain was very low and the mobile's output wouldn't come close to spec.