CDM 1550 VHF no power out-but aligns OK!

[com501]

OK, pounding my head on this one, and Motorola tech support is no help (they haven't been trained on it).

CDM1550. BAD PA, I replaced it (LDMOS). I did the complete alignment, voltage thresholds, bias settings, power out calibration. It all works peachy. When I exit the alignment and get the radio back to normal operation, no power out, at all.

When keyed, the radio briefly draws current and then the PA shuts down. Any ideas anyone?

[n7maq]

OK, pounding my head on this one, and Motorola tech support is no help (they haven't been trained on it).

CDM1550. BAD PA, I replaced it (LDMOS). I did the complete alignment, voltage thresholds, bias settings, power out calibration. It all works peachy. When I exit the alignment and get the radio back to normal operation, no power out, at all.

When keyed, the radio briefly draws current and then the PA shuts down. Any ideas anyone?

Have you checked the cpodeplug to be sure the time out timer is set correctly, also the TX power output setting? I doubt that is it, but it's best to start with the simple stuff. As we would say in the fire service about doors "try before you pry".

Jim

[com501]

Yup.

Also flashed radio with newest firmware, loaded default codeplug from Moto archive. Same problem.

What I DON'T have is a saved alignment file or a default alignment file.

[RADIOMAN2002]

You may have more problems than just the output transistor. You could have a bad TX/RX switch, check the RX sensitivity, if over 1 UV check the switch. The radio may be sensing bad swr and shutting down the PA. I would try turning the power out all the way down first, see if you can get the radio to put out something, once you get there, slowly bring up you power setting. If at some point it trips start looking at the switch.

[com501]

OK everybody. Reminder. The radio functions perfectly in all alignment tests.

It receives fine. It still transmits, except when it is NOT in the alignment mode. Under alignment mode, low power average setting is 26 watts. High power settings average 50 watts. When in the normal operating mode, the radio tries to TX, draws current, but defaults to about 600 milliwatts with an almost idle current drain.

[Bill_G]

Just for s&g's, reprog the radio for low pwr on one of the modes and see if it xmits. If it does, then your high pwr alignment might be a bit too high. If it doesn't, then we need to think about it some more.

[com501]

No it doesn't at any power level the software will accept. On any frequency within the bandwidth limitations of the radio.

It still aligns just fine. I have started with a brand new blank codeplug too.

I am considering replacing all the components in the RF circuit related to power monitoring and bias AND replacing the LDMOS again, with a new one, including the driver. Or, I could admit defeat and flat rate it. I would REALLY like to know what is wrong, since all El Paso will do is throw a new PMUD in there and call it good.

I have about 6 of these now, lined up on the shelf to repair. This is the ONLY ONE that has ever presented problems after replacing the final.

[Bill_G]

Then shotgun it. Definitely sounds like a power ctrl issue. In test mode that ctrl is disabled. You should be able to verify that by monitoring the drive and pwr set voltages. I haven't had a PA go bad yet. So no experience with this issue. But, after problems with Maxtracs and Desktracs, I habitually lower pwr on mobiles now before they leave the shop.

[com501]

Yup.

We set them lower also, but the customer programs their own, and no matter how many times I set the VHF CDMs to 45w when I get them in here for repair, the codeplug is set to 54w.

[com501]

Lately, I have gotten MANY of these in with no power out.

They all have the same symptoms, no power, low current, and when disassembled the PA is stuck to the insulating pad and no longer soldered to the board.

I tried just reflowing the PA onto the board, but they are all, invariably, bad.

[Bill_G]

Do you have a maintenance contract with these folks that you have to honor, or is it t&m? If it's contract, then they need to keep their mitts out of the radio settings. If it's t&m, then they can burn them up all day long to their loss. I usually shave 2db off. In a well designed and maintained system they will not miss it, and their equipment failures will reduce. I know some customers believe more watts equals more range, but with some careful explanation, and perhaps a demonstration, you can show them that 30w will talk as far as 50w, and keep their maintenance costs down.

[com501]

It's T&M. Thank God. I always optimize the radios before sending them out, and power reduction is ALWAYS on the short list, especially this customer who likes less than optimum antenna installations (beer cans and patch antennas)

[Bill_G]

VHF beer cans? Fosters? 12oz pop cans are pretty decent dual band UHF/800 antennas, but could never get a match below 440 without a lot of elevation. I've used patch on fed three letter acronym cars, but have to keep power down to prevent glass damage as well as odd vehicle problems above +45db input. You are certainly faced with a challenge if the customer is adjusting power up without thinking about the ramifications.

[Karfield]

I'll try to answer several things that have popped up in this thread as best I can.

1. The thing to remember is that the way the radio keys up in tuning mode is not the same as keying up in standard mode. You might try using the test mode and see if you have the same problem. (I'm going to guess that you will. In which case I'd suggest starting to look at a possible control head issue.) I know I've seen this problem before but I'm a hands on guy so it'll be a while for me to remember.

2. When you pull the board out of the housing I might suggest adding a little heat to the board instead of just prying up on it. I must have pulled at least a half dozen traces before my trainer told me to just heat the board up a bit. Yes, it's also not uncommon to have the PA get so hot it desolders itself from the board. I don't see much of that anymore but I remember the good ole days where I'd see that stuff daily.

[AEC]

There is a small pad on the back side of the CDM that requires pressure from a carbon impregnated 'button' on the cover of the radio.

There is a small switch that this rubberized button contacts on the board, near the rear of the chassis, if this pad is not in place, the TX will not work, only in test mode will the PA show power as it is under CPS control at that point and not the radio's frmware/CP.
Look for the red rubber pad on the cover, if it moved, the alignment of the 'button' may be off enough to prevent the PA from operating.

I ran into this problem a while ago, and that was the solution.....drove me crazy seeing power out, but when closed up...POOF! no more output,

The 'switch' is somewhat square in shape, and looks a little like a microwave filter by its design, but it is necessary.

[Bill_G]

I will tear a cdm apart to look for that. thx.

[com501]

I always bypass that stupid thing with a zero ohm SMD on the test point. Makes working on the radio with the cover off not a three-handed operation.

[AEC]

Anybody in the know understands ALL techs have FOUR hands while repairing any radio!

At times, I do wish I was an octopus....sure makes holding the soldering iron, vacuum pump and replacement part sooo much easier.

[Jay]

I may have one that has the same issue...Is there a better description you can provide or a picture? Thanks.

There is a small pad on the back side of the CDM that requires pressure from a carbon impregnated 'button' on the cover of the radio.

There is a small switch that this rubberized button contacts on the board, near the rear of the chassis, if this pad is not in place, the TX will not work, only in test mode will the PA show power as it is under CPS control at that point and not the radio's frmware/CP.
Look for the red rubber pad on the cover, if it moved, the alignment of the 'button' may be off enough to prevent the PA from operating.

The 'switch' is somewhat square in shape, and looks a little like a microwave filter by its design, but it is necessary.

[AlexR]

We had the similar problem with GM340 (with the same hardware)-
everything looked OK when in 'K and M tuning' but no RF output
in normal mode with PTT pressed. The Global Tuner program used
was too old for a newer radio. Solved by re-tuning K&M with
latest version of Global Tuner.

[Bill_G]

I'll be dinged. See what you learn from the other guys here? Very interesting. From the service manual (VHF theory of operation) -

Switch S3440 is a pressure pad with a conductive strip which connects two conductive areas on the board when the radio's cover is properly screwed to the chassis. When the cover is removed, S3440 opens and the resulting high voltage level at the inverting inputs of the current control op-amps U3402-1&2 switches off the biasing of U3401 and Q3421. This prevents transmitter key up while the devices do not have proper thermal contact to the chassis.

This discussion is repeated over and over in the different bands in slightly different ways with different part designators, but the reasoning is the same: to prevent thermal damage to the rfpa.

[AEC]

Oh darn!

The secret's out of the bag...

At least it's at a good home here on the board, where good information abounds.

I hope we can cover all aspeccts of similar trouble areas and make a sticky that can be added to as new data is obtained.

Yeah, that's the ticket!

[Bill_G]

Ha! Yeah, it's not too much of a secret if you rtfm, but I appreciate you telling us about it. I took one apart and showed it to the other guys in the shop, and nobody was aware of it. Our problem is two-fold - very few customers are conventional anymore, and those that are buy mostly portables with extended warranties. So, we don't even get to see them when they fail. The radios get put in a box and shipped off. This would have eventually come up to haunt us.

[RADIOMAN2002]

To change the subject matter a little can anyone verify that this switch problem can also apply to the HT-1250 series portables. I have 5 of them on my desk right now awaiting approval to send to Depot. It would make sense as to why I have so many with no power out.
Now if someone could come up with a reason why I have so many with overheating PA transistors with the radio turned off. My take on it is the charger is spiking the PA transistor, either blowing it up or shorting it.

[Al]

Unlike the Waris mobiles, the Waris portables do not have a PA shutdown switch. I've never heard of any problems involving bad PAs caused by chargers creating voltage spikes. I've seen many fail due to inadequate or no heat sink pad between the chassis and heat spreader soldered to the PCB. I've had them die of infant mortality occasionally. But most PAs fail because of a failed PCIC(power control IC) or because the power crept up over time from the original factory setting, or because a sales-type believing that 5.8W out would ensure customer satisfaction more than 5.0W set the power too high to begin with.

[RADIOMAN2002]

Well most of the failed radios are of the same generation and age, about 7 years old. ( we have newer ones fail, but they were under warranty) We have been trying to compile a list of possible causes, like the heat sink compound dried up, discoloration of the insulator and such. None of these radios have been touched since they went into service, no one has readjusted the power above 4w, which is what we run to extend battery life, so there has to be a common reason, but we havn't found it yet. My opinion is to trash these radios, they are between 5 and 7 years old and for about $75.00 more than flat rate repair we can buy new replacements with new battery, updated firmware, charger and a 3 year warranty. Unfortunately that's not going to happen.

[Al]

If I remember, the PA LDMOS in the Waris portables is a 7 watt transistor, so it should operate at 5-5.5W OK. Do you have good confidence in your test equipment accuracy when you set the portables for 4W output? I've got 200 or so of these Waris portables in a *heavy* industrial environment, and they really get beat and abused...but other than an occasional infant mortality I've never had one come in with a blown PA. With all the regulation and filtering in the M chargers plus the radio's battery acting as a filter for spikes coming in on the AC line, I'd doubt that that's the root cause of your problems.

[com501]

I think the PA LDMOS in both the early HTs and the early CDMs was bad from the get go. Every mobile failure I have had is an older LDMOS with a TANAPA that is not even available online to look at the board anymore. Same with the portables that have come through our place, much older units, factory aligned, and yet, still they fail. Usually we get the flat rate repair and the option is usually 'replaced board' with the usual kit number for a complete board.

Failure rates for newer TANAPAs is significantly less. This also explains my inability to align the older boards. I received a radio back today from the factory that was one of the older radios with a failed PA. They listed the new part number for the LDMOS as what they replaced, however, the radio had the newer TANAPA board replaced in it. So, looks like they are just board swapping for newer boards.


Hmmmm.....

[RADIOMAN2002]

Considering that we have over 1000 radios in our fleet, yes we do have the equipment and the personnel to take care of our equipment. I chose to make the change to realign the radios to 4w max to try and cut down on the PA failures, and extend the battery life. Most transmissions are less than 1 mile. The failures run about 2-3 a month, and are getting worse as the radios age. I am trying to come up with a replacement method that is easy. We have a table top hot air machine, but it doesn't have the umph to remove the PA without heating up the whole board and moving other components around. My next stab at the problem is to use a hot air gun to remove the can, and help give a little extra heat more quickly to get the heat sink off first, then remove the PA.

[com501]

You could always use the two-sided air debonder that Motorola recommends....

If you have lots of extra cash laying around, that is.

Flat rate works for me, particularly if the radio has other issues also.

[RADIOMAN2002]

Wish we had money, we are broke like most PS agencies. It practically took an act of congress to get the hot air machine we have. I prefer to damage the older boards during repair and replace them. Going to use flat rate for all radios 5 years and newer.