Hello
I have a CDM1250 VHF mobile 45w LDMOS PA.with no power out. Radio came in with a defective final. Replaced part and tuned transmitter per instructions in service manual. Radio now transmits in test mode per specs, however in normal operation zip. I can see upon PTT a rise in current draw which quickly drops. I'm thinking the ALC is causing a problem , but all checks so far have seemed normal. I have had a number of these types of failure that have gone to the depot for repair...... I'm tired of paying for repairs I should be able to do myself.
Any suggestions are appreciated.
Thanks
Steve
CDM series... no power out
Moderator: Queue Moderator
Re: CDM series... no power out
We've heard about this before. There is a switch contact on the main board that can cut off the xmitter.
From the manual : 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.
Hope that helps
From the manual : 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.
Hope that helps
Re: CDM series... no power out
Thanks Bill.
I've checked the switch and have gone as far as shorting it out, so i'm sure that its not the problem. I found an updated bias tuning procedure from what was originally printed in the service manual. This procedure is makes much more sense and explains the tuning completely. I can make the proper adjustments for voltage value and bias 2 adjustment. Bias 1 adjustment yields no change in current draw. I'm thinking that my problem lies in the bias of the driver stage. Because I can get 42 watts out during K&M tuning, I don't believe any other devices are damaged. I don't understand (yet... I'm determined to figure this miserable thing out) how the test mode "tricks" the PA to operate at full power. I can recall seeing a post about the test mode, but can't find it now.
The tuning procedure I'm using is from the help section on the global tuner.
Once again any help is appreciated.
Steve
I've checked the switch and have gone as far as shorting it out, so i'm sure that its not the problem. I found an updated bias tuning procedure from what was originally printed in the service manual. This procedure is makes much more sense and explains the tuning completely. I can make the proper adjustments for voltage value and bias 2 adjustment. Bias 1 adjustment yields no change in current draw. I'm thinking that my problem lies in the bias of the driver stage. Because I can get 42 watts out during K&M tuning, I don't believe any other devices are damaged. I don't understand (yet... I'm determined to figure this miserable thing out) how the test mode "tricks" the PA to operate at full power. I can recall seeing a post about the test mode, but can't find it now.
The tuning procedure I'm using is from the help section on the global tuner.
Once again any help is appreciated.
Steve
Re: CDM series... no power out
What version of the Global Tuner are you using? R02.15.00 is the latest. According to a post from AlexR in the thread that you referenced earlier, using the latest Tuner solved the problem for him.