HT750 will not trx

[jims123]

I have in bench a HT750 this unit will trx at first I replace the RF jack this was in bad shape due to a fall, no damage to PC board or components after 3 weeks the customer return with same issue. I removed the battery and install battery eliminator to check current draw it was good 1.2 amp means main PA is good I removed shields and check rf input to PA this was normal as all the voltages that control the PA TRX enable but no RF. I check PIN diodes and recheck receiver there all good what can cause a failure like this no RF..help please..

[Tom in D.C.]

First thing I would do is to use a wattmeter to check for RF level at the antenna jack.
Second thing I'd do would be to listen on another receiver for
the transmit signal from the HT to see if any signal is being generated inside the radio.
This assumes you are certain what the transmit frequency is.
Pulling 1.2 Amps and having no signal out doesn't make any sense.

[Al]

If the unit waas in "bad shape" due to a fall, be sure to check the 14 Mhz crystal for normal and on-frequency output. If the PA bias was set too high, the radio could draw 1.2 amps with no RF drive to the PA. I know you stated that the RX was OK....is it "really" normal i.e. on frequency and < 0.25 uV at 12 dB sinad?

[Karfield]

If I read things right it sounds as if somewhere along the lines the radio had been dropped on it's antenna one too many times. Along with the other suggestions you might want to look under the shield where the switching circuit is. It is possible that one of the coils or parts under there have cracked or popped off. I've had one or two units with this problem in the passed.

Also, check radiation as well as power out. There is really only one solder joint on the antenna nut that connects it to the PA and that is the upper inside solder joint if that's not soldered or has a torn trace your radiation will suck even if your Rx is halfway decient.

Another thing to keep in mind is that current draw is not really an indication of good or bad power out. A power meter or if you are really hurting a demod probe will help to determine how much actual power you have out of the radio.

With certain board models there is a set of parallel resistors in front of the PA, these will go bad and can also cause some wierd failures. I just use 0 ohm resistors and haven't had any problems.

[cusackmusic]

Doesn't plugging in the RF adapter dis-connect from the ANT port? If that got messed up too, it would not TX very well. Also, there is a small plastic isolator between the RF threads and Ground. If that's not there, it might be shorting out to the case ground? I recall seeing that once.

[jims123]

If I read things right it sounds as if somewhere along the lines the radio had been dropped on it's antenna one too many times. Along with the other suggestions you might want to look under the shield where the switching circuit is. It is possible that one of the coils or parts under there have cracked or popped off. I've had one or two units with this problem in the passed.

Also, check radiation as well as power out. There is really only one solder joint on the antenna nut that connects it to the PA and that is the upper inside solder joint if that's not soldered or has a torn trace your radiation will suck even if your Rx is halfway decient.

Another thing to keep in mind is that current draw is not really an indication of good or bad power out. A power meter or if you are really hurting a demod probe will help to determine how much actual power you have out of the radio.

With certain board models there is a set of parallel resistors in front of the PA, these will go bad and can also cause some wierd failures. I just use 0 ohm resistors and haven't had any problems.

Thank you I got with your input thank you again.