HT750 Tx problem

[OneManShow]

Hi, I have a AAH25RDC9AA3AN HT, with ver R04.00.01 firmware.

Went to program it on a T468, R463 freq. When I Tx, the Tx light would flash, then low freq error tone. Rx is OK.

Found that strange since it had been working on a T459, R454 pair.

Reprogrammed the radio to a lower freq and it would work.

Programmed the radio in 100 khz steps from 467 to 467.9 Tx. At 467.6, the radio would not Tx.

Before I try to "fix" the radio, is this a firmware problem? Has anybody run into this problem before?

TIA
Sean

[Nand]

This is not a firmware fault and isn’t normal.

I guess that the VCO needs to be aligned with the tuner software. Was the radio water damaged at any time or did someone play with the tuning perhaps?

Nand.

[OneManShow]

Hi Nand, I wasn't sure on the procedure for the VCO alignment. Mind you, I thought it was for Tx modulation. i.e getting the correct ratio of audio to the reference osc and vco.

I had left the radio for a while, since it was one of those rainy day repairs.

So, one rainy day, I went and did some reading and playing.

To check the VCO, I hooked up a variable power source to the i/p of the varactors to see if the VCO would swing through the range. OK there.

Looked at voltages around the synthesizer. I found that the voltage from the voltage multiplier circuit (U210,211,cr201 and assoc caps) was only 9 volts not the 13.3 called for in the book. Checked the parts, all seem OK. The only thing that was left was U201. It couldn't possibly be the problem I thought. At any rate, I lifted pin 47 of the board. (That was fun ) Turned on radio and now measured 14.4 volts on C228!

I ordered the VCO i/c and replaced it. The radio now works! Go figure.

Even though the synthesizer i/c was working, one small section of it developed a fault.

I hope that will help somebody down the road.

Thanks
Sean

[Tony Soprano]

Nice work.