Batboard

Hi - I have an issue with an HT1250 (403-470 range) and the use of a speaker mic. With no speaker mic attached the receive audio from the intrenal speaker sounds fine but when you attach a speaker mic the receive audio is distorted. I did verify the speaker mic on other HT750 and 1250 radios and the mic receive audio is fine on them so I am going with the mic itself is ok. I tried different "front" cases with the 1250 board which had different ribbon cables and that made no difference in the audio when the speaker mic is attached (internal speaker worked fine) so it does not seem to be a ribbon cable/flex issue.

I did look into audio PA issues with the TDA8547TS BTL chip on the board as well as potential shorting/component issues on the various zener diodes and caps going to ground on the pos and neg output leads on channel 2 of the amp output for the speaker mic to the 20 position flex connector on the board and I cannot find any problems that I can see - values of the caps, etc when removed one by one seem to measure fine. Changing the TDA8547 did not make a difference either. I looked into issues with coupling caps and the gain resistors and the values seem to match the schematic when they were removed one by one and checked. I looked into the operation of Q410 that does the external speaker select and the switching seems to be happening fine as far as I can tell - when the speaker mic is attached no audio comes out of the internal speaker. All VCC supply voltages were fine for the two channels and all grounds were connected as per the schematic for the TDA8547.

The strange thing is that if I take my oscilloscope probe ground (earth ground I am guessing) and put just that that part of the probe to where the positive inputs and SVR pin of the TDA8547 are tied together to C476 the receive audio comes out fine on the speaker mic. The radio was powered by a dc power supply set to 7.5 volts. It should be noted that when on a battery the distortion issue was the same.

Has anyone seen something like this before with the speaker mic? Maybe I am missing something stupid in all of this but who knows. The scope probe clue may mean its some sort of grounding issue but I cannot tell where that issue may be. It could be just a bad board and thats all you can do. Any info would be great - thanks...

You should read the radio and look at the microphone/external mic settings.

You have two gain controls, and I suspect your external mic gain level is set too high, probably above the range of 25.5 - 27.5.

Do some adjustments and try a lower level as the external mic you are using may have too much sensitivity for the factory setting.
Start at the simplest cause and work from there.

Ok thanks - I'll check this out. I guess the mic gain settings have some effect on the receive audio in some fashion. The radio originally would not power up but after replacing one of the regulators it came back to life. All worked fine it seemed except for this speaker mic receive audio issue.

Thanks again

No, mic gain setting do not affect receive audio (except over the air). I think he misread your post. I don't work on portables, but it sounds like you have the tools and know-how to chase this down. I suspect you will find something in the spkr audio path switching causing the problem.

Ok - thanks... It is a challenge and If I can't find out what the issue is then that's ok. I am with you on the audio path and I'll keep digging - maybe there is something I overlooked. Challenges are good sometimes and in any event I learned a lot about how these audio amplifiers work.

Thanks again....

486dx4 wrote:Ok - thanks... It is a challenge and If I can't find out what the issue is then that's ok. I am with you on the audio path and I'll keep digging - maybe there is something I overlooked. Challenges are good sometimes and in any event I learned a lot about how these audio amplifiers work.

Thanks again....

Did you ever think that the speaker inside the speaker mic might have a problem? If you have another radio to try it on, move the speaker mic to a second radio and see if the problem stays with the speaker mic RX audio.

You could have a wire inside the mic rubbing the speaker cone, there could be something between the front grill and the speaker cone rattling around, or the speaker it self could have gone south on you.

I did try the mic on a few other radios and the mic worked fine on each case. So the problem did not "move" to the other radios I tried the mic on. I also did try a GP300 speaker mic with one of those adapters and on that one radio the receive audio was distorted as well while on the others tried it works fine - same results. I really was hoping for a bad speaker mic as that would have been an easy fix of course. I'll keep plugging away on this - thanks...