Unfortunatly I have reached an imppass on my ATM3000. After building, checking, checking a second time and a third, the audio I hear coming out of this transmitter sounds overmodulated no matter how I tune the knobs. I've checked voltage levels on all points listed on the schematic and they all match up except for 2. The base voltages on Q4 and Q5 are both measuring at almost 15v instead of the 1.25v that is listed.
Unfortunatly I have reached an imppass on my ATM3000. After building, checking, checking a second time and a third, the audio I hear coming out of this transmitter sounds overmodulated no matter how I tune the knobs. I've checked voltage levels on all points listed on the schematic and they all match up except for 2. The base voltages on Q4 and Q5 are both measuring at almost 15v instead of the 1.25v that is listed.
PH6305845340
Now I've been looking into this a bit and unless I'm way off (witch I probably am). I belive I read in the manual that these 2 transistors handle modulation. I also looked into the data sheet on the PN2222 and apparently 15v to the base is way beyond its saturation point of 1.3v.
When I originally presented this problem to phill a few weeks ago he advised me to check a few resistors to make shure they were in their correct positions. I have checked multiple times and they all seem to be in order.
I even brought it into school to check the signal with an ocilliscope. When I try the tests with signals from the function generator all the signals come out overmodulated.
Thinking maby it was the transistors that were bed, I decided to replace them. This only made the problem worse. Apparently the ones I replaced them with wern exactly what was included in the pack.
I'm probably gonna have to send it back to phil to look at. In the mean time, any thoughts?
You are getting close to solving the problem.
Measure the emitter voltage and post what you get. Also measure and post the C,B,E voltages for Q1, Q2, and Q3.
From what you have done so far, it is possible that Q4 or Q5 are in backwards or there is something wrong with R30 or R31. Double check this.
Transistors are easy to substitute and the DC bias voltages will not vary much. The use of "not exactly the same from the package" will not cause the trouble you are having.
There is something grossly wrong with the biasing or there may be a bad part. If you post the voltages I asked for, it will really help narrow it down.
What follows has little to do with your problem. Some additional information about Q4 and Q5. This circuit is called a balanced modulator and it is not obvious how it works. Q4 and Q5 both are involved in the modulation, but so are Q2 and Q3. For those who are interested, Q4 and Q5 are multiplying the carrier by the audio to produce the AM sidebands (DSSC) and are acting basically as switches. If anyone wants to pursue this check out the MC1496 application notes.
Neil
As I'm still at work I dont have all the voltages in front of me. However I do remember that all the other voltages on the schematic that phil gives numbers for matched up. On Q4 and Q5 the voltages were 15-15-0 or was that 0-15-15. I dont remember off the top of my head.
On the test pionts I only get around 4 volts max.
On a side note, dam Neil you know your shit.
I got alot of good ideas. Problem is, most of em suck.
George Carlin (WINO)
In my post above I described the operation of a balanced modulator. The 1496 operates as described, but this does not correctly explain the SSTRAN circuit operation. Sorry about that.
Thanks to rfry for asking the right question so I could catch this.
Neil