HAMMERHEAD QUESTION

[daverave]

ive at last got my hammerhead revision D working but there is an annoying back ground noise on the audio...it started off as a tone but now its like an unmodulated radio carrier...can anyone throw any light on this problem ??? ive reduced the amplitude of this back ground audio problem with a large electrolytic capacitor across the power supply input but i can still hear it in the back ground !! if i use bench power supply or battery the problem is the same.

[6666]

Just a thought is the 7660 synced to the 555, C11, R37.

[daverave]

i have copied the original hammerhead revision D schematic and driven pin 7 of icl 7660 from pin 3 of the 555 oscillator chip...surely someone must know of this problem on this site cause the hammerhead has been a very popular design....

[Qiaozhi]

Just a thought is the 7660 synced to the 555, C11, R37.

On the HH schematic you can see a connection from IC5 (555) pin 3 - labelled N1 - that connects the TX oscillator to IC1 (7660) pin 7 via R37 and C11. This connection provides the synchronization.

Also, read the section at the bottom of page 2 of the manual.

[daverave]

i know about the sync between ICL 7660 & 555 which is connected...but i still get background constant tone in the audio threshold....i never got this on the surf pi but only on the hammerhead...ive reduced the tone by using a 4700 uf capacitor across the supply but the tone is still annoying and noticeable

[Qiaozhi]

i know about the sync between ICL 7660 & 555 which is connected...but i still get background constant tone in the audio threshold....i never got this on the surf pi but only on the hammerhead...ive reduced the tone by using a 4700 uf capacitor across the supply but the tone is still annoying and noticeable

Does the background tone go away if you disconnect the coil?

[daverave]

i disconnected the coil and the problem is still there...its not like a waveform tone....its almost like when someone keys a microphone on a radio with no modulation...like dead key sound....its more pronounced when the volume control is turned up.

[Qiaozhi]

i disconnected the coil and the problem is still there...its not like a waveform tone....its almost like when someone keys a microphone on a radio with no modulation...like dead key sound....its more pronounced when the volume control is turned up.

The only time I've seen this problem is when the audio oscillator IC (555) is too close to the TX MOSFET on the PCB, where certain versions of 555 timer will pick up the radiated noise. Changing the 555 to a different manufacturer can reduce the noise, otherwise you have to add a shield wire close to the 555. However, in the HH design, the audio and TX circuits are at opposite corners of the board. Is it possible that it is being caused by external wiring? Try moving the wires around to see if anything changes.

[daverave]

ok thanks Qiaozhi