PULSESCAN AUDIO QUESTION

[kt315]

DR
surf pi is done by old school guys... if you noticed i ask not schematic of surf, but the layout itself...
i set tantalums in critical points of a layout i do... on a case of the problem...

...i will take on board your advice kt 315 about caps around each ic
---
no no. set only ONE cap on ONE ic, and so go on every ic.

[Qiaozhi]

i did try some caps on the +5v/-5v lines for the vco pcb but made no difference...i cannot understand why a pulsescan board would have this problem being a commercial detector...i wonder how whites got around this problem in their later detectors...the hammerhead design has the same problem but i never got this with surf pi detectors...i will take on board your advice kt 315 about caps around each ic

I think your problem may be due to the audio oscillator IC. Try changing it for one from a different manufacturer. Personally I did not have a problem with Hammerhead, only when certain 555 timer ICs were fitted. Some seem to susceptible to pickup from the transmitter pulses.

[daverave]

I think your problem may be due to the audio oscillator IC. Try changing it for one from a different manufacturer. Personally I did not have a problem with Hammerhead, only when certain 555 timer ICs were fitted. Some seem to susceptible to pickup from the transmitter pulses.

Hi George....When you say audio oscillator IC are you refering to the AD 645 on the vco pcb ??? if so i dont have a spare one and they are expensive to replace....or are you refering to the 555 coil drive IC ????

[Qiaozhi]

Hi George....When you say audio oscillator IC are you refering to the AD 645 on the vco pcb ??? if so i dont have a spare one and they are expensive to replace....or are you refering to the 555 coil drive IC ????

I was meaning the audio oscillator. However, if this is not a 555 timer (as in Hammerhead) then it's probably a different issue.
Since you've seen a small improvement in the background hum by adding a large electrolytic, then this points to the noise entering the system via the supply.
Are you running this test from a bench power supply or a battery pack? It could be that the supply voltage itself is dipping whenever the TX pulses. You could try running the audio section from a separate supply, and see if that makes the background hum disappear. If so, then simply add a separate regulated supply to power that section.

[daverave]

its not easy with the pulsescan vco board as there are about four different input supply voltages so to run off a separate supply is not easy and practical....i thinking maybe when the 555 oscillator outputs then it may cause a supply reduction giving ripple on the supply line...but then the supply capacitor is 4700 uF so should be sufficent ??? but by adding a 1000 uF capacitor across the supply reduced the noise to a much lower level....so i wonder is a 4700 uF sufficent in a high power pulse detector ???

[Ferric Toes]

its not easy with the pulsescan vco board as there are about four different input supply voltages so to run off a separate supply is not easy and practical....i thinking maybe when the 555 oscillator outputs then it may cause a supply reduction giving ripple on the supply line...but then the supply capacitor is 4700 uF so should be sufficent ??? but by adding a 1000 uF capacitor across the supply reduced the noise to a much lower level....so i wonder is a 4700 uF sufficent in a high power pulse detector ???

Hi Dave,
The audio oscillator is the AD654 and the TX timer is a LMC555. Check that yours is not a standard 555 as these create a larger ripple current at the transitions. If the background tone varies when the TX frequency is changed, then it is definitely the TX that you are hearing. I have had this problem from time to time and it usually comes down to grounding. You have only got to get a small millivolt ripple on the headphone ground track for this to be audible.

At the output end of the main board there are 3 ground pads. One is marked A gnd. which is for the analogue circuitry but on the far left end is*gnd and Dgnd, both come together at the 4700uF capacitor. The *gnd is not used and the attached diagram shows all the connections. This arrangement had no audio background that I recall. Whichever audio ground you are using, you could try the other one and see if it is better or worse.



Eric.

[daverave]

Hi Eric...I changed over the ground connection of the screened cable that go's from the end of the main pcb to the vco pcb and no change....the tx oscillator IC is a 7555 which is soldered directly into the pcb so it must be the original...the noise changes pitch when the freq control is rotated...the 1000 uF has reduced the noise but still present at low volume.

[waltr]

its not easy with the pulsescan vco board as there are about four different input supply voltages so to run off a separate supply is not easy and practical....i thinking maybe when the 555 oscillator outputs then it may cause a supply reduction giving ripple on the supply line...but then the supply capacitor is 4700 uF so should be sufficent ??? but by adding a 1000 uF capacitor across the supply reduced the noise to a much lower level....so i wonder is a 4700 uF sufficent in a high power pulse detector ???

ESR

[dbanner]

Perhaps you might replace 4700uF with new one. Maybe 3x2200uF in parallel. Replace the 7555 timer IC with other CMOS just to see if makes any difference.

[dbanner]

Does your audio board have two tantalum capacitors? They are side by side marked as C2 and C8.

[daverave]

Hi walt & dbanner.....the 4700 uF in the pulsescan is a standard non ESR type...maybe i will change it for a low ESR type....i did even try a 22 uF tantalum across the 7555 but made no difference.

[Ferric Toes]

Hi Dave,
Maybe the TX ripple is on the -12V supply to the BC184 audio transistors. Try increasing the value of C12 (470uF) on the audio board. this and a 100ohm resistor decouple the supply to the transistors that feed the phones.

Eric.

[dbanner]

You may decouple whole day
The ripple won't be goin' away
You must first find source of hum
That sounds like fun(not)
Without a scope
There is no simply no hope
Trial and error
Will go on forever

[daverave]

Hi Dave,
Maybe the TX ripple is on the -12V supply to the BC184 audio transistors. Try increasing the value of C12 (470uF) on the audio board. this and a 100ohm resistor decouple the supply to the transistors that feed the phones.

Eric.

Hi Eric....ok will try and increase the value of C12.

[daverave]

Hi Eric....I put a 1000 uF capacitor across the 470 uF but didnt make much difference...so ive resigned myself now to using the detector has it is for now.