jpl995 1w PLL driver board very bad. (Greece)

[Elopid]

DO NOT USE:



I was given a 160w FM transmitter to repair recently as it had a very bad humming problem. The PLL driver was the jpl995 board as shown in the picture and the power amplifier is a Philips BLF244 driving an ST SD1460. STL receiver is Sharp C77G74.

I thought it must be down to the fact it works on frequency so I shielded the whole PLL driver board in a metal box and tested it (without the satellite receiver can connected) and the noise was gone, but as soon as the audio from the Sharp C77G74 was connected there was the most awful humming sound again!

First I moved the sat can over to the other corner of the box and although there was a slight improvement the hum was still loud, so I change the sat can and it made no difference.

I had to resort to changing the whole driver board with a frequency doubler and the problem was completely resolved.

These JPL995 1w PLL driver boards are no good, with just an audio socket on the rig it's fine but as soon as you connect the sat can it's unusable. Does anyone know the cause for this and if it can be resolved without replacement?

Also, the 2nd harmonic is far too high (approx -40dB) at the bottom end of the FM band, I strongly advise people to not purchase this board.

~ eLoPiD ~

[pjeva]

Could be many things. This is not state of the art driver, but i don't think that frequency is the issue. All of my stuff are at freqquency designs but they don't hum. Can be poor grounding or maybe different impedance between satcan and input of the driver. You can try to add opamp at the input, better cable from satcan to board, connect grounds of satcan, box and driver together as short as possible or all of the above...


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[Elopid]

Good morning Pjeva, thank you so much for your suggestions. I spent a good few hours trying many different things.
It's a strange one, as soon as you switched the rig on the weak signal generated from the PLL is fine and you can hear no hum just a nice clean quiet hiss from sat can, but as soon as the RF output goes in to amp a few seconds later the hum is there.

It sounds like a ground hum (for example) just like when the turntable ground isn't connected to the mixer, but it only happens once the RF output is into the power amplifier.

I have decided not to waste any more time on this board, the spectral output is not safe enough for the TX location, the PLL has since been replaced and the problem is now resolved.

Regards, EloPiD

[Albert H]

You may find that it's a power supply problem.

However, you say when only the exciter is running, it's quiet, that suggests that it's an RF feedback problem. It could be as simple as connecting a small capacitor (a few pF) across the audio feed to strip any RF off it as is goes into the modulator.

[Elopid]

Good morning Albert H. Thank you for your suggestions. The power supply is a 24v 300VA torriodal with bridge rectifier and 16,600uF in smoothing caps (10,000uF + 3x 2200uF).

I will not be using that driver but I have certainly taken note of what you've said for the future, should a problem like this ever re-occur.

With thanks, Elo.

[Albert H]

Quick question:

Do you have RF bypass capacitors across each of the diodes in the bridge rectifier? I find that 1 - 10n really helps get the RF off the supply rails.

[Elopid]

Hi, yes there are 4x 103 capacitors in parallel with the diodes of the rectifier.