just wondering if any of the more serious rig doctors could assist me with my problem I'm having.

I have always made my own rigs for my own personal use up to about 50-60 watts from the output pa stage , everything is perfect and the audio is totally clean until I try and enclose a torridal and a rectifier and caps into the same box as the rig itself.
I think that the field created by the transformer must be entering the oscillator or pll section of the rig.
is this the case , or is anyone more able to tell me what I'm doing wrong.
I normally would of powered the rig from a switch mode power supply module , but now its time to use up some of my stock of torridal transformers.
ive seen many builders who have shown me their finished articles and they are perfectly clean and have no hum at all. is it due to my transformer clearly being too close to the rest or is it something else.
I am using a torridal , quite large not sure on the va rating got to be between 200va and 300va
a large metallic bridge rectifier bolted down to the enclosure and one or two smoothing capacitors approx. value 20,000uf between the two

Even through toroids create less electrical noise than frame type transformers they can still cause problems. Try and keep your driver away from the transformer as much as possible.
If that doesn't work put the driver in a metal can.

Use voltage regulators to provide power to exciter board. Place for example a 15v regulator right next to the exciter and keep wires short. Also make sure that the oscillator part of you exciter has its own regualtor (for example 78l09) , have this regulator feeding just the oscillating transistor and resistors to varicaps.

Keep exciter as far away from transformer as possible.
Keep Wires short
Make sure you have plenty of smoothing capacitance on power supply
Decouple power supply wires as they enter and leave PCB's with 10nf 1nf caps etc...

Post up some pics and were be able to help

Hello Mr 123!
With that type of PSU it is inevitable that you will get that 50hz hum to a small degree!
The most common culprits are inadequate grounding, DC ripple and the audio side of your driver board! Its unlikely that 50hz is leaking into your RF stages because they are tuned to well above that frequency and reject any other than your carrier!
However DC ripple and grounding can cause undesired effects! as a general rule? the transformer output current needs to be around 1.6 times the total consumption of your rig (driver,PA and fans etc) and your smoothing capacitance around 1uf per milliamp!
Also try to connect all negatives to one point (eg the main housing/case)
And as analyser says! shield the driver board if possible? and always use coaxial style cables for all audio connections

I hope this helps!

radio-berlin wrote:Use voltage regulators to provide power to exciter board. Place for example a 15v regulator right next to the exciter and keep wires short. Also make sure that the oscillator part of you exciter has its own regualtor (for example 78l09) , have this regulator feeding just the oscillating transistor and resistors to varicaps.

Keep exciter as far away from transformer as possible.
Keep Wires short
Make sure you have plenty of smoothing capacitance on power supply
Decouple power supply wires as they enter and leave PCB's with 10nf 1nf caps etc...

Post up some pics and were be able to help

Mr Berlin is spot on too!

sinus trouble wrote:

radio-berlin wrote:Use voltage regulators to provide power to exciter board. Place for example a 15v regulator right next to the exciter and keep wires short. Also make sure that the oscillator part of you exciter has its own regualtor (for example 78l09) , have this regulator feeding just the oscillating transistor and resistors to varicaps.

Keep exciter as far away from transformer as possible.
Keep Wires short
Make sure you have plenty of smoothing capacitance on power supply
Decouple power supply wires as they enter and leave PCB's with 10nf 1nf caps etc...

Post up some pics and were be able to help

Mr Berlin is spot on too!

Glad I got your approval

I think all the advice given in the previous posts is good.
As has been said, start off with the basics of adequate PSU components, try to keep a little distance between audio / oscillator stages and the transformer, regulation for low level stages etc. These things should make sure you have fairly clean power lines.
I chased a problem like this for a long time, when I first started trying to build transmitters. This was before the days of the internet, and it was hard to get any info from anyone. Builders generally kept info closely guarded, and although I'd been told that it could be done, I'd never seen it done, and all my early unregulated attempts hummed badly - despite large reservoir caps, and what I thought was good attention to detail. This was also before the days of SMPSUs, and I ended up building a number of rigs with big analogue regulated PSUs, where the PSU probably generated more heat than the rig.
In my case, the main issue was with grounding. I tried to follow principles I'd learned from audio gear, which didn't work very well. I found that I needed to kind of treat the whole case as a ground plane, and pay particular attention to the high current side of things. The following generally seemed to work well for me: Tie the PA board to the case (electrically), as well as possible. You can bolt the board to the case - either directly, or via heatsink / bracket (paying attention to spacing of the power transistors; don't let the board pull down on the leads), or by using tags soldered to the board, and bolted to the case. You may have seen boards with straps from the power transistor bolts to the emitter leads. This also helps to tie the board ground plane to the case ground plane. Mount the rectifier bridge close to the transformer and the PA board, and the reservoir cap close to the bridge and the PA board (to keep all the power wiring as short as possible). After rectification and smoothing, take the leads from the reservoir cap; either to the PA board, at a point where it is tied to the case, or to the case, where it is tied to the PA board, then a +ve feed from the cap to the +ve rail of the PA. Take separate +ve feeds from the cap to the downstream regulators for driver board, fan(s) (better to have a separate regulator for fans if they are DC op), etc.
As I said, this is just what caused problems for me. I knew enough to use big enough transformers, reservoir caps, take precautions against induced hum etc, from experience with audio gear, but grounding practices I'd used in audio gear, didn't work so well.
Do also bear in mind, that radios can suffer from RF hum - in the presence of a strong RF field. This is less likely using a dummy load, but can still happen if the radio is close to the Tx; especially if mains powered.

One more thing, specific to transformer type supplies. If transformer is not screwed tightly to the box, it can vibrate at mains frequency when current it supplies is large. This mechanical vibration can be transfered to coil in oscillator and produce hum in modulated signal.
Happy new year to all!


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