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T E M 250 watt PA unstable

S

Studio1

A friend has just called me about a unit he has and I thought I'd try to get a heads-up on it from someone here who may have struck this before, as I may end up with it on my bench.

Apparently, with no drive connected, it is sitting there transmitting on some odd frequency.

It's obviously self-oscillating, but how?

He's replaced the two MRF174 outputs and swapped the power supply out. Still no joy.

Any ideas?
 
I assume this is an RF module? T E M doesn't come to mind of a Saturday morning.
Usually do - it - your self spurs are due to instability caused by unwanted feedback or poor filtering resulting in feedback. All of the little decoupling capacitors are suspect. Likewise the caps in the signal path may have changed value. No fun to find at all. If it's full of surface mount stuff and you've no rework station, might be better served to send it to the manufacturer or one of the aftermarket shops which specialize in RF modules.
 
Part of it is :)

The rest of it is a rack mount unit including power supply, interface etc.

Have a look here: http://www.tem-italy.it/prodotti_eng.htm - scroll down the page and look at the 7A100 TX - it looks a bit like this, perhaps a bit older. Yes it does have some SMD in it apparently. I really don't want to look at it but there are few RF engineers here as it is - and the guy who owns it has tried to contact TEM with no success.
 
Geez, there's little inmformation there. I've one of the European little radios, but the US vendor supports them well. o you at least have schematics and a manual?
 
If you've the roadmap, have a look at all the decoupling caps around the RF section, and the interstage coupling caps. Also the decoupling inductors in the Vcc feed to each device. If worst comes to worst, isolate each stage - often by cutting the trace to the next stage - terminate the stage, remove Vcc to subesequent stages, and see which one generates the spurs. Marti even put removable strips in their 450 amps so this would be easier to do. Chasing these things is no fun at all, they tend to be a cascade of multistage 'almost' oscillations. It helps if you can get a factory or reseller tech on the phone who has experience with the particular gear... often they know which specific component or components are most likely to creat a problem.
 
Thanks LJ - yeah it's as I suspected, work my way through it until I find the offending section/component.

I've made a note of your thoughts and comments tho as it's always nice to refer back to in later times.
 
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