Gentlemen, I'm new to the forum and I kindly request your expert advice. I have a Yaesu FT7800 that has had a diode removed so it could transmit in the marine band. After a few try outs it seems that the tx power is almost non existent. It seems that the rf transistor(s) are damaged. Can one just replace the rf transistors or do one need special skills and / or equiment to carry out this procedure? I thank you in advance for your help.

Regards,

Raul

I can't give you a specific answer as I've not seen the insides of this particular model of rig. However I suspect that many of the components in this rig may be surface mount devices which are small and fiddly to solder.

I'm no expert on SMD but there seems to be two schools of thought, those that say leave it to someone with the kit and others, normally those who work with SMD regularly, who say its no problem as long as you have a steady hand and some form of magnification. There is a general view that a fine temperature controlled soldering iron and in some cases a solder sucker is more or less the minimum to work SMD successfully at a DIY level.

Personally if it is a SMD then I wouldn't go near it, but then one fellow Ham at my local radio club on examining some of my kit construction skills described my soldering skills with the technical term c**p! Anyway I only have a chunky 25W Iron with a lumpy point that would probably spell certain disaster in my hands on something like SMDs.

However it should be possible to replace the rf transistors at a component level with equivalent devices all be it that depending on the type and mathod not a task that could be undertaekn by all. Pop the cover and have a look.

73

Thanks Theo D Lite,

for taking the time to answer my query. I have access to special soldering equipment and a magnifier to work with. I was wondering if I needed special equipment to align or tune the rf transistors after they are replaced. I have found literature on this radio model that specifies a procedure to adjust the output power of the radio by using the front panel controls. I would only need a watt meter to verify the exact output. I will try my luck at it and will post the outcome. thanks again.

Raul

Depending on radio / circuit, there may be some bias adjustments to be made when swapping the output transistor(s) or stage(s). This would probably be described in a service or allignment manual. I think when I've done this previously (some time ago on an FT817) I had to measure the current through the transistor base at receive and ensure it was within specific limits. This required an ammeter sensitive to the mA level and a plastic adjustment tool for the relevant DC bias adjustment.

Are you confident it's the finals that have died, and not a driver prior to the final stage? Could be worth confirming this as the price of a driver transistor is probably pence whereas the final RF transistors are likely to be somewhat more

If you're unsure, perhaps a local club or amateur equipped with the relevant test gear could lend a hand?


Steve M0SPN

Thanks Steve,
I think I will heed your advice and get competent help. I wonder if I would be better off buying a new rig. It's just that I find it a waste to put the radio away because of this problem. I'll report how it all pans out. thanks once again for your help, much appreciated.

'73's
Raul

I'd be cautious about regarding it as repaired without checking on a spectrum analyser.
I don't know the circuit of this unit but, generally speaking, I'd consider a power meter, dummy load and spectrum analyser as the minimum test kit for repairing the PA.

Did it work when it was first wide banded or did it stop straight away?

It might be worth putting the diode back.

Cheers

Tony

Hello Tony,

thanks for your reply,
The radio worked inicially but the power output deminished after a few tryouts. I think it would be best to source out where I could send the radio to be repaired and find out how much it would cost. the thing is I would have to sent abroad since there is no repair shop for amateur stuff on the island. I'm guessing I would be better off buying a new radio.

Thanks for your input.

regards,

Raul