FT847 failure on 4 meters after mod.

[G3TMZ]

Hi Guys, This is my first post so please bear with me. I recently did the 70mhz mod to my FT847 (C2) as outlined on this site. It worked pefectly for 2 weeks but now I appear to be back to square one. On both 70mhz and 50mhz the input current for 5 watts output is 18 amps. Thinking the output pair could have died, I tried the same test on HF and everything is normal.
I have not opened it up for the 2nd time yet and the cap I removed was the smaller of the 2.

Any help would be greatly appreciated. TIA.

[GI0GDP]

Haven't the drawings in front of me, but one would have to ask what is common to 50 and 70 MHz ?, pressumable it's the same LPF for 4 and 6? or is there one for each band, cant remember. but if HF is OK then it needs to be common thing here. Must take a look at the drawings to see.
Bet your glad it's not the PA devices!
regards
Geoff

[G3TMZ]

Thanks for the quick reply. I am presuming it uses the same LPF as there is no relay operation when switching between the two bands. Not being a duff PA transistor is a bonus but they are available in Hong Kong at $50 for 2 which is not too bad. A bit stuck as I don't have circuit.

Regards

Dave

[GI0GDP]

Hi Dave,
The 50 and 70 MHz signal is feed to LPF(6).
This is the common element here, from memory the LPFs are on a seperate board so you would not need to dig the PA out, the FT847 service manual is online. I would send it to you but it would take forever with this internet connection that i have at present.
regards
Geoff
of course it could always be something completley unrelated !

[G3TMZ]

Hi Geoff,

Many thanks for the info. I have finally found the manual on line and it looks like the filters are on the top RF board. I will let you know the outcome.

Thanks for your help.

Dave

[G3TMZ]

Hi Guys,

I have found the problem, L5022 in the LPF (6) has overheated and dropped off the PA module PCB. I've seen a similar post regarding overheating FT847 filters but can't seem to find it.

Has anyone any idea what could have caused this, any help would be appreciated.

Regards

Dave

[GI0GDP]

Hi Dave,
Good to see that the LPF was the cause, which logically it had to be.

Why did L5022 unsolder itself, i will give my reasoning but it needs to assume that with your newly modified 847 you enjoyed the sporadic E not only on SSB/CW but perhaps FM as well on 70 MHz.

Before going any further your reference to coils before was about L5006
which is NO longer regarded as the way to modify these sets on 70 MHz.
Your findings are relatively new, i think.

So the L5022 over heated due to resistive losses generating enough heat to melt the solder.
This is fairly common with some things i have made!
The way around this is to decrease the resistive losses in the coil by using thicker gauge wire and a coil form which is basically square ie diameter is the same as the length. This will decrease the resistive losses and hence the heating. This is of course "Q" of the coil, and for these sets i would imagine it's way<100.

The coil was never designed to handle this power on 70 MHz, where i imagine the resistive losses are higher than at 50 MHz for which it was designed for.

When you sort the coil out a small fan will aid the coil cooling.

Search on google for Marc Vlemmings it may clear up the coil in the PA as opposed to the LPF(6).
regards
Geoff
Of course you could always fit a bespoke filter and relays for 70 MHz
and switch it in from the "logic" transistor which was used to limit the power to 20 watts on 70 MHz
I think Marc quoted 100 nH for this coil.

[G3TMZ]

Hi Geoff,
Thanks for the reply. I did use it FM but I am "nearly" sure I reduced power.

Refitted the inductor but still no output and I dont seem to have a signal from the RF PCB to enable LPF (6) in the PA. Its a real pain to work on, going to make up some extension leads so I can see whats going on with the PA.

I will advise later !!

Regards

Dave