Re: FT 290 problem


Brian Flynn GM8BJF
 

Dear Colin,

I had exactly this problem with my 290 which I use as an IF radio for microwaves. When the set was cold (when out /P in GM!) the S-meter would "pin" and the the ALC would cut in full on Tx. I first supected caps leaking but found that it was more subtle. It was a bit of dodgey design by Yaesu!.

The problem lies at the base of Q1014 which amplifies the output of the AGC/ALC rectifier diodes. There is no bias resistor on the base of taht transistor and it depends on the leakage of the diodes to keep the transistor turned off on the absence of signal. The solution I found was to add a 47k resistor between the base and emmitter of Q1014. Presumably when the diodes are cold they are less leaky in reverse and the base charges up turning it on. My set has been fine since doing this and has been tested overnight in the refidgerator. It is most easily done adding an SMD resistor on the track side of the PCB. It also is a low risk fix to the PCB.

G3LLL first described this mod a in a PW article in August 2007. I have a scan of the original article and I think it is in the files section of this group if you search for it.

Hope that is of use.

73s

Brian,

GM8BJF.

--- In ukmicrowaves@yahoogroups.com, "colin" <m5fra@...> wrote:

My Mk1 290 has developed a fault - on switch on the S meter goes hard over and the TX will not work. After a period of time, depending on how long the radio was last used, the S meter gradually returns to zero and the TX O/P rises.

This is a common fault and the only cure I have seen is to remove Q1014 which kills the AGC. Not a good fix.

Has anybody any ideas on a proper repair?

Colin - G8FRA/M5FRA

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