Re: FT 290 problem


M5FRA - Colin <m5fra@...>
 

Bill, a very big thank you for the most coherent explanation of the
likely probelm. I have changed a few 10mfd electrolytics already, some
were visibly leaking others had a v.high ESR. Will have another look
before I write it off.

Colin - G8FRA/M5FRA

www.m5fra.org.uk

------ Original Message ------
From: "Bill Hall" <whhall@g3rmx.karoo.co.uk>
To: ukmicrowaves@yahoogroups.com
Sent: 15/11/2011 15:42:49
Subject: [ukmicrowaves] Re: FT 290 problem
Colin, here is more info for you.
If there is no negative 6.8 volt supply then Q1014 cannot take the gate of Q1015 negative and so it cannot be pinched off. This means that the S meter will always read full scale. The TX ALC problem is the same mechanism. The -6.8 volts is generated by Q4005 ICL7660 on the regulator board. This has probably stopped working because C4014 or C4015 have gone leaky. The extra current drawn is killing the DC-DC convertor. After a while the capacitor reforms enough to reduce the leakage and the convertor will recover and start working again. I would replace both capacitors if you confirm that this is the problem.

I hope this explanation helps.

bill G3RMX

--- 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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