FT 290 problem


colin <m5fra@...>
 

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


Bill Hall
 

I am not familiar with this fault but looking at the circuit diagram I would suspect the -6.8 volt supply. If this is low or zero volts it could cause the symptoms you describe. The most likely culprits are C14 and C15 going leaky. This is a common problem with 7660 DC-DC convertors.

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


Mansueto Grech <mansueto.grech@...>
 

Hi Colin,
I have the same problem. I tend to believe that the Electrolytic Capacitors in the AGC / ALC  are not good. I did try to replace them but I made the mistake and not used low ESR type. The problem with the TX is More or less cleared but on RX still does it. I believe that electrolyte leaked into the PCB, Which Looks like pressed cardboard, and again the problem on RX. Mine usually clears in 20Mins or less. But in Summer the problem is much less pronounced.

What I do now, and this is not a cure, I is I try to leave it on for a couple of hours every day.
Hope I was of some help
73's
and Good Luck. Please do keep us informed of your outcome.
Mans. 9H1GB


On 15 November 2011 07:40, 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



Bill Hall
 

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


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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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


Mansueto Grech <mansueto.grech@...>
 

Hi Coin,
Don't give up on the rig.
Writing it off is not the solution.
73's

Mans. 9H1GB

On Nov 15, 2011 5:18 PM, "M5FRA - Colin" <m5fra@...> wrote:
 

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@...>
To: ukmicrowaves@...
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@..., "colin" 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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