Yaesu FT-200 no RX after power up

[lolomovil]

Hello All Members,

I've a Yaesu FT-200 with a strange fault and i hope that the group members could help me to fix it. I've little experience repairing tube radios and transceivers. When i power up the set the RX is normal in all bands and modes, but after one minute the s-meter needle drift to the right and the RX is lost. I checked the audio output section and is ok. I downloaded the service manual and i checked the resistance values and they are ok (some are a little out of tolerance). Then i measured the voltages of the tubes and i find that the grids of the if amp section tubes V103 (6BZ6) and the V104 (6BA6) are at -19V. The cathode of V104 is at 0v. The plate and screen voltages appears to be normal. I checked the V3 Pre-Mix (6CB6) voltages and the screen voltage is 120v (the service manual indicates 40v). The power supply is fine because i checked the transceiver with another one with identical results. Finally if i power the set using a variac adjusted at 180V AC (i've 240V AC in my country) the RX fault disappears and the IF voltage values are normal. Could be a open capacitor or resistor problem? The difficult is guessing what capacitor or resistor must check?. I'm thinking in disconnecting the AGC bus removing the D-103 diode, to help to isolate the fault, but i don't know if it is a correct approach.

I hope to get help to fix this failure?

Regards
Manuel

[Station X]

High screen voltage on V3 could indicate that the screen dropper resistor R42 has gone low in value. It's something that's easy to check.

The same HT line feeds V7A's screen grid, so it'd be worth checking the voltage there

[Nuvistor]

It gives me the me the impression of an unstable IF/RF amplifier, which goes unstable after the valves have full warmed up, the high neg AGC voltage on the grids of the IF amplifiers is an indicator and works ok with reduced power supply which will reduce the gain of the stages below the instability. The S meter going high will probably follow the AGC but I have not checked that.

Just my thoughts.

[lolomovil]

Originally Posted by Station X

High screen voltage on V3 could indicate that the screen dropper resistor R42 has gone low in value. It's something that's easy to check.

The same HT line feeds V7A's screen grid, so it'd be worth checking the voltage there

Thank you for your response. I'll check R42 again, but i remember that the value were ok. I'll check V7A's screen grid as you suggested.

Originally Posted by Nuvistor

It gives me the me the impression of an unstable IF/RF amplifier, which goes unstable after the valves have full warmed up, the high neg AGC voltage on the grids of the IF amplifiers is an indicator and works ok with reduced power supply which will reduce the gain of the stages below the instability. The S meter going high will probably follow the AGC but I have not checked that.

Thank you for your response. I'll check the cathode resistor R120 and capacitor C121 as the cathode voltage is 0v at full power. With low power supply the cathode voltage is normal. Could the 0 cathode voltage be the cause of unstable IF amplifiers? Is a good idea remove the AGC diode to isolate the problem?

Regards.

[Nuvistor]

With -19 volts on the grids I would expect 0 volts on the cathode, the valve will be cutoff and passing no current.

[Nuvistor]

After checking those items that Graham suggested, if those do not cure the fault the next step is to determine why the AGC voltage is so high.

[Richard675]

Manuel, you could try freezer spray on likely R's in the AGC

[PeterRoberts]

Hi Manuel,

There is a Yahoo Group specifically for the FT200 https://groups.yahoo.com/neo/groups/FT-200/info and they were helpful when I was repairing a FT-200 a few years ago.

Your problem does sound similar to one of the faults I had - in my case V104 was leaky and replacing that with an EF93 (equivalent to 6BA6) cured the problem.

If you don't have any luck try the FT-200 Yahoo Group.

73,
Peter

[lolomovil]

Hello, i partially fixed the fault. As i stated the grids of the if amp were at -19 volts. Next i discovered that V2 grid (Rf amp) were at the same voltage, but after disconnecting R26 the if amps grid returned to the correct voltage. This carry me to the problematics C55 and C40 region so i replaced them (they were a bit out of tolerance). Next i powered the set and the fault was still there but the negative voltage were at -6V. As it seems that C55 is connected on the RF amp grid bus i started to check the PA section and decided to remove the PA valves V6 and V7 and for my surprise the fault disappeared. Next thing was install a 6KD6 spare valve that was on my drawer in one of the sockets (i don't have another pair of 6JS6C) and alternatively put the two 6JS6C in the another socket. This took me to discover that one of the 6JS6C valves produced the fault. I don't know what exactly is bad in the valve, but this fail produced the RX problem. A strange failure in my opinion. I must test more conscientiously in rx and tx but the set appears to work well.

So, next thing is get a pair of matched 6JS6C or 6KD6. Thats the question, could i use the 6KD6 valve instead of the 6JS6C?.

Thanks for all the suggestions and i hope that my experience has some value for another owners.

[orbanp1]

There is a mod to upgrade the finals in the FT-200 with 6146B output tubes:
http://www.wd8das.net/FT200-6146.pdf

Regards, Peter