Help! Dynaco FM3 heater choke

[billinchile]

I recently acquired a Dynaco FM3 through the internet.
Last night, while re-checking the V2 tube and then replacing its tube
shield I inadvertantly left the shield in contact with L5 heater
choke. When I turned the tuner on, there was a short, of course, and I
got smoke from the choke which now is quite charred.

What I need to know is, how can I replace the choke? Are original
replacements are available, or at least a generic substitute? The FM3 uses 5 or 6 of these things. The schematic doesn't give a value for it.

Thanks for any help you can provide.

Bill Gallagher

 

[ingvar ahlberg]

Hi Bill
If the choke is in series with heater value woldn´t be too critical. I have the kit manual so i´ll read through and see if value is mentioned, otherwise You could meassure one of the others and buy a standard taking heater current into consideration.

 

[billinchile]

Hi, Ingvar,

Thanks for your reply. Please understand you are talking to a novice. I'm not sure how to determine whether the choke is in series with heater values. Does it refer to the heater value of the V2 tube? I also have the kit manual and found no reference in int to the value. It appears that these heater chokes - L2 through 25 - are all the same. There is no reference to the kit builder installing them on the board, so I assume they came already installed. I am not sure how to measure the value of such a component. What value would it be in? I read something about milihenries, which I have no idea what unit this would be. Nor would I know where to buy a "standard" or how to "take heater current into consideration. I'm afraid someone with patience would have to walk me through the process.

Kind regards,
Bill


Hi Bill
If the choke is in series with heater value woldn´t be too critical. I have the kit manual so i´ll read through and see if value is mentioned, otherwise You could meassure one of the others and buy a standard taking heater current into consideration.

 

[ingvar ahlberg]

Ok Bill, i´ll look through what info i´ve got on the choke.What You should measure is inductance, that´s the (milli) Henry figure quoted. Not easily done without special instruments.
Are You shure there is anything wrong withe the choke, other than looks. The short would cause a spark, some smoke and a bit of smell but not neceserealy destroy anything. Did You try the tuner after "accident". If not, do, nothing will break even if the choke is faulty.

 

[billinchile]

Hi,
Well, the tuner still works. But there is this lingering burnt smell... I have located a source for the coils. Wouldn't it be wise to replace the burned one and have a few spares just in case?

 

[ingvar ahlberg]

Good it works, if You found coil replacement change the burnt one, even if still´s ok it feels better and You will be shure that laquer coat on coil is allright.

 

[billinchile]

I have been corresponding with a person who has 4 of these coils from a disassembled FM3 and who will sell them to me for $10 dollars for the four. Sounds pretty reasonable to me. I assume that all 4 are the same value. The FM3 has several heater coils, L1 through L5, I believe.
What do you think?

 

[ingvar ahlberg]

Sounds reasonable as they are original, a new would be a buck but it would be hard to find exactly the same as a stock product.
All are same value wich is not critical, somewhere around 10mH @ 500mA would surely work, they are there to prevent hf/mf interference as a filter together withc34-c38. The original will look better.