[Boatanchors] Comet Pro Trasformer Went South

[Bill Cromwell]

On 07/14/2013 09:06 PM, Joe Connor wrote:
>
> I picked up a Hammarlund Comet Pro at a hamfest today. I changed the filter caps, ran it on a variac at about 105V and got it playing. Then the power transformer went. I want to go through my diagnosis just to make sure I've eliminated any possibilities other than the power transformer. I also want to get any ideas anyone may have on a replacement.
>
> Diagnosis: The xformer started smoking with the unmistakable bad odor. B+ dropped badly and the receiver stopped playing. I pulled the rectifier tube and the xformer still overheated. I replaced the rectifier and disconnected the lead from the rectifier to the first filter cap/filter choke and it still overheated. Resistance between the filaments on the rectifier socket (with the rectifier removed) measured 0.1 ohms. The plates on the rectifier socket measure 194 ohms. Plate to ground measures 112 ohms and 82 ohms. I just want to make sure it's the power xformer. I have restored close to 100 boatanchors and this is the first power xformer I've had go south. I guess I was living on borrowed time (or else replacement of filter caps before the initial fire-up really does avert a lot of disasters).
>
> Replacement: Any ideas on replacement?  This set uses tubes with 2.5V filaments so I'm sure my options will be limited. This set is pretty beat up so I'm not sure how much I want to spend. On the other hand, without a new xformer, this set is just a big doorstop. There is space on the top of the chassis for a separate plate transformer, if something like that is possible.  Of course, if anyone has a junker laying around with a good xformer, I would be interested in buying it (along with a few other non-essential parts that are missing from mine)
>
> As always, thanks for your help and guidance.
>
>
> Joe Connor
>

Hi Joe,

I don't have a Comet PRO so I went to BAMA and pulled the schematic. I 
see there are several tubes (I counted 3) with screen bypass caps 
straight to ground. There are several other caps that could load down 
the B+ and others could apply a more positive potential to the grid in 
the following stage and make that tube add to the overload. Those caps 
are how old now? Just the filter caps might not be enough. I lost a 
radio (okay I repaired it) due to a runaway tube frying parts with 
positive bias applied to the tube's grid through one of those leaky beasts.

I bought 2.5 volt transformers from "Tubes and More" on the internet to 
run a SW-3. Yours will need a bigger one (the ten amp model) with all 
those tubes. Then all you need is the plate supply. If you can't find an 
antique replacement and have the room, as you say, that approach will work.

Good luck with your project.

73,

Bill  KU8H