[Boatanchors] Comet Pro Trasformer Went South

[hwhall at compuserve.com]

 If it were my next move, I'd disconnect all but the xfmr's primary and apply power. If the transformer doesn't get hot then you know you've a problem somewhere in the rcvr that is overloading the xfmr. If it gets hot just running by itself, you know the xfmr has windings gone bad such as shorted internally.? The resistance mismatch between the two rectifier plate leads (112 ohms and 82 ohms) seems to suggest a short on the lower resistance side. Ideally they would be almost the same resisitance. (Hope I'm wrong though!!)

Wayne
WB4OGM



-----Original Message-----
From: Bill Cromwell <wrcromwell at gmail.com>
To: boatanchors <boatanchors at mailman.qth.net>
Sent: Sun, Jul 14, 2013 6:56 pm
Subject: Re: [Boatanchors] Comet Pro Trasformer Went South

 
 
 
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 
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