[Milsurplus] RBM HF Receiver Help Needed

[hwhall at compuserve.com]

 Actually I spoke too soon, it could be any of the 4 trimmers inside T-502. C514, C515, C516 or C517.

>wouldn't the problem be limited to one band
No, because all of the trimmers are connected between R505 & ground via L505/506/507/508. If one goes shorted R505 will be between B+ and ground. Hope none of those coils got too hot, too. Only the main tuning cap section is switched inside T502.

IIRC, to access the T502 box you have to open the topside main tuning cap cover. Some solder connections to be undone there & the mounting screws I believe. You'll also have to pull the bandswitching shaft out. Disconnects near the tuning gear works and can be extracted backwards to the rear of the chassis.

Wayne

 

 

-----Original Message-----
From: Joe Connor <joeconnor53 at yahoo.com>
To: hwhall <hwhall at compuserve.com>
Sent: Fri, Feb 3, 2017 10:14 pm
Subject: Re: [Milsurplus] RBM HF Receiver Help Needed



Wayne, if C-517 shorted out, wouldn't the problem be limited to one band? Also, C-517 is inside T-502 and the problem is how to get into T-502. If I could only get inside or drop that box, I could text all those components.


Joe Connor
 
1


 
 
 
 On Saturday, February 4, 2017 12:08 AM, "hwhall at compuserve.com" <hwhall at compuserve.com> wrote:

  

 

Isn't C-517 one of those little trimmers that sometimes short out when the brass bit retaining the rotor fails?  I'd suspect that puppy first, from personal experience.
 

 Wayne
WB4OGM

 

-----Original Message-----
From: Joe Connor via Milsurplus <milsurplus at mailman.qth.net>
To: Milsurplus List <milsurplus at mailman.qth.net>; BoatAnchors QTH <boatanchors at mailman.qth.net>
Sent: Fri, Feb 3, 2017 9:46 pm
Subject: [Milsurplus] RBM HF Receiver Help Needed




I just finished my MF RBM receiver and it's working great. I just started the HF receiver and need some help. 


R-505 (the RF plate filter resistor) is smoking. I go to the plate of the RF tube and measure resistance. It's supposed to be 18.5K but I'm getting a dead short.


I disconnect the lead from R-505 that goes into T-502 (the RF assembly). Now I've got B+ on R-505. I disconnect the lead from T-502 to the plate of the first RF tube so that T-502 is completely out of the circuit, and test T-502: it reads a dead short, so it seems like the RF assembly is shorted. A likely suspect is C-518, a .02 paper cap that bypasses the RF plate, but that cap is inside T-502.


Does anyone know how to get into T-502 or how to drop the box? I just don't see any way to do it. 


Failing that, can I bypass the RF stage and, if so, how? Any other ideas of what might be shorted?


As always, thank you for your help.


http://www.virhistory.com/navy/rcvrs/ww2/rbm/RBM5_HF-schem.pdf



Joe Connor


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