[Milsurplus] RBM HF Receiver Help Needed

[John Hutchins]

Joe -

    Also be aware of the possibility of whiskers that may have developed 
across these trimmers or even the main tuning cap.

Hutch

On 2/3/2017 11:32 PM, hwhall at compuserve.com wrote:
> 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 
> <mailto:hwhall at compuserve.com>" <hwhall at compuserve.com 
> <mailto: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 
> <mailto:milsurplus at mailman.qth.net>>
> To: Milsurplus List <milsurplus at mailman.qth.net 
> <mailto:milsurplus at mailman.qth.net>>; BoatAnchors QTH 
> <boatanchors at mailman.qth.net <mailto: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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