[AMRadio] HRO-50

jeremy-ca km1h at jeremy.mv.com
Tue Nov 27 15:48:44 EST 2007

The very early HRO's were aligned with the coils that were shipped with it 
and also serialized to the receiver. Finding a matched set is very hard and 
commands big bucks. Ive enough to do on my 2nd run March 1935 version 
without worrying about matched coils (yet).

We got plenty of all era HRO's in for service and a jig was not used. Once 
you got the knack it was easy. The holes in the chassis are there for a 


----- Original Message ----- 
From: "D. Chester" <k4kyv at charter.net>
To: <amradio at mailman.qth.net>
Sent: Tuesday, November 27, 2007 1:56 PM
Subject: [AMRadio] HRO-50

> My pre-WW2 HRO's have a  hole in the chassis just behind the panel and 
> next to the main tuning capacator.  You can access the trimmer caps with 
> the coils plugged in.  I'm pretty sure the later models also have this 
> trimmer access. I use a  long 1/4" nutdriver.  It doesn't seem to detune 
> the coils, but be careful.  Some of those adjusting screws carry the full 
> B+.  I have been knocked on my bum while aligning HRO's.
> The low-end adjustment requires opening up the coil to move a one-turn 
> loop of wire, which changes the inductance of the coil.
> The only way I could think to make this adjustment other than what you 
> mentioned, would be to build up a jig where you could mount the coil with 
> the cover removed and gain access to the coil, while it is connected to an 
> oscillator tube or an rf amplier tube with an external signal generator 
> and output meter, to indicate when the rf coil or oscillator coil is 
> properly aligned.  Even that would be approximate, because the adjustment 
> may be off a  little when the coil is re-mounted into the shield and the 
> whole assembly is inserted into the receiver.  For the alignmentt jig, 
> perhaps a spare shield could be modified by cutting open the end to allow 
> access to the coil with at least most of the shielding in place.
> This would be a good project for National affectionados who regularly do a 
> lot of HRO alignment.
> Don k4kyv
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