[Boatanchors] [National] HRO-50T Help Needed

[Richard Knoppow]

----- Original Message ----- 
From: "Joe Connor via National" <national at mailman.qth.net>
To: "National Radio Equipment" <national at mailman.qth.net>; 
"Boat Anchors List" <boatanchors at mailman.qth.net>
Sent: Monday, November 17, 2014 6:18 AM
Subject: [National] HRO-50T Help Needed


I need some help confirming my troubleshooting on this 
receiver. I recapped it and aligned it several months ago. 
It worked great until last night. Here's the Rider 
schematic:
http://www.nostalgiaair.org/PagesByModel/395/M0012395.pdf

Last night, it suddenly lost strength and sensitivity. 
Signal injection showed that the problem was in the IF 
stages. An IF signal injected into the grid cap of V-6 (2nd 
IF/6K7) produced a strong signal. An IF signal injected into 
the grid cap of V-5 (1st IF/6K7) produced a weak signal. An 
IF signal injected into the plate of the 1st IF (V-5) 
produced a strong signal.
I checked voltages and there was no plate voltage on V-5. 
The plate voltages for V-5 and V-6 are fed through R-17, the 
plate filter resistor. R-17 has plate voltage as does the 
plate of V-6. A resistance measurement from R-17 to the 
plate of V-5 showed an infinite reading.
As I interpret those readings, it looks like an open primary 
in T-6, the 2nd IF transformer. Does that make sense? Does 
anyone have any other ideas? What puzzles me is how I can 
get a strong IF signal through the plate of V-5 if the 
primary of T-6 is open. I'm wondering what could cause it to 
open so suddenly.
If you all agree with my diagnosis, I plan to remove T-6 
and, hopefully, I will find a break that I can repair. Of 
course, if anyone has a spare T-6 lying around, I would be 
interested in buying it. I suspect, however, that will be 
like looking for the proverbial needle in a haystack.
As always, thank you for your help.
Joe Connor


     The IF fed into the plate of V-5 might be coupled by
the capacitance between the two windings and appear at the
grid of the following tube. I think it would depend on where
the coil opened up. If you are not getting B+ at the plate
of V-5 the coil is almost certainly open.  If possible open
the IF can and inspect the coils and terminals. Its possible
the terminal has opened up in which case you may be able to
re-solder it. I am not familiar with the IF transformers
used in the HRO-50 but I've seen others open up this way.

--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com