[Boatanchors] H500 Transoceanic Puzzler Solved

digital-conjurers at roadrunner.com digital-conjurers at roadrunner.com
Fri Jul 13 17:49:20 EDT 2012

To the group:

Thanks to all who contributed help on this; as fate would have it, it 
turned out to be a fluke partial short!

On this particular set, the 230 volt option was not made available; so 
where the switch for that would be was an under-the chassis 
"option-defeating-wire-around" arrangement consisting of some lugs/tie 
points, and a square of phenolic-like material to insulate the unused 
switch lugs from the chassis. (The line cord solders onto the pins of this 
unused option arrangement.)  Somehow it had developed an intermittent 
partial short to ground under the phenolic square insulating material.

I only discovered it when I accidentally put a bit more pressure than usual 
on the lugs while removing a cap for testing: applied pressure, the partial 
short went away, set behaved fine.  Pressure relieved, the short came back 
to it's partial-short state, and the hum came back.

It's the darndest thing in this line I've seen in 18 years, never seen it 
in a TXO.

May someone profit by this!  Again, tnx to all who responded!


Original Post:

OK, folks, after several weeks of troubleshooting this thing, I'm throwing 
the door open for some help.  In 18 years of working on vintage sets, this 
one has the distinction of being one of the most unpleasant.  Done several 
TXO's, and this is the most refractory set ever.

Problem: incurable audio distortion and hum under AC power.  (Clue: When 
one unplugs the set the problem goes away for 1/2 second before the set 
shuts down; audio is clear for that instant.  Thought it might be a power 
supply issue, but...)

Standard solutions tried, all check out:

1. Standard recap, wax and paper as well as electrolytics.
2. Replaced any resistor 20% or greater out of tolerance.
3. Verified all voltages
4. Replaced "unknown" diode (previous tech's work) with standard 1N4004 
with appropriate dropping resistor
5.  Verified all wiring several times
6. All tubes thoroughly tested on a Hickok 539B
7. Disc caps checked
8. Isolation from ground of chassis-mounted power dropping resistor

(Interestingly enough, the schematic that came with the set lists a 1U5 for 
the det/amp, while the schematic on nostalgiaair.com lists a 1S5.  Probably 
has nothing to do with it, just different production runs.  The set has 
what's called for, a 1U5.)

About all I am left with is chasing some stray, "fails-only-under-load" 
component (like an AVC cap with some resistance in it that does not show up 
under static voltage test with cap checker), or, or...a bad NEW cap? 

Posted a mp3 recording of the audio, and also made a copy of the schematic 
that _came with the set_ at www.moonlightsys.com/misc if anyone wants to go 
up there and  have a peek/listen.

I'd be very interested in opinions!

Thanks to the group in advance,


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