[Boatanchors] H500 Transoceanic Puzzler

[Kevin McDonald]

Hi Lin,
I figure you have already been through the power supply section but I found 
this out in the Ethervoid:
http://www.ohio.edu/people/postr/bapix/H500_2.htm
The author also seems to have had some trouble getting the power supply in 
shape.
Kevin N5OJF

----- Original Message ----- 
From: <digital-conjurers at roadrunner.com>
To: <boatanchors at puck.nether.net>
Sent: Wednesday, May 30, 2012 6:07 PM
Subject: [Boatanchors] H500 Transoceanic Puzzler


> 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? 
> (unlikely.)
>
> 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,
>
> Lin/KJ6EF
>
>
>
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