[Test-Equipment] Hickok 539B with shorts light problem

[Pat]

Kim:

Thank you!

I did find Schoo's information and the schematic from Bama. The cal 
procedure he outlines is where I discovered the shorts light wasn't 
working.  The only cause of the light not working that he mentions is 
dirty or broken switching.  I am, of course, hoping to find an open 
resistor. (Something I can fix) Finding the two undocumented 47 ohm 
resistors leaves me wondering what else is hidden somewhere.

I don't think it's a switch just because the general appearance of what 
I can see still looks good. That's not to say there isn't something just 
out of view.

Pat


On 9/27/2011 7:05 AM, Kim Herron wrote:
> HI Pat!
>
>      Don't know if this will help, but I'll pass it along anyway.  If you
> put Daniel Schoo into google, you'll get to his website and in there
> you'll find about 5 pages of repair and calibration info on the 539.
> It's still out there.  I was looking at it a few days ago.  I'm
> guessing that you have an unresolved power supply issue and his info
> does address some of those things.  Hope this helps.
>
>> All:
>>
>> I have a Hickok 539B with a non-functioning shorts light.  I have a
>> known "bad" tube that a BK600 and a TC162 flag as leaky that will not
>> fire the bulb. Even using a simulated short using a 100K resistor at a
>> socket will not fire the bulb. Fooling with the switches will not
>> aggravate the lamp into firing. The switches and interior are still
>> relatively clean.
>>
>> Here's what I've done so far:
>>
>> 1. tested the lamp by using a fixed series resistor and a variac, no
>> problem, bulb is okay firing at about 60 volts
>> 2. checked the three fixed resistors shown near the lamp in the
>> schematic and tested the 250k sensitivity pot and toggle switch, all okay
>> 3. checked plate supply, okay
>> 4. checked screen supply, okay
>> 5. general visual inspection for any cold solder joints, none found
>> 6. replaced paper caps with mylar, none of the originals found leaky
>> 7. none of the postage stamp caps were replaced
>> 8. There were two 47 ohm 1/2 watt resistors embedded in the shorts
>> switch assembly that don't show on my schematic, one was discolored,
>> both were replaced
>>
>> The tester works but reads high due to the bias meter reading too low.
>> Using a DVM to set bias gives a plausible reading.  I can measure a
>> resistor across a tube socket on the gm meter but it is off about 100k.
>>
>> Is there anything that commonly goes bad after 50 years in this shorts
>> test circuit that I have over looked? I've probably got about 20 hours
>> of spare time into this thing already. I've not really been able to
>> follow the schematic back to the voltage source. Perhaps someone already
>> knows how to check that source.
>>
>> Pat
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>