Re: {Collins} Very strange problem discovered during LAB alignmentonKWM-2 RE

----- Original Message ----- From: <kc9cdt@xxxxxxx> To: <wcarns@xxxxxxxxxxxxx>; <david.hallam@xxxxxxxxxxx>; <jsternmd@xxxxxxx>
Cc: <collins@xxxxxxxxxxxxx>
Sent: Thursday, July 07, 2011 8:58 PM
Subject: Re: {Collins} Very strange problem discovered during LAB alignmentonKWM-2 RE

Bill, do I get her out of there?
And are you saying to take her apart?
Never worked on a Mechanicle filter before..I thought they were not sericable?

I think in your original post you said you had already worked all the screws in the chassis. If I am wrong and you have not done that yet do it, it cures lots of problems. The idea is to isolate the problem. If nothing responds to poking but the mechanical filter it may well be the problem. The filter itself is not repairable, its in a sealed can. About the only way of proving the trouble is the filter is to find or borrow another filter and see if the problem goes away. There is probably a way of by-passing the filter with a simple resistor network. It won't have any selectivity but will provide a signal path when the filter is removed. Someone on the list may know how exactly to do this. The idea is to see if the voltage you are reading remains steady with the filter out of the circuit even if its not the right value. Mechanical filters do go bad so its worth investigating. If it turns out not to be the filter then other possibilities can be followed up but right now you are saying that tapping on the filter makes a significant difference. It is also possible for this to be a bad tube, try tapping on the tubes in the IF chain while the chassis is flat and when its on its side to see if anything happens. It is _possible_ although unlikely that there is either something loose in one of the tubes or that something is sagging. Again, a small chance but you have tried all the more obvious things. If you have not poked at the wiring try that. I use a chop stick but something like one of those wooden coffee stirring things will work. Its possible that there is a solder joint that has gone bad and is barely making contact. You may have to poke at things that don't seem likely to find it. Also, carbon comp resistors can split without the crack being obviously visible, they sometimes act like this. If you don't see anything obvious when inspecting get someone else to look. It is easily possible to overlook something obvious that someone else will spot right away. When I worked for Hewlett-Packard we had a rule that if you couldn't find a problem in a reasonable time get someone else to look. I remember working on some signal generator with a power supply problem and not being able to find the problem, my mate looked and immediately pointed to an exploded power resistor in the middle or the chassis. It was as obvious as a skunk on a white sheet but I missed it for half an hour.

Richard Knoppow
Los Angeles

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