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Antique Radio Technical Forum
Troubleshooting a screw?

Posted by Peter G. Balazsy on 04/28/2009 03:33

Here I was tonight gathering up the loose odds and ends of a nice old Emerson ED354 that I basically had completed last fall. It was awaiting cabinet work so it sat all winter.
One of my customers who tells me he does furniture refinishing offered to help do a couple cabinets for me at $25 each... so I gave him two to do.

Well he brought them back today ( not my level of quality... but hey for $25 I couldn't make him feel bad) and anyway...that's what prompted the need now for me to get this Emerson reassembled.

I saw that it still needed a new power cord though but everything else had been done. So I hooked up a temp power cord and fired it up as a test while my iron warmed up to solder on that new cord.

Well it came to life quickly and sounded beautiful... so I shut it off and put on the new AC cord.
But when I turned it back on.... There was no signal anymore.

What?

So I quickly checked that everything is in order.. antenna connection... B+ ...yes, yes yes..
Plenty of static.. but no signal.

Fire up my scope to see the oscillator... sure enough that's fine!
Still... no stations coming in!

Fire up the signal generator to 455khz modulated... connect it to the ext. antenna terminal lug on the loop antenna ... and boom!.. plenty strong tone!

Okay I've got IFs aligned and signal getting ALL the way through right from the antenna.
BUT no stations are coming in off the air like it was before I put the power cord on!???

What the heck kind of a problem is this I thought?

So I now changed the signal generator to 1-Mhz with modulation and connected it right to the antenna... and sure enough as I turn the tuning dial.. the signal is there right where it should be on the center of the dial....

Ok.. so that means that EVERYTHING is in fact working.... yet I still can not get a signal off the air!

What gives here?

So I clipped another antenna lead to the grid of the 6sa7 RF stage... but I still got no signal... except a slight weak one with volume all the way up.. and that is a nearby 50kw station... how can this be??

So fiddling around measuring voltages on the RF stage .... and each time I touch pins, the leads make static sounds that are real loud... but the signal is still down in the mud!

just shot-gun fiddling...I clipped the external antenna right onto the oscillator coil and now I can get some stations... but way off frequency of course as I'm killing the oscillator.. but there is at least medium strength signals coming in now.

So I stopped and said... wait a minute I should be able to just use my brain and think this through... but just like Curly from the 3 stooges says.. " I'm trying to think... but nothing's happening"!!

This was now the craziest thing I ever had to deal with.

So while trying to think I started replacing a few wires here and there just to make sure I didn't have any internal breaks or cold joints... and nothing is making any difference.

I was drawing blanks. I had no clue what to do.
So for no good reason I tweaked the RF trimmer cap on the tuner a tad... and the signal came in loud and clear. But I just lightly touched it.
Then I turned it in and out and as it went in.. the signal died and backing out the signal returned.

Why?

Then I look at the other trimmer for the oscillator right next to it and I notice that the screw head looks a little different.
Sure enough... someone had replaced this trimmer screw... and it was just a few threads longer than the other one!
So as it got turned downward the tip would ground out or short the tuner cap!
So all the time before as the radio had played the screw must have been one tiny hair's width away from shorting... and it chose this moment to touch and drive me nuts!

I cut it shorter and the radio plays like a dream.
Who'd a thunk it?



Troubleshooting a screw? 
Peter G. Balazsy 04/28/2009 03:33 
Dennis Wess 04/28/2009 09:33 
Marv Nuce 04/28/2009 12:49 

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