EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

Ok gang...so I have an AW 23 that I fully restored about 4-5 years ago. Played great. For lots of reasons, it hasn't been run much since then. I ran it up on the Variac this weekend, and here's what happened: AM and first SW band are dead, and the osc VR tubes don't light. On SW2 and SW3, osc VR lights up and the set plays great. Good audio on the phono input too.

So I take the tuner apart and check the osc circuit. The grid resistor is significantly off value, so I replace it. Clean contacts (which were still very clean), and still nothing. Voltage is only slightly below the VR threshold on AM and SW1, but just low enough for the tubes not to fire...and no signal. I'm stumped at this point.

Thoughts and suggestions appreciated!

Kent

Views: 139

Comment by David C. Poland on June 1, 2015 at 4:04pm

So the problem is ahead of the IF amp.

There is some switching in the RF section.  How about the switch in the mid chassis coil can controlled by the the band switch shaft from below? And the first switch on the wave change shaft?  Loose wire or bent contact.?  Associated RF coils for BC and SW1 have continuity and not shorted to ground?

Checked for B+ voltage sag  on both sides of the 10K resistor feeding B+ to the VR's to confirm a drain is after the VR's? 

Check for pot metal deterioration on the castings that supports the Coil wheel, that might cause the wheel to misalign and fail to properly lock in mechanically. Pressing on the coil wheel assembly have any effect?

Comment by Norman S Braithwaite on June 1, 2015 at 6:00pm

I agree with Dave that it is likely a switch contact problem.  One potential cause of the oscillator voltage sag is strong parasitic oscillation that causes to much current draw through tubes in the RF or IF sections of the receiver (affecting B+ on the power supply side of the 10K resistor).  Oscillation may due to a bad switch contact leaving an unconnected wire in the circuit to act as an antenna.  But, before putting time into switches, swap out the oscillator tube to make sure the problem is not a due to a misbehaving tube.

Norman

Comment by Bill Liers on June 1, 2015 at 6:16pm

I was also going to suggest changing or for test, removing the oscillator tube. If the problem is caused by a parasitic oscillation, it will stop. How far does the Osc. B+ drop when Bdcst is switched in?

Comment by Kent King on June 2, 2015 at 8:47pm

Thanks for the ideas...I didn't have time to look at it last night, but did talk with Dave on the phone for a few minutes. He suggested checking the switch that is under it RF cover on top of the chassis (driven from the wave change). I thought we might have something, using a dental mirror, it did not look like the contact was touching on AM, so I gently bent it and got a clear contact. However, it made no difference.

Bill...to answer your question, the osc B+ voltage on the two working bands is 138 VDC with a nominal B+ of 235 VDC. When I switch to the "dead" bands, it drops to 110 VDC. I have also swapped the osc tube and the converter tube as well. Nothing has worked so far. I'm going to try to take more time with it later this week. I'll keep everyone posted. Ideas still appreciated.

Comment by Bill Liers on June 2, 2015 at 9:28pm

Kent,

If the problem is a parasitic oscillation, you will find that the voltages return to normal (or are at least the same between bands) when the oscillator is pulled out. Otherwise, something partially shorting the regulated line. I assume the B+ stays close to 235 when the problem happens.

keep digging...

Comment by Norman S Braithwaite on June 2, 2015 at 11:28pm

This is very similar to a problem with which I am helping Bill DeRyke.  I will review my correspondence with him and report.  But before sinking more time into this, have you removed the tuner and made sure the machine screws electrically connecting the solder lugs are tight?

Norman

Comment by Kent King on June 3, 2015 at 7:42am

I will do that this evening and see if it helps. Thanks. I do appreciate all the help.

This set played great a few years ago, although I did move it to my new radio space last fall. Old electronics...who knows?

Comment by Bill Liers on June 3, 2015 at 9:39am

As my Grandpa used to say, "everything works til it doesn't".

Comment by Kent King on June 3, 2015 at 10:35am

Norman...the solder lugs on this set are all riveted.Should I drill them out, clean them and then set them with screws? Or leave as is? The set works on the two upper SW bands, which is where I'd expect more problems than on the MW band...

Comment by Norman S Braithwaite on June 3, 2015 at 2:38pm

The solder lugs connected to the tuning capacitor stators are fastened using machine screws through phenolic.  The phenolic shrinks allowing air between the solder lug and the head of the machine screw thus allowing surface corrosion and resistance to develop.  It does not take much resistance in RF circuits to foul performance.  I agree that the problem, being limited to the middle two bands, tends to indicate other problems but a combination of resistance in RF connections and RF alignment of the individual bands may be the culprit.  If the chassis is relatively free of corrosion, I would not worry about the riveted ground returns at this time.

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