Antenna Coil Problem on AW23

[Joe Miller]

Hi everyone,

I'm up to my ears working on my AW23 (same one I've already got so much help with--much appreciated).

Anyway, I've powered it up and the only way I get any sound is if I put my finger on the grid cap of the RF amp (6D6).  The local station comes in loud and clear when I do that, but nothing when I connect the attenna.

Obviously this sounds like a problem with the antenna coils.  Fortunately, Bruce has shared an AW23 parts chassis with me that I can disassemble to see what's what.

I have found the switch that rotates when the band switch rotates under the can that covers the AM antenna coil. It looks like all the antenna coil secondaries (am coil under one can and the other 3 under another can) have continuity and the switch wiper does connect up to the RF amp grid cap cap.  So the secondary side of the coils seems to be okay.

I have traced the antenna wiring from the antenna post to a coil that surrounds the other 3 antenna coil primaries under their can.  I can't tell how the antenna wiring connects to that coil and I can't get a meter on it.  The schematic indicates that each of the antenna coil primaries are switched via the band switch (I assume like the secondaries are), BUT, I can't find any such switch. 

Anyone know how these antenna coils are configured?  I believe the problem must be somewhere with the connection from the antenna post to the grid of the RF amp?  Any other ideas?

Thanks,

Joe

[Norman S Braithwaite]

Joe:

The antenna switch is located on the bandswitch immediately behind the front apron of the chassis.  The AM antenna coil primary circuit can be checked by placing an ohm-meter across the antenna terminals with the AM band selected.  If open, possibilities include an open antenna coil primary (not uncommon) or the antenna circuit switch being 180-degrees out of phase (uncommon but possible if the bandswitch had been disassembled at some time.  The AM antenna coil primary is located inside the AM antenna coil form.

Norman

[Thomas Day]

Joe:

One thing that happened to me when I was cleaning up my Allwave 23 is that the wire that comes from the antenna coil in the center of the chassis to the RF tube had damaged insulation which allowed the wire to short out to the chassis. This happened AFTER I had the chassis back in the cabinet and working. This wire comes out of the large can and connects to a terminal strip between the RF tube and the tuning capacitor. There is a capacitor on this terminal strip. The wire is connected to the right end of the strip and then to the tuning cap. The other end of the capacitor connects to the grid of the RF tube. I was able to clip the damaged wire, solder and insulate a new wire and feed it thru the can to the terminal strip without taking the chassis out of the cabinet. 

There is also a switch at the bottom of the coil that moves with the band switch.

[Joe Miller]

I think I just found my problem.  The bevel gear assembly appears to have been out of sync with the bandswitch by about 2 teeth.  That was enough to not connect the antenna coil primaries when in the various bandswitch positions.  I was able to loosed the bandswitch enough to align it perfectly,  Now the antenna coils show continuity in all 4 band positions.  Haven't yet powered it up to see if it plays.  Crossing my fingers that this is the fix.

Will also check the wire you mentioned Thom.

Thanks,
Joe

[Joe Miller]

Primaries of the antenna coils all show continuity now. (one ohm or so for BC and the 1st two SW bands, the highest frequency SW band shows about 46 ohm).  The bevel gear misalignment was the problem with the primaries.

But, I had problems with the antenna secondary for the BC band. Was able to bend the wiper of the rotating switch for the secondaries a bit and voila, now I have connection through all the secondaries for all bands.

Found the grid cap wire to the RF amp connected to the wrong side of input capacitor--fixed that and powered it up on variac.

The set is super quiet--no hum and no background static.  The local station comes in super--loud and clear.  That's the good news.  That's about all that does come in.  One other station comes in faintly with volume, selectivity and sensitivity at max.  On SW, I heard one station very faintly.

So, I have a senstivity problem to troubleshoot.  Tomorrow, I will:

• Check all tube plate and screen voltages
• Maybe replace the rotating coil assemble with a spare that I have in an old parts set (the assembly appears to be in good condition.)
• Remove all the grid cap cans to verify I don't have a broken grid cap lead.

Some questions:

• Am I missing something obvious?
• What voltage should Isee on the AVC bus with a strong station tuned in?
• What voltage is needed to deflect the tuning meter full scale?
• Anyone every align one of these sets?

I'm not giving up until this baby blasts me on all bands.

Joe

 

 

[Norman S Braithwaite]

Joe:

You should check the alignment of the antenna switch primary.  The resistance across the antenna terminals should be 46-ohms in the broadcast band and low for the shortwave bands.  It is clear that someone has disassembled the bandswitch and has improperly reassembled it some time in the past.  I would proceed as follows:

1)  Remove or substantially loosen the portion of shaft between the bevel gear and the bandswitch knob to disengage this shaft from the coil turret and the antenna switch primary (right behind the front apron of the chassis).

2)  Place the antenna switch primary in the broadcast band position (rotate for 46-ohms measured across the antenna terminals).

3)  Replace the coil turret and rotate to the broadcast band position (blank coil points to side of chassis - it should only seat fully if installed in the correct direction).

4)  Rotate the removed or loosened shaft such that the flat faces the top of the chassis (broadcast band position).

5)  Reset the removed or loosened shaft making sure not to rotate the antenna switch primary in the process.

6)  If you had to bend the antenna switch secondary wiper (above chassis and within the broadcast band antenna coil shield), you may have to readjust it after correcting alignment of the rest of the bandswitch components.

7) CAREFULLY rotate the bandswitch through its range while observing the antenna switch secondary and other components to verify no conflicts.

8) Apply power and report back!

Norman

[Joe Miller]

Norman,

You are a genius--I don't know how you were able to diagnose my problem and propose a detailed solution off the top of your head, but I am impressed!

I made the alignment as you suggested and yes the bandswitch was off.  I now can get stations on BC and the SW bands, I think the marine is working too (sounds like it is) but just no stations to tune to.  Thank you so much for your spot on diagnosis and solution.

I have a few loose ends yet to finish up:

• I'm not seeing a  lot of movement on the tuning meter.  It moves a bit but never over half way across the dial even on a strong station.  What voltage should be on the AVC bus?  My Phantom had a loose grid wire and wouldn't move the target on the eye.  Had low AVC.  So I don't know whether or not I have a gain problem and low AVC or whether or not I need to adjust the pot in the tuning meter circuit?  The fixed resistor in the cathode of the tuning meter circuit is 1000 ohm rather than 750 ohm and the adjusment is set at 75 ohms.  My first check though is to see if my AVC is in spec once I know what the AVC  spec is.
• I have the Scott phono (schematic is in the Scott technical manual I got from Kent).  When the phono switch is in phono, the phono circuit is wired to the cathode of my tuning meter amp.  When the phono switch is not in phono, it provides a short across the phono terminal on the radio and ground.  As wired this would short out the tuning meter.  So this doesn't seem right.  Norman, on the schematic you sent me for this 5 knob set, the phono is shown injecting on the cathode of the first AF.  When in phono position,  the phono is in the cathode circuit, when not the cathode is shorted to ground.  I think this is the right configuration for my set, but my first AF has been modified due to the failed interstage transformer (resistance has been added in the cathode circuit--I've not been able to trace out all the wiring yet).  Penciled in on the schematic you sent me, the phono is shown injecting on the high side of the volume control which I think would work fine, except when I take the phono switch out of the phono position, which would short the volume control to ground.  So, I'm looking for ideas on how to connect up the phono?

Sorry for the long winded discussion, but I am very excited about getting this thing working.  The cabinet is a beaut and the phono is a neat extra.

After all this is resolved, I need to make a decision on doing an alignment--but that's a story for a different day.

Again, my hat's off to you Norman,

Thanks again,

Joe

[Norman S Braithwaite]

Joe:

Thanks for the complement.  I have worked on so many AW-23 chassis (including AW-27 and Quaranta tuners) that I am modestly familiar with the circuits and have become familiar with problems associated with the age of these receivers such as corrosion in ground returns and incorrect prior repairs.  Believe it or not, this alignment problem turns up on a repeated basis as well as a short-cut miswire in the IF AVC diode circuit that effectively defeats the IF AVC.  If your  tuning meter moves at all, your chassis does not have this miswire.  The most likely cause of anemic tuning meter response is IF alignment.  The AW-23 will work well even when the IF is not well aligned.  It will work even better when correctly aligned.  Before considering alignment, please print and review the AW-23 tuner chassis instructions that I have posted on this site.  It is not productive to align the tuner chassis unless the problems identified in that document are addressed.  The range of tuning meter motion in good working and correctly aligned AW-23 receivers is seldom more than 2/3 of the width of the dial.

The hand drawn phono input on the schematic that I provided came with the schematic.  It appears that someone noted the location for injecting the signal from a high impedance phonograph (crystal cartridge) at some time past.  The original location for injection of a phonograph signal is for a low impedance cartridge (early magnetic).  Without a better understanding of the modifications of the audio circuit in your receiver, I cannot comment on where you may inject the signal on your receiver.

Norman

[Joe Miller]

Norman,

Where is the document posted?  I don't see it.

BTW, I think I have a crystal cartridge so I will rig it to inject on the high side of the volume control.  Hope it works

Joe

[Norman S Braithwaite]

Joe:

Here's a link... http://ehscott.ning.com/photo/albums/aw23-restoration-checklist

It is especially important to loosen and re-tighten the set screws in the pot-metal collars holding the IF trimmers on the variable selectivity shaft and to loosen and re-tighten the screws holding the solder lugs to the under side of the tuning gang.

Norman

[Joe Miller]

Still giving this set the once over--it plays but I want to make sure everything is in top shape such that I get maximum sensitivity.  All the plate and screen voltages are dead on except for the RF amp which I haven't checked yet since I have to take the coil rotor out to get to it. All the grid leads are okay although the 250K resistors range from 217k 343K ohms,  A few questions:

• Are the grid lead resistors that have drifted okay or do I have to replace them? (I assume they are close enough)
• The cathode voltage on the noise suppressor is out of spec (supposed to be 9.5 volts but is -4.4 volts.)  I'm not sure how his circuit works.  Does it have any impact on the sensitivity of the radio?
• I have very little tuning meter deflection.  The grid voltage is 9.2 volts which is in spec, but it only changes to 7.5 v on a strong local station.  I can't tell what the milliamp rating is on the tuning meter to get full scale deflection.  Anyone know what it is? I assume it is 8 ma and with about 9 volts on the cathode and about 1000 ohms in the in the cathode circuit, the meter would be at full scale.  I assume a strong station will tend to cut off the tube and drop the cathode current to zero....but it's not happening.
• The schematic shows several resistors that are single digit numbers and for example look like 2 with the ohm sign--are they using the "ohm" symbol to mean megohm? I'm reading 2 megohm between the AVC bus and the grid of the 2nd detector.  I'm assuming the "1 ohm" is really 1 megohm.
• Still trying to figure out my selectivity control.  It isn't ganged to any bass control (cap selection).  Mine has a 5 position switch mounted on the back of the chassis that holds these caps for adjusting bass.  It is adjusted manually independent of the selectivity control.  Anyone ever seen one like this?

That's it for now, still working my way down the AW23 checklist...RF diode, IF diode checks are next.

[Norman S Braithwaite]

Joe:

I might replace the 343k resistor but anything within 20% should work fine.  The first instruction for alignment is to adjust one of the 600-ohm bias resistors under the chassis for 9.5-volts on the cathode of the noise suppressor.  This voltage is important for correct operation and alignment.  Make sure the measurement is made with the sensitivity control at maximum sensitivity (full clockwise) and the selectivity control at maximum selectivity (full counterclockwise).  As you have surmised, 1-, 2-[omega] represent megaohms.  I would not worry about tuning meter operation until after full alignment of the receiver.

It sounds to me like you are missing a short link between the selectivity control shaft and the low pass filter behind the selectivity control shaft.  Post a photo and if missing I can provide the link.

Norman

[Joe Miller]

Norman,

I can't get +9 volts on the cathode of the noise supressor. I get -9v to -10 volts there. The -4.5 volts coming in on the resistor string is at spec. I replaced the 76 tube with another good one but no change in cathode voltage.  I assume the grid of the 76 must be too negative not letting the plate voltage "overwhelm" the negative bias on the cathode from the resistor string.  I read -5 volts on the grid of the noise surpressor.  Are we sure its supposed to be a positive voltage there?

On the selectivity shaft,  I was able to remove the switch with caps from the selectivity switch from my parts chassis.  That was a job, since the link could not be removed due to clearance problems.  Had to drill a large hole in the chassis and pass it out through the hole.  Not sure yet how I am going to install in the good set.  I don't see an easy way to get the clearance I need to install the link on the selectivity switch.  May need to do some more drilling?