Some receiver repair advice please.

[Ed/KB1HYS]

Not exactly Ham related but it is a sweet SWL rig. I'm restoring a Zenith 10S566 floor model. The audio that comes out was awsome with two 6V6s in a PP audio section to a BIG speaker - very nice. 

seems there is a problem somewhere in the RF section.  The Oscillator is running, but she's deaf except on 900khz (which is ~2x the IF).  Weird.
I can get some RF through if I couple the signal through directly to the grid of the RF stage, so everything is working from there back.  Audio is still good, there's just nothing being delivered to the detector.

Ok, I pulled the Detector tube (6j5) and still got audio... Pulled the inverter and STILL got audio... something is detecting the RF that aint supposed to.

Nothing on the higher bands, not even static.

I'm just looking for suggestions as I have been digging in this thing all weekend and either it's right in front of me and I can't see it, or it's something I'm overlooking.   I'd love to get this baby working again, like I said it sounds sweet, and looks good too.

[w3jn]

What freq is the oscillator running on?  Bet it's WAY off.

The reason you're still getting audio with the det and inveter toob pulled is (a) the signal's being detected in the first audio state and (b) the inverter just cuts off signal to one of the 6V6s.

Get the IF stages right first.  Then attack the oscillator tracking.  Then align the RF stages.

[w4bfs]

I am sure you have already recapped and reresistored this receiver ... I would pull all the tubes ahead of the if stage and inject known 455 khz ....if you get a normal response (strongly peaked at 455 and deaf otherwise) with good buildup of agc you may have run into a receiver design that depended on the charracteristics of orginal parts (I hit this once on restoring a 30's AK) for 'normal' operation ... I found that distributing smaller values of bypass caps (say .01 uFd) instead of one .05 with several tubes using it generally improved performance and made alignment easier ... the trap in the mixer plate is suggesting this .... I have only done a few Zeniths but they will respond to a good technique ... hope to get one with an eye one of these days ...73...John

[N3DRB The Derb]

I'd bet anything you have a open coil or changed value rx associated with yer 6A8.

[KM1H]

Recheck the PS wiring, the filter caps do not go to ground.

Then do a stage by stage socket voltage and resistance check if the PS is OK

Carl
KM1H

[Ed/KB1HYS]

I did find the coil for the 455kc shunt was open,(the one adjacent to the 1232 RF amp tube on the schematic, it's supposed to shunt anything close to the IF to ground before it reaches the mixer). and given one of the local stations is on 900kc that may have been overloading the following stages.

I've got to find a replacement or repair to see if this helps the rest of the issues.

the thing has a boat load of what looks like 38 gauge litz wire wrapped on a small cardboard tube and coated in parafin. I may just wind a new one with some fine wire I have and a sewing machine bobbin...

[N3DRB The Derb]

yep open coil.  that 6a8 does more stuff than just be a detectorz. you will have to count the turns coming off the form. Ed, tht thing is yelling
PARTS CHASSIS at you.   

[Ed/KB1HYS]

Actually I did think of salvaging the audio section to use as a driver for a pair of 811s in a modulator. the push pull 6V6s and phase splitter ciruit would have been Saaaweeet.   

Save those old radios boys. Thar be Gold in them thar hills... er chassis....

[flintstone mop]

Well you start the old fashioned way of looking for DC volts on the plates, screens,cathodes of the toobs and then generate audio to the center pin of the valium control and then start working backwards with whatever the I.F. is to a modulated RF carrier up front.
Is she deaf or just a little hard  of hearing??
Those Zenith radios were a World Class radio. Great sound in those BIG speakers

Fred

[N3DRB The Derb]

ed, the good news is that there are a ZILLION of those radios around. and that coil my not be super critical but if they use litz wire they usually are. 

[Ed/KB1HYS]

Fred,

She's deaf unless I tune in the 900kc local station. That comes in Loud and clear.

I've got to replace the open coil first, and then trouble shoot the problem.

I've got some old coils w/cores fromTV's  close to the same size. Time to fire up the ol' Grid Dipper...

[N0WVA]

I wonder how many of these old radios are still being chunked into the landfill? Quite a few, I bet.

[Ed/KB1HYS]

craigslist frequently has them, though some people thing they are worth big bucks usually they can be had for
< $50 or so, sometimes free for the taking.  They are nice. If I had the room I'd have quiet the collection.

Maybe one in every room... 

[N3DRB The Derb]

I'm going to use a RCA 6T2 as the bedside receiver. have to get all the ham gear back going and get back on first. it's about 5 down on the project list now.....headed down.....

best one I ever had was a Philco 200-X console. had variable bandwidth:

from philcoradio.com..

MODEL 200X
This is a historically significant set. While most people today have been conditioned to believe that AM has very poor frequency response, this was not always the case. In fact, this very model was designed to receive a wider frequency audio range than the normal AM receiver of the time - 50 to 7500 cycles.

While keeping in mind that most vintage radios can outperform any modern AM receiver, average AM sets of the time would only reproduce an audio frequency range up to 4000 cycles.

The Philco 200X was the industry's first high fidelity radio, thanks to its variable bandwidth control which made high fidelity reception possible when conditions were right; the control could be adjusted to narrow the audio bandwidth when necessary, also.

It only received the AM band (540-1720 kc), and used 10 tubes.

Original selling price: $200
Number made: 3,500

no other radio I've ever owned has come anywhere near the sound and tone of that 200X. No Zenith shutter dial could touch it that I've ever personally heard.

[Todd, KA1KAQ]

Litz wire coils?? But...but...I thought only the SX-88 used Litz wire for the coils?  (reference to a dead thread)

I've always wondered how much the larger enclosure has to do with resonating, Derb. Even the Philco floor model I got started with years ago sounded great (single ended audio 6F6), and better than many smaller sets with higher tube counts. Recently swapped out the bedside Zenith for the Halli SX-62B driving an old Dukane school speaker though, and it sounds better. Think the Zenith just needs a good going through.

You'll get a lot of good enjoyment out of that radio, Ed. A tad moreso for reviving it yourself.

[The Slab Bacon]

Ed, Todd,
        You will find Litz wire coils in many older receivers. I have repaired oodles of them. Repairing the fine litz wire is usually a pain in the ass. the wire is so fine that it doesnt take much to corrode it off somewhere. If a radio has been cat-pissed or severely mouse-pissed that will usually wipe out the Litz wire if it comes in contact with it. Sometimes just the ambient moisture in the air is enough to make that stuff fail. Take an ohmmeter to each winding of each coil / transformer before going any further and replace / repair whatever is necessary. Many times you can swap an IF can from another receiver as long as the IF frequency is within tuning range of the bad one.

If all of the coilz have continuity, start making voltage tests to all of the tubes from the circuits in question. Voltage / resistance checks will usually show up open resistorz or shorted capz.
 
                                                         The Slab Bacon

[Ed/KB1HYS]

Good advice, that's how I found this bad coil, just following the signal path back from the ant input.

highlighters on schematics are very usefull here.  I usually blow up a schema to 11x17 size and mark it up with highlighter(s), volts and resistnaces if they fit, wire colors too and lots of notes in margins too before it's done.  Tube socket pin numbers on this one was one detail that was missing, so I wrote those in. Also if there are components I am not familiar with, like these molded resistors having the body-tip-dot color coding, I make notes about those too. (this is one reason BIGGER is better, the other is tired eyes).

I did like the fact that the old paper caps were marked with values and part numbers easy to read, unlike the nice new ones... oh well reliability over readablity right?  I thought about restuffing but that is TOO much for components under the chassis. This isn't a smithsonian restoration.

One thing that threw me on this schematic was the resistor values lister used an "M" to indicate "thousands" I remember thinking "why did they use so many High Meg-ohm resistors???"

The toughest part for me is figuring out the switch positions with the old pictoral switch wafer representation.
 

[The Slab Bacon]

Ed,
    When I was doing the antique radio thing years ago. (thatz what financed a lot of my initial ham gear) I had fixed literally hundreds of them for cheap-assed collectors that wanted them to play but didn't want to spend a penny more than they had to.

I did the overwelming majority of them without ever seeing a skizmatic of them. I would just go from the basing of each tube and make sure I found somewhat reasonable voltages on the plates and screens. Then make sure that none of the control grids show a positive voltage, and the cathodes had a DC path to ground.
that was usually enough to get them up and running somewhat. Having a skizmatic on hand was a definate luxury. I only ran into a few that I really ended up NEEDING a skiz to repair, but it was a nice luxury if you had one.

Those old dogbone style resistors use the same color code for the numbers as do the later ones. However the placement is a bit different. Just remember the keyword is "BED" just like the one you lay on. It stands for "body, end, dot". The body and end are your first 2 significant figures, and the dot is the multiplier. Itz just that simple.

                                                   The Slab Bacon

[WD8PHW]

Hi,

Visit here:  http://antiqueradios.com/forums/viewforum.php?f=15

Post a "Wanted"  ad, and I'd be surprised if someone doesn't have your coil. 

Visit the other forum topics too.  You'll find it a wealth of information!

Also, search the net for antique radio parts.  There are a few suppliers out there. 

Lastly, the chassis in your '566 was used in 5 different 1941 Zenith models...making your chances even better.

Please, don't part it out!

73,

Greg

[Ed/KB1HYS]

hi and thanks for the pointer!

No plans to part it out. Unless there is a catastrophic failure (like a power tranny or???)

[WD8PHW]

Some more advice!   

The rectal fire tubes are 6x5's, not 1 but 2.   From what I've read, they are prone to cathode/plate shorts.  To protect your power tranny, you can do a few things.  First, fuse the secondary.  You could also use a "contemporary" 6X5GT, with an "X" plate design.  Again, from what I've read, these are much less prone to failure!  You could also use a solid state substitute, by making your own or buying one. 

Visit the ARF, and search 6X5.  You'll get the entire scoop!   

[Ed/KB1HYS]

Yes, I've read about those 6X% and their ability to fry power trannies.

I was thinking of moving the B+lines to two unused pins and putting in ss diodes. I'd leave the tubes with lit fils for "effect".

[w4bfs]

could do that ... I would check fil voltage and if high, would also use it to buck him down five % or so