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 Post subject: Zenith 1945 8H034 Restoration
PostPosted: Apr Wed 15, 2020 9:37 pm 
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Joined: May Sat 04, 2019 6:11 pm
Posts: 53
I have started on my second restoration. The first one was a Zenith TO H500 and you can view the details in that thread. I purchased this Zenith from the original owners family and he remembers listening to it when he was a child. Reading about this model it is clear that it has an interesting story behind it. I also think it is an attractive table radio that I plan to keep for my own use. It will be my first attempt at wood restoration, a area in which I have no expertise or knowledge. However, just like the Zenith TO I am highly interested in learning how to do it from the members of the forum.

The original owner told me that the radio lights turn on but it did not receive anything. So when I got it home I hooked it up and using a variac and monitoring the current draw I slowly brought it up to 110v and sure enough the dial light came on but no sound. Not a hiss, crackle or buzz. Nothing. I quickly found out that the tuning dial string was broken as the dial would not move. I powered it down and started to remove the chassis from the wooden cabinet.

Internally it is good shape compared to pictures of others I have seen on the forum and it looks to be all original. I powered the chassis up again and could see the glow of the tubes and could not detect anything burning. Still no sound from the speaker at all. I powered it down and removed and checked the tubes and found that a couple needed to be replaced. They were original Zenith tubes and I placed an order for new ones. I also ordered the needed capacitors from a different source.

The tubes arrived quickly and I am expecting the capacitors any day. I installed the tubes and powered it up again and still no sound. I powered the radio down and will wait until I have replaced the capacitors. I guess I was not going to be so lucky as to only need to replace a couple tubes.

I started to ponder what things I might check that could cause No Sound other than bad tubes, capacitors or resistors.

I had seen on the forum a quick way to test if the speaker is working by using a 9v battery to see/hear if the cone moves. So I did that and it appears the speaker itself is functional. I also read that while powered up if you touch the center terminal of the volume control you might hear a crackle and that would be an indication of where the problem might be. I tried it using a insulated screw driver not wanting to risk touching it with my finger. There was no sounds from the speaker. I also tapped on most of the components and parts with a wooden chop stick carefully but no sound from the speaker.

It appears from the radio information online that this model came in two different speaker configurations. One with a permanent magnet speaker and one with a field coil. With this being only my second restoration I can't say for certainty that I know what a field coil speaker looks like but from the pictures I believe my version has the permanent magnet speaker. I did quite a bit of reading on the forum and internet as well as looking over the circuit diagram to come to this conclusion. So that is my first question.

1. In the pictures below is this a permanent magnet speaker version?

While examining the radio closely I could see several pieces of wire that were cracked and missing some of the insulation. I suppose these pieces are rubber insulation and I read that it can be a problem and needs replacing or jacketing with insulation. So my second question.

2. Looking at the wires going to the transformer is this indeed rubber insulated wire that is failing? Ignore the two temporary pieces of black shrink tubing that I installed to protect the wire from shorting out against the speaker metal when powering it up to check its operation.

The brown wire (others are yellow and green) is the worst and has a break down near where the wires go thru the metal chassis and at several places under the chassis. It will need to be replaced and maybe some others. If you look closely where the brown wire goes into the transformer you can see there is no longer any insulation and I cannot see any contacts that this wire is soldered to on the underside of the transformer.

3. What is the best method for replacing the wires into the transformer with this condition?

While waiting for the capacitors to be delivered.

4. Are there any other checks I could make to possibly determine the cause of the "No Sound" issue?

Thanks, John

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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Wed 15, 2020 11:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11771
Location: San Jose, CA USA
Your speaker is a permanent magnet speaker.

The failing insulation is rubber. Here you have a number of options. For now, if you simply make sure none of the exposed wire is touching anything it shouldn't, you're OK to proceed with other troubleshooting. For a more permanent repair, you can slip spaghetti over the original wire (either having removed what's left of the rubber or not), or you can cut off the wires close to the transformer and attach new wires, making sure to insulate the exposed joints with heat shrink tubing or some other suitable insultator.

Since your radio seems to be completely dead, I would check voltages. For now, just check the voltages shown on the schematic for the rectifier, audio output tube, and 1st audio tube. What you find may clarify immediately what the problem is.

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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Thu 16, 2020 2:50 am 
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Joined: May Sat 04, 2019 6:11 pm
Posts: 53
Thank you for the reply and suggestions.

Well I went to measure voltages on the suggested tubes and when I flipped over the chassis and turned the power switch on and turned all the way up a loud buzz started to come out of the speaker after the short tube warm up period. Turning the volume up or down did not effect the intensity of the buzz. As I started to measure voltages on the Rectifier tube at one point the loud buzz went away and was replace with a low buzz. I thought maybe I had touched a component and caused a short. So I poked around the rectifier and the components and wires around it and it would come back to a loud buzz and then a short while while later go back to a soft buzz. I could not seem to find a specific point, wire or device that I cold touch that would immediately change the intensity of the buzz.

Might this be a result of bad filter capacitors which I will be replacing once the shipment arrives?

John


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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Thu 16, 2020 3:16 am 
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Posts: 11771
Location: San Jose, CA USA
My apology for forgetting that you mentioned that the original caps are still in there. I would do nothing further until those are replaced.

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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Sat 18, 2020 10:53 pm 
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Joined: May Sat 04, 2019 6:11 pm
Posts: 53
The capacitor replacements have arrived so I have been looking at how I want to approach the Filter Capacitor replacement and measuring resistor values to see which of those need to be replaced. This has brought up three questions.

1) Is it normal on Zenith circuit schematics of the time to use M to designate thousands? For example I see written on the diagram 47M and I believe that is 47k ohms = 47000 ohms. I think this is correct from my measurements but I want to check for sure. On the diagram they write nothing to designate ohms and for meg ohms they write MEG.

2) The current Electrolytic is a Zenith part #22-1366 (40 mfd@150V, 40 mfd@150V, 40 mfd@25V). I was planning to re-stuff the capacitor can with three axial electrolytics (47 mfd@160V, 47 mfd@160V, 47 mfd@50V. Does that look acceptable?

3) I was planning to re-stuff using the method I have seen on the forum where holes are drilled in the base of the current electrolytic beside each terminal and the wires are passed thru these holes and soldered to the current lugs. Today on close examination with a magnifier it appears that the terminals on the bottom side of the electrolytic can might have connectors that pull apart. I tried to look up this specific part number to see if there was a detailed view that might tell me if these pull apart. I could not find anything to guide me so I have posted a picture of two of the terminals to see if anyone knows if they can be pulled apart? If so I might rethink my approach of re-stuffing the can.

Thanks, John


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Cap terminals.JPG
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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Sat 18, 2020 11:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11771
Location: San Jose, CA USA
In the 1930s quite a few companies used the Roman Numeral M (Mille means "thousand" in Latin) instead of the more modern K. So you're right about that.

Your replacement electrolytics should be fine.

In my experience, those lugs do not come off easily. I just clip the leads on the inside of the wafer, and pass the new leads through holes to solder on the terminal outside of the can as you are planning. The internal wires are strips of aluminum which cannot be soldered to.

Restuffing seems to be popular lately, based on a number of recent threads. I rarely do it; I just install the new capacitors under the chassis. But restuffing can be a nice touch, so go for it if you like!

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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Sun 19, 2020 2:55 am 
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Joined: May Sat 04, 2019 6:11 pm
Posts: 53
Thank you for the confirmation Tom.

I decided to proceed with the re-stuff of the can method. It has been easier than I anticipated. I used a Dremel tool to cut thru the electrolytic can and then sawed thru the inside material and leads with a hacksaw blade. Cleaned out the bottom well of the connector with a small screwdriver and used the heat gun to remove the material from the can. The material was mostly paper with a small bit of tar holding the end to the inside top of the can. It came out easily with the heat gun.

Examining the bottom well and it seems Zenith was nice enough to make running the leads of the new axial capacitors easy. The black coloured material right beside each connector tab is a soft and it is easy to push the axial lead right thru it. That brings the positive capacitor lead right alongside each the tab on the underside of the chassis and allows for easy soldering to the existing connection points.

For the negative connection I plan to place the three 40 mfd capacitors together in a bundle with the negative leads up. I will twist them all together along with a piece of 20 ga wire and secure them with a bit of solder. The 20 ga wire will run from this common negative point down thru the void between the three capacitor bodies and thru the hole to the negative (ground) on the underside of the chassis. Zenith was nice enough to already provide this center hold that is large enough to fit a insulated piece of 20 ga wire.

It is a very close fit but the old can body will slip over the bundle made from the three new capacitors effectively hiding them from view.

This does leave me with a couple of questions.

1) Is there anything I should put in the bottom of the capacitor can well to support, imobolize or insulate the bottom (positive) end of the three capacitors?

2) I have some 2 inch wide silver metal conductive tape which I can wrap around the capacitor can hiding the cut line made by the Dremel tool. Unfortunately there was no brown cardboard sleeve over this capacitor can like I have seen on some other zenith radios. Is there another way to secure the can and make it look nice at the same time?

3) I just want to confirm that I have the correct 25V terminal/capacitor identified so I do not hook it up where one of the 160V capacitors belongs. I am actually using a 40 mfd 50V axial capacitor instead of the original spec of 25V. Please check my yellow marked up circuit showing what I believe is the 25V capacitor. Am I correct? I am able to trace a wire going from this terminal on the underside of the chassis to pin 8 of the 25L6GT power amp tube and confirmed it with a continuity check.

Regards, John

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Cap Well.JPG
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Attachment:
25V Cap.JPG
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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Sun 19, 2020 5:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: San Jose, CA USA
1. You probably don't need to add anything to immobilize the capacitors inside the can. The positive leads of the caps, soldered to the terminals will do a good job of that. If you feel a need to add anything further, some hot glue or silicone caulk can do the job.

2. For the "slice the can" method, I'm not aware of any good tricks to make is look as though it hasn't been sliced. Metal tape is about as well as one can do. Another way to open the can is to file or grind off the little overhang lip on the very bottom at the outer diameter. Then when you reassembly, you can use epoxy and hide the joint pretty well since it faces down.

3. Your connection looks correct. The low voltage cap goes to the cathode of the output tube.

This should all work fine when you've got it in place. :)

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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Mon 20, 2020 9:32 pm 
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Joined: May Sat 04, 2019 6:11 pm
Posts: 53
I have not started installing the replacement capacitors yet. I first wanted to follow the entire circuit diagram to identify each component and trace every connecting wire while also testing each of the resistors. I was quite surprise with what I found and I wonder if the location and number of out of spec resistors tells the story of what might have caused this radio to have stopped working?

Just in general, I was surprised at the poor connection wiring in the chassis. Many connecting wires are two long, run in odd directions and are often bunched together touching and pushing against resistors, capacitors or tube pin connections. The biggest problem is the vinyl wiring that has been used extensively. I avoided touching or moving many of these wires for fear that the insulation would crumble and fall off exposing the conductors to a possible short circuit. It appears the brown colored wire is the worst and crumbles easily with very little movement. Unfortunately, it appears the brown wire is used to carry B+ around the chassis.


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circuit1.JPG
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Here is a picture of the brown wire that drops down from the speaker coil on the top and thru the chassis to a B+ distribution point. You can see that it has lost the insulation in several places and does not seem to be dressed well and is longer than it needs to be. This seems to be how if came from the factory.

If you examine my marked up circuit below you can see that virtually every single first resistor that comes off the B+ rail is out of spec and in some cases far out of spec. I am wondering what event might have caused such a group failure? This is more a question for my own learning about the failure modes in these radios. I will be replacing all the resistors marked in blue that were out of spec along with all the capacitors except ceramic and mica.
I was not able to find the 680K resistor that feeds the pin 6 grid screen of the RF 6AG5 tube. I will assume it is also out of spec like the rest of the 680K resistors and see if I can find it once I start changing components. It might be that I just cannot see this part of the chassis quite obstructed with the FM circuit components and the wafer switching mechanism.

I think I will approach the vinyl wiring issue by replacing all the brown wire and any other pieces that I can see have crumbling insulation. I really should do it all but that would be a huge job as it also feeds into the IF cans and around the FM section. Does this approach sound reasonable?

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circuit2.JPG
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One strange thing I found involves the factory modification to help with FM drift. See the picture of the suggested changes. On my radio the 20MFD 150 capacitor is installed but not the 2.2 M and 1K resistors. This causes me to wonder what happened? Any ideas?

Thanks, John


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circuit3.JPG
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 Post subject: Re: Zenith 1945 8H034 Restoration
PostPosted: Apr Wed 22, 2020 5:20 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 28398
Location: Annapolis, MD
You have all the right instincts...

When you start replacing wiring, be sure to check against the schematic at every step.

One caution:
Wiring layout is mostly art and a bit of LUCK. What you don't see on the schematic is all the extra capacitors and inductors that came FREE with the radio. What counts is that it presumably works correctly in the original configuration---if you make radical changes, you might have trouble. What is typically important is to ground things the the chassis in the same place as original. I typically DO shorten wires that seem excessively long---but keep the same basic routing.

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"Even if you don't understand Ohm's Law, you are still required to obey it."


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