Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently May Fri 29, 2020 5:31 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 36 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sun 05, 2020 9:56 pm 
Member

Joined: Dec Wed 25, 2013 7:57 am
Posts: 4299
Location: USA
aJJ wrote:
...The voltage data appears to be within spec in general, BUT I would expect to see plate voltages to be higher. Is the radio on a variac? I would investigate this further

- A weaker converter tube may not work at higher frequencies (i.e. AM or but not FM), especially if plate voltage is low...
I apologize. I read the chart backwards. I think these plate voltage readings are ok


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Mon 06, 2020 3:16 am 
New Member

Joined: Mar Tue 31, 2020 5:16 pm
Posts: 15
So I've had a bit of a breakthrough tonight. Or probably something I was missing before. With the switch in the FM position I was getting some intermittent static. Whenever I would hear the static I would move the tuner around carefully to see if I was actually picking anything up. Eventually, I heard it. An actual FM station. It's very very weak. I have to have the volume almost all the way up to hear it at all, which really intensifies the buzz from the speaker. It's the only station I get, and it's a local station, so if I'm getting anything else, it's likely being drowned out by the speaker buzz. I guess my next step is to go through the schematic to understand how the signal is being amplified and check that whole path. But I'm excited.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Mon 06, 2020 4:49 am 
Member

Joined: Dec Wed 25, 2013 7:57 am
Posts: 4299
Location: USA
Congratulations. It's little breakthroughs like this that get some people hooked with this hobby. Please keep us posted.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 07, 2020 12:11 am 
Member
User avatar

Joined: Jun Sun 03, 2012 12:40 am
Posts: 1705
Location: Cromwell, Connecticut
Looking at your voltages, there are 3 things that stick out.

The 6SH7 Limiter grid should be around -1 volts. Either R9 the 47K is bad, the tube is bad, or their is leakage through C10 and L21. It would be easy to swap the two 6SH7 tubes to see if their area any changes. The limiter is supposed to provide a steady signal to the discriminator and is important.

The screen voltages are way low on the 1st and 2nd IF tubes. They are both fed through separate 68K resistors with .001 bypass caps C8 in both cases. If these components are bad, this will effect the screen voltage. These tubes make most of the gain in the radio. Check everything around these tubes. The plate voltages are good, which is why I’m not looking at the source. Screens should be 130 volts per Schematic.

The 6SB7Y tube has high screen voltage, and we are not sure the oscillator is running on FM. I’d say to do the second radio test. Grab a known good FM radio preferably with digital readout. Set the second radio to 98.3 and turn up volume. Tune the Zenith around 90 on the dial. Sweep it above and below to see if your 2nd radio hears it. Radio should get quieter as the Zenith Oscillator nears 98.3. If you don’t hear it, then the Oscillator is not working on FM.

I don’t remember if you tested the tubes, but now may be the time. Weak AM and no FM could be tubes.

As for the hum with the volume up all the way, that may be normal. The audio stages are running full gain, and no one runs a radio wide open. If there was a usable signal there you’d likely not hear it. In fact, if FM was working with at least good IF amps and Limiter, I’d expect to hear some hiss like any other radio not tuned to a station. This is amplifier noise.

Give it a try and poke around some more.

_________________
Tony

People may not remember how fast you did a job, but they will remember how well you did it.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 07, 2020 3:06 am 
New Member

Joined: Mar Tue 31, 2020 5:16 pm
Posts: 15
Thanks very much for the thorough reply.

Quote:
The 6SH7 Limiter grid should be around -1 volts. Either R9 the 47K is bad, the tube is bad, or their is leakage through C10 and L21. It would be easy to swap the two 6SH7 tubes to see if their area any changes. The limiter is supposed to provide a steady signal to the discriminator and is important.


The 47k checked out fine. C10 was replaced, however I'm not entirely sure of the voltage rating of what I used. I used what I had around and used a couple in parallel to make that capacitance and raise the voltage. So it's entirely possible there's a problem here. I did try swapping the tubes and I do get a difference in grid voltage, but still not to -1V. In fact, it basically went to 0. And the 2nd IF grid screen went up to 160V. I'm not exactly sure what you mean about leakage on L21 (the capacitor leak voltage to L21?) but I checked resistance and it's .5 ohm.

Quote:
The screen voltages are way low on the 1st and 2nd IF tubes. They are both fed through separate 68K resistors with .001 bypass caps C8 in both cases. If these components are bad, this will effect the screen voltage. These tubes make most of the gain in the radio. Check everything around these tubes. The plate voltages are good, which is why I’m not looking at the source. Screens should be 130 volts per Schematic.


The 68k resistors had already been replaced when I took my measurements. The originals were reading about double the value. Both caps were replaced in the original re-cap.

Quote:
The 6SB7Y tube has high screen voltage, and we are not sure the oscillator is running on FM. I’d say to do the second radio test. Grab a known good FM radio preferably with digital readout. Set the second radio to 98.3 and turn up volume. Tune the Zenith around 90 on the dial. Sweep it above and below to see if your 2nd radio hears it. Radio should get quieter as the Zenith Oscillator nears 98.3. If you don’t hear it, then the Oscillator is not working on FM.


I tried this out and it went a little strange. I didn't have a digital radio, so I dialed in as close to 98 as possible. As I swept through the dial on the Zenith, at three different points, it tuned the other radio to a local station that comes in strong. At one point, it silenced the radio completely. Unfortunately, I don't know what frequencies I was dialing on the Zenith. I have it apart without the dial face on.

I tried to test the tubes a long time ago with a tester I borrowed from a co-worker. None of them tested good, which lead me to believe I was doing something wrong, or that the tube tester didn't actually work. Now I know that the tubes may not be 100%, but they work.

I actually just ordered a pair of 6SH7 tubes because they're cheap and could rule that out.

And yeah, the speaker hum was expected at full volume. I was just using that to illustrate how little I could hear the station.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 07, 2020 10:54 am 
Member
User avatar

Joined: Jun Sun 03, 2012 12:40 am
Posts: 1705
Location: Cromwell, Connecticut
The fact that you can swap the 6SH7 tubes and get different results, screen voltage went up, is a sure sign you have issues with your tubes. If the tube tester showed tubes being bad, there is some merit to it with the symptoms you are getting.

We know the rectifier tube is good since you have good B+. This tube could be used to check the tube tester. If it too shows bad too, then I’d question the testers validity.

As for the leakage comment, C10 would have to be leaking and the discriminator transformer would have SMD. The combination would cause enough leakage to raise the grid. Swapping tubes kind of ruled it out. You could measure the voltage on the grid pin with the tube pulled to rule out leakage.

While it’s good to replace capacitors with new units, not something sitting around in unknown condition, and change resistors, troubleshooting a radio with potentially bad tubes is an exercise in frustration.

_________________
Tony

People may not remember how fast you did a job, but they will remember how well you did it.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 07, 2020 8:20 pm 
New Member

Joined: Mar Tue 31, 2020 5:16 pm
Posts: 15
Thanks again. I'm no longer in possession of the tube tester and working from home, so I won't be able to get it any time soon.

Good to know I have a working rectifier though, that's one of the more expensive tubes.

Quote:
As for the leakage comment, C10 would have to be leaking and the discriminator transformer would have SMD. The combination would cause enough leakage to raise the grid. Swapping tubes kind of ruled it out. You could measure the voltage on the grid pin with the tube pulled to rule out leakage.


What should I see on this pin with no tube in the socket?

Most of the component I used were bought specifically for this project. It's the unanticipated stuff I've been pulling from my "stock" which is unused, but I haven't always had the right voltage or wattage so I've had to make what I needed, but this is only temporary. I'm making a list of what I need for the correct permanent replacements so I can order all at once.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 07, 2020 9:29 pm 
Member

Joined: Dec Wed 25, 2013 7:57 am
Posts: 4299
Location: USA
Brawndo91 wrote:
...I tried this out and it went a little strange. I didn't have a digital radio, so I dialed in as close to 98 as possible. As I swept through the dial on the Zenith, at three different points, it tuned the other radio to a local station that comes in strong. At one point, it silenced the radio completely. Unfortunately, I don't know what frequencies I was dialing on the Zenith. I have it apart without the dial face on.
For this oscillator test, you don't need a radio with high precision readout. If the is result is repeatable and that your other radio is tuned anywhere close to ~10mhz above the Zenith (i.e. Zenith at 90mhz, other radio 98-101mhz) it's fine. Since you've determined you can hear a station, even just barely, the oscillator tube is working, but perhaps weak on FM band.

I apologize if this has been convered already. Is there a chance someone got in there and tried to tweak any of the alignment adjustments? If the antenna or IF alignment is off, even a little bit, would result in the signal not coming through. (This is not to suggest you should start turning screws just to see what happens)


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 07, 2020 10:15 pm 
New Member

Joined: Mar Tue 31, 2020 5:16 pm
Posts: 15
This radio originally belonged to my grandparents. I have no reason to believe that anyone messed with the screws, and I haven't myself. Tbone gave some advice that led me to believe one of the tubes is bad, so I'm waiting on a replacement and will see what happens then.

I also still haven't ruled out a contact issue with the band switch, even though I said before that I had. I'm finding it very difficult to clean them beyond just a spray with contact cleaner. I can't get a brush in there, nor can I get sandpaper in between. However, I've been checking the continuity between the disc and the contacts and I haven't found a problem there, so maybe they're okay after all?

I've noticed some other odd things while poking around. When I put my test lead on certain tube pins, the radio gets louder, others will make it quieter. Also, when I was getting the one FM station, if I put my lead on L3, the FM detection coil, it plays a little stronger. This would happen on another coil, but it escapes me which one.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 07, 2020 10:54 pm 
Member

Joined: Dec Wed 25, 2013 7:57 am
Posts: 4299
Location: USA
- Use extreme care when poking around. Usually people would use non-conductive plastic or wooden sticks to poke to discover loose joints, not metal

- Also inserting test leads with the set powered on is potentially hazardous. Chances are sooner/later an inadvertent short, shock, or damaged meter would happen. Some of us experienced it first hand. I personally unplug the set (not just power off), attach leads with clips, verify it's secure, then step back and plug the set in :)
Brawndo91 wrote:
...Also, when I was getting the one FM station, if I put my lead on L3, the FM detection coil, it plays a little stronger.....

- Is this the coil you touched?

- For copper contacts that you can see, oxidation can be cleaned with just small amount of fluid. If it's black/carbon, then you would need to scrape it. If sandpaper or a thin metal instrument


Attachments:
Screen Shot 2020-04-07 at 2.45.18 PM.jpg
Screen Shot 2020-04-07 at 2.45.18 PM.jpg [ 107.43 KiB | Viewed 241 times ]
Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Wed 08, 2020 12:19 am 
New Member

Joined: Mar Tue 31, 2020 5:16 pm
Posts: 15
Yes, that's the coil, specifically the joint labelled D.

For the switches, one of them has a bunch of black on it, which I've been attempting to clean off with sandpaper, but it's really tight and tough to get in there. However, when checking continuity between the disc and contacts, they come up fine.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Wed 08, 2020 1:09 am 
Member

Joined: Dec Wed 25, 2013 7:57 am
Posts: 4299
Location: USA
Brawndo91 wrote:
Yes, that's the coil, specifically the joint labelled D.

For the switches, one of them has a bunch of black on it, which I've been attempting to clean off with sandpaper, but it's really tight and tough to get in there. However, when checking continuity between the disc and contacts, they come up fine.

Ok. Since you're looking in this area. (From the schematic) this is what the circuit would look like when the radio is switched to "2" (FM). Take your antenna or a wire about 3' long, connect to 1. FM antenna terminal; 2. 6AG5 pin 1; then 3. 6SB7Y pin 8. See if that weak station's signal changes at each of those points. It might reveal something.


Attachments:
Screen Shot 2020-04-07 at 4.58.44 PM.jpg
Screen Shot 2020-04-07 at 4.58.44 PM.jpg [ 65.74 KiB | Viewed 238 times ]
Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Thu 09, 2020 2:00 am 
New Member

Joined: Mar Tue 31, 2020 5:16 pm
Posts: 15
So I tried this out and I should mention something before I give my results. I was no longer getting the one FM station shortly after I posted about it. I'm not sure what I moved around, but I think I really need to take a look at the contacts, etc.

Now when I would put the wire on the points you mentioned, I would get some static sounds when I first touched it or moved it. But with the wire on, I would move the dial around and get nothing.

A new symptom popped up though, which I don't believe has anything to do with this wire test. On AM, I was getting what sounded like a car horn on most of the station. This varied in intensity, sometimes sounding like an organ, or not present at all toward the end of the dial. Before this I had also noted that certain parts of the dial would, whether a station or static, would kind of pulsate.

I have a few theories. There's the possibility that something was near shorting and my moving things around caused it to happen. Another is that I'm getting intermittent contact somewhere, and again, moving things is making trouble, or the third is that lately I've had this radio powered on for longer periods of time than it's seen in 50 or so years and the heat is bringing out gremlins from hiding.

In the meantime, I'm still waiting on two new tubes, which I'll certainly report back on after the swap, and I'll be checking some more for shorts or bad contact areas.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Thu 09, 2020 5:19 am 
Member

Joined: Dec Wed 25, 2013 7:57 am
Posts: 4299
Location: USA
That test was meant to move the antenna physically closer to the oscillator is in case there's a poor connection somewhere.

I think you're correct to go look for a poor physical contact, a mistake, or short somewhere. Often something like that would escape detection even with close examination. So keep looking; that's how some of us do it.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 14, 2020 4:37 am 
New Member

Joined: Mar Tue 31, 2020 5:16 pm
Posts: 15
I took a short break from posting here to continue poring over this thing and while I found a couple things, I'm still not far from where I started. I mentioned that I'd ordered a couple 6SH7 tubes to replace the limiter and 2nd IF tubes. The limiter seems to be closer to what it should be, the 2nd IF is basically the same, but it didn't seem to be a problem before.

On the limiter, my new voltages are: P - 85V, GS - 62V, G - (-50mV). For context, the schematic reads P - 60V, GS - 58V, G - (-1V).

I also noticed a low voltage on the 6AG5 RF tube, which I traced to a high resistor R2 (around 2x value). The odd thing is that I'd checked this resistor before and it read correct, or close enough, and the plate voltage I had written down before was 262V but out of the blue it was significantly lower. With that resistor replaced, the plate now starts around 280V or more, then over time goes down to around 268-270V (I had leads clipped on). Also, the grid screen on this tube reads 108V which is below the schematic's 132V. On all the other tubes, the screen voltages read higher than the schematic.

In other news, I mentioned before that I was no longer getting the FM station, but I found that one of the wires I'd replaced a while back, which went to one of the tone control switches, had broken from it's solder joint. I think the tone change this caused had led the already very weak FM to be completely inaudible. When I resoldered that wire, I got the station back.

(Side story, feel free to skip: I spent a long time tracing the schematic to navigate the 6 tone control switches to figure out which one I was dealing with and where that wire should go. I had replaced it, along with the all the other wires to the tone control switches, years ago. The schematic has all the tone switches labelled - treble, bass, etc. But this was no help because I didn't know which was which. So I carefully followed everything, using surrounding components for context, figured it out, soldered it, had it playing like before, and patted myself on the back for solving the puzzle. Cut to tonight, I decide to give the plastic switch operators a good cleaning, and find out that they're all labelled. Now this radio sat in my parents basement for years and for a long time I had no idea what these switches were for. It wasn't until I brought it to my house and started researching that I learned what they were. Just to illustrate how long those switches have been dirty enough that there wasn't even a hint that there might be useful information underneath. Information that would have saved me a lot of time.)

One other peculiarity, or maybe this is totally normal - another thing I mentioned before was that when I had a test lead on L3, at the contact that goes to the RF tube plate, the radio would play a little louder on FM. I found that without my hand on the lead, this isn't the case. I also noticed that if I walk away from the radio (with or without the lead on) it will play louder. Maybe it means nothing, but I want to give as much information as possible.


Top
 Profile  
 
 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Tue 14, 2020 5:17 am 
Member

Joined: Dec Wed 25, 2013 7:57 am
Posts: 4299
Location: USA
I'll let others comment on the measurements....

I think you're doing right kind of work. That careful inspection and finding loose wires and other problems can sometimes get a radio back to where it should be. So keep at it.

The behavior where your body position in the room affects FM reception is common. You can keep experimenting to see if you can improve reception (i.e. different lengths of wire in different positions).


Top
 Profile  
 
Post New Topic Post Reply  [ 36 posts ]  Moderators: Marcc, Norm Leal Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: chriswch2003, ZombieElvis and 13 guests



Search for:
Jump to:  


































-->


Privacy Policy :: Powered by phpBB