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 Post subject: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Fri 03, 2020 7:16 am 
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It's a Zenith console, model 9H079.

I re-capped it about 7 years ago and couldn't get it going. Poked around every once in a while and then a few weeks ago it was playing AM radio for the first time in 50 years (according to my father).

The AM seems a little weak. I have to have the volume knob nearly all the way up for it to be at a listenable volume. And even the strongest stations aren't what I'd call "loud". The FM is completely non-existent. I did my best to rule out any broken connection or switch issues. There's absolutely nothing coming out of the speaker except the slight hum it normally has (another problem?).

I tried making an antenna according to the instructions in the service manual, but still nothing. The fact that I'm not even getting static leads me to believe that it's not a reception problem.

Now, I did take apart one of the IF cans and found what I think might be the silver mica disease I keep reading about. I took a picture, but didn't want to go too far into it, so I hope it's illustrative of what I was seeing:

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There's sort of a dark grey haziness around the outside of the disc. I don't get any of the "thunderstorm" symptoms I'd read about, but a few scratches on AM, and of course, no FM.

I'm afraid to replace them because I read that these are often hand-picked to get the alignment to work, and I don't have the equipment to do an alignment. But I also read that it's possible to scrape them or clean them with a pencil eraser. Any opinions on this?

Onto the matter of the resistor, I was checking some of the values with my meter and most were okay, but I came across a couple that read way off (according to my cheap meter that I've ordered a replacement for). The schematic shows that they should be 68M. My meter is saying 110 when on the 2000k setting, which I thought at first might be 110M, but I'm not so sure. I think my meter sucks. And I don't think the resistors match the schematic. They're blue, grey and likely orange, but maybe yellow:

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The picture looks a little more yellow than orange, but I think it was a little more orange. Either way, not 68M.

To cap it all off, I've been having trouble reading the plate voltages. I'm touching a lead to the plate pin and the other to the chassis, but I'm getting very little reaction from my meter. The filaments are heating up, and reading 6VAC. The plate pins will show maybe .1 or 0. Again, possibly my meter, but the schematic has numbers like 60V or 235V. And yes, I'm on DC for these. When I measured the voltage across a couple of the transformer secondary wires, I read 730VAC. These voltages aren't on the schematic, but that one seemed high.

Any suggestions on the above issues are very much appreciated. I'd love to get the radio working completely and then I can move on to the record player, which will be another adventure....


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Fri 03, 2020 1:47 pm 
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Location: Cottage Grove, Minnesota
Quote:
Onto the matter of the resistor, I was checking some of the values with my meter and most were okay, but I came across a couple that read way off (according to my cheap meter that I've ordered a replacement for). The schematic shows that they should be 68M. My meter is saying 110 when on the 2000k setting, which I thought at first might be 110M, but I'm not so sure. I think my meter sucks. And I don't think the resistors match the schematic. They're blue, grey and likely orange, but maybe yellow:


That resistor is 68K ohms which is the same as 68M ohms on older schematics. I believe everyone has run into this nomenclature issue, on older schematics M ohms was used to represent what we now call K ohms. Resistors in the Meg ohms range usually have the "Meg" ohms suffix in the schematic. In Roman numerals M =1000 and in metric K =1000.

I'm not sure if your measurement of the resistor is 110 ohms or 110K ohms, either way I would replace it, especially since you have already removed it.

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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Fri 03, 2020 4:43 pm 
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Thanks for the explanation. I wondered why I was seeing so many "high value" resistors on the schematic. And why only a couple were "meg" instead of "M". I have some 68k's so I'll replace them both.


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Fri 03, 2020 5:57 pm 
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I have seen worse SMD, but you do have some.

The fact that you have no alignment equipment, means you should stay away from rebuilding any IF cans. Any one you touch and maybe replace caps in will need to be re- aligned. Even opening one up and cleaning the mica may likely cause the need for alignment as well. This is my suggestion.

Instead, I would focus on capacitor/resistor replacement. This can be done with minimal impact. I would also focus on the power supply and audio sections to make sure they are working OK, so good voltages at each tube etc. With AM somewhat working, low audio could be an audio stage. Also, test and or swap tubes, and clean band switches.

While I don't discount SMD as an issue, I would rule out everything else first.

Schematic here: http://www.nostalgiaair.org/pagesbymodel/480/M0025480.pdf

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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Fri 03, 2020 6:08 pm 
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Thanks. I'll leave the IF cans alone for now. I put it back together and in the radio and I still get AM, so I guess I didn't throw it off too much, if at all. I didn't actually take the discs out, just lifted the top plate/pin off to see them.

I cleaned all the band switches with contact cleaner, working them back and forth after I sprayed. If you have any other suggestions for this, I'd be grateful. I've also replaced all the wires with old crumbling insulation to rule out shorts.

The power thing is where I'm a little lost. Like I said, I was trying to read the plate voltages, but I feel I'm doing something wrong because I'm getting nothing on my meter. I only get AC voltage on the filament, which is correct according to the schematic, though it doesn't list the voltage. I don't get any DC on the plate pins except for fractions of volts. I tried checking out the rectifier, but things seem to get weird when I put my test leads on it. The voltage doesn't stay constant. I'll have to check around there again later this evening, because I don't remember exactly what I saw.


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 12:55 pm 
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If I know more about your experience, I could maybe better answer your questions.

For starters, it’s a good idea to make a print out or two of your Schematic. A 11X17 copy is the best. I like to mark the tube PIN numbers on the Schematic. This helps when measuring voltages, because the tube sockets have a numbering scheme.


I also highlight components as I replace them. It forces you to identify the connection points and curtails errors.

So for measuring voltages, there is AC and DC. The tube filaments in this radio are 6.3 volts AC as measured across the filament pins of each tube, for tubes beginning with a 6. The rectifier tube 5Y3GT has a 5 VAC filament. Again measured across the filament pins.

DC voltages appear to be measured with the meter - lead to the metal chassis, and the + lead to the desired test point. The meter is set to DC volts.

Hope this is a start. I’ll check back in later after morning chores. I have no place to go anyway.

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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 3:30 pm 
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Until you can get a good voltage reading at the power supply don't jump to any conclusions about what is wrong.
Check out your meter first, measure some know DC voltages like 9V batteries, 12V car battery and maybe the power supply in an other radio that is working OK.
There is also a simple test that is easy to do. With the radio warmed up and on, touch the center wiper of the volume control with your finger. Don't touch anything in the radio with any part of your body. There should be a loud hum from the speaker. If you hear that then the problem is not in the audio stages.
Checking the plate voltages will likely be of primary importance to find the problem.
Some of these old schematics are not easy to decipher
.

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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 3:58 pm 
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Is the speaker plugged into its socket? You won't get B+ on the tube plates if it's not.


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 4:52 pm 
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I have the Schematics in 600dpi pdf files if you’d like. Send me a PM with your email address, and I’ll get them to you.

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


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 5:05 pm 
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for starters, it’s a good idea to make a print out or two of your Schematic. A 11X17 copy is the best.

In addition to the suggestion above, I also fasten the schematic to cardboard to stand up at back of bench, and also easier to write on.

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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 6:01 pm 
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Brawndo91 wrote:
...poked around every once in a while and then a few weeks ago it was playing AM radio for the first time in 50 years (according to my father).....
There's a good clue here. If you simply "poked around" and got AM going, maybe a similar thing is going on with FM part of the circuit. Perhaps something is/was loose or leads touching/shorting.


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 9:54 pm 
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Didn't expect to see all these replies. I'll try to answer all the questions, but first an update.

My new meter came yesterday afternoon and it actually works properly for checking the DC volts at the plates, etc. I checked every voltage shown on the schematic. They all read a little high, but for the most part, consistently high. I'll post them all if asked.

There was one that was way off which I think is called the limiter tube. The plate voltage was really low, and the grid was coming up at something like -220 when it should be -1. I followed the schematic and zeroed in on a bad resistor and a possible short. I took care of those and the plate voltage is under control, but there's now almost no voltage on the grid pin. It's only showing around -50mV.

So still no FM, but I'm going to keep following the power around and try to suss out the bad components. Or could it be a bad tube? Again, I can post all the voltages if it will help.


Last edited by Brawndo91 on Apr Sat 04, 2020 11:50 pm, edited 1 time in total.

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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 10:18 pm 
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For tube voltages, they only need to be close, within 20%. I think you're on the right track. There may be more bad component or short. Take your time, you'll find it.


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 11:11 pm 
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I didn’t see what the grid voltage of the 6S8 tube is supposed to be. The cathode is listed as -1.2 as is the grid of the 6K6. Can you verify which tube grid is wrong?
This Schematic is busy to say the least.

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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sat 04, 2020 11:45 pm 
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Sorry, I wasn't looking at the schematic when I replied. It's the Limiter tube that I'm having problems on. The 6SH7 on the right. I now have the plate at 70V and the grid screen at 55V. Grid is reading -50mV. It's supposed to be -1V.


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sun 05, 2020 1:05 am 
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Tbone wrote:
I didn’t see what the grid voltage of the 6S8 tube is supposed to be. The cathode is listed as -1.2 as is the grid of the 6K6. Can you verify which tube grid is wrong?

Probably neither. I also cannot tell what the grid voltage on the 6S8 is supposed to be. At THIS point, it's not especially important. The 6S8 is pretty busy. It's a triode with two diodes sharing a common cathode AND another separate diode in the same envelope. The triode is being used as the First AF AMP. One diode is the AM Detector. The other two diodes are the Discriminator (FM Detector). It's common layout in vintage AM/FM receivers. It gives you a fair amount of functionality while minimizing tube count.

Tbone wrote:
This Schematic is busy to say the least.

Not really. The REAL problem is that it is drawn upside down. The next problem is that it is a multiband AM/FM receiver, well, one AM band and two FM bands,.

What I would do:
Label all the screen and plate pins with their measured voltages on a copy of the schematic. If the voltages are within about 10% of nominal, they are probably good. Repair any obvious faults. Trace the broadcast band AM signal path. Remember, the 455 KHz output of T3 goes straight to the AM detector in the 6S8. It bypasses the limiter.

Now, it is time to start eating the elephant. (How do you eat an elephant? One bite at a time.)

If you have or can borrow some basic test equipment, inject an audio signal into the top of the volume control. When that works, start alignment on the AM circuits, working from the audio stages forward. ignore the FM circuits until you have the AM circuits working.

The schematics give you an idea of how much gain you can expect from each stage.

John

Edit: Whoops, I lost track of the original poster.


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sun 05, 2020 8:19 pm 
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So with my new meter I've taken down all the voltages I'm reading on the tubes, compared to the schematic.

I wish I could make a table, but I'll try my best to format so it's easy to read.

TUBE________PIN________SCHEMATIC_______READ
6K6GT------------P--------------245V---------------312.5V
PWR. AMP-------GS-------------270V---------------324V
-------------------G-------------(-1.25V)-----------(-1.4V)

6S8GT------------P--------------115V---------------140V
DISC. DET.-------K-------------(-1.2V)------------(-1.5V)
1ST AUDIO

6SH7-------------P---------------60V----------------66V
LIMITER---------GS--------------58V----------------53V
-------------------G--------------(-1V)------------(-80mV)

6SH7-------------P--------------255V---------------301V
2ND IF----------GS--------------58V-----------------53V
-------------------K--------------1.2V----------------1.4V

6SG7GT----------P--------------252V---------------301V
1ST IF-----------GS-------------110V---------------115V
-------------------G-------------(-1.6V)------------(-1.2V)

6SB7Y------------P--------------262V---------------304V
CONVERTER-----GS-------------85V----------------141V
-------------------K---------------0V-----------------22mV

6AG5-------------P--------------246V---------------262V
RF---------------GS-------------132V---------------108V
-------------------G------------(-1.6V)-------------(-1.2V)

5Y3GT------------P-------------315VAC-----------350VAC - BOTH PLATES READ SAME
RECTIFIER-------F--------------322V--------------369V


So there are a couple of obvious problem areas, like the grid pin on the 6SH7 limiter, and the grid screen on 6SB7Y converter. Are there any others I should be concerned with?

I've been checking out the resistors mainly, but aside from what I've replaced, nothing is reading too far out of tolerance.

There are some capacitors that I hadn't recognized previously, because they look so much like the dogbone style resistors. I don't really have a good way of checking them. Are these a type that should be replaced or should I leave them alone?

And when replacing capacitors, how close should I be to the original value? Most of the caps I used were bought specifically for this project, but I have some others, just not in the exact values, so I've had to "make" some to replace some caps I missed last time, like the sneaky domino types that hide the paper caps.


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sun 05, 2020 8:42 pm 
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Have you checked all the coils and transformer windings for continuity? No point in applying power until these are good.

Ted


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sun 05, 2020 9:23 pm 
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Brawndo91 wrote:
...There are some capacitors that I hadn't recognized previously, because they look so much like the dogbone style resistors. I don't really have a good way of checking them. Are these a type that should be replaced or should I leave them alone?

And when replacing capacitors, how close should I be to the original value? Most of the caps I used were bought specifically for this project, but I have some others, just not in the exact values, so I've had to "make" some to replace some caps I missed last time, like the sneaky domino types that hide the paper caps.
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

Not about fixing what's wrong, but some general ideas in response to your question:

- Good practice: Instead of "see capacitor, replace capacitor," a more disciplined way is to use the schematic/parts list. This way you don't miss any and reduce chance of replacing one with the wrong value

- In general, only paper and electrolytic capacitors need to be replaced. They deteriorate over time even in a box or radio in storage; that's why many advocate replacing them all. Other types tend to last a very long time

Here's a visual guide in identifying capacitor types and a more detailed discussion of which need replacing: https://www.radiomuseum.org/forum/repla ... itors.html

- Replacing higher value capacitors (usually 0.001 mfd or above in vintage radios), "close enough is good enough." Example: 0.05 -> 0.47; 0.002 -> 0.0022; 0.3 -> 0.33. You can put two or more in series/parallel in case you have one close enough. Example: two 0.0022 in parallel is 0.0044; two 0.022 in series would equal 0.011

- In general, with the exception of candohm type, resistors are very robust. Most radio circuits keep working even with some resistor drifting way out of tolerance. Resistors do smoke or otherwise become damaged due to failure/short of something else. You can usually see obvious signs of heat damage. For the initial round of restoration, if the meter shows a resistor even remotely close to the original value, I'd leave it alone (i.e. "if it ain't broke, don't fix it")

- Pay attention to avoiding/finding mistakes or poor workmanship. They're the #1 cause of a restored radios not working

- Don't overlook simple things like dirty or loose tube pin/sockets; oxidized switch contacts

Hope this helps


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 Post subject: Re: Zenith with AM but no FM and resistor confusion
PostPosted: Apr Sun 05, 2020 9:46 pm 
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Usually Lurking wrote:
Have you checked all the coils and transformer windings for continuity? No point in applying power until these are good.

Ted


The power transformer is fine and secondary resistance is pretty close to the schematic. All IF coils have continuity and resistance doesn't read too far off. I haven't really check the rest of the coils, but I'll make that my next step.

Quote:
- Good practice: Instead of "see capacitor, replace capacitor," a more disciplined way is to use the schematic/parts list. This way you don't miss any and reduce chance of replacing one with the wrong value

- In general, only paper and electrolytic capacitors need to be replaced. They deteriorate over time even in a box or radio in storage; that's why many advocate replacing them all. Other types tend to last a very long time


This is how I went about things in the beginning. I only replaced the paper/wax and electrolytics. I didn't learn until later that a couple of the domino types could hide paper/wax inside. It may have been my replacement of one of those that got the AM working recently.

Quote:
- In general, with the exception of candohm type, resistors are very robust. Most radio circuits keep working even with some resistor drifting way out of tolerance. Resistors do smoke or otherwise become damaged due to failure/short of something else. You can usually see obvious signs of heat damage. For the initial round of restoration, if the meter shows a resistor even remotely close to the original value, I'd leave it alone (i.e. "if it ain't broke, don't fix it")


I left the resistors alone until recently, when I checked some and found most of them to be fine, but a few were reading 2x their value or more. And to be totally sure, I checked with a leg lifted to rule out any capacitance messing with the reading. All the others, I left alone.


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