Having a problem with my 'Automatic' button selectors. I cant seem to get anything. I selected a station on BC, switched over to auto, pressed the corresponding button, and turned the slug in and out. Also played with the padder nut. Are there any tricks or diagnostics I can do? Thanks.

You aren't getting anything at all??? Make sure that the switches (button and knob selector) are working properly. Make sure that the wires are connected properly. The coils are for the oscillator, and the condensers are for the antenna. Certain buttons favor certain frequency ranges. You should be able to get reception for each of the buttons. Call letters can be attractively printed on your computer. I typed in the font I wanted and then dragged the letters to form vertical rows. Then I shrunk the text so that it looked appropriate. After printing a sheet of call letters that I wanted, I coated the sheet with about three coats of varnish. After it dried, I cut out the letters. You can use lacquer, too, which dries faster. Varnish will yellow, and might look better with the off white buttons.

Thomas

Thomas, I am not getting anything at all, not even static, on the automatic selection. I cleaned all of the contacts and the buttons work well. I would think that the condenser padder for the automaic function would be a crital asjustent if it ultimatly ties in to the RF and wavetrap circuit. Maybe I can try some capacitance(100pf) in series with automatic switch and antenna circuit. I had to put some Capacitance in the variable cap off of the wavemagnet to get the radio to work in BC.

Thomas,

What has worried me about this radio is the fact that in the BC position the voltage on the 7B8 oscillator grid (pin 4)should be minus 8 volts but Dave stated using a VTVM he only measured minus 0.5 volts. The other voltages on the tube were close to stated. I suspect there is not any voltage on pin 4 in the auto position either. There is discussion on another post close to this one about this radio, may be the next one down.

Radiodoc

:You aren't getting anything at all??? Make sure that the switches (button and knob selector) are working properly. Make sure that the wires are connected properly. The coils are for the oscillator, and the condensers are for the antenna. Certain buttons favor certain frequency ranges. You should be able to get reception for each of the buttons. Call letters can be attractively printed on your computer. I typed in the font I wanted and then dragged the letters to form vertical rows. Then I shrunk the text so that it looked appropriate. After printing a sheet of call letters that I wanted, I coated the sheet with about three coats of varnish. After it dried, I cut out the letters. You can use lacquer, too, which dries faster. Varnish will yellow, and might look better with the off white buttons.
:
:Thomas

I am glad we have Thomas in on this thread. I have read some of his articles on Zeniths of this vintage and he knows his stuff. Yeh -.5 and -.8 is a stretch and I did use a vtvm, DVOM, and scope on dc so it is in fact off but.... the set is working on SW and BC, no automatic or MW. I have ordered some other 7B8's to try.

Dave,

I believe I asked in a previous post about what the voltage on pin 4 was with the radio working in SW. Could you please humor this old communications techie and measure it and let me know so I can sleep at night?

Radiodoc

:I am glad we have Thomas in on this thread. I have read some of his articles on Zeniths of this vintage and he knows his stuff. Yeh -.5 and -.8 is a stretch and I did use a vtvm, DVOM, and scope on dc so it is in fact off but.... the set is working on SW and BC, no automatic or MW. I have ordered some other 7B8's to try.

I know some things. I'm not an expert at all. I also learn a lot from other people on here. Together we all come up with much of the information everyone needs, and decide which stuff is incorrect or not needed.

I'm certainly not a Zenith expert. I've worked on a few, but there are a ton that I haven't worked on.

T.

Thomas is very knowledgeable, dont think different. He has talked about shielding for along time and people have asked, why shield stuff, it didnt come that way. I can tell you that shielding, especially on wavemagnet radios is a BIG plus. My radio would not align at all without shielded antenna wires.

Ok, radiodoc, you really wont get any sleep tonight. I might not either. still -.5 BC, -1 to -3 on SW from 49 to 19 meters moving dial, nothing on MW(no wonder I cant get anything on MW). Like I told thomas, the only way I could align this 7-s was to shield the wavemagnet wires(aluminum foil). C21 looks ok, no short, I may replace it anyway.I am back to thinking about pin 4 again.

Dave,

With minus 1 to minus 3 volts on pin 4 in SW the oscillator is running on that band. Not very strong but running none the less. I believe you said you tested C21 for leakage. Might substitute a known good 500 pf cap just to see what happens.

Radiodoc

:Ok, radiodoc, you really wont get any sleep tonight. I might not either. still -.5 BC, -1 to -3 on SW from 49 to 19 meters moving dial, nothing on MW(no wonder I cant get anything on MW). Like I told thomas, the only way I could align this 7-s was to shield the wavemagnet wires(aluminum foil). C21 looks ok, no short, I may replace it anyway.I am back to thinking about pin 4 again.

Radiodoc, I could not sleep last night so I messed with C21 on the auto circuit. It measured .67nf on my meter, close to original value. I didnt have a mica in that value so I tried some ceramics at .33 and .58nf (330pf and 580pf). Nothing. I cranked up the volume and heard a very very faint station on the last button but the padder and slug did nothing. About nothing on MW, notice that Q trimmer(MW) is attached to the wavemagnet and avc. I truly believe that all of my sets problems relate to the wavemagnet and circuitry.

p.s. exactly how does the oscillator create that negative voltage?

:p.s. exactly how does the oscillator create that negative voltage?

Dave,

In operation some energy from the osc coil/C1 tuned circuit is applied to C2 forcing electrons off it onto the grid pin 4 and are drained off thru grid leak resistor R3 developing a voltage across it. There may be others on here who are more proficent at describing how this all works.

Radiodoc

The C1 var. cap located near the C2 you are refering to gets disconnected during the automatic mode. The top of that loop antenna connects to the push button condensers. The reason I mention that is that in this mode c1 has nothing to do with c2. My new intent is to take a measument on the oscilator C1 cap when the dial is set to say 600khz and then that is what should read for the 1 buttons padder.

:p.s. exactly how does the oscillator create that negative voltage?

Dave,

You may want to check out:

http://www.du.edu/~etuttle/electron/elect27.htm#Penta

Maybe this will help explain better. Look under oscillators and mixers.

Radiodoc

Great article. I wish someone made a cartoon version of that for the common repair hobbyist to understand. If I completly understood that theory on the negative grid bias I could track down the problem. With my knowledge I have to replace parts or tinker.

:Great article. I wish someone made a cartoon version of that for the common repair hobbyist to understand. If I completly understood that theory on the negative grid bias I could track down the problem. With my knowledge I have to replace parts or tinker.

Dave,

You mentioned about a cartoon version. I have an old radio book somewhere that shows electrons as little black stick people running around the circuit. One I remember shows electrons running toward a radio frequency choke and at the front of the choke is a black squiggle guy wearing a sheriff's badge with his hand up. This figure was to depict the opposition a choke coil has to RF current flow. Basically any amplifier can be turned into an oscillator by taking part of the signal from the plate and feeding it back to the grid in the right proportion causing the tube to cycle on and off resulting in a frequency being produced. You mentioned you had ordered some 7B8 tubes. The problem may be as simple as a tube whose emission or other characteristics have changed so that it no longer will work.

Radiodoc

I ordered another 7B8 but you know I am so sfraid of messing with this set. Its very delicate. Just removing the tin foil from the antenna wires messes up the whole set. If this thing works when I remove it from my bench in the garage into the cabinet in the house it will be a mircle. I may insert the new 7B8 and if the negative voltage is not better then -1 I will forget it and button it up. I dont want to disturb any of the trimmers or padders unless I am sure that tube was 100% of the problem.

:I ordered another 7B8 but you know I am so sfraid of messing with this set. Its very delicate. Just removing the tin foil from the antenna wires messes up the whole set. If this thing works when I remove it from my bench in the garage into the cabinet in the house it will be a mircle. I may insert the new 7B8 and if the negative voltage is not better then -1 I will forget it and button it up. I dont want to disturb any of the trimmers or padders unless I am sure that tube was 100% of the problem.

Dave,

I believe if it can be found why there is not enough voltage on pin 4 the radio will work again. With a few exceptions once defective parts have been replaced, the radio should work properly again without adding parts to the circuitry unless it was recommended by the manufacturer. Servicing communications in the past, I found that sometimes it was better to put the radio away for a little while and do something else and come back to it later. Don't give up, when you find what the problem was and the radio comes to life, it is a wonderful feeling. I find that feeling is still just as wonderful today as it was 40+ years ago.

Radiodoc

Radiodoc, my nos 7B8 tubes came and the problem is still there: -.5 on pin 4. Just for the fun of it I tried a sheild over the 7B4, no output at all. The wires going to the wagemagnet had to be covered with foil. Sheiled pre-fab wire did not work. Here are some interesting measurments I took:
C1 (variable condenser disconnected) cap. readings
550khz=.84nf
1000khz=.39nf
1500khz=.24nf

NOW the automatic button condensers
button1=23-85nf
button3=13-54nf
button5=9-31nf

Way off but the funny thing is that if you use series capacitance formula, the component required to get where I need (C1 replacement) is the same as C1. ie 85nf(button1)+.85(new cap) = .84(what c1 was). Very strange but its late so I need to think about this.

I can match up this capacitence to emmulate C1, I just hope the slugs on the oscillator side of the buttons are in tune. C6 padders could be trouble.

Dave,

Did you replace C2 (50MMFD) and/or C5 (.00025MFD)? When trying the new tube did you temporarly remove the 100pf cap you had installed. If the service data is correct, C6 is only used in pushbutton and the SW bands, unless I looked at the schematic wrong.

Radiodoc

:Radiodoc, my nos 7B8 tubes came and the problem is still there: -.5 on pin 4. Just for the fun of it I tried a sheild over the 7B4, no output at all. The wires going to the wagemagnet had to be covered with foil. Sheiled pre-fab wire did not work. Here are some interesting measurments I took:
:C1 (variable condenser disconnected) cap. readings
:550khz=.84nf
:1000khz=.39nf
:1500khz=.24nf
:
:NOW the automatic button condensers
:button1=23-85nf
:button3=13-54nf
:button5=9-31nf
:
:Way off but the funny thing is that if you use series capacitance formula, the component required to get where I need (C1 replacement) is the same as C1. ie 85nf(button1)+.85(new cap) = .84(what c1 was). Very strange but its late so I need to think about this.
:
:I can match up this capacitence to emmulate C1, I just hope the slugs on the oscillator side of the buttons are in tune. C6 padders could be trouble.

Radiodoc, I replaced every part in this radio. C2 and C5 were replaced with new Silver Micas. I did notice that the old micas were not close to value, that could explain why the radio did not work when I matched the schematic and parts list. Wouldnt that be a bummer if the parts list was wrong. The 100pf cap I added was to C1 in parallel, variable condenser, which is disconnected in auto mode( yes I disconnected it with the new 7B8, same voltage). C6 padders are now in effect when in auto mode. I need to put new caps in parallel with the automatic padder condensers to match what C1 was doing. That will be fine and dandy for the rf section but I am worried about the slugs and C6 settings on the oscillator section. Radio works pretty well, about 8 stations on BC, 5 on 49 meters, none on MW or auto. Here is an idea why I had to add 100pf to C1. That double gang variable cap does not have built in padders, looks like it does but it does not BUT there is a piece of material that fits where the padders usually go(top and bottom) made of insulation material (cardboard). Well, if that goes bad or captures moisture you will have a short to ground.

Radiodoc, also notice that in auto mode all of the caps and inductors are gone on the C2 side of 7B8. That line just goes to the buttons. Strange but I bet at some point I may need to tweek C6 to adjust for that loss.

Dave,

Just a wild thought. When you replaced C2 and C5, you didn't get them swapped did ya?

Radiodoc

:Radiodoc, also notice that in auto mode all of the caps and inductors are gone on the C2 side of 7B8. That line just goes to the buttons. Strange but I bet at some point I may need to tweek C6 to adjust for that loss.

One of these days I am going to take a look at all of the parts again to see if I screwed up. The radio works ok now so I am afraid to touch it. One thing though, even without the 100pf I added to C1, that capacitance is no where close to the automatic button padders. The padders are 10 times larcer then the C1 capacitance.

Dave,

It is always good to closely and systematically check ones work when the repair either made the problem worse or still resulted in a dead or poorly working set. One thing I look for is if someone else made repairs and did it wrong before I got it. I believe there was a post on here where the person found someone had attempted to replace the bandswitch in the radio he had acquired and wired it incorrectly. When the wires were put in the proper place, the radio then worked ok.

Radiodoc

:One of these days I am going to take a look at all of the parts again to see if I screwed up. The radio works ok now so I am afraid to touch it. One thing though, even without the 100pf I added to C1, that capacitance is no where close to the automatic button padders. The padders are 10 times larcer then the C1 capacitance.

Radiodoc, not so good news, I tried matching the automatic button capacitors to the C1 condenser and it did not work, even tried messing with the C6 padder. Nothing at all. The radio gets most of its BC stations in the correct place on the dial and the SW band works so while the going is good I am going to button it up and hope that it works in the cabinet. Knowing my luck it wont.