I cant seem to get a good wavetrap adjustment for clear stong signals on the complete scale on my 7-s-530. One adjustment of (E) will get great reception from 540khz to 1100khz, a quarter turn of (E) will get 1100khz+ stations but will loose the lower scale stations. There is no happy balance that I can acheive. Any ideas?

Hi, Dave. Here's your schematic: http://www.nostalgiaair.org/PagesByModel/333/M0025333.pdf

The wavetrap (labeled "E") was essentially a kludge to prevent a potential problem that no longer exists: long-wave maritime signals coming in close to the set's IF frequency. You can just disconnect it.

:I cant seem to get a good wavetrap adjustment for clear stong signals on the complete scale on my 7-s-530. One adjustment of (E) will get great reception from 540khz to 1100khz, a quarter turn of (E) will get 1100khz+ stations but will loose the lower scale stations. There is no happy balance that I can acheive. Any ideas?

Without a wave trap on this radio it would never work, trust me on that, its the heart of wavemagnet consoles. This is a tuned antenna radio; ie the (E) trimmer connects to the grid of the mixer and works with the centertap trimmer on the antenna to match the opposite side of the mixer: the oscillator.

Adjust for minimum output with a 455 signal

Dave

:I cant seem to get a good wavetrap adjustment for clear stong signals on the complete scale on my 7-s-530. One adjustment of (E) will get great reception from 540khz to 1100khz, a quarter turn of (E) will get 1100khz+ stations but will loose the lower scale stations. There is no happy balance that I can acheive. Any ideas?

Yep.

Adjust it to the IF and your radio will work well all over the dial. My Zenith 6-G-601 has the same thing. If you adjust it to some portion of the band, it's going to cut that portion out (or if you adjust one part for maximum sensitivity, the other part may be cut out). Adjust this circuit for minimum sensitivity at the IF, and you'll have fairly even reception over the entire band. My portable can pick up TONS of stations in just about any environment without static. This wave trap also helps eliminate image signals.

Thomas

I tried that adjustment and I while it did get uniform across the dial, I picked up nothing. When Manually adjusted(E) I can pick up stations really strong and clear, I only can get half of the dial. Would be nice to figure out a way I could fool the system by changing the wavetrap as it entered 1100khz+ on the dial. The bottom line is that if I mess with the trimmer(E) I loose 540-1100khz. Would (F) trimmer bring in stations on the high end or is that for something else.

Well, if you adjust the whole radio correctly (I assume that you are doing this, but perhaps you aren't...), it should work quite well. Again, here is a similar set-up that comes with instructions.

http://www.nostalgiaair.org/PagesByModel/130/M0025130.pdf

Thomas,

If this is the radio I think it is from an earlier post, the voltage on the oscillator grid (BC band) was 0.5 volts minus instead of the approximately 8 volts minus called out on the schematic. Not sure the oscillator is working or working very little. I suspect that receiving some stations when adjusting the wavetrap that there is some feedback possibly via the wavetrap and other elements of the mixer tube demodulating the input signal, and changing the wavetrap adjustment is like adjusting the main tuning cap. I could be all wet on this, though.

Radiodoc

:Well, if you adjust the whole radio correctly (I assume that you are doing this, but perhaps you aren't...), it should work quite well. Again, here is a similar set-up that comes with instructions.
:
:http://www.nostalgiaair.org/PagesByModel/130/M0025130.pdf

Very good point. Probably is something wrong with his set.

T.

Thomas, what is your thought on shielding antenna lead wires to the wavemangnet. I noticed a big performance increase(stations align where they should on the dial) with aluminum foil on the lead wires, take it off and the stations scatter. Now this 1941 radio never had shielded antenna lead wires from the factory. I tried shielded 6 conductor and the RF section did not work. Go figure.

Not sure. Shielding increases capacitance, so you don't necessarily want that. The wires on my portable Wavemagnets are separated by a cloth belt. The two wires are mounted on each edge of the belt. This keeps distance uniform. Spread the wires on your Wavemagnet apart. If they're all twisted together, capacitance may increase in ways you don't want it to.

If your oscillator has a low voltage at the grid, though, you need to attend to this problem. The voltage should be around 5 volts negative (connect your negative lead to the grid and your positive lead to whatever the cathode connects to, though if it connnects to a coil, connect on the other side of the coil--B-, but not necessarily the most negative point in the radio). If the voltage isn't around -5 volts (it varies over the dial....sometimes gets higher and sometimes gets a bit lower), then your oscillator needs work. Your meter, by the way, will of course read +5 volts because you have the negative lead connected to the grid of the oscillator, which is supposed to be negative. Unless your meter is equipped with a negative reading scale, you must connect it as I said, and then read the voltage on the positive scale.

People say that I tend to generalize. Well, there are a lot of different things that can contribute to poor oscillator function. Without being too unscientific, I say that you must go through everything in the oscillator section and look for trouble. Condensers must have ABSOLUTELY NO leakage what-so-ever. If there's a mica looking condenser in the grid circuit, or anywhere in the circuit for that matter, be sure to check it, too. For values under .001 MFD, your meter needle might not normally visibly swing up and down for the charge-up cycle. However, any condenser that is leaky will swing the needle up and not let it fall back down. If the needle lingers up by even the most microscopic amount, replace the condenser, especially in grid circuits.

After you have made absolutely sure that there are no leakage paths in the condensers, chech your resistors one more time for drifting. Try replacing the oscillator tube with a new one, too.

Make sure that all tubes are shielded well. Shielding the wiring in the oscillator/converter section can be tricky, and generally Zenith did a good job with whatever needed to be done originally, so you probably shouldn't add shielding there. Keep the plate lead of the converter tube (final plate, not the oscillator plate/2nd grid) away from all other grid and plate leads. Sometimes Zenith put a spring coil shield over this wire. Oscillation can occur if the wire isn't shielded properly. Be sure to keep all plate and grid wires away from eachother as much as possible, though if everything looks as though it hasn't been altered before, consider it to be routed properly, for if it wasn't, the radio wouldn't have sold in the first place.

Thomas

Thomas, you are right on the money. Shileding does add resistance and thats why shielded wire killed my antenna. I used aluminum foil over the wires and the radio performed much better. I think I will separate the wires like you mentioned.

You said "shield ALL tubes". My schematic shows the 1232(rf) and the 7a7 IF tubes being shielded only. I once tried to shield the 7B8(mixer) and it killed the radio.

ALL of the passive components have been replaced and triple checked in addition to the rubber wire being replaced.

Now for the hard question no one can seem to answer, HOW does the oscillator create a negative voltage in scientific terms. If I understand that I can trace it down.

p.s. forgot to mention that I use a VTVM for voltage checking and it has a negative voltage scale so I connected it to pic 4 of the 7B8(grid) and chassis ground(cathode). Not sure if it moves with gang caps but will check.

Just use a regular 20,000 ohm per volt meter. You don't need a negative scale. If you connect the meter with the negative on the grid, and assume that all positive voltages you see are actually negative voltages on the grid.

As to why the grid goes negative, well, I really don't know. Someone on here might be able to tell why. I assume that the frequency induced onto the grid coil is what puts the voltage on the grid. The oscillation is alternating current, though....that's what's strange. I would think that any positive shift would off-set the negative shift, and create zero volts DC. Another theory of mine is that any positive shift actually shifts the grid positive, and so it then becomes a plate. It conducts and becomes negative again. That would upset the shape of the wave, though, which, I believe, is more rounded like a sine wave (though not perfect). Anyway, I don't really know. Norm or someone on here should probably know. I've never really read about it in any of my radio books. Even the really technical ones don't seem to get into detail on that subject, or maybe I missed something.

Regarding shielding the mixer, you should be able to shield the mixer and still have it function. Shield whatever tubes Zenith originally shielded. Shielding all RF tubes is ideal, but isn't always necessary.

You say that you replaced all of the wiring. Well, be absolutely sure that the new wiring is connected properly, and be sure that it is routed properly. Keep grid wires away from plate wires, and check to see if the plate wire from the converter was originally shielded.

And finally, check the tube, and try several brand new ones if necessary.

Thomas

Or rather, I should re-word my theory.

Another theory of mine is that any positive shift actually shifts the grid positive, and so it then becomes a plate. It conducts, and becomes neutral again. The negative swing of the signal will not be conducted away, though, since a negative grid will not conduct to the cathode. Perhaps the voltage builds up, since the negative cannot be conducted away. I know that there is usually a resistor around 100K that keeps the grid from going too negative.

Whatever it is, the negative voltage is definitely created by the oscillation.

Be sure to check those mica condensers. Did you remove them and check them????

Thomas

Thomas, I think I have found the answer for the negative voltage on the converter.

http://en.wikipedia.org/wiki/Grid_leak

But why is the negative charge so big in oscillators? It is enough to drive an ordinary meter, where-as other circuits require VTVMs unless the tube is cathode biased.

T.

Dave,

Didn't you mention in an earlier post that the SW band worked and you had minus one volt or more on pin 4 (oscillator grid)?

Radiodoc

:p.s. forgot to mention that I use a VTVM for voltage checking and it has a negative voltage scale so I connected it to pic 4 of the 7B8(grid) and chassis ground(cathode). Not sure if it moves with gang caps but will check.

Radiodoc, yes -.5 to -1 on pin 4 I believe on all bands, dont think voltage changed with bands, I could be wrong.

Thomas, when I restored the radio I replaced all of the parts including all MICAS. Triple checked them and their locations. BUT...last night I replaced them with the old ones.Dont have a meter that will check .00025mf. Thats pretty far out there. Did check for leakage with VOM. What about carbon composition resistors vs. film resistors. I replaced all of the resistors with film. I have heard that carbon composition are better for freq. applications.

Thomas, the resistor that goes from the plate of the converter to gnd is a 47K resistor on my radio. Do you think if I changed that, it would provide more negitive voltage to the grid?

If that's what Zenith specified, then that's probably what you should use (47K). It seems almost as though you installed condensers for C2 and C3 that are too small.

In my experience, and from what I read on here, the oscillator grid voltage should be several volts negative, not just 1 or less.

Carbon film or carbon composition resistors are both fine. Your problem is more likely due to a faulty component (including the tube or the coils...and make sure that the oscillator secondary has continuity....with the way it is wired, a break can still allow it to work somewhat), miswire, or wires placed poorly.

T.

C2 and C5 mica capacitors which are in the oscillator circuit are correct values(per schematic). Not sure what you are talking about C3 because it is in the antenna area, not mixer (maybe you mean R3) that you are talking about. As far as shielding the plate wires, this radio does not use 6__ tubes for the RF and IF it used 7__'s which dont have the cap on top. 10-s zeniths use those. These wires are under the chassis and would be a major choir to shield. There were a few wires I did not change and they all attached to the oscillator.

Ok, I did some experiments tonight. I changed the 47K resistor across the converter filament to a 1meg and the negative voltage came up to about -.9 volts. The (F) trimmer on the oscillator does nothing to that voltage. Any ideas?

I did mean C5...sorry about that. Regarding the trimmer, it won't affect voltage much at all. Tuning over the entire broadcast band will only affect the voltage slightly.

Did you check the continuity of the oscillator coil secondary? It is capacitively coupled to the oscillator plate (grid 2), and may still work if broken due to capacitive coupling between the two parts of the broken winding, but performance will be weak due to this.

Regarding shielding the plate lead of the converter (lead that goes to the 1st IF transformer primary), none of the Zeniths I own (portables), and none of the Zeniths I have worked on have top cap tubes, though the top cap is usually for the grid, not the plate. I am talking about the final plate, though. In my portables the plate wire is shielded by a close wound spring that covers the entire length of the wire. Without the shielding the oscillator performs poorly. If your radio didn't have it to begin with, then it probably doesn't need it.

Also, regarding oscillator plate (grid 2) vs. converter plate, the oscillator plate is in fact grid 2. Grid 2 serves as both a plate and a grid. The final converter plate is the plate that encircles the entire unit (there may be a shield around the outside of this, too, which looks like a plate, and is connected to the cathode or the supressor grid).

Thomas

:Thomas, this is what I thought you meant by shielding the plate wires. Example is a 10-s

http://www.nostalgiaair.org/PagesByModel/678/M0024678.pdf

All of the RF/IF wiring is tight up against the chassis and tought to get at. There was only one long wire in the set that had a spring that goes to the phono switch but I dont think it has to do with the converter, not shown on any schematic, and I reinstalled the spring.

I will check all ohms on oscillator coils tonight in addition to bypassing the selector switch. Let you know the results soon.

Thomas, I took some readings. Dont know what you mean about primary and secondary coils. The oscillator looks like a transformer in schematic form but in reality the coils attach and it's one piece of wire. I measured on c2 and c5 before and after the caps on all selections and it looked good. .5ohms before caps and 10k thru the caps. I did notice that the voltage is about 30 volts of pin 5 of the 7b8 and pin 3 of the 7a7 (they direct connect) Dont know if that is the problem. Any idea what c6 padder do?

That is no good. Probably is your problem. Check R8 and C4. Remove both of them from the circuit if necessary. The screen grids of these two tubes should have a voltage higher than 30. Make sure you are using a 20,000 ohm per volt meter or better.

I looked at the schematic you showed me, and what it shows is exactly what I was talking about. It shows the PLATE wire of the converter with a shield. The grid wire, which goes to the cap on top of the 6A8 is not shielded, and usually does not need to be shielded. The plate wire goes to one of the pins on the bottom of the tube. This wire can be shielded on any radio if you wish, no matter what the tube is. Generally the trouble without the shielding is howling and irratic oscillation--not the trouble you are having.....but I thought that it might solve your problem. The low voltage on the screen grids is more likely your problem, though. Give R8 and C4 a look. If neither of these is faulty, then give the circuitry ahead of this a check. You shouldn't be checking the capacity of C4 so much as its leakage. Capacity accuracy here is very unimportant so long as the capacitance is significant and somewhere near what it should be.

Thomas