I have a Philco 118 cathedral that I recently restored and it works for the most part, but it does have an issue.
When aligning the radio the IF alignment and the RF alignment for the BC band went good.
The alignment for the SW band went good, but here's the problem.
As I tune lower on SW the frequency starts to be off.
Here's some measurements I just made.
Sig gen freq---Radio Dial
10.8MHz-------10.8MHz
10MHz---------10MHz----10.5MHz
9.5MHz--------9.3MHz----9.9MHz
9MHz----------9.3MHz----8.75MHz
8.5MHz--------8.75MHz---8.25MHz
8MHz----------7.75MHz---8.25MHz
7.5MHz--------7.25MHz---7.75MHz---7.6MHz
7MHz----------6.75MHz---7.25MHz---7.1MHz
6.5MHz--------6.75MHz---6.2MHz---6.6MHz
6MHz----------6.25MHz---5.7MHz---6.1MHz
5.5MHz--------5.71MHz---5.6MHz---5.15MHz
5MHz----------5.19MHz---5MHz
Any idea what could be causing that problem?
At first I thought it was the 3,000pF cap that is put in parallel with the trimmer cap for the oscillator low frequency which is adjusted on the BC band only, but putting a cap in parallel with it only shifted the frequency and I was left with the same problem.
EDIT:
Failed to mention earlier that the shadow meter coil is open (getting it rewound), but I did substitute a 1K resistor for it. Not sure if that would be causing the problem.
Welcome to the Phorum. Those readings look pretty good to me. Maybe some of the experts will comment on how close you can get.
Thanks.
I would think that I should be on the exact frequency or at least that the frequency should only be picked up in one place on the dial unless this is just how the radio works and if so why even have the SW band.
Now I do not have the shields for the RF and IF tubes as the radio only had one shield when I purchased it which I installed over the oscillator/mixer tube.
Don't know if the RF tube being unshielded would cause my issue.
I have the same problem with my 118. Dial calibration off at the lower end of the SW band. Also tried playing with the value of the padding cap, but if I got the dial calibration close, then the ant and RF tuned circuits did not track and the sensitivity suffered. Finally I gave up and just adjusted for best tracking and sensitivity, and accepted the dial error.
It really seems that the SW band dial was poorly calibrated, because when carefully aligned, the tracking of the Ant, RF, and Osc tuned circuits is very good on SW despite what the dial indicates.
Do you have the problem of receiving one frequency at two or three spots on the dial in the SW band?
EDIT:
I also noticed that I can readjust the SW antenna trimmer at 5MHz and the signal level of WWV increases.
To me that seems like a tracking error in the RF stages, but it would have to be something that only affects the SW band.
Discovered there are two settings of the SW oscillator trimmer that will provide a peak at 10.8 MHz. One setting causes a major drop in signal around the middle of the band and the other setting does not. Think that explains my antenna trimmer issue. I had the SW oscillator trimmer set to the wrong peak which in this instance was the peak that produced more negative AVC voltage.
Also I put a 2700pF mica cap in series with the 3000pF cap and it brought WWV in at just under 5MHz on the dial. I am thinking maybe 100pF or so of capacitance in parallel with the 2700pF cap might just get it right.
Yes, it is not unusual to hear a SW station in two places on the dial with this set. The 118 uses an IF of only 260 kHz, so you will hear the image freq of a received station 520 kHz away. This is not much of a problem on the BC band, but at higher SW freqs the problem becomes more evident.
The 118 was really designed as a modification of the earlier BC only model 18, and making it receive SW was an afterthought. Later SW radios used a 465 kHz IF which improved the image rejection, putting the second signal 930 kHz away where it could be attenuated better by the Ant and RF tuned circuits.
There is no way to trim the RF amp plate circuit on SW, so you really have to compromise the osc freq a bit so it tracks. I basically went back and forth between the SW osc and ant trimmers to get the best sensitivity, but I favored the high end of the band because that's where most of the listenable signals are.
The 118 receives SW pretty well, but don't expect the performance of later sets specifically designed as all band radios.
What I don't quite understand is why Philco used a 3000pF mica cap that gets switched in parallel with the oscillator low frequency trimmer when from tests I did by putting a 2700pF cap in series with the 3000pF cap which brought the capacitance down to 1421pF got WWV at 5MHz much closer to 5MHz and checking the frequencies at 1MHz intervals between 5 and 10MHz shows they are almost spot on as well when compared to my RF generator's frequency setting.
Pretty sure that Philco would not have used a cap that was so far off the proper value and have that much tracking error on the dial on the low end of the SW band.
That said I still think maybe I have something wrong that lowering the value of that cap fixes unless all Philco 118s are off by that much at 5MHz. If all are off then it makes me wonder why the 3000pF cap was even used unless they used that cap in other models and figured they could save having to stock another part and that the resultant tracking error wouldn't matter.
EDIT:
It appears that some value of capacitor between 1421pF and maybe 2200pF will get the tracking spot on.
If that works I need to figure out why a lower value of capacitor was required.
An 1800pF cap gets it real close so I'm thinking that maybe 2000pF will be just about right.
If it works maybe someone else who is restoring a Philco 118 can try the same value just to confirm if I have an actual problem or not.
What Mondial said makes sense, the model 118 is an updated version of a model 18, some model 18s had the standard AM broadcast band alone, and others had AM broadcast and the old fashioned police band which was basically an extension of the prior above 1500 KC, much like a model 60 would have. So Philco being Philco, when they wanted to come up with a new model, rather then engineering and tooling up for a new chassis they took the model 18 and modified it's front end to allow for a proper shortwave band, probably by their logic hard core shortwave listeners could go out and buy a model 16 , those that were casual shortwave listeners but would more then likely listen to dance bands or soap operas could buy the 118.
Regards
Arran
I agree.
Well I tried a 1900pF cap I had and it was a bit too much.
Concluding tests for the night I have so far found 1800pF to provide the best overall dial tracking.
I may experiment tomorrow with adding series resistance to the original 3000pF cap just to see what effect if any it has.
EDIT:
Just thought of something this morning which didn't occur to me last night. Given the SW oscillator trimmer has two settings that provide a peak could the LF oscillator trimmer also have two settings that have a peak and maybe I just set it on the wrong peak like I did the SW oscillator trimmer?
I also have yet to replace the grommets for the tuning cap so perhaps that could be causing some of the issue.
Its much more difficult or impossible to set the broadcast band oscillator to the wrong peak. A mentioned previously, the radio will respond to two different frequencies. One 260 kHz below the osc freq and one 260 kHz above. On the broadcast band, the osc is always above the receive freq, so to receive 1000 kHz you need a 1260 oscillator signal. With a 1260 kHz osc, you can also receive the undesired image at 1520 kHz, but since it is so far away from the desired 1000 kHz, it is eliminated by the antenna stage tuned circuits.
So to tune to the wrong osc peak, you would have tune it 520 kHz lower than normal which would be out of the possible tuning range of the trimmer. But its a different story on the short wave band. Say for example you want to receive WWV at 10 MHz. You need an oscillator signal of either 9.740 MHz or 10.260 MHz. Either will work just as well, depending on which was chosen by the designers, as it has to track with the ant and RF tuned circuits. But it is easy to tune the osc to the wrong peak in this case, as the freqs are only a few percent different and well within the range of the osc trimmer.
With the 118, I believe the SW band was designed with low side injection, that is the oscillator runs 260 kHz below the desired receive freq. By changing the padding cap from 3000 pF to 1800 pF, you may have effectively converted the circuit to high side where the osc operates 260 kHz above the receive freq. This may improve the dial calibration, but may disturb the tracking with the ant and RF tuned circuits. Check the SW sensitivity with the 1800 pF padder, as the Ant and RF stages may no longer be synchronized. See how the shadow meter responds and see if you have the same deflection with a known SW signal as you did previously before the padder change.
As I recall, I played with the padder value to get the dial cal right on, but then lost sensitivity due to mistracking with the ant and RF circuits, so I went back to the original padder.
Unfortunately the shadow meter is open circuit, but a guy on the Antique Radio Forums is gonna rewind it for me. I substituted a 1K resistor for now.
Concerning the oscillator adjustment I find one peak which gives more AVC voltage which indicates a stronger signal and the other peak gives less AVC voltage which indicates a weaker signal.
The setting with the stronger signal causes a notable reduction in signal as I tune near the middle of the SW band, whereas the setting with the weaker signal doesn't cause any drop in signal over the whole SW band. Also based on my digital frequency display either setting seems to be low side injection as the display when set for -260KHz IF offset displays the correct frequency.
All changing the cap value seems to do is alter the LO frequency some which requires re-adjusting the SW oscillator trimmer and antenna trimmer at 10.8 MHz
When you tune to the osc peak which gives more signal strength, where does max signal occur? At the low end or high end of the band?
Did you happen to check that 3000pf cap for value or leakage? I have been finding a number of mica caps failed lately. Had a similar problem with a 116 receiving multiple signals like yours and it was a bad mica cap in the oscillator.
Gregb
I didn't check it for leakage as I don't really have a setup for that yet.
Will have to try it with the 1800pF cap and see if I still receive multiple signals.
Ok I have an important update.
I found three grommets from a defunct record player that with one washer each was able to be used to replace the rubber grommets for the tuning cap. I did realign the RF section after doing that.
With my particular Philco 118 using 1800pF in place of the 3000pF cap gets the dial as close to being properly calibrated as possible.
I was able to pick up some hams on the 7MHz ham band so the sensitivity seems to be there.
For now I will leave it as is.
What I need to do is see how good the sensitivity is using the original 3000pF cap.
Also if the dial is off by the same amount on most of the Philco 118s maybe someone could figure out how much it is off and create a new dial scale with the necessary corrections made to it. That way the dial will be correct and no one else will have to go through thinking their radio has a problem when it is simply just that the dial scale is off.
What I wonder is was the dial scale off like that on SW when the radios were new? To me it would seem like Philco wouldn't allow that sort of mistake to make it to production.