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 Post subject: Philco 118 question
PostPosted: Feb Thu 22, 2018 5:20 am 
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Is it possible to replace the trimmer caps in a Philco 118 cathedral with regular trimmer caps of the correct capacitance range?

That would make some easier to adjust as the SW OSC one in particular is touchy about its adjustment and probably really should be a lower capacitance range.

Also maybe it would fix a dial tracking issue.

Chassis fully restored and going through the alignment the AM band had near perfect dial tracking and the SW band was slightly off.

All of the sudden I could no longer get the AM band to track properly, but as I recall the SW band now tracks properly.

Could it be an issue with a trimmer cap or is this a case of an RF coil issue some earlier Philcos were known for?

Really want to get this solved so I can use the radio.


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 Post subject: Re: Philco 118 question
PostPosted: Feb Thu 22, 2018 2:00 pm 
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Caps may be dirty and thus erratic. Can you take them apart, clean mica and plates with alcohol and cotton swabs, and reassemble? Take note of how many turns to fully close each one from their present aligned position before disassembling, so they can be returned to approximate alignment on reassembly.

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 Post subject: Re: Philco 118 question
PostPosted: Feb Thu 22, 2018 2:19 pm 
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I'm not sure what a "regular" trimmer cap is....they come in a variety of shapes and sizes.

Within reason, anything with the right capacitance will work.

Dial tracking will always be a compromise. It is affected by the L/C ratio, the shape of the tuner curve, and the use of padder capacitors. Unless a trimmer is defective**, changing it to another type will not affect tracking.



** in this context, "defective" would mostly likely mean leakage. While this would affect the Q of the tuned circuit, I'm not sure it would affect tracking.

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 Post subject: Re: Philco 118 question
PostPosted: Feb Thu 22, 2018 3:11 pm 
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Could indeed be dirty.

Don't think I can take them fully apart though.

I do know the trimmer for the SW OSC only requires a little movement to affect the frequency quite a good bit so I may at least look at replacing that one with one of a smaller range .

Also one of the insulators broke on one of the tuning cap trimmers that insulates the nut from the metal piece that is one plate of the trimmer. I used some sort of insulative material to replace it with, but forgewt what I used. That didn't happen until after the AM band would not properly align.

All caps that should be replaced have been replaced and all non-wirewound resistors were also replaced before even attempting an alignment.

Also had the shadowmeter coil rewound as it was open.

Sensitivity also drops as I tune lower on the AM band so that tells me the stages are not properly tracking each other.

One arm of the spider is broken, but my solution is one of two things.

1. Put a cap in series with the voice coil to form a high pass around 200Hz so the cone don't move much if any and install a 15" Jensen speaker (came from an Emerson console with three Jensen tweeters) in a proper open back box with a choke in series to handle frequencies below 200Hz.

2. Disconnect the Philco's voice coil and just use the Jensen.


pixellany wrote:
I'm not sure what a "regular" trimmer cap is....they come in a variety of shapes and sizes.

Within reason, anything with the right capacitance will work.


By regular I mean the type adjustible with a screwdriver versus the special tool for the Philco trimmers.

pixellany wrote:
Dial tracking will always be a compromise. It is affected by the L/C ratio, the shape of the tuner curve, and the use of padder capacitors. Unless a trimmer is defective**, changing it to another type will not affect tracking.

** in this context, "defective" would mostly likely mean leakage. While this would affect the Q of the tuned circuit, I'm not sure it would affect tracking.


What I need to do is align the AM band best I can then set the radio to the lower end of the AM band then vary my RF generator frequency and using my scope check the output of the various stages involved with the tuning cap and see what frequency they all peak at and if either or both of the other two sections do not peak at the same frequency of the oscillator + the IF frequency then I know the faulty stage. I will also do the same test at the higher end of the AM band. Once I get the individual stage isolated I can figure out what is wrong with it.


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 Post subject: Re: Philco 118 question
PostPosted: Mar Mon 05, 2018 11:04 pm 
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I had a squirrelly trimmer once on a high short wave setting. It turned out the fix was to put a small insulating washer between the screw head and the moveable plate. This cut out the metal-to-metal static and allowed smooth adjustment.

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 Post subject: Re: Philco 118 question
PostPosted: Mar Mon 05, 2018 11:22 pm 
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With mine it is more like the trimmer cap is too large of a value as one has to make very careful slight movements when adjusting that one.

A forum member was selling a proper tool for the Philco trimmers and I got one ordered so maybe that will help the alignment.

Don't know why the trimmer is so sensitive like that. Perhaps it was more difficult to produce a smaller value trimmer back then so they just stuck in the smallest value they could reliably make.


Now when doing the initial alignment when the AM band dial tracking was proper I was getting a peak at one setting of one trimmer, but after the problem with the dial tracking cropped up I no longer get the peak at that same trimmer setting.


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 Post subject: Re: Philco 118 question
PostPosted: Mar Sat 31, 2018 6:01 pm 
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There are two tube shields missing one is for the audio tube I think and one is for a RF or IF tube.

Could that cause the alignment issue I am having?


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 Post subject: Re: Philco 118 question
PostPosted: Mar Sat 31, 2018 10:06 pm 
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Tube Radio wrote:
There are two tube shields missing one is for the audio tube I think and one is for a RF or IF tube.

Could that cause the alignment issue I am having?
Yes---and worse. Missing shields means unwanted feedback---which, as a minimum, changes the gain and AVC response of the amplifier stages. In the limit it will cause oscillation which will make alignment impossible.

Echoing another recent discussion, trying to do the "real" alignment does not make sense unless the set it in the right configuration and working properly.

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 Post subject: Re: Philco 118 question
PostPosted: Apr Sun 01, 2018 4:15 am 
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I still don't quite get why the alignment was going perfectly then all the sudden I couldn't get the dial tracking right on the AM band and sensitivity gets lower as I tune lower.

To me that means either the RF oscillator or converter stage is not tracking the others properly.


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 Post subject: Re: Philco 118 question
PostPosted: Apr Sun 01, 2018 1:31 pm 
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At a minimum, I would put a shield on the RF tube before attempting an alignment. You could even use something as simple as aluminum foil connected to ground as a temporary shield.

You may have a bad trimmer, but I would rule out the tube shield first.

I see no reason you couldn't put a replacement trimmer in. I have had one or two flakey trimmers in radios which prevented proper alignment. Not a common failure, but they can fail.

Assume you have ruled out open coils in the front end?

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 Post subject: Re: Philco 118 question
PostPosted: Apr Sun 01, 2018 1:53 pm 
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Tube Radio wrote:
I still don't quite get why the alignment was going perfectly then all the sudden I couldn't get the dial tracking right on the AM band and sensitivity gets lower as I tune lower.

To me that means either the RF oscillator or converter stage is not tracking the others properly.

I think that "tracking" is being used ambiguously here. To me, "dial tracking" means the accuracy of station locations on the dial. This is affected only by the LO adjustments and, to a lesser extent by the IF frequency. Once those parameters are set, then a separate issue is the extent to which the RF adjustments allow you to peak up the response at various points on the dial. Almost all the radios I have worked on can only be peaked up at one dial setting. This is why alignment instructions typically specify setting the LO at the low end and high end, and then peaking the RF at something like 1400.

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 Post subject: Re: Philco 118 question
PostPosted: Apr Sun 01, 2018 2:16 pm 
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I'm using dial tracking right.

If the three stages were so far off at the low end of the AM band then I might get a peak elsewheres at the low end which would not be a true peak.

Now I did just add a preamp to my frequency display so I might be able to use that to see if the LO is tracking properly across the whole AM band.

At this point I'm thinking maybe it's a coil because some philcos did have a problem with the coils due to age.

Now using a scope and the frequency display how can I tell if all three stages are tracking properly where the tuning cap is concerned?


Now once I get the radio right the plan is to make a cabinet that can hold a 15" Jensen speaker and sit the radio on that and use that as the speaker given the original has part of the spider messed up. I'll either then use the original above 200Hz so the cone don't move much and the Jensen for the bass or use the Jensen exclusively.


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 Post subject: Re: Philco 118 question
PostPosted: Apr Sun 01, 2018 3:52 pm 
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"three stages"---if you mean Antenna, RF, and LO---then you are not using the definition I quoted.

In the typical set, the three sections of the tuning cap and not going to "track"--IOW, they won't all peak up in the same place. If one of them is WAY off then,. yes, there is a problem.

You should be able to quickly isolate which on is off---yes, a bad coil would cause a problem like this.

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 Post subject: Re: Philco 118 question
PostPosted: Apr Sun 01, 2018 7:53 pm 
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They should from what I understand all three be at peak at whatever the dial is set to on a properly aligned radio.

What I mean is the RF and IF both putting out the largest signal they can when fed a signal or receiving a radio station and they doing it at the same frequency meaning

for example:

I shouldn't have to tune to 650KHz for the RF to be at peak, but 700KHz for the IF to be at peak with an unmodulated 675KHz signal being fed to the radio from an RF generator. Both should peak at 675KHz with the LO running at 675KHz which usually means the sensitivity is the same or close enough to where the loss in sensitivity is not noticed and the stations should agree with the dial setting across the band.

At least that's what I've learned and experienced when doing alignments and I know that's what I had with this radio very briefly.


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 Post subject: Re: Philco 118 question
PostPosted: Apr Sun 01, 2018 8:21 pm 
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Tube Radio wrote:
They should from what I understand all three be at peak at whatever the dial is set to on a properly aligned radio.
.

No!!

Once you tune a station, you only have 2 adjustments to peak up---antenna and RF. In the typical radio, they will only be peaked at one place on the dial----try it on some random set.....

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 Post subject: Re: Philco 118 question
PostPosted: Apr Mon 02, 2018 3:56 am 
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I don't quite understand.

I've always thought those adjustments were to ensure the everything tracked each other properly so that everything would pass the maximum RF signal at any given frequency on the dial and that the frequency listed to adjust those adjustments was the frequency where it was found that everything would track properly across the whole tuning range.

Why have a tuned RF amp if it would only peak at one frequency? Wouldn't that make the sensitivity at that one frequency be higher than the whole rest of the band?

By peak I mean no further adjustment would cause the stage to pass a higher RF signal level for a given input RF signal level.


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 Post subject: Re: Philco 118 question
PostPosted: Apr Mon 02, 2018 2:18 pm 
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Sorry if I wasn't clear.

Take the simple case--AA-5 with a 2-section tuning cap---one for the LO, and other tuning the inout RF stage. The RF tuning tries to peak up the incoming signal at a frequency equal to the difference between the LO setting and the IF (high-side LO). Once the basic curve is cast in metal, one or more adjustments are provided to "dial it in". In most basic AM radios, this is a single trimmer adjusted somewhere near the top of the band.

The only profound point is that you can typically only get it exact at one or two points on the dial. The only way to do better is to have an adjustable L and/or a mechanical adjustment.

Try it: Take a simple radio and peak up the RF at 1400. Then tune a signal at 1000 or so, and then tweak the RF trimmer. I think you will find that the peak comes in a slightly different place.

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 Post subject: Re: Philco 118 question
PostPosted: Apr Mon 02, 2018 2:56 pm 
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I'm bringing home my RF generator from work today since I got it properly sussed out and will try the alignment again so I can give an accurate result of what I'm getting.

Perhaps with the Phinco alignment tool I'll get better results.

If not I may remove the trimmers and clean what I can then try again.


EDIT:

I wound up fabricating a temporary shield then proceeded with the alignment.

Didn't solve the problem.

I aligned it the best I could and for an RF generator frequency of 1,000 KHz the dial has to be set at 1,050 KHz.

I know that's not right at all and I am not sure how to fix it.

Now with the oscillator trimmer I can tighten the trimmer all the way down then back it off some and get a peak. I can then continue backing it off and get a null then get an even larger peak that keeps increasing the farther I back off the adjustment nut.

Any ideas there?

EDIT 2:

Think I finally got it :D :D :D

I tried the larger peak and found a spot where the AVC voltage started to decrease so I adjusted to the larger peak then readjusted the IF and RF trimmers on the tuning cap then adjusted the 600 KHz oscillator trimmer.

Here's what I get.

Attachment:
Measurement 3.png
Measurement 3.png [ 13.38 KiB | Viewed 603 times ]


I do believe I can live with that.

Unfortunately I cannot verify sensitivity given the output voltage of my RF generator varies with frequency.


Now to do the SW band alignment.


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 Post subject: Re: Philco 118 question
PostPosted: Apr Mon 16, 2018 1:49 am 
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Concerning the SW band I can adjust the oscillator trimmer to a point where it self oscillates at around 10.8 MHz.

I can tweak it to be proper, but there is a dead zone on SW where it seems like nothing comes in not even static.

Not that worried about SW as I have my SP-600 and R-390 for SW, but I do prefer the SW be working properly since it does have SW.


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 Post subject: Re: Philco 118 question
PostPosted: Apr Mon 16, 2018 2:10 am 
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Sounds like you are making progress. :D
How does that shadow meter work with the rewound coil?

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