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 Post subject: FM dial not matching
PostPosted: Jun Wed 12, 2019 10:39 pm 
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I got ahold of this GE 10 transistor p1818D radio and the only problem I could see was dirty pots. I cleaned them and spent the rest of the day trying to put the chassis back in the case. I was going through stations on the AM when I first got it and everything seemed to match very well. Today when listening to FM I saw that the dial was basically flipped! When listening to a station on 107.5 the dial shows around the 88 mark. AM is still showing up on the dial fairly well. No idea what could cause this. I haven't done any work besides cleaning the pots.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Thu 13, 2019 12:37 am 
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Location: Mountains of Mourne. Ireland.
Service information for P1818B (blue) can be found in Sams TSM-59 (transistor radio series).
I don't have it. →
$7.00 shipped, ebay: https://www.ebay.com/itm/Sams-Photofact ... GTR=1#shId

Image
Image
https://www.radiomuseum.org/r/general_e ... p1818.html

Post subject: FM dial backwards.
viewtopic.php?p=297710#p297710

Greg.

ref: TSM-59 https://www.samswebsite.com/en/photofac ... sid/TSM_59


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 Post subject: Re: FM dial not matching
PostPosted: Jun Thu 13, 2019 3:38 am 
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This can be caused by the oscillator trimmer (or coil) being a bit misadjusted. The reason it "flips" is because you start to pick up the image of the station when the oscillator tunes beyond the normal range.

If you check carefully, you'll probably find that the stations are in order (probably running a little higher on the dial than they should), except at the very bottom of the dial.

The image of a station will usually be 21.4 MHz off of where it should be on the dial (two times the IF frequency). Since the FM dial is only 20 MHz wide (88 - 108 MHz) normally images are not possible. But if the actual tuning range shifts downward, such as 85 - 105 MHz due to a misadjusted oscillator, then you can start to receive stations 21.4 MHz above. For example, if the dial is shifted too low so that the bottom of the dial is 85 MHz, then you could also receive a station at 106.4 MHz at that position on the dial.

Image reception will usually be quite a bit weaker than normal reception (since the front end isn't tuned to the image frequency) but really strong stations will still get through.

Check the sequence of stations you are receiving over the whole dial. If you find everything shifted upward, with a few images at the bottom of the dial, then an adjustment of the FM oscillator trimmer will fix. If the oscillator is that far off, the odds are a complete alignment would probably be beneficial, although if you don't want to tackle all that, just start with the FM oscillator trimmer adjustment. You'll find it moves stations up and down the dial. Getting stations to the right position on the dial will eliminate image reception at the bottom.

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 Post subject: Re: FM dial not matching
PostPosted: Jun Thu 13, 2019 4:51 am 
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When tuning around where I believe 107.5 would be I can pick it up slightly there. I haven't work on a whole lot of sets and this is the first transistor set. I assume the trimmers are the four screws in the clear plastic. I do not have a signal generator which I talked about in a previous post so I do not know how to proceed from here.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Thu 13, 2019 6:11 am 
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Since we don't have access to the service literature at the moment, if you're brave, you can do the following:

There are four trimmers on the back of the tuning capacitor. These four are:

1. AM oscillator trimmer - moves AM stations up and down the dial
2. AM RF trimmer - affects signal strength of AM stations, especially at top of dial
3. FM oscillator trimmer - moves FM stations up and down the dial
4. FM RF trimmer - affects signal strength of FM stations at any point on the dial

With some experimentation, you'll be able to identify all four simply by observing what they do when you make a small adjustment on each one. The oscillator trimmers have a big effect; even a small turn will move you off a station so that you need to retune it either up or down the dial. RF trimmers will be more gentle in their behavior.

Once you identify which one is the FM oscillator trimmer, use it to move stations to the correct position on the dial. Some further touch up of the other trimmers may be needed after that, but we'll take it one step at a time.

BUT, before you try this, it's very worthwhile to understand whether the phenomenon I describe above (image reception near the bottom of the dial) is really what's happening. To understand things better, can you do the following:

1. Tune in about 10 FM stations all the way up and down the dial
2. Identify the stations (if necessary, use another radio that works properly) and note where they come in on the dial of this radio, and what their actual frequency is (Google the stations if you don't happen to know their exact frequencies)

If you find that most stations are shifted up the dial a bit from where they should be, except for those at the very bottom, which are far below where they should be, then the problem is as I described. If the behavior is different, just post here what you find, and we can take a close look and try to figure it out.

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 Post subject: Re: FM dial not matching
PostPosted: Jun Thu 13, 2019 10:56 pm 
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Okay I went through the dial and it seems like the spread of stations and the order they are in seem really out of wack. When tuning in a station the distance I can turn the dial and still here that station seems really wide as well.

Here is a list of station I confirmed with a digital tuner and where they are at on this GE set:

107.5 is in the middle of the 88 on the dial
95.7 is on the 8 in 98 on the dial
98.7 is on the last 1 in 101 on the dial
100.5 is on 4 in 104 on the dial
98.7 is on the 1 in 108 on the dial

I have not messed with the trimmers yet but I will if it is the best way to proceed with this list of stations.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Fri 14, 2019 1:09 am 
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Tom Albrecht wrote:
...There are four trimmers on the back of the tuning capacitor. These four are:

1. AM oscillator trimmer - moves AM stations up and down the dial
2. AM RF trimmer - affects signal strength of AM stations, especially at top of dial
3. FM oscillator trimmer - moves FM stations up and down the dial
4. FM RF trimmer - affects signal strength of FM stations at any point on the dial


I've done this procedure to map out each trimmer on a transistor radio, but I would recommend it only if you're fully confident of what you're doing.

I found some photos on an eBay listing. https://www.ebay.com/itm/VINTAGE-G-E-PO ... 4326298190

The 4 trimmers are here. Unlike many older tube radios, these are very very sensitive adjustments and one wrong move, it may be difficult to recover.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Fri 14, 2019 5:24 am 
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Looks like the behavior of the tuning doesn't follow any obvious pattern. Given that situation, I also would not expect an adjustment of the trimmers to solve the problem. I don't think it's all that hard to adjust the trimmers on these Japanese tuning capacitors, but that's not really the question on the table here given the behavior being seen.

Another possibility is that the stopping point is not correct. The way these Japanese tuning capacitors are constructed allows for continuous rotation all the way around, and if the mechanical stop isn't working, the range can be all screwed up.

Take a close look at the insides of the tuning cap as you tune from 88 to 108. Is it clear that the plates go from fully meshed to fully unmeshed over this range, or does the fully meshed (or fully unmeshed) point occur somewhere between the two ends of the dial? If so, there is some serious mechanical problem with the tuning cap.

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 Post subject: Re: FM dial not matching
PostPosted: Jun Fri 14, 2019 12:32 pm 
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One hopes a metal (iron based) screwdriver is not being used to align?

I would look carefully to see if you have not damaged any wiring from the antenna etc. Printed circuits should always be checked for cracked tracks & "dry" (bad) solder joints. In this case around the resonant FM coils.

Marc


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 Post subject: Re: FM dial not matching
PostPosted: Jun Fri 14, 2019 5:24 pm 
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Nothing wrong with using a ferrous tool to adjust trimmers. Coils, of course, are another matter.

And of course, no alignment has been done here.

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 Post subject: Re: FM dial not matching
PostPosted: Jun Fri 14, 2019 11:36 pm 
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I'm confused on what to look for in meshed and unmeshed. I do not know where it should be closed or open. I can see the plates moving and the dial moves the full way.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Sat 15, 2019 3:23 am 
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In my experience metal can sometimes affect that type of trimmer where there are coils in proximity to it. IF's can be affected in a similar fashion, however, in tube radios the next danger is top trimmers where the trimmer on the primary is often alive & can be shorted.

Marc


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 Post subject: Re: FM dial not matching
PostPosted: Jun Sat 15, 2019 3:44 am 
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Are you familiar with larger air variable capacitors found in most U.S. and European antique radios? It’s easier to see all the parts in those and how they work. This works the same way, but instead of a small empty gap between the fixed and moving plates, there are thin sheets of plastic to prevent the plates from touching each other. That allowed them to make the whole unit much smaller with more closely spaced plates.

Basically there is a stack of fixed plates and a stack of moving plates which mesh into (overlap with) the fixed plates as you turn the shaft. At the bottom of the dial, the plates should be fully meshed, for maximum capacitance. These will gradually unmesh as you turn the shaft, and should be fully unmeshed (for minimum capacitance) at the top of the dial. If you look through the transparent plastic housing as you turn the tuning knob, does it look like it is working properly?

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 Post subject: Re: FM dial not matching
PostPosted: Jun Sat 15, 2019 6:45 pm 
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Inside the that plastic thing with four screws is the miniature version of this. Never seen one out of adjustment except that time when I was 8-years old and wanted to know what these screws do to the radio.

Just speculating; when you were cleaning the radio, is it possible some cleaning fluid got inside?


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 Post subject: Re: FM dial not matching
PostPosted: Jun Sun 16, 2019 1:40 am 
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[quote=
Just speculating; when you were cleaning the radio, is it possible some cleaning fluid got inside?[/quote]

+1 Anything in there that can change capacitance, even in the trimmer, will cause issues. There is also the possibility that something was loose, or damages in the FM section or pushed into something else & shorted

Marc


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 Post subject: Re: FM dial not matching
PostPosted: Jun Mon 17, 2019 9:42 pm 
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From what I can tell the tuner is moving properly. Doesn't really seem to have a problem moving and meshes correctly from what I see.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Mon 17, 2019 11:34 pm 
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FM alignment is typically a bit more complex than AM. The RF and Oscillator adjustments tend to interact. The most common procedure is to set the signal generator at the desired frequency and then adjust both trimmers until the signal is peaked. If the only adjustments are the 2 trimmers, then this should work. When there is also a coil adjustment available, things can get confused in a hurry.

Try tuning a local station near the top of the dial--i.e. around 104. Peak up the RF trimmer, and then note where it is on the dial. If it is too low, then tweak the oscillator trimmer for a slightly higher C. This means the tuner has to go to a LOWER C to tune the station---this puts it higher on the dial. At the new setting, peak up the RF trimmer.
Then repeat the process until the station is in the right place---doing this in small steps is called "walking" the adjustment.

When the station is in the right place, then see where all the others are.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Mon 17, 2019 11:57 pm 
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Did you spray anything into this Polyvaricon tuning capacitor?

Greg.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Thu 20, 2019 4:06 am 
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I will mess with the RF sometime soon.

I did spray contact cleaner into the tuner and volume control because the pots were dirty. Sadly, I did not look at the FM dial placement before doing this.


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 Post subject: Re: FM dial not matching
PostPosted: Jun Thu 20, 2019 7:39 am 
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Contact cleaner inside this type of tuning capacitor can be a real problem. Since you've got a stack of metal plates and plastic sheets, there are lots of spaces for the liquid to capillary into, and it basically doesn't come out. It won't really evaporate on its own, because it's trapped between the plates and sheets. I'd be tempted to remove it, and then heat it to try to get the liquid out, but I'm not sure how much temperature one of these can stand.

Great catch, AJJ and egg. Maybe one of you has an idea how this liquid is going to find its way back out of this tuning capacitor?

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