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Midland 79-893 tx clarifier adjustment?

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jhutch62

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Hello,
I was wondering if anyone on the forum could help me with my Midland 79-893 SSB CB radio? I was talking skip yesterday and all the DXers said that I was off frequency and that I needed to turn my clarifier to the right. Well, I tried that but there responses were garbled due to the fact that they were now off frequency, relative to my receiver. I've done some extensive reading on the subject and looked at several unlocking clarifier mods but I couldn't find any pertaining to my radio, plus, I really don't want to mod it, I would just like to adjust it back to factory specs. I've tried searching through the CB Tricks website and had no luck.

I guess my question is: is there a separate adjustment for the tx clarifier on the board and does anyone have the service manual for this radio?

Thanks.
 

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prcguy

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So Cal - Richardson, TX - Tewksbury, MA
Sounds like your radio currently has no mod to enable the clarifier on tx and your radio is off frequency in general. Doing a tx clarifier mod will probably allow you to tune everyone in and everything will work ok but unless you adjust all the crystals in the synthesizer your clarifier will always be off to one side.

Since this is a 23 channel radio made in the 70s, all the crystals have drifted off frequency over time and they usually go low in frequency. I used to work on similar models and I believe yours is before they used a single master oscillator with one crystal, and you probably have a bank of them that cover maybe 4 channels for each crystal and that's mixed with another crystal.

If there are some small value capacitors across the crystals it might be possible to reduce their value by a few pf to get the basic radio back on frequency. Most CB radios were not designed to have the crystals adjusted separately via a trimmer but a repair manual or old Sam's Photofact will have all that info.

If there are adjustments for the crystals its not diffacult to align and there is probably a tx clarifier mod out there somewhere.
prcguy

Hello,
I was wondering if anyone on the forum could help me with my Midland 79-893 SSB CB radio? I was talking skip yesterday and all the DXers said that I was off frequency and that I needed to turn my clarifier to the right. Well, I tried that but there responses were garbled due to the fact that they were now off frequency, relative to my receiver. I've done some extensive reading on the subject and looked at several unlocking clarifier mods but I couldn't find any pertaining to my radio, plus, I really don't want to mod it, I would just like to adjust it back to factory specs. I've tried searching through the CB Tricks website and had no luck.

I guess my question is: is there a separate adjustment for the tx clarifier on the board and does anyone have the service manual for this radio?

Thanks.
 

RFI-EMI-GUY

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Is that a 40 channel radio?

Is this your model number? (Below). If so there are only a few adjustments to make. I would be concerned if the 10.24 MHZ crystal has drifted as there doesn't seem to be an adjustment. I think the designers intended only the carrier oscillators need be adjusted.

^^^^^ correction, the 10.24 is adjustable as well. Start with adjusting it so that you have 10.240000 MHz. If you can find a service manual for a model with same PLL. Otherwise lacking specific test points and instructions, use an AM shortwave radio to receive 10.24 MHZ, loosely couple the receiver to the PLL chip (to receive 10.24000) and also an accurate signal generator to 10.24000 MHZ. Use a zero beat method to adjust the crystal to agree with the generator. Do same for the other crystals at their marked frequencies. ^^^^^

^^^^^^update. Lou Franklin's The CB PLL Data Book, has all your answers about the PLL used in this radio Model! ^^^^^^^^


http://www.cbtricks.com/radios/midland/79_893/

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jhutch62

Member
Joined
Jul 29, 2008
Messages
34
Location
Harrison, Michigan
Yes, the 79-893 is a 40 channel radio, I got this picture from the internet and I noticed the 23 channel dial after posting this thread. The Midland 13-893 is a 23 channel radio and is identical to the 79-893. I'm thinking maybe the dial is just a poor replacement.
 

JayMojave

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Hello jhutch62: You need to have the clarifier Receive and Transmit tied together, as there is always someone OFF Frequency. There are arguments against this, but I am right as usual.

Since the old days of Channel 16 Lower side being used, there again will always be stations OFF frequency, but not as far off frequency as they use to be. So the Clarifier needs to be modified to allow operation in both receive and transmit. It sure makes things easier.

The LSB frequency adjustment is CT2 for 7.7975 MHz at Test Point (TP2) and USB CT3 for 7.8025 MHz again at Test Point 2 (TP2). But these adjustments require a accurate frequency meter and the tech to go with it. As said before the years on the radio will throw the crystal frequency off some but hopefully it can be adjusted to back on frequency. It needs a Good Tech to go thru it checking the oscillator frequencies and alignment. The Clarifier Mod should not be too expensive.

I have modified my Motorola Radios for having the clarifier to track both receive and transmit, and another modification to reduce the frequency coverage to allow a smooth and easy clarifier tune in of off frequency stations. Usually the Clarifier has separate transmit and receive voltage feeding it, it needs to be feed with a constant voltage not effected by receive or transmit allowing the Clarifier to track both receive and transmit.

Jay in the Great Mojave Desert 110 F degrees to day, yeah the upper desert is were yeah want to live... No traffic, No nit noid neighbors wanting you to paint your trailer a certain color, get to shoot out the back door, Neighbors think your towers and antennas are cool!
 
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RFI-EMI-GUY

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Jay, what is the test point for the main 10.24 MHZ PLL oscillator. He should check that as well.

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JayMojave

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Hello RFI-EMI Guy: I am NOT a day in day out radio bench Technician, but have worked some radios and such. So there are smarter guys out there than me.

I looked all over for the 10.24 Mc Osc circuit and didn't see any, wondering what I am missing here, then saw the radios PLL Osc circuit, which is separate at:
http://www.cbtricks.com/radios/midland/79_893/graphics/midland_79_893_om_pll_sch.pdf

The 10.24 Mc Osc is a 11.2842, 11.2850, and a 11.2858 Mc Osc, for AM, LSB and USB for the transmit function I think. So the radio needs to have all 5 of these Osc's checked for being dead nutz on frequency and of course alignment. Again a high quality frequency counter and Tech is needed to make this all right again.

I have a old Heathkit Frequency counter with a terrible expensive 10 Mc Time Base Reference Osc, that always checks dead nutz on the 10Mc WWV radio station. So I have corrected a few radios back onto the right crystal frequency as age has lowered there operating frequency some. Some are able to be brought back, and some need new crystals. But I have make darn sure I am not adjusting the wrong frequency as the oscillators and frequency are very close to each other on the diagram. So don't put me a home just yet.

Jay in the Mojave
 

RFI-EMI-GUY

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Hello RFI-EMI Guy: I am NOT a day in day out radio bench Technician, but have worked some radios and such. So there are smarter guys out there than me.

I looked all over for the 10.24 Mc Osc circuit and didn't see any, wondering what I am missing here, then saw the radios PLL Osc circuit, which is separate at:
http://www.cbtricks.com/radios/midland/79_893/graphics/midland_79_893_om_pll_sch.pdf

The 10.24 Mc Osc is a 11.2842, 11.2850, and a 11.2858 Mc Osc, for AM, LSB and USB for the transmit function I think. So the radio needs to have all 5 of these Osc's checked for being dead nutz on frequency and of course alignment. Again a high quality frequency counter and Tech is needed to make this all right again.

I have a old Heathkit Frequency counter with a terrible expensive 10 Mc Time Base Reference Osc, that always checks dead nutz on the 10Mc WWV radio station. So I have corrected a few radios back onto the right crystal frequency as age has lowered there operating frequency some. Some are able to be brought back, and some need new crystals. But I have make darn sure I am not adjusting the wrong frequency as the oscillators and frequency are very close to each other on the diagram. So don't put me a home just yet.

Jay in the Mojave
It is indeed the PLL master crystal. I was asking about a test point, because I see none. That is why on my lengthy post I describe an indirect way to adjust the frequency. If the OP simply hooks a frequency counter to the oscillator(s) and doesn't read off a safe test point downstream after some buffering, the test lead will load down the oscillator and either stop it or cause the resulting adjustment to be way off. Usually the maintenance manuals are good at telling a bench tech to do this. I use my indirect method whenever the manuals are lacking.


Also 110 degree weather isn't when you want to adjust a radio oscillator! Or do much of anything! Just marvel that a radio works at all at that extreme..


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JayMojave

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Yo Yes the Test Point for all 3 of the 11Mc oscillators is TP502 at the top of the page, and the frequencies are tripled to 33 Mc so that would allow monitoring the output frequency.

No 110F is out side, inside is around 70F. Just going out to the mail box you need a quart of cold water.

Ok there it is the 10.24 Mc Reference crystal in the upper rh side of the PLL diagram and CT501 is the adjustment for that crystal. Not seeing a test point on the PLL tho, I think TP501 coming off IC501 is the test point for the PLL output frequency. I am sure the service manual would give the out.

Looking at the diagrams Id say the radio is worth the money to have professionally aligned and again modify the Clarifier to track both receive and transmit.

Jay in the Great Mojave Desert No International Airport yet, No on/off Freeway ramp yet, No Fiber Optic Cable TV yet, Paving the roads is out, But summer BBQ's and camping is on...
 
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