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DX-979 Channel Mod Question

SLR_65

Active Member
Dec 27, 2014
104
16
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Hi Guys,

Years ago my brother was into freebanding and had a really nice setup, a full Yaesu FT-101E station. As a young teenager I had a lot of fun talking DX with it and later I did get a ham ticket and put that equipment to full use.

As time went on, I went to college, got married, etc. and I got away from radio and my brother sold his setup years ago.

Recently I've had some renewed interest though and thought I may give a look at freebanding, or possibly getting a ham ticket again.

I found a good deal on a used Galaxy 979 CB and matching FC-347 frequency counter, I liked it because it can freeband and be converted to 10 meter use if I go that way.

There seems to be basically one mod method for the 979 and I'm curious type, I don't like to just follow the directions, I like to understand what it is I'm actually doing and the instructions for the 979 channel mod are a bit perplexing to me and I was wondering if someone could help clear things up for me?

Some of this could be found by experimentation, but my radio isn't here yet so I'm just looking at the schematics and board prints right now.

My understanding of how the frequency selection works is:

The 40 position channel switch has 7 output lines, each of these output lines can either be set high (designated by a 1) or low (designated by a 0).

The RCI8719 PLL chip is controlled by those 7 lines coming from the channel selector, what frequency the radio operates on is controlled by how those lines are set.

For instance, the channel selector output lines are designated as P6, P5, P4, P3, P2, P1 and P0 and they connect to the PLL chip on pins 10, 11, 12, 13, 14, 15 and 16. By having them set as 1100000 channel 15 is generated.

By overriding the inputs from the channel selector we can set the PLL control lines to create channels other than those normally allocated. The override is done by installing switches and tying their contacts to the PLL power leg or to the board ground and the PLL input line we wish to override and force to a certain state.

For instance, the channel selector is set to channel 15 as per the previous example which sets the control lines to 1100000, if we would connect P6 (pin 10) to ground then the power being fed to pin 10 via the channel switch to set it high would be shunted to ground and the lines would be set as 0100000 which would generate channel 77 (27.775). If we would also ground P5 (pin 11) at the same time then the lines would be set to 0000000, which would generate channel 45 (27.445).

In another example, if the channel selector was set to channel 12 then the lines would be set as 1011101. If we would connect P5 (pin 11) to power we would force it high and the lines would be set as 1111101 which would generate channel 42 (27.425). From what I gather, if the line is set as low by the channel selector power is simply not fed to it, but it's not connected to ground so it won't shunt the power we are injecting to ground, the line will simply go high.

My understanding is that we can force any channel within the design parameters to be created by simply setting those 7 lines correctly.

With the above in mind, the channel expansion method posted on several internet sites pretty much makes sense - the channel selector is set to a certain channel and then control lines are grounded or powered as required to modify the control settings for that channel to generate the new channel, the only part that doesn't seem to make any sense to me is where a couple resistors come into the equation.

The channel expansion method that seems to be the only one around is in a gif attached to this message.

3245-1419734902-4f1d2e14c15778f8dbf2043282e1f734.gif


I don't understand what part the 4.7K resistors play.

Doing a little research I found that in digital circuits leaving a line unpowered may result in the line picking up noise and it's value floating rather than remaining at what is considered the false, or zero, level. To combat this floating pull up or pull down resistors can be used. From what I gather, pull up resistors are normally ran from constant power through a fairly high value resistor to the line to keep a comparatively low constant level on the line. Pull down resistors are usually placed between the line and ground to help sink any stray noise. Two different ways to do the same thing.

The modification instructions say because this PLL chip doesn't have internal pull up resistors we have to provide one by soldering it across the pc trace we cut. Looking at the schematic, it appears to me that this circuit is using pull down resistors (and capacitors, probably to help suppress spikes?), they are R327 (P6), R326 (P5), R325 (P4), R324 (P3), R323 (P2), R322 (P1) and R321 (P0) and they are on the traces between where the lines come in from the channel selector on J33 and where they connect to the PLL pins. If one cuts the traces where instructed to, they are still in the circuit and should still be functional.

Cutting the traces where instructed simply disconnects the line from the channel selector.

I've attached a gif of the mod that shows where the suspected pull down resistors are at.

3246-1419734918-7f25639f435bca02ad2e56930293b8da.gif


In normal operation, P6 (pin 10) is always high so the P6 line from the channel selector would always be powered so running a resistor to it could provide the pull up function, however P5 (pin 11) is low from channel 1 to channel 14, so it wouldn't always have power and thus it wouldn't be able to provide the pull up function all the time. But again, I don't think this circuit uses pull ups, I think it uses pull downs.

While we're discussing the cut traces, I don't like to cut traces. It's just a pain as they're always in cramped quarters it seems, and you have to scrape the lacquer off to get a good solder joint and stuff. It's just not a job I enjoy doing. In this case, the trace we are cutting is fed by a wiring harness from the channel selector though J33, I would think it would be preferable to simply cut the wiring harness to install the resistors and switches. Even if you needed to use the resistors just solder them in series and the switch connections below them, then no cutting or soldering on the board at all.

Also, one of the complaints about the channel mod for these units is that there is no modification bypass so you can tune through the normal 40 channels just like an unmodified radio. The mod for the Galaxy 959, which also uses the RCI8719 PLL chip and also seems to use pull down resistors (they are in a resistor network rather than separate ones like on the 979), simply puts single pole single throw switches in line with the P5 and P6 channel selector wire harness feed to the board, opening these switches puts P6 and P5 low (well, floating low, not forced low with a ground connection). I'm wondering if a similar mod wouldn't work for this radio? You wouldn't get as many channels, but you would get most of the upper channels.

I'm wondering if you used a double pole, double throw, center off switch and hooked PLL pins 10 and 11 to the top two poles, then the wiring harness P6 and P5 feeds to the center two poles and then didn't connect PLL pin 10 to the bottom pole, but did connect PLL pin 11 to the corresponding bottom pole if it wouldn't work? I don't know if pin 10 in this circuit responds to just floating low as opposed to being connected to ground, but the 959 uses a similar circuit and it seems to work. (One note here - doing some research it seems there are two versions of the RCI8719 PLL chips, the -97 version and the -99 version. The -97 has pin 10 tied high internally so it can't be forced low, the -99 is not tied high interanlly and can be forced low. One mod page I read noted the -99 needs a pull down resistor and suggested using a 10K resistor connected from pin 10 to ground, but again - I think there already is a pull down resistor in the circuit, R327, though I don't know it's value as I don't have my radio yet and it's not listed in the service manual.)

.....ic pin 10 11
chan sel P6 P5
.....ic pin --- 11

In the top position the lines would be connected as normal and the radio would operate as normal, in the center position both pins 10 and 11 would be disconnected, i.e. they'd be low. In the lower position pin 10 would not be connected but pin 11 would be.

I think this would give:

Switch in upper position: normal operation

Switch in the center position: 27.455 to 27.765 on channels 15 through 40 with the normal skips due to the A channels.

Switch in the down position: 27.775 to 28.045 on channels 15 through 40 with the normal skips due to the A channels.

Anyway, this is my first post and it's a long one, sorry about that! I can't wait to get my radio and do a little playing, before then though I'd like to line the modifications up I want to do and I'd like to understand what I'm doing, so if anyone could shed any light on the subject I'd sincerely appreciate it!

Thanks,

Steve
 
Last edited:

Well, what's the fun in that?! LOL!

Actually, I ran across the Farmer Dave Mod and I think I like it better than the Lescomm mod. My subscription to the Farmer Dave forum hasn't been approved yet though, so I dunno costs and stuff.

Along these lines though...I program in C and have used the Arduino microcontrollers in a few projects and I'm thinking this may be a good candidate for an Arduino project.

This hobby is about the journey and the projects as much as it is the end result for me.

Thanks for the reply and product recommendation!

Steve
 
The idea of putting the resistors there is to be able to control the state of pins 10 and 11 without totally "floating" the pins. In some cases, some PLL IC's will become unstable on those particular pins if they are left completely "floating". That's why unused pins are generally grounded.


Hope this helps!

~Cheers~
 
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Hi ExitThirteen,

The author of that mod says the resistors are pull up resistors, and yeah, the function of a pull up, or pull down, resistor is to keep the input from floating.

What's confusing me is:

*) Pull UPs are usually put between power and the pin, in this case it bridges the cut trace instead of going to power and the chip side of the cut. Now power is coming from the channel selector P6 all the time, so that resistor could function as a pull up, but P5 coming from the channel selector is low when on channels 1-14, so it couldn't always function as a pull up.

*) To keep inputs from floating you can use either a pull UP or pull DOWN resistor (and the choice of UP or DOWN affects the design of the rest of the circuit). The author of this mod seems to think pull ups are needed, but if you look at the schematic, and the pc board, it appears to me that there are pull DOWN resistors implemented in the circuit (R326 for P6 and R325 for P5), and if you cut the traces where indicated, or cut the P6 and P5 wires in the wiring harness feeding J33, those pull downs are still in the circuit, so the pull UP resistors I don't think are appropriate or needed for this circuit.

I find it very curious, I can't wait for my radio to get here so I can investigate this stuff a bit further!

Thanks for the reply!

Steve
 
Hi ExitThirteen,

The author of that mod says the resistors are pull up resistors, and yeah, the function of a pull up, or pull down, resistor is to keep the input from floating.

What's confusing me is:

*) Pull ups are usually put between power and the pin, in this case it bridges the cut trace instead of going to power and the chip side of the cut. Now power is coming from the channel selector P6 all the time, so that resistor could function as a pull up, but P5 coming from the channel selector is low when on channels 1-14, so it couldn't always function as a pull up.

*) To keep inputs from floating you can use either a pull UP or pull DOWN resistor (and the choice of UP or DOWN affects the design of the rest of the circuit). The author of this mod seems to think pull ups are needed, but if you look at the schematic, and the pc board, it appears to me that there are pull DOWN resistors designed in the circuit (R322 for P6 and R325 for P5), and if you cut the traces where indicated, or cut the P6 and P5 wires in the wiring harness feeding J33, those pull downs are still in the circuit, so the pull UP resistors I don't think are appropriate or needed for this circuit.

Here's the schematic for the relevant part of this circuit, what are R321 through R327 if not pull down resistors?

3247-1419751934-fee53496041490aa21b0ff670837db34.jpg


I find it very curious, I can't wait for my radio to get here so I can investigate this stuff a bit further!

Thanks for the reply!

Steve
 
Interesting, the schematic shows R322 going to pin 10, but the top and bottom side illustrations clearly show R327 going to it. Oh well, no doubt to me that they're pull DOWN resistors.
3248-1419753515-5755d4ea5af6c276084a822ffc078ade.jpg
 
I'm looking at a 979 right now, and R321-R327 are NOT installed in circuit. Therefore, there are no pull-up/pull-down resistors in circuit. That may be why they have an "X" next to them on the schematic.

~Cheers~
 
Ah Ha! So practice varies from application! Can't wait to get my radio!

Sooo, is the board drilled for them and they're not installed? If so, we could just install a pull down resistor?
 
I wonder how they keep the pins from floating? I wonder if pull ups or pull downs are installed at the other end of the wiring harness coming from the channel selector?
 
That I am not sure of.... I'm not real familiar with these new RCI-8719 PLL's internally, I haven't done a lot of research on them. When I've done extra channel mods, it's just habit for me to bridge a 4.7k ohm resistor across cut traces so that particular pin isn't floating.

I think the author of the channel mod is accounting for the lack of a pull-down resistor in circuit, and that's why there is a 4.7k installed across the cut. Had there been resistors at R321-R327, the 4.7k may not be needed.

Since the state of the pins are defined by the channel selector, there is technically no need for pull-up/pull-down resistors in circuit. Why install them if they're not needed? Well, in today's "if we don't have to install these resistors, we can save 5 cents per radio" mentality, it's all about saving money. Sadly.

~Cheers~
 
I can't find much on the internals either, but I'd think if pull up/downs were in the chip then we wouldn't need to add any. But if they're not in the chip then they should be in the circuit somewhere and since they are not installed where indicated on the schematic then the only other logical place would be at the other end at the channel selector.

I'm really wondering though if the circuit wasn't designed for a chip without internal pull up or downs and then a chip with them was used, hence they didn't populate the board with them because they didn't need them, if that's the case then we probably don't need them either.

From what I gather, these type circuits are designed to use either pull up or pull downs and it appears to me this one was designed for pull downs, so this mod author's use of pull ups may not be appropriate (doesn't mean it won't work though, obviously it does), and like I said...the pin 10 one will work as P6 is always held high by the channel selector, but the pin 11 one won't as P5 isn't always held high by the channel selector, on channels 1 through 14 it's low, and if it's not grounded and is just left unpowered then that resistor can't function as either a pull or pull down resistor.

It's true that the state of the pins is defined by the channel selector, but is it powered and not powered, or is it powered and grounded? If it's powered and not powered then it needs a pull up or down resistor somewhere as unpowered is when things float. If it's powered and grounded then still there should be a pull down somewhere.

According to the service manual, a 959 should have a resistor and capacitor array to function as pull downs, are they installed in those radios? It uses the same chip, so if so then I'd say the 979 has pull downs in the channel selector somewhere, and it may as it's a different part number than the 959's. But if the 959 doesn't have those resistor and capacitor arrays then I'd guess the pull up or down is internal to the chip.

When my radio arrives I'm going to test it to make sure it works, then I think I'll channel mod it, but I'll cut the wires to J33 instead of traces. I'll put the resistors in and make sure it works, if it does then I think I'll remove them and see what happens. If that creates a problem then I'll see if the R321-327 spots are open on the board, if so I'll put a 10K in them. Why 10K? I'm a creature of habit and I come from the Arduino microcontroller world and we always use 10K for pull up/downs there.

It's just curious to me that the 959 has the same chip and it doesn't require these resistors, and it appears these boards are designed for pull down resistors and these are functioning as pull ups...and at that, one only works if the channel selector is positioned above channel 14.

Strange indeed...I know, just mod it and go on, it works...but I'm the curious type and I always want to know the how and why!

THANKS for taking the time to check a radio and post back, I sincerely appreciate it!

Steve
 
If the RCI-8719's are internally the same as an MB8719 (minus the 5V PLL control voltage), they should have pull down resistors internally. Altho from what I see in a 949 (same as a 959 minus the frequency counter), there is a pull-down resistor array present, and you do the P5/P6 mod AFTER those resistors. That being said, there wouldn't be a need to use a 4.7k resistor on pins 10 and 11, since you're cutting the wire in between the main PCB and the channel selector.

I do find a it a little odd that the 979 does not have these resistors present. Only thing I can think of (without diving into the channel selector circuit to see if there's a ground wire) is that if there is a pin that needs to be pulled "low", it is grounded thru the channel selector, eliminating a need for a pull-down resistor.

The 949/959's use the exact same PLL as the 979.


~Cheers~
 
I haven't found a data sheet for the RCI8719, but I found several references on the net when doing a "RCI8719 pull down" search. From what I gather the MB8719 indeed had 1K internal pull down resistors, but the RCI1879 does not, so it needs external pull downs. I found a couple threads where guys were told to check the pull downs in the channel selector.

Soooo, I'd say since the 959 uses the same chip and has the resistor array by the PLL populated, but the 979 uses the same chip and doesn't have the resistors by the PLL populated and the mod author was putting a resistor in that there are external resistors somewhere, and I'll bet they're in the channel selector and when you cut the trace you're cutting the PLL pin away from it's pull down resistor.

I don't think bridging the cut trace with a resistor is the best approach though, I'd think putting a resistor in the unpopulated pcb spot would be the better approach, lacking that I would put one in but tie it to ground rather than to the other side of the cut trace.

If you get a chance, could you see what value the resistor array is in the 959? I think emulating what it's doing is probably the best approach here.

THANKS!!!

Steve
 
Found a thread where a guy replaced an RCI1879 with an MB1879 (he just had the older chip on hand) and had problems. After troubleshooting, including directly addressing the chip, it was found the problem was the coupling of the internal pull downs with the external ones....that were in the channel selector...so yeah, it appears manufacturers do sometimes build them in the channel selector and I'm guessing that's what the 979 is doing.

Can't wait till my radio gets here so I can investigate this a bit further.

None the less, I'm not sure bridging the cut traces is the best approach, I would think going to ground would be better (or again, just populating the missing components on the board if the circuit board has the traces and the holes are there).
 

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