DX-979 Channel Mod Question

[SLR_65]

OK, doing a little more research...

*) The MB8719 had internal 1K pull down resistors.

*) The RCI8719 does not have internal pull down resistors, but schematics for radios using that chip show 1K external resistors in the circuit.

*) The 979 schematic shows the pull down resistors but they are not populated on the board. The 959 schematic shows them and they are populated.

*) There's no doubt there has to be pull downs in the 979 circuit somewhere, my guess is they are in the channel selector switch circuit, I'll investigate that further when my radio arrives, but the 959 and 979 use different channel selectors so I imagine that's the difference.

*) Reading up on pull down resistors it seems they are usually in the 3.3 to 10K ohm range, however in speed sensitive circuits 1K is common. So 1 to 10 K is common, exactly what value to use depends on a lot of factors in the circuit and a lot of times in practice is determined by experimentation.

*) In the 979 channel modification circuit the 4.7K resistor installed on P6 isn't doing anything. Normally P6 is high, so no floating issues. When the switch is set to expand channels P6 isn't simply disconnected, it's tied to ground, so again, no float issues. The 4.7K installed on P5 does function as a pull down when the channel selector is on channels 1-14 and it's unpowered. In this case the 4.7K coupled with the 1K pull down in the channel selector gives a total of 5.7K resistance and the unit still functions fine.

*) Placing those resistors in series though I don't think is the best approach as it depends on the setting of the channel selector to complete the pull down circuit. Simply placing a resistor connected to the line and then connecting it to ground I would think would be a better approach as then the pull down is always in circuit as it was designed to be and it more closely emulates what's done on the 959 radios (i.e. because the pull downs are on the board near the PLL rather than in the channel selector we can open a line from the channel selector to place it low and it won't float because the pull down resistor is still in circuit, there's no need to connect the line to ground to force it low and keep it from floating).

*) It appears to me we have two choices, if the 979 circuit board has the traces and holes for the unpopulated pull down resistors then simply installing a pull down resistor where it was intended would solve the problem. What value to place there is a question, but I'm thinking anything from 1-10K total resistance on the line will work. There's 1K resistors in the channel selector we can't do anything about, so if we place a pull down resistor in the unpopulated locations we need to take them into account. Basically this would put two resistors in parallel with each other when the line between the channel selector and the PLL are connected, and just one resistor when the line is isolated to pull it low. It seems any value between 1 and 10K total will work. If we place a 10K resistor in the unpopulated area the resulting total resistance is about 900 ohms, just shy of 1K and I'm sure close enough. If the circuit isn't stable when we have the line isolated then I would suggest trying a 4.7K as we know 5.7K total works. Using a 4.7K would give about 825 ohms total resistance when the line is not isolated and the channel selector pull down is in play. 825 is probably close enough to work too. Placing resistors in the unpopulated area would be easy to do, but requires soldering on the board and I know some don't like to have to do that. The other approach would be to cut into the wiring harness coming from the channel selector that connects to J33. One could install the resistor and jumper them to ground there (use some liquid tape to insulate the bare resistor leads) and install the switches there - just be sure to install the switches on the channel selector side of the resistors so they remain in circuit when the switches are opened and the lines are isolated. Once the resistors are installed the circuit is basically the same as the 959's so if you wanted to install two separate SPST switches the channel map would operate the same as the 959 and you'd gain the lower channels.

If you wanted to do the 10 kc mod it would also work, but you wouldn't need to install a pull down resistor because the switch switches between power and ground and the line is never left floating, however it may not be a bad idea to install the pull down resistor just in case that moment between switching betwen power and ground generates some noise. If you cut the wiring harness cutting the circuit board trace wouldn't be needed either.

I think I have a handle on this...I'll play with my radio when it arrives and report the results, if things work as I think they should, then I'll take some pics and down some drawings and post back the updated mod procedure.

Thoughts, comments...speak up, you may save my radio!

Steve

 

[unit_399]

The 4.7k ohm resistors are properly referred to as isolation resistors. Their function is to isolate the logic state being applied to the pin by the channel selector from the logic state being applied by the freq expansion switch. Without the resistor, if the channel selector was in a position where pin 10 was at logic 1, and you applied logic 0 via the switch, you would create a dead short. The resistor across the cut trace prevents this from happening

- 399

 

[SLR_65]

Hi Unit399,

That makes more sense, but the author of that mod specifically stated they were pull up resistors, so that's what threw me.

With the 979 mod though, they are cutting the trace from the channel selector, so the state of the line beyond that cut to the chip pin should always be at zero, and they're connecting the switch directly to the chip, again beyond the cut trace, so there should be no danger of creating a short - the input from the channel selector is simply out of the picture.

I also prefer the approach used on the 959 where pins aren't grounded, they're simply disconnected to create a logic zero and the pull down resistors take care of the float, that also helps avoid the possibility of creating a short.

I really think the better the method is to leave out the 4.7 k resistors that jump the trace cuts and simply install proper pull down resistors to ground and then just mod it like the 959s.

We'll know soon, tracking shows my radio on the way!

 

[ExitThirteen]

In all honesty, any of the Galaxy DX 9XX series radios are poor to run, especially on SSB. They drift all over the place. If you get bit good by the CB bug again, I'd highly recommend getting a good older CB radio, something from the late 70's into the early 90's. Anything with the uPD858 PLL or MB8719 PLL will be good from that era.

~Cheers~

 

[SLR_65]

We'll see how it goes, the Galaxy's are too badly reviewed and the 979 seems to score higher than the 959s.

Old equipment comes with it's own set of problems, aging caps, hard to find parts, etc..

If I get bit good I'll probably just opt for a ham rig.

 

[SLR_65]

Hi Guys,

My radio arrived!

Unfortunately it didn't come with the power cord (insert Homer Simpson DOH! here!). I'll see if I can run one down in the next day or two.

I lifted the lid on the radio though and have a couple observations.

*) It has the RCI8917-99 chip, so should be no issues there.

*) The pull up resistors are indeed not installed, but the capacitors are. The holes and solder pads are there also.

*) Unfortunately the channel selector sits over the area we would need to have access to to install the pull down resistors on the top side as normal, but we have full access to the bottom, so installing the resistors on the bottom of the board would be the easiest route (it'd be very simple, there's ampul access to the bottom of the board).

*) Once the resistors are in place then the mod is pretty much like the one for the 959s, just install spdt switches in the P6 and P5 lines - note the P6 / P5 wire colors are different than the ones used on the 959s, P6 is white and P5 is grey on the 979.

*) Once modded the switch settings and channels should be the same as the 959.

I'll be confirming that a 10K pull down resistor will indeed work shortly. I'll take some pics and stuff too.

For those who don't like to slice and dice their wiring harness...taking a quick look, the wiring harness appears to be a 10 pin JST PH series socket/connector. I actually have some of those somewhere complete with wiring pigtails...I bought them off of fleaBay a while back (I play with Arduino microcontrollers for a hobby). I'll dig them up and confirm that's what it is, and then I'll probably leave my harness in tact and slice and dice on one of those pigtails so it's a plug and play sort of thing. (though you can't unplug the harness from the board easily because the channel selector board is in the way, there should be enough slack for most to be able to patch a set of switches in without issue...I just prefer to not slice and dice if I don't have to, and I have the stuff to do it plug and play, if I didn't have it though I would be slicing and dicing.

I'll do a better update after I've had some hands on play time with the radio.

Steve

 

[sdmahr]

Hi Guys,

My radio arrived!

Unfortunately it didn't come with the power cord (insert Homer Simpson DOH! here!). I'll see if I can run one down in the next day or two.

I lifted the lid on the radio though and have a couple observations.

*) It has the RCI8917-99 chip, so should be no issues there.

*) The pull up resistors are indeed not installed, but the capacitors are. The holes and solder pads are there also.

*) Unfortunately the channel selector sits over the area we would need to have access to to install the pull down resistors on the top side as normal, but we have full access to the bottom, so installing the resistors on the bottom of the board would be the easiest route (it'd be very simple, there's ampul access to the bottom of the board).

*) Once the resistors are in place then the mod is pretty much like the one for the 959s, just install spdt switches in the P6 and P5 lines - note the P6 / P5 wire colors are different than the ones used on the 959s, P6 is white and P5 is grey on the 979.

*) Once modded the switch settings and channels should be the same as the 959.

I'll be confirming that a 10K pull down resistor will indeed work shortly. I'll take some pics and stuff too.

For those who don't like to slice and dice their wiring harness...taking a quick look, the wiring harness appears to be a 10 pin JST PH series socket/connector. I actually have some of those somewhere complete with wiring pigtails...I bought them off of fleaBay a while back (I play with Arduino microcontrollers for a hobby). I'll dig them up and confirm that's what it is, and then I'll probably leave my harness in tact and slice and dice on one of those pigtails so it's a plug and play sort of thing. (though you can't unplug the harness from the board easily because the channel selector board is in the way, there should be enough slack for most to be able to patch a set of switches in without issue...I just prefer to not slice and dice if I don't have to, and I have the stuff to do it plug and play, if I didn't have it though I would be slicing and dicing.

I'll do a better update after I've had some hands on play time with the radio.

Steve

Look up the P5/P6 mod. I've done this on all my Galaxy's and it works good. Much less work. You'll need to keep a road map since your model doesn't have a freq counter.

 

[SLR_65]

The P5 P6 mod doesn't work on this radio as is because they moved the pull down resistors to the channel selector, so that's why somone came up with the different mod. It does work but one problem is there's no way to turn it off and have your normal 40 channels. Looking at the circuit though, it appears we can put pull down resistors back on the board, they shoud be able to coexist with the ones in the channel selector without affecting operation. Once those resistors are installed then yeah, the P5 P6 mod will work. I'll confirm the added resistors will work shortly when my parts arrive.

 

[PU2PLL]

Hi all,

This is not a reply but is a request so I need the 01010101 table of the RCI8719 used in the Galaxy DX-979 radio that shows binary codes and frequencies from channel 1 (26.965MHz) up to channel 80 (27.855), here in Brazil this is an official modification.

73,
Fernando - Call sign PU2PLL - Brazil

 

[Unit-192NC]

What's the final verdict? Yes or no? Did the resistors on the board and cutting get p5 and p6 work?

 

[Shadetree Mechanic]

Bump, good info for for Texas Ranger 797.