Galaxy DX 2547 mind of its own

[North Jersey Noise Maker]

I have a DX 2547 that after a few hours of operation, the transmit comes on and gets stuck on its own and won’t release unless I power off the rig. If I turn it on immediately after turning it on, it will still be stuck in transmit. If I let it sit for a few minutes, it will be a few minutes before it does it again. As is with if I leave it off for hours, it works for hours after, which leads me to believe it’s heat related. First time it happened, I quickly unplugged the mic thinking it was a short in the mic, no difference. After long sessions of scenario testing, it will even do it with out the mic attached so I’ve narrowed it to radio internally. Was thinking relay, but being a newer radio, none were found. Some background, as I bought the radio second had for peanuts at an estate sale a while back is 1.)the modulation regulator circuit was bypassed and someone had golden screwdrivered the pots for max output. All that has been reversed by a master radio tech and the radio has been returned to a better than factory tune and alignment and the radio has been operating perfectly for about a year.
I know there are many strong opinions out there about Galaxy radios, myself not too fond of their q.c. quirks, but I love the old school form factor of a desktop base station and these are the only ones you can get now a days.
Any insite would be greatly appreciated so I have a direction to start with as my tech is amazing, but also expensive
Thanks.
73s
Kyle

 

[midnight special]

since the 2547 is the same as 959 and 949 here ya go,, https://web.archive.org/web/2011071...i-bin/database/galaxy_db.cgi?details=1&key=68

 

[staticcity2]

I have a DX 2547 that after a few hours of operation, the transmit comes on and gets stuck on its own and won’t release unless I power off the rig. If I turn it on immediately after turning it on, it will still be stuck in transmit. If I let it sit for a few minutes, it will be a few minutes before it does it again. As is with if I leave it off for hours, it works for hours after, which leads me to believe it’s heat related. First time it happened, I quickly unplugged the mic thinking it was a short in the mic, no difference. After long sessions of scenario testing, it will even do it with out the mic attached so I’ve narrowed it to radio internally. Was thinking relay, but being a newer radio, none were found. Some background, as I bought the radio second had for peanuts at an estate sale a while back is 1.)the modulation regulator circuit was bypassed and someone had golden screwdrivered the pots for max output. All that has been reversed by a master radio tech and the radio has been returned to a better than factory tune and alignment and the radio has been operating perfectly for about a year.
I know there are many strong opinions out there about Galaxy radios, myself not too fond of their q.c. quirks, but I love the old school form factor of a desktop base station and these are the only ones you can get now a days.
Any insite would be greatly appreciated so I have a direction to start with as my tech is amazing, but also expensive
Thanks.

73s
Kyle

bad keying diode. D-71 in that radio i believe. if you dont have the diode on hand try 2 1n4148 diodes in series instead. hope this helps.
73's from the Mountain Man.

 

[North Jersey Noise Maker]

That’s exactly the first thought I had midnight special! I pulled the pcb and checked continuity and then re flowed everything just to be safe, but sadly, that wasn’t it.
Today when I got home from work I started exploring staticcity2’s suggestion and I had some 1n4148s sitting around in a donor carcass so I soldered them up. The operation went well so now I have it on, staring at it waiting for it to get warm. It’s gonna be a long night
Thanks for the input. I love this blog. Never disappointed! Great group of folks out here

 

[nomadradio]

2 1n4148 diodes in series

Sometimes it takes three. Don't know why, but the 9-Volt regulator can drift up or down and affect this. Never hurts to see that the center leg of Q37 shows between 9 and 10 Volts DC. If it drifts too far, the transmit/receive switching circuit can get twitchy.

73

 

[brandon7861]

Sometimes it takes three. Don't know why, but the 9-Volt regulator can drift up or down and affect this. Never hurts to see that the center leg of Q37 shows between 9 and 10 Volts DC. If it drifts too far, the transmit/receive switching circuit can get twitchy.

73

I believe that is what the 3 in kb362 means. 3 junctions. The kb262 has 2 junctions inside it. I think there is a kb462 as well, but I am not sure.

 

[Handy Andy]

It needs a voltage drop big enough to know the difference - so since many of todays radios are now using Schottky diode types - the voltage drop across those types is not enough to make the op amp sense the difference well enough to be reliable.

So that's why the 2 diodes (or more) back to back in series (inline) are needed to make the 4558 TX/RX toggle work.

 

[brandon7861]

It needs a voltage drop big enough to know the difference - so since many of todays radios are now using Schottky diode types - the voltage drop across those types is not enough to make the op amp sense the difference well enough to be reliable.

So that's why the 2 diodes (or more) back to back in series (inline) are needed to make the 4558 TX/RX toggle work.

I dont think the opamp is "sensing" the voltage, (it looks like) the opamp is providing the voltage.

I thought that side of the dual opamp was for the RB timing and the output routed to the two transistors Q31 and Q33 to do the RB TX/RX switching. Or is that doing something else there? The zener on Q31 appears to ensure the op amp voltage output is high before turning on the RX transistor and that the KB362 on Q33 ensures that the voltage from the opamp is low before turning on the TX path. I believe its just a crude schmidt trigger type thing. If the kb362 loses a junction or three, the voltage from the opamp required to turn on Q33 will be much higher and could cause false/random triggering of the TX side.

Just put three junctions of whatever normal silicon you have in there (in series) and it will be fine.

 

[brandon7861]

Spent some time thinking about it. You need three diodes to replace the kb362, NOT just two.

Using the 959 schematic, that side of the opamp (configured as a non-inverting comparator with hysteresis for TX/RX switching, the other half being the mic amp) has R182 and R184 putting 4v on the positive input and R180 and R181 putting 3.2v on the negative pin. R183 provides positive feedback (hysteresis) to prevent jitter. The PTT switch pulls down the positive input of the opamp via D76 and R185 which makes pin 7 go low during TX. With the opamp output going down to nothing, Q33 can turn on.

Since Q33 (the PNP TX switch) has 8v on its emitter, it would require a voltage lower than (8v - Q33's be junction drop - three drops in the kb362 = 5.6v) from the opamp to turn on.

Since Q31 has a zener in front of its base, the opamp must produce more than (Q31’s be drop + 5.1v from the zener plus a little for the resistances = a bit higher than 5.6v) to turn on the RX path.

If you were to use only two silicon diodes to replace the kb362 (which has 3 diode drops), the TX path would turn on before the RX path turned off! When the opamp begins to go low, with only two diode drops present, the TX transistor starts to light up when the opamp hits 6.2v. Remember, the RX isn't shutting off until the opamp output drops below 5.6v. Yes, its a fast transistion from 8v to 0v coming from the opamp, but still, its 5 cents man... Thats cheap insurance!

For those interested in the roger beep side of things, with the RB switch actvated, C159 charges via the same PTT path through D76 (while D77 keeps the RB oscillator dead during TX audio). When the PTT is released, the cathode of C159 remains low for a brief moment while it charges via R185. This keeps the TX on for just a brief moment while the RB comes back alive and squeeks in a chirp before C159 settles and the opamp flips back. D60 goes to the LD on the PLL, and if that goes low, the opamp will not switch into TX.

Edit: Galaxy must not care because, upon looking in my radio graveyard (desk drawer), I see my old mosfet DX2547 used two 4148's in series there. They must not have cared about the TX coming on before the RX shuts off. I wish I remember what put this one in it's grave... Maybe that opamp is fast enough to where the RX is still off before the RF gets to the output? Who knows, doubt it. Here I thought I had a KB362 to put on a curve tracer and solve this mystery (and another about it being a stabilizer, story for a different thread). I will have to see if the 959 in the truck has one. If it does, I'm going to compare it's curve with a set of 3 4148's and see what the deal is. Either way, it is bloody obvious that the TX will start to come up before the RX is turned off with only two diodes there, so proceed with caution.

Edit2: This is all bringing back a vague memory of my old 2547 making the speaker pop when keying, I bet that missing diode drop was why.

 

[Handy Andy]

Its' a lot to digest - isn't it?