Galaxy DX 2547 strange alignment results

Hi Gang,
I have a 2547 on my bench in which the owners complaint was that it would randomly stop transmitting. After reading up on this radio I did the following:
- powered up the radio and performed an initial test. The radio did transmit but was keying up to 5 watts (AM) and driving backwards on modulation. Side band was transmitting around 10 watts (dummy load). Receive seemed ok.
- my prior research found that this radio (like other Ranger/Galaxy/RCI) has solder issues. I inspected every solder joint and re-flowed about half of the board. I could tell by the solder joints and the heat sink paste that the driver and final had been removed and/or replaced at some time.
- Something in the radio was rattling when you shook it. After some shaking while holding upside down a 1000 ohm 1/10th or 1/16th watt resistor fell out. It was formed as if it were standing in place instead of flat on the board. I looked at the board for a while and could not figure out where it could have been installed. I have not poured over the schematic yet. I have no idea what to assume except that it's the little things that drive me nuts.

Now the results and need for some input.

The PLL alignment went fine.
The transmit alignment had some issues.
- On the bias alignment TP7 and TP9 set to 50ma fine. TP7 and TP8 call for 100ma. It was reading 580ma on receive and 785ma on xmit. I could only lower it to 600ma on xmit but that was at the extreme point of the VR. Why would that happen?
- The remainder of the transmit alignment when fine but the radio still drives backwards even when AM hi power is set to 3.8 watts. Is that tied to the same bias issue?
- The receive alignment went fine except that the SSB receive is not great. -3db sinad @ 2 volts AF (sig-gen at .5uv).

One last thing to figure out, where did that resistor come from? HAHA

Any thoughts on these issues?

Thanks as always men!!!!!

Larry

tecnicoloco said:

start on page 19 in the service manual and look for the 1K resistor parts

http://www.cbtricks.com/radios/galaxy/dx2547/graphics/DX2547sm.pdf

R62,64,67,72,98,115, 116,118,122,123,136, 137,138,160,164,166, 167,179,186,189,205, 206,214,232,240,320, 261,270,271,291,292, 295,303,237,238,D104 1K Ω 1/16W R88,192

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Good thought. I've already looked at that and it was not from the main board,. in order to look at some of the other boards it will require some more work to take it apart. The strange thing is, it does show signs of being soldered at one point but not like it was "in place" at the time. I am wondering if someone replaced it along the way and dropped it in the radio and left it there. All of the features work except for the clarifier light and the few strange results I shared so I am a little stumped. I wish folks really understood why it takes longer than expected to repair a radio when you find issues like that. That one resistor will leave me concerned that the radio is not "perfect" when I return it if I cannot figure it out. So goes the battle against the golden screwdriver........

Ok Men,
Here is the update. Amazingly, I did find that the driver (Q49) diode was in backwards. The final (Q47) was missing the .1mfd cap (C211) like in the mosfet version. I decided to pull the driver and final as it is tough to get to the diodes as they are mounted under the transistors. What I found was that the driver is an IRF520 and the final is a 1969. I have not seen a version of the radio where there was both a mosfet and a traditional transistor. I will try to determine if the radio was originally a mosfet version or not but I am wondering if anyone has seen this or has an opinion. If I put 2 mosfets back in it is there anything else specifically I should do? Also, would the alignment be the same as the non-mosfet version?

Thanks as always!!!

Larry

Lkaskel said:

Ok Men,
Here is the update. Amazingly, I did find that the driver (Q49) diode was in backwards. The final (Q47) was missing the .1mfd cap (C211) like in the mosfet version. I decided to pull the driver and final as it is tough to get to the diodes as they are mounted under the transistors. What I found was that the driver is an IRF520 and the final is a 1969. I have not seen a version of the radio where there was both a mosfet and a traditional transistor. I will try to determine if the radio was originally a mosfet version or not but I am wondering if anyone has seen this or has an opinion. If I put 2 mosfets back in it is there anything else specifically I should do? Also, would the alignment be the same as the non-mosfet version?

Thanks as always!!!

Larry

Click to expand...

One more quick update....the main board is an EPT0696011A which is showing it as a mosfet version so someone put the 1969 in it for some reason. CB Tricks did not have the alignment details for the mosfet version so I am going to use the one for the DX939 with the same board number unless someone has a different opinion. I'm lucky that I just bought a dozen 520's from DigiKey.

Handy Andy said:

You're going in the right direction.

With the 1969 - that isn't the usual upgrade, more like a downgrade. But if they did have issues with keeping '520's in it before, this may have been their approach to using a lower power upgrade Bipolar to operate either an Amp or driver into an amp - so this is more of a backwards approach - to each their own.

Surprised it worked this long - '520's usually blow up Bipolar's they drive into.

You've got some work ahead of you...

You will have to rework the bias - revert back to the MOSFET design - and the 5.1V zeners their caps, 4.7K and 47K resistor divider networks and even the Variable pots used to trim their bias'.

Now if they say they had issues with it powering down, or it quit working on them, then the power supply is really at fault, they simply put in a less current munching part to keep the radio operable.

IF you have trouble we're here if you need help or advice...

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Thanks Andy!!!

I did notice that the diodes were 4148's and the schematic calls for zeners. I am ordering some 1 watt 5.1, 5.6 and 7.5 volt zeners for this repair. I did read your post on upgrading to the 7.5 volt for better swing. I may do that if I can get the radio working well with the original ones 1st. I need this to be stable because the owner is an hour away.

Lkaskel said:

Thanks Andy!!!

I did notice that the diodes were 4148's and the schematic calls for zeners. I am ordering some 1 watt 5.1, 5.6 and 7.5 volt zeners for this repair. I did read your post on upgrading to the 7.5 volt for better swing. I may do that if I can get the radio working well with the original ones 1st. I need this to be stable because the owner is an hour away.

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Hi Everyone, it sure has been a while since I could sit at the bench!! The 2547 is back. The MOSFET conversion is not that hard as long as you have the parts. I needed to order some 1 watt resistors and zener diodes but I had everything else. I did a quick radio check tonight and got good reports so the owner should be happy.

Thanks again for all of your input!!

loosecannon said:

Lkaskel,

just so you know, there are versions of those boards that had the diode markers silkscreened on in one direction for bipolar transistors, and when they switched to mosfets they used those holes but put in a diode that was backwards from the silkscreen on purpose.
LC

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HAHA!!!! I found that out........ Those diodes are hard to remove with the FET's in place.....

It's working now

Thanks Cannon!!!! Be Well!!!!