Galaxy DX-2547 suddenly virtually no output

[Adamf]

Hi all, I have a DX-2547 that I use every so often and have not used it in a couple of weeks and find that output is extremely low. On AM it puts out about 1/2 watt dead key and swings to like 3 watts with modulation. SSB swings to about 3 watts as well. It used to put out like 8-10 watts dead key and swing to 15 or so on AM and SSB swung to like 20 watts. I am fairly handy and not afraid to try anything, can someone tell me what most likely the problem is or how I can check and/or what to replace?

Thanks for any help...Adam

 

[newoldguy]

sounds like finals are bad

 

[Adamf]

HI all, so my radio has 2pcs IRF-520, does anyone know if I can use an IRF-520N because I have some of those? Thanks, Adam

 

[Adamf]

Hi all, so the radio has 2 finals, IR520's and I replaced those 2 but no change? Can anyone help? Thanks, Adam

 

[nomadradio]

Do you have any way to make a measurement inside the radio?

Starting point is to make sure the modulator circuit is feeding full voltage to the driver and final transistors. Adding a second final doubles the load on the modulator transistor. Might be okay, might not.

The metal tab on final or driver transistor should have at least 12 Volts DC on it under the conditions: USB or LSB, mike gain at zero, mike keyed.

73

 

[Adamf]

Hi, thanks for answering. Yes, I have a Fluke DVM and can measure no problem. The Final I guess is the the IRF520 but I don't know what the driver part number is? With the DVM do I simply take the positive leave of the meter and touch that to the tab and then the negative lead to ground in LSB or USB and measure while keying?? Thanks again for any help. Adam

 

[nomadradio]

The circuit ground in most CB radios is not the chassis. The negative pin of the DC power socket is connected to the circuit board's ground. Meter's negative lead can go there.

That radio was produced over repeated production changes. Oldest one did not use MOSFETs. Sounds like your radio has one IRF520 driver and one IRF520 final. All of them made since 2007 are built this way.

The positive probe goes to the metal tab surface. Be careful not to 'bridge' that tab to any nearby stuff with the probe.

Choose one sideband mode. Won't matter which. Mike gain at zero, key the mike and see what the DC reading is.

73

 

[Adamf]

Hi, without keying mic voltage is 13.23VDC, when I key the mic as told voltage drops to 12.96VDC. I am probably wrong but was told that both IRF520's are finals, this radio has dual finals but I am not sure what I am talking about. Please tell me if these readings tell you anything. Thanks, Adam

 

[Handy Andy]

Ok, INPUT voltage to the board seems ok...

But...

You also need to verify the VOTLAGE to the Final and Driver - they WILL BE DIFFERENT than the input voltage depending on mode. AM it's lower than the input, in SSB modes, they should be about equal.

​

 

[Adamf]

Hi, thanks again for continuing on with me. For the negative lead, I am not sure exactly where you are saying for me to touch the probe to on the transformer, sorry? Also, would I test voltage keying up in AM and SSB, yes? Thanks again, Adam

 

[Handy Andy]

For the negative lead, I am not sure exactly where you are saying for me to touch the probe to on the transformer, sorry?

Hmmm...

You are here, asking for help on how to fix a radio.

If you cannot understand this...

​

Then you really should NOT try to fix something you don't understand.

We cannot accept liability for mistakes on a radio that is no longer made.

 

[Adamf]

OK, that is clearer, I'll test that way keyed up in AM and SSB, try not to give up on me but if you do I appreciate your help you have given me. Adam

 

[Adamf]

OK, again Andy, if you can help that would be great if not I understand but here are findings:

UNKEYED:
AM 7.02VDC
SSB 13.10VDC

KEYED:
AM 6.87VDC
SSB 12.92VDC

Shorting bar was in place, not removed..

Thanks, Adam

 

[Handy Andy]

Thank you.

Voltages look high on AM side, but SSB side is normal.

AM side is the one that "loads" the AM Regulator by a given amount of current "pull" which would drop down voltage to a level, that can be adjusted by AM PWR (AM Carrier Pot)

So since AM Power is high, check to even see if the FRONT PANEL RF Power knob is able to control the power level you still have in the radio - can it adjust it?

When you are more comfortable - re-test your voltage using those same test points, and adjust your RF Power Knob

• - we'd like to know what your voltages are when
• the RF Power knob,
  • set to the LOWEST POWER setting reads (set to) what is the voltage
  • Set RF Power Knob to "midpoint" and what is the Voltage...
  • Set RF Power Knob to Full Clockwise (Full Power) Setting and what is the Voltage.

Why?

This can help "build a Power curve Profile" so intuitively we can decipher what could be happening inside.

Failed Caps?
Failed Part causing a SHORT?
Failed Part blown causing an OPEN condition?

If power is low and doesn't move much, the issues are one direction...

IF the Power "tracks" but is only low power curve - the Issues are another Direction...

IF the Power knob doesn't control ANY of the power curve - then the Issues are again, in another direction.

 

[Adamf]

Thanks Andy and RF Power control does adjust smoothly from o watts to just under 1/2 watt deadkey. Will test voltages as asked now. Thanks, Adam