Checking Bias on a Galaxy DX949

Adjust proper values.
1 or 2 finals radio?

No change means broken potentiometer or bias diode or final itself.
Mike

Have you tried by checking gate voltages?

Best to start here, at the INDEX

http://www.cbtricks.com/radios/galaxy/index.htm

Why?

Check your serial number...

Questions? Refer to the link...

• There is not much there, so people default to the Bipolar one thinking they'll be OK - Well they can be but...its' a BIPOLAR adjustment.

Now as far as the "Bias" - you still need to check current side of that "Voltage" measurement...

Why - because of TOO MUCH range of variability of the MOSFET IRF520 's they use in those things...

What am I talking about?

Setting a voltage on a part that has known variability - is not the WISE-est' method - for if you think you're setting to 3.65 volts - the part may already be toast by the time you even get the covers back on...and power it up again.

This is why that mA reading has to be done...

We're not quizzing you on "Knowledge of Datasheets and How To Read Them..."

But since you're playing around with a radio...you're smart enough to understand this...


For best success, use the mA setting and trim your respective pot for current draw, THAT IS WHEN YOU KNOW the MOSFET is just ready to TURN ON. It's the best method.

Now...as far as TP? and Why no adjustment?

Well they may have soldered jumpers in place instead of the Mirror Board...

OR...

Has the Final been Jumpered? - As in Red Wire On Bottom (RWOB - Per CB Doctor)

36 mA is low...and no adjustment - usually signals open - you're seeing the BASE (A reference, Not Gate) reference current draw thru the circuit itself - not the Finals SOURCE to DRAIN (which would change the mA reading) This indicates something else is not right...check soldering job there - you may have discovered a fault and you need to fix it.

The current Draw is necessary else the Gate voltages would rise and stay put - so the Radio needs a place to dump current and power from the Bias when the radio is running along in TX mode. It's a stability issue.

Single Final? Use 55mA
Dual Final? Use 70mA <<-- NO MORE THAN THIS!
Driver? Use 50mA

If i recall, you cant set the mosfet bias the same as the older transistors. My new out of box DX 959 with mosfets had exactly 4.20volts on the leg of that resistor both driver and final.

KD8HMO said:

If i recall, you cant set the mosfet bias the same as the older transistors. My new out of box DX 959 with mosfets had exactly 4.20volts on the leg of that resistor both driver and final.

Click to expand...

So, when you key the radio using (If applicable) the mirror board removed and/or any jumpers to TP7 (FINAL) or TP8 (DRIVER) from AM Regulation TP9 (Origin) - you have what mA reading in SSB LSB/USB no audio?

Because, if you post this as cast in stone - your 4.20 volts reading - you may be buying a lot of people some new MOSFETS' ... are you sure you want to post this reading without verifying the mA is correct?

I wouldn't - and I have seen people escorted off premises due to that type of check. It's just too costly to a business' bottom line to repair or replace a radio and to remedy the damaged reputation - from someone that presumed all radios and MOSFET's are created equal.

Well, I can tell you that it was set from the factory that way. If i set those settings down to the other specs, i can only get about two watts dead key. It makes sense, I read those mossfets are 5v devices. It would be nice if the manufacturer would update their service manuals.

Isn't 4.2v just a HIGH for a MOSFET?!?
Supposed to be 3.65v for the driver and 3.75v for the final.
That is what the service guys said at Galaxy last time I contacted them some 10 years ago.

If it is bipolar transistor; then the driver is supposed to be 35mA and the final is supposed to bet at 50mA tested in SSB mode with mic gain set to zero on ch 20 IIRC.

???