Looking for an RF coil for Galaxy DX 2527 (RCI 2950)

[OldTech03]

How many of those people that are off frequency when you are listening to sideband are off frequency because their clarifier is unlocked? That is very difficult to know, but considering how common a modification that is, I would bet it is quite possible.

Anyway, back to repairing this monster. I now have new finals in the thing, and new diodes, but I am getting no power again. I have checked, and the diodes are soldered to the proper place and are not touching anything but the pads they go to, I don't see any solder bridges, and the capacitors are in the right place as well.

I have gone through the previous procedures explained. I again get no amp reading between TP2 and TP3, but I do get .46 amps between TP1 and TP3. When I put my oscilloscope probe on the collector and base of Q48, I get a very weak 28 Mhz signal, which is the frequency I am transmitting on. I don't seem to see any signal on the emitter. This signal also never seems to get to the finals.

Can someone help me understand how this signal is supposed to go from driver to finals, and shouldn't I be seeing a signal on the emitter of the driver?

===============================================================
You should see a marked difference between the base and the collector of Q48 if you don't see a fair amount of gain there you need to become familiar with what level of signal you should see there I could tell you but you can also look at almost any dual final radio you have around that is working and take notes as to what you should see on the scope.

 

[OldTech03]

Also, remember signal generators can be utilized in these kinds of circuit issues to inject signals at specific points that will allow you to see if the final section is capable of amplifying a signal and at the same time giving you clues as to where signal is being lost if indeed signal is being lost and/or not making its way to the final section. I'm guessing that a significant number of benches that have signal gens are never utilized in this way. Most only want a gen to align receivers and they are much, much greater tools than that. as is the case with many of the test instruments on a bench.

 

[Handy Andy]

Hey PoDuck, sounds like something is starting to work...

Ok, to get .46 is good news, here's a little help...

TP3 - your AM Regulator
TP2 - The Finals
TP1 - The Driver.

Keep Test jumper strip disconnected - and put a dummy load on the rear antenna jack.

When checking bias circuits - you will need to be in TX mode...unkey the radio in between steps to prevent further damage...

Ok, you say TP3 to TP 1 - you get .46 Amps! Ok, leave that one alone for now, let's see that TP 2 - TP2 is in the areas of all that work you did (SIGH) maybe there is where the problem is, it's not pulling any current so that means a open circuit.



If it was a short, you'd lose TP3 and the ammeter would blow a fuse inside itself to protect the meter.

So let's think this thru, you have the Driver pulling some idle current - good.

But Finals - nothing. Ok, basics here look for the obvious...
Open traces (foil popped like a fuse?)
Burnt parts (resistors darkened black, Caps bulging and cracked)
Do a Continuity Check - From TP 2 back towards the Finals - each component in the chain performs a function...

The Bias circuit in these things isn't too complex - it takes in power from a source, usually the TP - puts it thru a power resistor divider network, thru a cap to remove RF and noise, and then a jumper with a ferrite bead onto the Bias diode and Base lead gets the RF from the Pre-driver - to Driver then onto the Finals...



Note the graphic above, there are highlited sections - TP 3 feeds TP2 (Finals) and TP1 (Driver)

Back probe from TP2 - and J6 thru the Toroid and verify the winds are all soldered and not open... The Toroid is wound a little differently than a regular coil - so it feeds two from one - a Bifilar type of winding.

So, for simplicity sake, test TP2 all the way back to the CENTER LEGS of the Final Transistors. If that shows short, it means they are connected. Now to show any current draw, that means the BIAS circuit needs to power them to adjust the idle current - you see that at the Driver - the Finals need this too...then you'll need to meter voltages of the bias circuit for the Finals - starting at that R257 a 150 ohm resistor. Set up for Voltage 20V is best for now, and hook up black lead to known good ground on the main PCB - like a coil can case - would work - and the Positive tip to all the power parts like R257 and R255 one side will show a voltage - the other side will also show a voltage only one side will have more "power" than the other...work your way back thru the chain R256 is an 18 ohm resistor - one side will have voltage - the other won't - it is connected to board ground - same as your black lead - no power...R253, another 18 ohm resistor - results will be the same...

Now, if you find that one side of R257 and R255 are showing power in TX mode, but NOTHING or little power - if any - on the other side of the resistor - when you test in TX mode - STOP and recheck the work, for it may indicate you have a dead short - check assembly.

Hope the hints above help.
:+> Andy <+:

 

[Handy Andy]

Some more for you...

The Driver - TP 1 - draws 1/2 an AMP? Or were you looking at the mA reading and it showed 46?

Because if you're drawing 1/2 amp - keep the test point jumper off and figure out if the Driver has shorted itself to case ground (Collector to Emitter) or you have a short in the TP1 to Drivers CENTER leg somewhere, lift unsolder the center leg and do that TP1 to TP3 again, if it's no longer drawing a 1/2 amp - the Driver may be the cause it's shot - replace...and reset trim pot (Vr11) to mid point so you can re-check.

(More than likely it VR 11 needs to be fully turned down - clockwise - so the "dimple" that is the wiper arm, is fully towards the side case of the radio - you will see that as voltage across the bias diode will drop off to nothing. Then SLOWLY return it to mid point watching the voltage reading - at about 0.4 to 0.5 volts ACROSS THE DIODE - STOP - go no further...it's set...you can verify the mA reading on TP3 to TP1...)

And to help out OT-03 - see if you even have 8 volts going to the Bias network R257 and R255 - if you don't then Q38 (Final TX Bias power) may be the fault in all of this...

IF you need any reference materials - try here...
http://www.cbtricks.com/radios/galaxy/dx2527/index.htm

:+> Andy <+:

 

[Low_Boy]

Could there be a bad trace some ware in that path.

 

[nomadradio]

A half of an Amp would be normal for the driver in AM mode.

You did switch to a SSB mode before making that measurement, didn't you?

73

 

[PoDuck]

===============================================================
You should see a marked difference between the base and the collector of Q48 if you don't see a fair amount of gain there you need to become familiar with what level of signal you should see there I could tell you but you can also look at almost any dual final radio you have around that is working and take notes as to what you should see on the scope.

I do see a difference, but the signal on the collector is still so weak that it doesn't give a very good trace on the oscilloscope. It looks like stair steps.

As for looking at another dual final radio, I don't have any other dual final radio to look at.

 

[PoDuck]

The Driver - TP 1 - draws 1/2 an AMP? Or were you looking at the mA reading and it showed 46?

I misspoke. It is .046 amps, or 46 mA. I don't know why I tend to drop zeros when I am doing that. I think I debate between writing it out in milliamps or amps, and combine the two by dropping the zero.

Anyway, I will check all your tips when I get to the shop tomorrow. Thanks.

 

[PoDuck]

Could there be a bad trace some ware in that path.

There was a trace that lifted on the collector of the driver, but I repaired it with some copper foil and epoxy, and tested that the trace conducted through to R305. Otherwise, I don't see any other problems with traces.

 

[PoDuck]

A half of an Amp would be normal for the driver in AM mode.

You did switch to a SSB mode before making that measurement, didn't you?

73

Yeah, the alignment instructions say to have it on LSB, which is what I did.

 

[nomadradio]

Whups, should have read the thread's updates.

You did correct that large reading to 0.046 mA.

That reading should be perfectly legit.

73

 

[PoDuck]

Okay, back at the shop, and to give an idea of what I'm seeing, here is what is on the collector of Q48.

Here is the base of Q48


Here is what I see at the base of Q46


Here is the base of Q47


These are all in AM, with no modulation. The only thing that is anywhere near correct is the frequency on Q48.

Anyway, I'm going to check out all the advice I was given over the weekend and I'll post results later.

 

[PoDuck]

Hey PoDuck, sounds like something is starting to work...

Ok, to get .46 is good news, here's a little help...

TP3 - your AM Regulator
TP2 - The Finals
TP1 - The Driver.

Keep Test jumper strip disconnected - and put a dummy load on the rear antenna jack.

When checking bias circuits - you will need to be in TX mode...unkey the radio in between steps to prevent further damage...

Ok, you say TP3 to TP 1 - you get .46 Amps! Ok, leave that one alone for now, let's see that TP 2 - TP2 is in the areas of all that work you did (SIGH) maybe there is where the problem is, it's not pulling any current so that means a open circuit.

View attachment 24720

If it was a short, you'd lose TP3 and the ammeter would blow a fuse inside itself to protect the meter.

So let's think this thru, you have the Driver pulling some idle current - good.

But Finals - nothing. Ok, basics here look for the obvious...
Open traces (foil popped like a fuse?)
Burnt parts (resistors darkened black, Caps bulging and cracked)
Do a Continuity Check - From TP 2 back towards the Finals - each component in the chain performs a function...

The Bias circuit in these things isn't too complex - it takes in power from a source, usually the TP - puts it thru a power resistor divider network, thru a cap to remove RF and noise, and then a jumper with a ferrite bead onto the Bias diode and Base lead gets the RF from the Pre-driver - to Driver then onto the Finals...

View attachment 24721

Note the graphic above, there are highlited sections - TP 3 feeds TP2 (Finals) and TP1 (Driver)

Back probe from TP2 - and J6 thru the Toroid and verify the winds are all soldered and not open... The Toroid is wound a little differently than a regular coil - so it feeds two from one - a Bifilar type of winding.

So, for simplicity sake, test TP2 all the way back to the CENTER LEGS of the Final Transistors. If that shows short, it means they are connected. Now to show any current draw, that means the BIAS circuit needs to power them to adjust the idle current - you see that at the Driver - the Finals need this too...then you'll need to meter voltages of the bias circuit for the Finals - starting at that R257 a 150 ohm resistor. Set up for Voltage 20V is best for now, and hook up black lead to known good ground on the main PCB - like a coil can case - would work - and the Positive tip to all the power parts like R257 and R255 one side will show a voltage - the other side will also show a voltage only one side will have more "power" than the other...work your way back thru the chain R256 is an 18 ohm resistor - one side will have voltage - the other won't - it is connected to board ground - same as your black lead - no power...R253, another 18 ohm resistor - results will be the same...

Now, if you find that one side of R257 and R255 are showing power in TX mode, but NOTHING or little power - if any - on the other side of the resistor - when you test in TX mode - STOP and recheck the work, for it may indicate you have a dead short - check assembly.

Hope the hints above help.
:+> Andy <+:

Okay, TP2 to center leg of both finals shows continuity and relatively no resistance. The resistors, 255 and 257, I switched out to 180 Ohm resistors as per your previous instruction, but they both are showing 8 V on one side and .45 V on the other. The rest of the resistors in that area seem to be fine.

Am I wrong in thinking I could make some headway if I follow the signal path from the driver to the finals and test for signal? I'm not sure which path the signal takes from the driver to the finals. It seems that since I do get a signal from the driver, however bad it is, it should conduct until wherever the problem is, and I should be able to follow it, if I knew the path. It might be that it is being pulled down somewhere, or it could be an open circuit, but either way, that is what I am thinking about.

 

[OldTech03]

I can tell you that you should be seeing a lot more voltage at the base of the driver than you are on the Rigol if those readings are with the probe set at 1x then you start checking from pre-driver forward I would think there should be at least 1.5 to 2 volts at the base of the driver but 2.0 mV is nothing. Just guessing but you should see around 400 to 500 mV at the predriver if not more.

 

[PoDuck]

I can tell you that you should be seeing a lot more voltage at the base of the driver than you are on the Rigol if those readings are with the probe set at 1x then you start checking from pre-driver forward I would think there should be at least 1.5 to 2 volts at the base of the driver but 2.0 mV is nothing. Just guessing but you should see around 400 to 500 mV at the predriver if not more.

Okay, that helped me fix something. I had the probe set for 10x, but the scope had the probe set for 1x. After fixing it, I got some better results.

Base of driver Q48


Collector of Q48


Base of Final Q46


Collector of final Q46


Base of final Q47


Collector of Q47