Galaxy DX 88 VCO Problem

[timrim]

VCO voltage at TP2 is 8V. I have replaced the PLL, what else should I check to trouble shoot problem?

NOTE: VCO adjustment never allows voltage to go below 8 volts.

Thanks in advance,
Tim

[The Defpom]

The 8V is being generated by the PLL, to try and control the VCO frequency, which is obviously not correct, so I would be looking at the VCO and the downmix frequency that returns to the PLL.

Do you have a frequency counter which can be connected to the VCO test point and downmix test point, so that we can try to figure out what may be happening ?

Once we know what frequency the VCO is generating, and what the downmix frequency is we can start to work out what may be wrong.

[timrim]

Thanks for fast response, I will try to locate the test point and let you know what frequency I get.


EDIT: VCO Is drifting at ~19Mhz
Looking for Down converted signal now

Tim

[The Defpom]

You want TP3 (VCO output, which is right in front of L43), and pin 2 of the PLL, do the test on CH1, of the normal CB band.

[timrim]

Pin2 of PLL 3.165 Mhz

16.725 at TP 3 in front of L43

[The Defpom]

OK, try adjusting the VCO so that the freq at TP3 is 16.270MHz, and then re-check PLL pin 2, you should get 2.71MHz

Again, make sure that this is done on CH1 of the normal CB band.

[The Defpom]

As the difference between the VCO and downmix frequencies are correct (13.560MHz) I suspect that the PD output from the PLL may not be getting to the VCO control varactor diodes, which is how the PLL controls the VCO frequency.

If you cannot tune the VCO down to 16.270MHz, then check the voltages at R118, it should be a bit less than the voltage at TP2.

[The Defpom]

If the voltage is not present at R118, then I suspect that either C100 has shorted or R117 has failed, possibly both.

[Rick]

This may help.
A common fault is C101 and C102, which I have seen fail quite a few times in the 3600 chassis.

[The Defpom]

After doing some more checking, I believe that the PD voltage to the VCO increases with frequency, so an 8V output from the PLL PD (TP2) is trying to make the VCO generate a higher frequency, this could be caused by a few things.

Firstly if the voltage checks out at R118 , then it could be that the PLL is trying to produce a high frequency from the VCO, this could be due to an incorrect binary code on the PLL, possibly from a bad binary adder, you would need to check the binary code on the PLL, on CH1 on CB band it should be 271 (I believe CB band is band E on your radio, but I am not sure), it is important that you are on the correct channel and band for this troubleshooting, otherwise things can be backwards.

If the binary code checks out, then there could be a short circuit to the trace where the PD voltage is used, check the voltage on both sides of R116, the PLL side should be higher.

[The Defpom]

Thanks for joining us Rick, I was thinking of the same parts, but it was stated that there was 8V at TP2, which kind of rules out one of them being shorted, as it would drag down the voltage at TP2.

What symptoms have you seen when these parts have failed before ?

Actually something just came to mind, if the resistor installed at TP2 is in backwards, so the wrong bare lead is used, then the TP2 test point can be on the PLL side of the resistor, instead of the VCO side, which would mean that it measures OK... So the best way would be to measure the voltage at R117 instead !

[timrim]

OK, try adjusting the VCO so that the freq at TP3 is 16.270MHz, and then re-check PLL pin 2, you should get 2.71MHz

Yes, that is correct!

Voltages at R-118 is 8 and 7.7
Binary Codes to PLL are correct.

Initial Check On R-116 PLL side is actually 0 and other side is 8V
Removed, and reinstalled R-116...8V on PLL side 3.4 Volts on VCO Side Radio worked for a few minutes and quit again. Both sides of R-116 Are 8 volts with no noticeable difference.




NOTE: I should have mentioned it before but the Clarifier is not affecting the frequency output at all.

[The Defpom]

The clarifier controls a oscillator which is mixed with the VCO freq, to give the downmix freq, so the PLL shifts the frequency to compensate, so at this stage it would be normal for the clarifier to not appear to be functional, as the loop is broken.

Check for any shorted traces, as it looks like the PLL is trying to reduce the VCO frequency as it should (PD output as 0V on PLL side of R116) but an 8V source is somehow getting into the VCO's varactor circuit.

[timrim]

Initial Check On R-116 PLL side was 0 and other side is was 8V (May not have made proper contact with probe?)
Removed, and reinstalled R-116...8V on PLL side 3.4 Volts on VCO Side Radio worked for a few minutes and quit again. Both sides of R-116 Are 8 volts with no noticeable difference.


I'll examine all traces.
EDIT---Traces seem to be ok, getting tired so I will recheck again later.



Rick: I will check those, I have replaced the varactors already!

EDIT: Rick, Replaced those Tantalum Caps. Radio still not operating!

[The Defpom]

You will probably need to retune the VCO again once you have the fault corrected, keep an eye on the voltages to see if the PLL behavior changes, if it does, try retuning the VCO, you should get around 3.2V at TP2 when the PLL is controlling the VCO correctly on CH1 of the CB band, it will be higher on higher channels, and lower on lower ones.

[Rick]

Reading between the lines, it looks more as if the fault is with the loop osc mixer (IC10).

If the loop osc is the KIA6410 it maybe best to change it to a TA7310P, as the KIA6410 is known to go faulty.

You say you have replaced the varicap, is the replacement varicap the right one?

[GRTS]

The easiest way to troubleshoot the PLL/VCO/Mixer circuit is to break the loop and inject your own voltage.

Unsolder IC5 pin 7.
Inject 3.0 volts at TP2.
Connect freq. counter to TP3.
Adjust L17 for 16.270mhz on the freq. counter.
If that's bad look at TR25 and TR26.
Should have 16.270mhz on TR25 (emitter/collector) and TR26 (collector).
If that's good put scope on IC10 pin 7 and look for 16.270mhz
Also check IC10 pin 2 for 13.560mhz
If IC10 pins 7 & 2 are good, check IC10 pin 6 for 2.71mhz.
If that's good then IC5 PLL is bad.
If that's bad then replace IC10.

This is called the VCO loop test and works for troubleshooting all Galaxy Radios with hard to find PLL/VCO/Mixer problems.
There are changes to the test points, ICs and output frequencies depending on the EPT# of the main PCB.

[The Defpom]

Thank you for your input, that is a technique I have used myself on occasion, it can narrow down a problem when you are not sure of where it is.

Things that appear to be working correctly in this case are: loop oscillator, downmix freq. and mixer IC.

From the voltages mentioned it appears as though there is a fault with the PD into the VCO, which is preventing it from controlling the VCO freq.

The technique you mention is something I had in the back of my mind, I was thinking of lifting R116 and injecting a voltage in there, what I tend to do is hook up a 50K pot, across the PLL DC supply, with the wiper going to the VCO, that way the voltage can be easily varied to see if the VCO is responding correctly.

[The Defpom]

There is something that we have overlooked !

Have you checked that the 10.240MHz frequency is present on the PLL, pin 3 ?

If the reference oscillator is playing up, then the PLL will not have a frequency to compare the downmix frequency to, which could cause all kinds of weird behavior, I have heard of problems with Galaxy radios where the 10.24 crystal has needed to be re-soldered, I have even needed to do it myself a couple of times, even though the soldering looked acceptable.

[timrim]

The Defpom: Yes, Pin 3 has 10.240 Signal


GRTS:

Using your troubleshooting method, I have the following results:

IC10 Pin 2 13.560 Mhz Looks Ok

IC10 Pin 6 3.4 Mhz and drifting a bit Way off

IC10 Pin 7 16.270 Mhz Looks Ok


TR25 and 26 Have the 16.270 signal


So, Because Pin 6 of IC-10 is wrong, that means that IC-10 is bad?

[The Defpom]

From those results IC10 would look suspicious yes, the interesting thing is that earlier on you confirmed that the downmix freq to pin 2 was correct, which would indicate it was OK at that point.

If you have another TA7130 available then I would just replace it to rule it out at the very least.

[timrim]

If earlier it was correct, perhaps it was with the PLL controlling the VCO, In these tests, I was using a seperate variable power supply to control the vco.
I think I will swap it out and see what happens.


I just swapped out IC-10. No change, still not working. Pin 6 is 3.491 Mhz
And that was tested with the PLL controlling the VCO.

[The Defpom]

I would also check pin 9 of IC10, as pin 7 is the input to its amplifier, and pin 9 is its output, which then runs to L22 for the buffered VCO output, which runs to the freq. counter, and feeds back into IC10 for the mixer part to produce the downmix freq. (mixed with the 13.560Mhz oscillator)

[The Defpom]

The difference between the VCO frequency, and the downmix frequency at PLL pin 2 should always be equal to the loop oscillator frequency, in your case 13.560MHz (+- a bit dependent upon clarifier position).

So for a VCO freq of 16.270MHz the downmix will be 2.71MHz, it should not matter what supply is providing the DC to the VCO, as long as it is a very clean DC supply.

On my diagram pin 6 of IC10 is not connected to anything, so it may be best to just use the PLL pin 2 for that check, as that is where it matters.

[The Defpom]

Something else that may pay to check is the DC supply on pin 1 and 18 of the PLL, just in case C90 is failing / failed, preventing the PLL from operating correctly, as it could be picking up its DC through the binary inputs instead, which could make it do all kinds of weird things.

[timrim]

Pin 2 of PLL is 2.71 Mhz .... Pin 6 of IC-10 is where I get 3.4

Pin 9 of IC-10 also has correct frequency

Just Replaced IC10 and still not operating.

I use the following schematic:
http://www.cbtricks.com/radios/galaxy/d ... dx88hl.pdf

Pin 6 of IC-10 is used.


Hope this helps. At least we are ruling out things.

EDIT: Pin 1 of PLL is 8 Volts and pin 18 is zero.

[Rick]

What is the frequency at pin 3 and 4 of IC10?
Pin 3 is the none used pin.

[The Defpom]

Thanks for the diagram link, my diagram had a line missing from pin 6, I didn't think it looked right !

[The Defpom]

Here are some voltages for you to check, just in case it throws up something.

IC10
pin1 - 2.7V
pin2 - 2.1V
pin3 - 1.5v
pin4 - 2.7v
pin5 - 0V
pin6 - 5.4V
pin7 - 2.1V
pin8 - 7.4V
pin9 - 5.4V

You may get some slight variation to these, but as long as they are close to them things are probably OK.

[GRTS]

If you have 16.270mhz on the input of IC10 pin 7 and 13.560mhz on the input of IC10 pin 2 then check that 16.270 output is on IC10 pin 9 going to L22
L22 has a 16.270 tap that goes back to IC10 pin 4.
Check that L22 is aligned for max output at IC10 pin 4.
The 16.270mhz on IC10 pin 4 is mixed with the 13.560mhz on IC10 pin 1.
The difference (2.71mhz) will be on the output of IC10 pin 6 which then passes through LPF L50 (22uh choke) to C105 and back to PLL IC5 pin 2.

You should also verify the voltages on the pins IC10 to see if something associated is causing wrong output.

Pin voltages for IC10

1 - 2.7v
2 - 2.1v
3 - 1.5v
4 - 2.7v
5 - 0.0v
6 - 5.4v
7 - 2.1v
8 - 7.4v
9 - 5.4v

Late post....
LOL, least our voltages agree!!

[The Defpom]

I think that we need to go back to a systematic approach...

Set the radio to CH1 on normal CB band, check the voltage at PLL pin 7 and note it down.

Lift one end of R116 to disconnect the PLL PD output from the VCO.
Use a variable power supply (I recommend using a pot across the PLL DC supply at pins 1 and 18 with the wiper as the output, and feed it into TP2, to control the VCO frequency manually.

Hook up your counter to TP3, and set the voltage into TP2 to 3.2V, set the radio band to normal CB band adjust the VCO coil (L17) so that you get about 16.270 MHz on the counter.

Now reduce the voltage to 2.1V and set the radio to band "A", you should get about 14.470MHz (or something like it, it wont be exact).

Now increase the voltage to 5V, put the radio onto its highest band and check the frequency on the counter, it should be much higher than 16.270MHz, if this all checks out, then the VCO PD input is functioning correctly.

You will not need to adjust the VCO coil again, as it is now correctly set (which removes a variable)

Put the band switch back to CB band and adjust the voltage to give 16.270MHz on TP3, the downmix at PLL pin 2 should be 2.71MHz, if this is correct then IC10 appears to be OK.

Check the voltage at PLL pin 7. now reduce the voltage into TP2 so that the VCO freq. is less than 16.270MHz (say 16.250MHz), and recheck the PLL pin 7 voltage, it should be at its maximum (8V), now increase the voltage into TP2 so that the VCO freq. is more than 16.270MHz (say 16.300MHz), and recheck the voltage at PLL pin 7, it should be at its lowest (probably about 0V but could be a bit more), if the PLL pin 7 voltages are changing then the PLL is correctly seeing the downmix freq. and is trying to correct for it, which means that the PLL is probably OK.

If the PLL voltage is not changing, then it could be either a faulty PLL, a bad 10.240 reference frequency (could be noisy), or incorrect binary code.

[GRTS]

I think that we need to go back to a systematic approach...

Set the radio to CH1 on normal CB band, check the voltage at PLL pin 7 and note it down.

Lift one end of R116 to disconnect the PLL PD output from the VCO.
Use a variable power supply (I recommend using a pot across the PLL DC supply at pins 1 and 18 with he wiper as the output, and feed it into TP2, to control the VCO frequency manually.

Hook up your counter to TP3, and set the voltage into TP2 to 3.2V, set the radio band to normal CB band adjust the VCO coil (L17) so that you get about 16.270 MHz on the counter.

Now reduce the voltage to 2.1V and set the radio to band "A", you should get about 14.470MHz (or something like it, it wont be exact).

Now increase the voltage to 5V, put the radio onto its highest band and check the frequency on the counter, it should be much higher than 16.270MHz, if this all checks out, then the VCO PD input is functioning correctly.

You will not need to adjust the VCO coil again, as it is now correctly set (which removes a variable)

Put the band switch back to CB band and adjust the voltage to give 16.270MHz on TP3, the downmix at PLL pin 2 should be 2.71MHz, if this is correct then IC10 appears to be OK.

Check the voltage at PLL pin 7. now reduce the voltage into TP2 so that the VCO freq. is less than 16.270MHz (say 16.250MHz), and recheck the PLL pin 7 voltage, it should be at its maximum (8V), now increase the voltage into TP2 so that the VCO freq. is more than 16.270MHz (say 16.300MHz), and recheck the voltage at PLL pin 7, it should be at its lowest (probably about 0V but could be a bit more), if the PLL pin 7 voltages are changing then the PLL is correctly seeing the downmix freq. and is trying to correct for it, which means that the PLL is probably OK.

The band switch position has no effect on anything if you lift R116 and you're injecting your own voltage.
Band switch only controls n-codes to the 4-bit binary adders on the band board which sets n-codes to the PLL.

Timrim said on earlier post he has 2.71mhz at IC5 pin 2.
The 3.4mhz on IC10 pin 6 is because your counter can't lock on the freq. mixing but if you check it after L50 you'll see it's good (2.71mhz) there too.

Having the 2.71MHZ at IC5 pin 2 tells me your VCO/Mixer are working.
This normally indicates a bad PLL.

I think you mentioned replacing IC5 PLL but can you verify which PLL was in the radio originally?
Was it MC145106P or RCI-145106? They are not drop-in replacements.

[The Defpom]

When switching between certain bands the radio switches in some extra capacitance across the VCO coil (L17) and buffer coil (L18), to broadband the VCO, to ensure that the L17 tuning is correct the bands need to be switched.

The broadband circuit consists of C80, R109, D33 (VCO), C77, D123, R107 (buffer), R108, R140, C219 (switch supply).

The broadband tuning is enabled by switching to 8V in bands A-D, and is turned off by switching to 0V on bands E-H

[timrim]

GRTS, it was a MC145106 and I replaced it with the same.


The Defpom, I will complete the systematic approach again.

[James Unit 198]

Hear are some faulty parts i have had in the 3600 PLL section.

C101, C102 O/C.
R124 gone hi.
Bad cap in L22.
C225 turn in to a 1.3M resistor.
Tr25 low gain.
D30 near to short 830 ohm
C108 O/C.

I use the GRTS method for troubleshoot the PLL/VCO/Mixer circuit.
This is the best method can usually find the faulty part in minutes.

I also find look at the unused pin 3 on IC10 can be very helpfully.

James.

[timrim]

After re-doing the GRTS troubleshooting steps, I came to the conclusion that the VCO was properly functioning. I put in another PLL, because the first one I put in was removed from another radio, instead of a new one. (Yeah, I know) Turned the radio on and all is good. It was the PLL all the time. Thanks to all your help, it is now working 100%. I will never forget this PLL Troubleshooting technique, I will prove most valuable to me in the future. My old method was simply alignment check, program check, and replacing the suspected part(s). This method proves beyond a reasonable doubt where the problem lies.

Thanks guys, you're a great group!

[timrim]

New Update:


Radio operates, however, there is an issue that when I tune certain channels, it sounds like a guitar string strumming. When I switch to SSB, it sounds like a guitar or rubberband.
Both of these noises only last for a second or less.
If I touch the vco area I can also make the noise. I made sure all the wax was added after repair. Its almost like the vco is slow to respond.

Any Ideas?

[GRTS]

New Update:


Radio operates, however, there is an issue that when I tune certain channels, it sounds like a guitar string strumming. When I switch to SSB, it sounds like a guitar or rubberband.
Both of these noises only last for a second or less.
If I touch the vco area I can also make the noise. I made sure all the wax was added after repair. Its almost like the vco is slow to respond.

Any Ideas?

Just a guess but you might try replacing the two varactor diodes in the VCO.

Also cap C101 can cause that problem.

Weak 10.240mhz signal to the PLL can make the radio chirp too.

Verify resistor R121 has been updated to a 1K.

[timrim]

Thanks, I will check those. Other than that, the radio now works good.

[timrim]

It was C-101. Thank you again!