Those are some strange readings you are seeing on your counter. Per the service manual the high VCO should free run at around 32 MHz. Operating range is 63.05 to 78.05 MHz.
On Wed, 1 Jan 2025 17:24:38 -0400
"Jim Harris via groups.io" <radiove1rb@...> wrote:
Gary the frequency counter is hook to 5/24. I will check the input to the
two 5 volt regulators and let you know. I do have a scope but it only goes
to 30 mhz but I know where I can get a good 100 MHz one. Will get that as
soon as I can and have a looks at those outputs. Many thanks Gary for all
your help otherwise I would be working in the dark.
Jim VE1RB
On Wed, Jan 1, 2025 at 4:45?PM Gary Follett via groups.io <xntrick1948=
[email protected]> wrote:
If that oscillator is showing anything real (~152 MHz) then it is being
selected. This selection is enacted by the application of +10 volts on
U502. The VCO board has no other information about the band selected by the
band switch.
The only other information the VCO gets from the outside world comes in
via pin 5/9 (500 KHz Reference) and pin 5/11 (this is roughly 500 KHz as
well, and is the result of the 40 MHz Reference, the PTO and the VCO signal
divided by N (N being the selected number produced by the band selection
logic) and this is the signal that closes the PLL loop.
However, even with the loop open and unlocked, the frequency at the output
of the VCO should never exceed 80 MHz,
Where are you measuring the 152 MHz?
If grounding RFC505 on the away end does nothing, try grounding the other
end. If still no change, then ground the physical junction at CR511A, B and
CR510 A,B. This should pull the frequency down a lot. If not, ground the
Drain of Q504 through a capacitor of about 0.01 uF. That should clearly
kill any oscillation and remove any output from the VCO.
Then humor me on this one - measure the voltage on the inputs of both 5
volt regulators U501 and U502 when a low band (less than 15 MHz) and a high
band (greater than 15 MHz) are selected by the band switch. If there is a
switching issue, perhaps BOTH VCO¡¯s are on when High Band is selected. I
would have a hard time deriving what would happen in this condition but I
am quite sure things would not go well.
This is when an oscilloscope would be handy. You could look at the
waveform at the output of the VCO, at pin 5/24, the output of the VCO to
the radio. You should see a nice sine wave of roughly constant amplitude
when either VCO is selected.
Gary
W0DVN
On Jan 1, 2025, at 1:50?PM, Jim Harris via groups.io <radiove1rb=
[email protected]> wrote:
Gary I tried that ground on RFC505 and no change. Is it not getting band
information?
Jim VE1RB
On Wed, Jan 1, 2025 at 3:02?PM Gary Follett via groups.io <xntrick1948=
[email protected]> wrote:
If you are reading 152 MHz, the you should ground RFC505 on the end away
from the varactor diodes. The frequency should come down.
Gary
W0DVN
On Jan 1, 2025, at 11:58?AM, Jim Harris via groups.io <radiove1rb=
[email protected]> wrote:
Jim maybe this will help. I touched up several solder joints although
they didn¡¯t look good but not cracked. I switched the input to the counter
and it is reading 17 to 18 mhz
<IMG_1166.jpeg>
as the frequency is jumping around so is not locked. I wonder if that is
a clue?
Jim VE1RB
On Wed, Jan 1, 2025 at 12:52?PM Jim Harris via groups.io <radiove1rb=
[email protected]> wrote:
Good ideas Jim and I will get at doing that today. It is windy and
raining here quite hard. A good job inside hi hi. I will let you know how
it goes. Many thanks for the suggestions and help.
Jim VE1RB
On Wed, Jan 1, 2025 at 12:24?PM Jim Shorney via groups.io <jimNU0C=
[email protected]> wrote:
At this point I am suspicious of that 1.3 MHz reading. That may be a
spurious reading. It seems to me that the VCO should not be able to run
there. What is the level of the signal? The frequency determining
components are L502 and the varactor diodes. Resist the temptation to try
to adjust L502. As a long time tech told me back in the dim past, when you
have a failure "it is never the alignment".
At this point I think I would inspect the solder connections in the
high VCO circuit under magnification and good lighting. You could have a
failed joint or via. Check the voltages on the three JFETs. C156 or C517
could be suspect. You have a working low VCO to compare voltage and
resistance readings to.
On Wed, 1 Jan 2025 07:41:18 -0400
"Jim Harris via groups.io" <radiove1rb@...> wrote:
Yes Gary I did the ground on RFC505 when looking at the frequency on
the counter and it does drop a lot. So I guess you are saying the High
VCO is oscillating at 1.3 MHz . If that is the case there must be
something wrong on that board unless the info it gets is wrong. Hope
this helps.
--
73
-Jim
NU0C
--
73
-Jim
NU0C
Jim Shorney