[Yaesu] FRG-8800 PLL problems. Help needed!

[Clive Wallis]

Many thanks Ed for your comments.

> No knowledge, but you will have to give a better description so that we
> can "think" about the symptoms.

I didn't want to jump in too deep at first!  I was hoping that it might
have been a well known problem, and some one would have come up with a
quick cure.

> For example, what do you mean by "jumps"?  For example, does it receive
> 1234.4 correctly, but when you step one more time, it goes to 1234.9 (or
> 1235)on the display....

No

> or, do you mean that the display goes to 1234.5 but the set receives on
> 1235?

Yes.  The display works perfectly, but the received frequency jumps. 
Thus it is impossible to set the RX to receive some frequencies
(accurately).  

> In either case, in order for this to happen every time you cross the *.4
> border, it's got to go back to working correctly somewhere.  Can you
> determine where that is?

The problem repeats every KHz.

> For the display, test this way.  Set the dial to 1234.0.  Then go up in
> frequency and record what the display does every time it changes.  Stop
> at 1235.0 and this should give you 11 readings.  Let me know what they
> are.

The display increments correctly in steps of 100 Hz.

> The next part is more difficult because it requires you to determine the
> actual receive frequency for each of those steps.

The steps are smaller than 100 Hz.  then there is a big jump which makes
up the shortfall, every one KHz.

Since my posting I've made a little progress. The problem seems to be in
the second loop VCO, which provides the fine tuning.  The frequency is
controlled by a simple D/A converter, which doesn't provide enough
voltage to change the frequency by the required 1 KHz.  

I've looked at the circuit and there is a variable resistor VR 01 which
controls the output voltage.  On the circuit it's 10K.  On the parts
list it's 20K.  Measurements on the circuit indicate that it's about 2K.

I've managed to clip a variable resistance into another part of the
circuit which increased the output of the D/A.  This has helped, but I
get a nasty click when the D/A changes from full output to zero. 
Perhaps setting up the frequencies accurately, and removing the test
leads, will help.

If you have a technical manual, I wonder what value is given for VR 01?

BTW I think the significance of the change at 0.4 KHz, is the offset for
SSB & CW.  On AM it changes over between 0.9 and 1.0   

-- 
73     

	 Clive    G3CWV

         Hitchin, North Hertfordshire, UK.