All,
I have another project. This one is a Grant XL with an MB8719 PLL. The owner said it was working fine, made a strange noise through the speaker while receiving then receive was gone.
I found this unit had been highly modified. I.e. 10 turn clarifier, extra channels on 2 of the front panel switches and a variable power potentiometer in place of the S/RF-SWR-Cal knob.
I found the 10 turn potentiometer was bad. No resistance on either outside terminal to the wiper in any position. It was wired directly to 8vdc and grounded to the top of the 11.325 crystal. I replaced the 10k ohm 10 turn potentiometer with a new 20k single turn pot and added in the missing R174 and R175 resistors. It still has 8vdc on the wiper in TX and RX. The problem is after aligning the oscillator(s) the clarifier will slide up but will not slide down at all. Example: Aligning the AM oscillator at 27.185, the clarifier will slide up to approximately 27.245 but will not come back down to 27.185 much less slide below channel 19.
In looking at the schematic (as found on CB tricks) I noticed what I think may be an error. If not, this may cause the problem I am experiencing.
Below is the area of concern.
In comparison to the 148GTL schematic below;
I was leaning towards a bad varactor diode prior to see this strange routing of R174 and R175 on the Grant schematic.
Any thoughts on this?
73’s
David
I have another project. This one is a Grant XL with an MB8719 PLL. The owner said it was working fine, made a strange noise through the speaker while receiving then receive was gone.
I found this unit had been highly modified. I.e. 10 turn clarifier, extra channels on 2 of the front panel switches and a variable power potentiometer in place of the S/RF-SWR-Cal knob.
I found the 10 turn potentiometer was bad. No resistance on either outside terminal to the wiper in any position. It was wired directly to 8vdc and grounded to the top of the 11.325 crystal. I replaced the 10k ohm 10 turn potentiometer with a new 20k single turn pot and added in the missing R174 and R175 resistors. It still has 8vdc on the wiper in TX and RX. The problem is after aligning the oscillator(s) the clarifier will slide up but will not slide down at all. Example: Aligning the AM oscillator at 27.185, the clarifier will slide up to approximately 27.245 but will not come back down to 27.185 much less slide below channel 19.
In looking at the schematic (as found on CB tricks) I noticed what I think may be an error. If not, this may cause the problem I am experiencing.
Below is the area of concern.
In comparison to the 148GTL schematic below;
I was leaning towards a bad varactor diode prior to see this strange routing of R174 and R175 on the Grant schematic.
Any thoughts on this?
73’s
David