Anyone got a schematic for the MSF5000 VHF VCO and/or uniboard?
I'm having the exact same problem as post http://batboard.batlabs.com/viewtopic.php?f=1&t=79420
This is a new to me station. When I first got the station home I read the code plug and verified the station received (.2uV @ 12db SINAD) and transmitted (110W) on its original frequencies. I pulled the VCO's and modified them for the lower range to reach down to 144MHz. When I put them back in I programed channels at 136MHz, 138MHz, 144MHz, 148MHz, 152MHz, and 156MHz. The RX VCO seems to lock fine on all frequencies, but the TX will not lock except on 138MHz but then the TX VCO LED flickers at around 20Hz or so. No errors are present when tuned to 138MHz, no beeps, but if the station tries to go into transmit all the PA light quickly flash off and on and it does not produce any RF, but still does not produce any errors or beeps. If I take a screwdriver and short the outer case of the VCO to the RF deck housing the VCO will lock on all the programmed frequencies with a solid steady green light, and the station will go into transmit and produce RF. I did not get any reading on Meter5 but need to check and verify the meter panel and cable and connections are good and correct.
I have lifted the uniboard and check and reset the VCO cable at both ends. Reassembled everything with the same results. All connections "look" good and clean. I plan on checking them with a ohm meter tomorrow.
Questions:
1. How can the VCO housing not be grounded by the coax shield? The coax looks prefect. Is the RF circuit isolated?
2. Should be black wire be grounded to the chassis? Does the uniboard ground to the chassis?
3. Will it hurt to run the station with the VCO locking knobs in "transit" position which grounds the VCO housing to the RF deck. Since the VHF VCO does not contain a tuning cavity like the UHF units there wouldn't be (or less of) a concern of vibration modulation.
4. Is there a reason Motorola isolates the VCO on plastic?
This is a VHF CXB station, the VCO are not adjustable, the VCO are not sprung, they sit on a plastic ring which isolates them from the RF deck.
This problem likely existed when I got the station before I modified the VCO's, but it worked OK because the VCO locking knobs were set to transit position grounding the VCO to the RF deck.
MSF5000 VHF CXB VCO locking problem
Moderator: Queue Moderator
Re: MSF5000 VHF CXB VCO locking problem
1. If the coax shield has cracked or become unsoldered, the ground may no longer be good. Check with an ohm-meter. The other end of the coax goes into a connector on the Uniboard.
2. The Uniboard is grounded through several pins that go to the Interconnect board under the RF Tray. It probably does NOT rely on a mechanical ground to the casting. The black wire should go to ground when the Uniboard is plugged in properly. The station will operate properly with the RF Tray cover removed.
3. It will not hurt to run the station with the locking knobs in "transit". The VCO DOES contain a coil inside; it's just not adjustable with a tuning core like the ones used on higher-frequency stations. Vibration can still affect it.
4. I'm guessing the plastic ring around the VCO replaced the plastic posts and springs in the corners of the VCO. It isolates the VCO from the chassis to prevent modulation by fans and also keeps the VCO somewhat in the center of the casting well. They wouldn't have done it if it wasn't necessary.
Bob M.
2. The Uniboard is grounded through several pins that go to the Interconnect board under the RF Tray. It probably does NOT rely on a mechanical ground to the casting. The black wire should go to ground when the Uniboard is plugged in properly. The station will operate properly with the RF Tray cover removed.
3. It will not hurt to run the station with the locking knobs in "transit". The VCO DOES contain a coil inside; it's just not adjustable with a tuning core like the ones used on higher-frequency stations. Vibration can still affect it.
4. I'm guessing the plastic ring around the VCO replaced the plastic posts and springs in the corners of the VCO. It isolates the VCO from the chassis to prevent modulation by fans and also keeps the VCO somewhat in the center of the casting well. They wouldn't have done it if it wasn't necessary.
Bob M.