Batboard

I have a Spectra VHF with a A4 head. THe head quit working and when you turn it on and off, only the 2 small lights, monitor and TX , on the right hand side light momentarily and then go out. Replaced the main trunk unit and all is fine.

I thought it was the head at first. Anyone else run across this ?

Thanks.

JD wrote:I have a Spectra VHF with a A4 head. THe head quit working and when you turn it on and off, only the 2 small lights, monitor and TX , on the right hand side light momentarily and then go out. Replaced the main trunk unit and all is fine.

I thought it was the head at first. Anyone else run across this ?

Thanks.

Check the two aluminum electrolytic caps on the control head's PC board for signs of leakage (easily spotted by evidence of corrosion around their perimeter, affecting surrounding components).

Failing that, you'll need the service manual and an O-scope to check for the various waveforms in the DC/DC converter that drives the display.

I will add that the display tubes have been known to go bad over time (they are, after all, a type of vacuum tube). If the caps are OK (you should replace them anyway if they're over 10 years old), and the converter checks out, the tube is your likely culprit. Replacements are only available from Motorola, as it is a custom part made for them by Noritake.

Happy tweaking.

JD wrote:I have a Spectra VHF with a A4 head. The head quit working and when you turn it on and off, only the 2 small lights, monitor and TX , on the right hand side light momentarily and then go out. Replaced the main trunk unit and all is fine.

JD says he replaced the "trunk unit and all is fine" That would mean the radio it self, correct.

Could be the radio is telling the control head to turn off the display? Or in fact the caps have done their dammage which is usally one or two positions in the display are inop.

Motorola made some componet changes in the display voltage generating circuit, aka the switching circuit.

The head is good, I placed a new head on the orogonal transceiver and it did not work either. I then replaced the transceiver and it worked with the original head. Sorry, I was not clear on describing the problem.
Thanks.

You call it the main trunk unit, but reading your post it occurred to me it might still be in a dash mount configuration with the control head mounted directly to the radio?

If this is the case then you should try swapping out the radio interconnect board (inside the radio behind the control head). I cannot see why one of these would quit working (they are simple), but if you can isolate the problem to this board then you still have a good radio.

If it is a remote mount with a DB-25 connector and cable running to the control head, then the only thing that comes to mind is a special power configuration. In the special configuration control head power is run from the DB-15 ignition connector on the rear of the radio drawer then through the cable to the control head. This type of power hookup is sometimes used by people that want their radio to power up/down with the vehicle ignition switch. If the voltage in the cable is really low then there is no telling what the control head would do, but the problem could follow the radio drawer and not the control heads.