P.A.Slab failure on VHF radios.
Sometime ago a club member brought a dual band (2 metre/70 cms) FM radio with no transmit output on 2 metres but ok on 70 cms.This was an Alinco radio with seperate PA modules for each band.Most of the PA slabs used in VHF/UHF radios have 4 terminals plus theheatsink which is ground.These are (L-R) rf input, 12 volt to driver stage, this is usually varieddepending on power setting, 12 volt to output stage and finally rf output so to check these you need an rf voltmeter of some description as well as a multimeter.On putting the radio on 2 metres while connected to a power meter and dummy load and keying to transmit no rf output was seen.I opened up the radio and connected my rf millivoltmeter to the rf input of the 2 metre PA slab, rf was seen and a frequency counter confirmed it was on the correct frequency, also it could be heard on a nearby radio.12volts was available on both the 12 volt terminals so I then put my rf millivoltmeter to the output terminal, no output at all so it looked as if theslab had failed, this one was an “SAV17” which is 50watts output.These at the time were about £70 each (ouch!!) a search of the 'net produced an article about repairing them so I removed the slab and opened it (the plastic cover on this one comes off quite easily) on careful inspection I noticed the substrate was cracked removing the 12 volt supply to the PA transistor so I repaired it by bridging the crack. On reassembling the radio (with heatsink compound under the slab which wasn't there before!) it now worked fine.I've since had another radio with the same problem, different make of radio but the same slab and another club member has since repaired one with the same problem (different make of slab) so it seems it's worth checking this before condeming a slab which may be repairable.
Tony g0mqg
Editors note: A 'Slab' is a kind of hybrid of several conventional components soldered to a circuit board or ceramic circuit and then the whole thing encapsulated in resin or similar to make a complete module, commonly referred to in radio circles as a Slab.
73 de Tony G0MQG
