Ok Fella's
To save time wasting emails, I have no bias control on the junctions of CR517,518/pin 13 of the UA739 Op Amp.I can't see the voltage swing as specified (fig 2-12 TR7 Service Manual) to control either the Hi or Low VCO's on the coarse tune line . The Hi and Low VCO's are functional and operate when the bandswitch is changed ie low VCO 0-15MHz, Hi VCO 15-30MHz, however, the Hi VCO is oscillating around 82MHz, a little high.. Is the op-amp output bias (pin 13) eventually controlled by the PLL chip? or the VCO bias line? or is that line purely for monitoring? Or is there something else I'm missing??? The VCO bias line (pin 19 on the VCO board) is showing no voltage swing when the band switch is changed. Or do I try and buy another VCO board?? Basically from what I can figure, if the OP Amp voltage doesn't swing when the appropriate VCO is selected it will not supply the correct mixing freq to the translator and up-converter boards causing RX failure, The IF section is fine and PBT no problems from the 2nd mixer down have injected 48.05MHz into the input of the 2nd mixer to prove operation. 40MHz & PTO injection is OK into the Translator, VCO injection (48-78MHz) is obviously incorrect due to the main VCO fault. I can see why it's such a good radio when you get down to this level, they were very clever to design a radio of this complexity in that period with a 48MHz 1st IF, but it's a pig to fault find, any idea's?? Cheers Frank.. Frank H Woolfe, Dip EE. VK5MFW 36 Coolaroo Street Woomera South Australia 5720 Ph 08 8673 7725 Mb 0408 100 707
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