Down the Alice In Wonderland rabbit hole. This Cobra 135XLR had a Troy Radio DDS installed. Looked okay to me. The tech who did the job reported "It wouldn't receive. Pressed on the main circuit board and it sprung to life, wouldn't shut down again after that." Or something to that effect.
It arrived deaf and mute. No transmit, except for what leaked out of the transmit mixer into another radio on the bench. And very weak receive. A fault shared by transmit and receive should be caused by some function that's shared, right?.
The incredibly weak receive seemed baffling. The output of the 5351 DDS chip was connected to TP311, the output of the VCO buffer transistor. I reasoned that insufficient drive level from that point to the two mixer circuits would hold back receive and transmit signals. And then I discovered that the factory VCO circuits were still running. Yeah, two VCOs. One for AM and LSB, the other one for USB. Removing C305 and C307 took them out of the picture. Two local-oscillator signals together made the 'scope trace at TP311 way too fuzzy. I moved the output of the DDS to the gate of Q303, where the original VCOs fed into. Figured it would improve the level at TP311. Receiver is still weak, and still no transmit.
Got flustered and rechecked alignment. For those playing along with the home version, the schemo is attached. It required a few hundred microVolts to hear anything at all. The alignment tuning slugs upstream from the IF crystal filter all showed normal peaks. The next one downstream following the crystal filter is T5. Turn the slug, no response. Doesn't change signal level. This points to trouble in the circuit attached to T5. The transistor that drives into T5 is Q4. DC voltages on Q4 are all wrong. Base and emitter voltages are way high. Collector voltage is way low, barely higher than the emitter voltage. As if it were shorted C-E. Pulled Q4, but it's not shorted. Checks perfect. The base voltage with Q4 removed is still way high. Long story short, the PLL-unlock lockout is active. That circuit is required for the radio's design to be FCC-legal. Rule says it has to shut down the transmitter if the PLL loses control of the radio's frequency. Prevents transmitting outside channels 1-40. Some radios also shut down the receiver as well. This one does. Well, this radio's PLL has four (count 'em) four chips. Soon after this model arrived on the market, single-chip PLL circuits became common. This radio was only a little ahead of its time. Turns out that the PLL unlock-protect circuit is active. The main PLL chip was removed, just like for any other Troy Radio DDS install. But the out-of-lock signal doesn't come from that chip. It comes from two of the other three PLL chips. Turns out the best fix to disable the out-of-lock-disable feature is to cut one foil trace where the two unlock signals come together. It's the collector of a PNP switch transistor Q22. In the factory schemo, it's directly above the symbol for the mike's pickup element, near the bottom of the diagram. This view is with the front of the radio facing you.
A closer view of the collector trace.
I cut the trace with the power on. As soon as it was severed, the receiver roared to life. Previously had to turn the volume full up to hear much at all. Turns out now it transmits, too.
Wahoo!
I know a fella up in Chicago who thinks the 23-channel 135 sounds better with a DDS than this version.
I have no way to know, but this radio proves to be more broad banded than I expected. Good thing I'm not prone to tearing out my hair. Disabling this function is easy for a single-chip PLL, just find that foil pad when the PLL chip is removed and jumper it either to ground or to the chip's power pin, whichever logic state a properly-locked chip would produce on that pin.
Sure won't miss this detail again.
Poor choices come from lack of experience.
Experience comes from making poor choices.
73
