Cobra 132XLR Frequency alignment challenge

[Lkaskel]

Hi Everyone,
So, on the bench is a Cobra 132XLR for an alignment. The frequency alignment is a little different than most radios I have worked on. It starts with a frequency check (no adjustments. It's looking for 5.12 MHz and its reading 5.11814 MHz. You then adjust L303 for 17.055 MHz. Done. This is where the challenge begins. Adjust R324 for 34.9875MHz. In the center position it reads 34.9783 MHz and will only adjust up to 34.9795 or down to 34.9758. I can adjust R322 to get more of a move but still not even close. The AM adjustment is about 10K off as well. The only thing that seems to be able to bring it into alignment is to adjust L303 to get the USB and AM adjustments in line. The problem is that when they are in line the transmitt is way out. Like 13MHz out except for channels 1,2 and 3. I'll admit that I am out of my ability to understand what is happening here. Anyone have any thoughts? The alignment instuctions are attached for reference.

Thanks as always!!!

 

[nomadradio]

This is where the challenge begins.

This radio was built by Toshiba. It has a PLL, but the radio's block diagram more resembles 23-channel CB designs.

Toshiba chose not to use a SSB filter with a symmetrical band pass shape. Just about every SSB CB built since those days will either have two carrier crystals, one above and one below the filter's passband or one crystal that gets its frequency shifted above and below that passband with tuneable trimmer coils. This selects either the upper or lower sideband. The filter also serves to further attenuate the carrier, above and beyond what the radio's balanced modulator and balance trimpot can do.

But this method presented a problem for a 23-channel crystal-synthesizer radio. You have to move the radio's local oscillator up or down the opposite amount that the two carrier crystals' frequencies are shifted apart. Gotta keep the radio on channel center when changing modes. This is simple to do with a PLL, so most everybody has done it this way since 1979 or so. But making all of your synthesizer crystals EACH accurately shift up or down 3 kHz is a headache nobody tackled. Yes, the "set of four" crystals in the 23-channel synthesizer became two sets of four in some radios, this provided the local-oscillator shift, but at the expense of more crystals and more-complex channel selector circuit.

Toshiba had a different idea. This radio uses only one carrier crystal. It generates USB only. The PLL feeds an approximate 16 MHz local-oscillator frequency below the channel frequency and adds it to the 11.275 MHz carrier frequency to get the 27 MHz channel frequency.

To get LSB they double the 11.275 MHz carrier crystal to 22.55 MHz. This gets added to the (approx) 16 MHz VCO output from the PLL. The resulting sum of those two frequencies runs from 38.55 to 39.99 MHz.

What this does is to change the relationship between frequencies coming down the antenna and the frequency of the crystal filter in the radios IF stages. Since the LO frequency is now above the channel, when an incoming signal's frequency goes up. the difference between the LO and the IF is reversed. A higher frequency coming in from the antenna makes the difference between it and the LO smaller, a lower frequency feeding into the 11.275 MHz crystal filter. The same reverse-frequency relationship also applies to the transmit signal. This inverts USB to LSB.

Now that everyone is comatose and snoring away, The trimpots in this radio are supposed to shift the PLL's clarifier-circuit voltage for both sideband and AM. You want your AM carrier in the center of the crystal-filter's passband. But the carrier you feed into the sideband modulator has to be shifted to the lower side of the passband. Toshiba chose to use the clarifier circuit to shift the PLL between sideband and AM.

Trouble is, the trimpots will interact. One trimpot they tell you to set for sideband also affects AM. The pot that sets how wide the clarifier's range of frequency change also affects the canter frequency with the knob centered.

Add to that the procedure they give you doesn't take this into account. Getting this sort of radio on frequency becomes a "lather, rinse, repeat" operation. Naturally this assumes that the PLL crystal that those trimpots are connected to is on frequency. If not, you'll end up with a compromise with one sideband mostly centered and the other one not so much.

Best of luck.

73

 

[Lkaskel]

This radio was built by Toshiba. It has a PLL, but the radio's block diagram more resembles 23-channel CB designs.

Toshiba chose not to use a SSB filter with a symmetrical band pass shape. Just about every SSB CB built since those days will either have two carrier crystals, one above and one below the filter's passband or one crystal that gets its frequency shifted above and below that passband with tuneable trimmer coils. This selects either the upper or lower sideband. The filter also serves to further attenuate the carrier, above and beyond what the radio's balanced modulator and balance trimpot can do.

But this method presented a problem for a 23-channel crystal-synthesizer radio. You have to move the radio's local oscillator up or down the opposite amount that the two carrier crystals' frequencies are shifted apart. Gotta keep the radio on channel center when changing modes. This is simple to do with a PLL, so most everybody has done it this way since 1979 or so. But making all of your synthesizer crystals EACH accurately shift up or down 3 kHz is a headache nobody tackled. Yes, the "set of four" crystals in the 23-channel synthesizer became two sets of four in some radios, this provided the local-oscillator shift, but at the expense of more crystals and more-complex channel selector circuit.

Toshiba had a different idea. This radio uses only one carrier crystal. It generates USB only. The PLL feeds an approximate 16 MHz local-oscillator frequency below the channel frequency and adds it to the 11.275 MHz carrier frequency to get the 27 MHz channel frequency.

To get LSB they double the 11.275 MHz carrier crystal to 22.55 MHz. This gets added to the (approx) 16 MHz VCO output from the PLL. The resulting sum of those two frequencies runs from 38.55 to 39.99 MHz.

What this does is to change the relationship between frequencies coming down the antenna and the frequency of the crystal filter in the radios IF stages. Since the LO frequency is now above the channel, when an incoming signal's frequency goes up. the difference between the LO and the IF is reversed. A higher frequency coming in from the antenna makes the difference between it and the LO smaller, a lower frequency feeding into the 11.275 MHz crystal filter. The same reverse-frequency relationship also applies to the transmit signal. This inverts USB to LSB.

Now that everyone is comatose and snoring away, The trimpots in this radio are supposed to shift the PLL's clarifier-circuit voltage for both sideband and AM. You want your AM carrier in the center of the crystal-filter's passband. But the carrier you feed into the sideband modulator has to be shifted to the lower side of the passband. Toshiba chose to use the clarifier circuit to shift the PLL between sideband and AM.

Trouble is, the trimpots will interact. One trimpot they tell you to set for sideband also affects AM. The pot that sets how wide the clarifier's range of frequency change also affects the canter frequency with the knob centered.

Add to that the procedure they give you doesn't take this into account. Getting this sort of radio on frequency becomes a "lather, rinse, repeat" operation. Naturally this assumes that the PLL crystal that those trimpots are connected to is on frequency. If not, you'll end up with a compromise with one sideband mostly centered and the other one not so much.

Best of luck.

73

I'm sorry, as I was reading this I was getting a can of gas and a match . What you explained regarding R322 "the range" of the clarifier makes so much sense to me after I spent a lot of time in the rinse/repeat mode. Let me ask you this. I I adjust L303 (the 17.0555 adjustment) to get the frequency set for the USB and AM 34MHz does that throw other functions out of sort OR is it ok to make that adjustment to L303?

Thanks as always Nomad!!

 

[nomadradio]

So here's the problem. The setting of L303 is affected by the position of the clarifier knob and the trimpots., But the setting of the trimpots is affected by L302. When one zigs the other one zags.

Some time before I swore off working on this Toshiba chassis I was given a piece of advice to simplify it all. Noting that the transmit side of the clarifier circuit is a single trimpot activated during transmit only, the suggestion was to unhook both CW and CCW ends of the clarifier control. This takes all the trimpots out of the picture. Remove CD304, the source of transmit-only clarifier voltage. Connect the input of a 78L08 3-terminal regulator to a wire terminated at the filtered B+ at the emitter of Q27. It's easiest to mount the 78L08 regulator across the outer two lugs of the clarifier control. A 10uf cap should also go across the output and ground pins of the 78L08. A ground wire goes to the CCW lug of the clarifier, preferably connected to a ground foil near the PLL chip. Now the clarifier is active all the time, transmit and receive. The 8-Volt regulator gives the control more range to the right end of the knob's travel, since the clarifier was originally powered by 6.2-Volt zener diodes. Channel center will be off to one side of 12 o'clock for USB and the other side of center for LSB, but it will have enough range to prevent this from being a problem. L302 should allow those two 'center channel' positions of the clarifier knob equal distance either side of dead center.

It's been too long since I tried that trick, but it solved some seemingly-stupid alignment-acrobatic puzzles.

73