I have a very nice Hy Gain 23 ch VIII that works on Am and USB. but LSB is dead. I recapped the entire radio and cleaned everything, bit still no LSB..
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23 ch hy gain VIII no lsb
- Thread starter demco satelite
- Start date
Don't have one of these, so I hope I'm not too far out in left field. There's a 9.2 Volt source for LSB that comes from the mode switch. If that's not present, no LSB. So that's where I'd look first.
Service info in Sam's CB-136 or here:
I'm assuming this is the crystal synthesis rig and not some in between super rare 23 channel PLL version.
Service info in Sam's CB-136 or here:
I'm assuming this is the crystal synthesis rig and not some in between super rare 23 channel PLL version.
A quick look at Sam's CB-148 shows what appears to be a nearly identical mode switching setup that both radios share. There's a 9.2 V source off the mode switch that's just for LSB. If you can believe Sam's, and if the assembler didn't make a mistake that day, it should be a white wire with a yellow stripe.
I put both away for now. May dig them out later. My midland 13 898b also shares the board but it works great.
A CB with a 40-channel PLL usually has two crystals to generate a carrier for the sideband modulator. One crystal's frequency is above the filter passband and gets you lower sideband. The other crystal's frequency is below the passband and gets you upper sideband. If one crystal quits, that sideband dies on transmit and receive both.
This 23-channel radio uses a different trick to get both sidebands. Only one carrier crystal is used. The channel-selector crystals are mixed to produce a local oscillator below the channel frequency, around 16 MHz. This frequency is added to the 11.275 MHz upper sideband signal to get the 27 MHz channel frequency. To get lower sideband, the 11.275 crystal feeds into a frequency doubler circuit. This 22.55 MHz gets added to the 16 MHz local oscillator from the channel crystals. The resulting 38 MHz local oscillator is above the channel frequency and this reverses upper sideband to lower sideband.
Kinda convoluted, but that's how they chose to do it.
When the carrier-crystal frequency changes going from LSB to USB, the local oscillator has to 'jump' the opposite difference in frequency to stay on the channel center. This is easier to do with a PLL than with a bank of crystals. Odds are that the tuned circuits in the crystal synthesizer have developed a fault. Following the 38 MHz LSB local-oscillator signal from the channel-selector section is where I would start.
73
This 23-channel radio uses a different trick to get both sidebands. Only one carrier crystal is used. The channel-selector crystals are mixed to produce a local oscillator below the channel frequency, around 16 MHz. This frequency is added to the 11.275 MHz upper sideband signal to get the 27 MHz channel frequency. To get lower sideband, the 11.275 crystal feeds into a frequency doubler circuit. This 22.55 MHz gets added to the 16 MHz local oscillator from the channel crystals. The resulting 38 MHz local oscillator is above the channel frequency and this reverses upper sideband to lower sideband.
Kinda convoluted, but that's how they chose to do it.
When the carrier-crystal frequency changes going from LSB to USB, the local oscillator has to 'jump' the opposite difference in frequency to stay on the channel center. This is easier to do with a PLL than with a bank of crystals. Odds are that the tuned circuits in the crystal synthesizer have developed a fault. Following the 38 MHz LSB local-oscillator signal from the channel-selector section is where I would start.
73
I probably couldn't do that as I have no proper test gear and no techs want to work on old stuff around here. Most are lazy ass peak and tune guys.
Just making a point that some tasks can't be done without some minimum kind of tool. Tasks like troubleshooting a crystal frequency synthesizer in a 49 year-old radio.
Even if you identify a failed part you might find it tough to track down a replacement. The industry stopped using crystal-only frequency synthesizers almost 50 years ago. Finding someone with experience in that technology is one problem. Finding someone willing to learn how it works to troubleshoot it might be harder.
73
Even if you identify a failed part you might find it tough to track down a replacement. The industry stopped using crystal-only frequency synthesizers almost 50 years ago. Finding someone with experience in that technology is one problem. Finding someone willing to learn how it works to troubleshoot it might be harder.
73
There are two tools that would be immensely useful. An oscilloscope with a times-ten probe to identify if a desired signal is present. Or not. And a ham or shortwave receiver that can tune in and eavesdrop on the various mixing frequencies inside the radio. If it has a S-meter, this can provide a way to peak tuning adjustments that can be tricky to get right. Parts of this radio will have multiple frequency signals combined together. Peaking an adjustment for just one of those mixed signals can be misleading using only the scope to watch the result of making adjustments.
A missing signal, in your case LSB, calls for a way to "follow" a signal through the radio from where it starts to where it goes. No other good way to identify where it stops. And why.
73
A missing signal, in your case LSB, calls for a way to "follow" a signal through the radio from where it starts to where it goes. No other good way to identify where it stops. And why.
73
Yup. No techs in my area that will help me as they are all about peak and tunes or of they work on radios, they only want to work on comms gear. So I'm on my own.