Bummed that my Roadtalkers Xtal is out of whack

[MKwrench]

It's a 934.38270700 "Slantface Roadtalker 40. I've wanted one of these for a while, so when one popped up for peanuts I had to grab it. AM worked alright but showed only about 75% Modulation on a 3 watt carrier.
Transmitting on the upper sideband sounded close to being in tune , but real bright and brassy. Lower sideband sounded extremely low and distorted, definetly off.
I started the alignment from CB246 which I'll include below. All goes well untill steps 4 and 5 where my frequency counter has a hard time capturing the measurment. On a hunch, I removed the cage shields and replaced all of the electrolytics on that side of the radio. Nothing improved. But, when using a counter with less resolution I can measure and adjust without a problem. The rest of the process goes without an issue (even with the sharper counter). It's easy to gather that my filter has drifted off center, because when all is done I have simular audio issues. So I tried the "zero beat at 1Khz" route, Now I have an okay USB but the LSB sounds like I'm speaking through a pillow. I even tried "backwards" by setting offsets 1st. One thing I noticed is that the ofsets seem to screw up the Receive too, I see in the schematic that they are always lit up in their mode so I dont know if this is normal.
I'm kinda bummed that I can't get it just right.........
Whatca think? Do I need a new Filter, or are still a few flaming hoops I can try jumping through?

 

[brandon7861]

T304's primary is lightly coupled to T303 via a 1pF cap. Maybe the probing of T304 pulls it down too low to measure? Could you try probing at Q308?

Did the filter drift or did the shape of the passband change from an internal failure? Maybe you could wind a couple 4:1 transformers and sweep it with a nanoVNA to see what it is doing.

 

[nomadradio]

Hmm. Does leave the question of carrier-crystal frequencies up in the air. We're accustomed to poking the center pin of a coax jumper near the carrier oscillator and using a calibrated ham HF radio to eavesrop and use the 1 kHz tone method. If you hear the pitch of the receiver's noise change when the counter probe is unhooked, that suggests the probe's capacitance is upsetting the trimmer setting.

Once you're sure the carrier crystals are set where the alignment says to, that leaves only the crystals inside the filter. Performing a "sweep" measurement of the filter's frequency response is not a simple proposition. Just the same, if the carrier trimmers are set properly, the filter is the only remaining culprit. Crystals do drift with age, whether in an oscillator or inside a passband filter.

73

 

[doffo]

I got a few of the Slant Face SSB radios that need to be recapped. They work but the SSB slowly fades away on receive, and the other one take a bit for the audio to actually go through when you key up the mic.

Capacitors are basically 50 years old soon. Hope ya get it fixed.

 

[MKwrench]

T304's primary is lightly coupled to T303 via a 1pF cap. Maybe the probing of T304 pulls it down too low to measure? Could you try probing at Q308?

Did the filter drift or did the shape of the passband change from an internal failure? Maybe you could wind a couple 4:1 transformers and sweep it with a nanoVNA to see what it is doing.

Thanks for getting back with me, I don't not have a VNA or a spectrum analyzer that I could do this with. If I did it would be my first time trying a sweep on a filter. These are tools that are next in line to treat myself with.

 

[MKwrench]

Hmm. Does leave the question of carrier-crystal frequencies up in the air. We're accustomed to poking the center pin of a coax jumper near the carrier oscillator and using a calibrated ham HF radio to eavesrop and use the 1 kHz tone method. If you hear the pitch of the receiver's noise change when the counter probe is unhooked, that suggests the probe's capacitance is upsetting the trimmer setting.

Once you're sure the carrier crystals are set where the alignment says to, that leaves only the crystals inside the filter. Performing a "sweep" measurement of the filter's frequency response is not a simple proposition. Just the same, if the carrier trimmers are set properly, the filter is the only remaining culprit. Crystals do drift with age, whether in an oscillator or inside a passband filter.

73

I'm going to try monitoring while removing the probe. Just a satisfy that part of my curiosity. I did try to monitor at different spots too. I might button this one up until I get a parts radio or a VNA to put my mind at ease.
Anyway I found that If anyone wants to pump up that modulation percentage from 75/80%, to 100% Do not cut D405 or D406 as suggested online. Putting a 10K ohm resistor under the cathode of D406 will keep your carrier from pinching while still getting 100% out of it.

 

[MKwrench]

This Roadtalker is back on the shelf until I feel more capable of verifying the integrity, bandwidth and center of the filter. Otherwise if a parts radio comes along I'm just going to Hail Mary it with the suspected goods.

The adjustments behaved in a simular way in one of my 2-button Washingtons and a filter swap from a parts radio patched it up, but now my "irrational need" to have everything in working order buggs me every time I see that parts radio who works great *now* other than a muffled LSB transmit.

Back to the Roadtalker, The offsets have a huge effect on the receiver too. That why I had a hard time with the "zero beat" technique. If TX was in, RX was out, if RX was in TX was out, plus always had Tinny USB and fluffy LSB. LSB hammers the ALC, and USB barely moves the meter. Is this offset thing normal on this chassis? I don't notice it on other radios, might be chasing my tail if this is normal.

 

[MKwrench]

Theres a Neon Bulb with a 150pf in parallel before the SWR bridge. Does anyone know what thats about? .... A safety device maybe?....