Uniden Madison USB/LSB problem

[David Springmire]

Hello,
Really hoping someone with more knowledge can give some guidance. I have a Uniden Madison It had the extra channel mod switches and an open clarifier so I started by getting everything back to stock. Next, aligned it by the service manual for a President Madison since I couldn't find the Uniden version manual. The receive aligned fine and works well (good sensitivity). But in TX it sounds off frequency in both USB and LSB. The voice sounds high in USB and low LSB. I've aligned the PLL and carrier osc. but no luck. The carrier osc. for USB and LSB are 7.8015 MHz and 7.7985 MHz respectively. They do align to those numbers using a counter and a scope probe but got the results indicated above. So, I used a spectrum analyzer trying near field measurements with the same results. The clarifer has been corrected and it only works in receive as it should. Does someone have any idea as to what's causing the problem? Thanks so much!

 

[Cable Guy]

Did you use scope probes to set the vco? Scope probes can load down the circuit and cause misalignment. An rf probe is ideal. When aligning the carrier oscillators, and the vco is exact, I sometimes use a tone generator app on my phone to generate a 1 kilohertz tone, feeding through the mic, then check/adjust the carrier oscillators while looking at an external frequency counter coupled at the antenna port/dummy load. For cb channel 20 in USB, adjust for 27.206. LSB is adjusted to 27.204. AM for 27.205(not modulated).

 

[David Springmire]

Did you use scope probes to set the vco? Scope probes can load down the circuit and cause misalignment. An rf probe is ideal. When aligning the carrier oscillators, and the vco is exact, I sometimes use a tone generator app on my phone to generate a 1 kilohertz tone, feeding through the mic, then check/adjust the carrier oscillators while looking at an external frequency counter coupled at the antenna port/dummy load. For cb channel 20 in USB, adjust for 27.206. LSB is adjusted to 27.204. AM for 27.205.

Thank you very much for the reply, Yes, I did use a scope probe with a frequency counter for both the vco and carrier oscillators in both x1 and x10 modes. I suspected like you mentioned the probe was loading the circuit so I also tried a piece of 50 ohm coax with a stripped back center conductor and measured only near field (without actually touching the circuit) but got the same result. It seems to be shifted about 300 to 500 Hz. I will sure try your method and get back with you (it would eliminate another variable). Thanks so much!

 

[Cable Guy]

But you did the vco/loop with the near field probe too, right? Just checking the basics.

 

[David Springmire]

Thank you, yes with both the pll/vco loop and the carrier oscillators.

 

[Cable Guy]

Okay, thanks. Do you know when your counter was last calibrated? If it's off some when setting loop oscillators then the error gets compounded after adjusting carrier with the same discrepancy. Everything looks okay, but sounds way off, especially on sideband. There are some great people here and vast experience and knowledge, just give them a chance to help.

 

[David Springmire]

You make a good point. My counter hasn't been calibrated for at least a few years I picked it up on ebay after my old Kikusui failed I purchased another Kikusui but a different model this is a FCO-1120. I thought I'd at least try and get it "close" so I bought a 10 MHz GPS disciplined oscillator and adjusted it to within 1 Hz. Interestingly enough, the scope and spectrum analyzer both have counter functions and while they don't correlate exactly they're within about 20 Hz of each other. I like your idea of measuring at the antenna and measuring TX frequency that would allow for any other errors. I'll give it a try and let you know. I really appreciate the help and look forward to communicating with you and the other members. While I've worked in electronics many years I'm not near as well versed in radio knowledge you all are. I really appreciate all your help.

 

[David Springmire]

Tried your trick of using CH 20, USB 27.206 and LSB 27.207 MHz adjusting only the pll/vco with the probe in x10 and counter and that seems to get it spot on! Next I'll check the receive to confirm the carrier oscillators are correct and sensitivity is good. I'll use it as a new method from now on. Thank you so much Cable Guy!

 

[Cable Guy]

Glad you got it sorted out, and you're very welcome. I try to do it by the book but sometimes you just gotta do whatever you can. Come back if you have any issues or questions.

 

[loosecannon]

the "1 khz tone SSB tuning" trick should only be used on the IF section (7.8mhz).

the 34 mhz pll/vco section should be tuned using a freq counter at the TP.

this method works pretty well and the reason you can't just set those cans for exactly 7.7985 and 7.8015 is because the crystal filters in the radio are not necessarily perfectly centered in the passband.

IF tuning is much easier if you have an SDR you can listen to and just tune the cans for most realistic voice tone.
LC

 

[David Springmire]

the "1 khz tone SSB tuning" trick should only be used on the IF section (7.8mhz).

the 34 mhz pll/vco section should be tuned using a freq counter at the TP.

this method works pretty well and the reason you can't just set those cans for exactly 7.7985 and 7.8015 is because the crystal filters in the radio are not necessarily perfectly centered in the passband.

IF tuning is much easier if you have an SDR you can listen to and just tune the cans for most realistic voice tone.
LC

Just a quick question, I noticed after getting the pll adjusted for good transmit that the clariifer is slightly off (about two o'clock) is there something I'm missing? Thank you! I'll invest in a SDR too.

 

[Cable Guy]

Making sure the clarifier is at 12 o'clock when you begin vco alignment is a good idea. Is it transmit or receive that is off, or both?

 

[David Springmire]

Oh, I see ok. The transmit is fine now just that the clarifier is at two o'clock to get best receive.

 

[Cable Guy]

After you set the vco loop oscillators, you will need to readjust the carrier oscillators as well I think. Is this a mb8719 pll radio same as a cobra 142gtl?

 

[David Springmire]

Oh Ok yes you're correct I notice following the SAMS is a little misleading. The PLL settings in some manuals don't include the LSB/USB PLL settingings in SSB TX, only RX while others do shw both. This chip shows only receive. The transmit is fine now just that the clarifier is at two o'clock to get best receive rather than twelve o'clock as expected.