Cobra 29 XLR (Japan model) 10.240 fluctuates

[Lkaskel]

Hi Gang,
I hope everyone is doing wel and has a restful Memorial Day weekend!!!

I have a Cobra 29XLR 40 channel radio that all functions work on and appears to have never been worked on. I also re-capped it. When doing the synthesizer alignment the first 2 test to perform to do is adjust L5 for max on the scope and then adjust VC1 for 10.240 (both as measured at TP1). When I am looking at TP1 on the scope the wave form jumps a little but does adjust with L5. When TP1 is connected to the freq counter the reading fluctuates randomly up or down as much as 100Hz but does adjust with VC1. I replaced the crystal wth one from a known working radio and I have the same result. Has anyone seen this before?

Thanks!

 

[loosecannon]

try changing VC1 itself.
LC

 

[nomadradio]

Dirty trimmer cap. This is one of the reasons I abandoned using a direct connection for frequency measurements. A counter is fine to measure things that stay stable, but not so much for erratic circuits. Using either a calibrated ham receiver, or service monitor lets you listen to how the trimmer cap responds when you turn it. If you hear scratchy noises and random jumps in tone pitch when turning the trimmer slot, it's either time to see if contact cleaner will quiet it down, or just replace the trimmer cap.

Learned the hard way to squirt the carrier and PLL trimmer caps in Uniden SSB radios with cleaner and spin them until the noisy behavior goes away, and the pitch change when turning the trimmer screw is smoothed out. A couple of minutes to warm back up, and it will stay on the frequency where you set it.

The sideband operators always seemed to be happier when the radio's frequency settings stayed calibrated. One good chuckhole on the trip home and one of those dirty trimmers could jump a kHz or so.

73

 

[Lkaskel]

Dirty trimmer cap. This is one of the reasons I abandoned using a direct connection for frequency measurements. A counter is fine to measure things that stay stable, but not so much for erratic circuits. Using either a calibrated ham receiver, or service monitor lets you listen to how the trimmer cap responds when you turn it. If you hear scratchy noises and random jumps in tone pitch when turning the trimmer slot, it's either time to see if contact cleaner will quiet it down, or just replace the trimmer cap.

Learned the hard way to squirt the carrier and PLL trimmer caps in Uniden SSB radios with cleaner and spin them until the noisy behavior goes away, and the pitch change when turning the trimmer screw is smoothed out. A couple of minutes to warm back up, and it will stay on the frequency where you set it.

The sideband operators always seemed to be happier when the radio's frequency settings stayed calibrated. One good chuckhole on the trip home and one of those dirty trimmers could jump a kHz or so.

73

Hey Nomad,
I did clean the trimmer with Deoxit and also replaced it with a new 40pf trimmer and the frequency still fluctuates. I had to head out of town again so I will be back on this radio mid week. I am interested in your method of troubleshooting with a radio or service monitor. I am assuming that you would listen to them while transmitting on the radio under repair. Is that correct? I'll keep everyone updated as I make progress.

Thanks as always!!!

 

[nomadradio]

Not while transmitting, just 'eavesdropping' on each internal frequency. I'll set my radio for LSB and the display frequency 1 kHz above the desired target for the crystal trimmer. I have a tone box with a crystal-controlled 1 kHz. Turn it on, tune in the target radio's internal frequency and you'll get an audio beat note between the monitor radio's speaker audio and the sound from the tone box. You can get down to 1 Hz this way, without waiting for a counter to stabilize. A little like tuning a piano with a tuning fork.

73

 

[Lkaskel]

Not while transmitting, just 'eavesdropping' on each internal frequency. I'll set my radio for LSB and the display frequency 1 kHz above the desired target for the crystal trimmer. I have a tone box with a crystal-controlled 1 kHz. Turn it on, tune in the target radio's internal frequency and you'll get an audio beat note between the monitor radio's speaker audio and the sound from the tone box. You can get down to 1 Hz this way, without waiting for a counter to stabilize. A little like tuning a piano with a tuning fork.

73

Well, ok then. That makes snese. I just got got home (a few days early) and I spent a few minutes at the bench. I was looking at the schematic and was wondering why I would not just take the frequency measurement at pin 10 of the PLL. Many radios are done that way. Well, don't you know, the frequency was stable there. Hmm. I know that service docs are not always accurate and sometimes thats because the manufacture makes changes. This radio has the PC-198 board in it and seems to be somewhat accurate compared to the Sams manual. One thing that got me was the alignment docs reference 2 test points (10 & 11) that are not in the layout, schematic or on the board. They were for the 455KHz alignment and the 23.5MHz alignment. Is the 23.5 for the noise blanker?

Thanks again!!

 

[nomadradio]

Is the 23.5 for the noise blanker?

Yep. The blanker is fed directly from the antenna in this radio, and the assumption is that noise at 23.5 is the same as what's entering the main 27 MHz signal path.

73