Transmit Frequency Potentiometer

[QRP 725]

I have a question about adjusting the transmit frequency for a Cobra 148 and Realistic TRC-451.

My radios were off frequency, so I adjusted the transmit frequency potentiometer in each radio. For example VR3 in the Realistic. I only have a 5 digit frequency counter, and used that while transmitting on AM to get me in the ballpark ( I just ordered a 6 digit counter). I then used a second radio with headphones to listen to myself on LSB while adjusting the transmit freq.pot. to match my voice. My clarifiers are not unlocked.

Am I doing this correctly by just adjusting the one transmit freq. pot? What I did seemed to work for AM and SSB.

 

[Robb]

You will need a 8 place freq counter to align any radio. Should've asked before you purchased.

If you are going to learn to align a radio, you need to follow the Service Manual instructions.

 

[QRP 725]

Robb, I do know that the 8 digit is far better. I did read the Service Manual for each radio. That is how I learned of VR5 in my Cobra 148. If I sound good while on SSB on a second receiver and others say I sound fine, isn't that acceptable? I'm not a tech for sure and never will be. This is just a hobby for me.

 

[Robb]

Sorry; It is not 'far better'. Has nothing to do with that. It is about accuracy and what the Service Manual calls out for accuracy. When the Service Manual states that a freq is to be held to +/-10hz, you simply cannot do that with a 7 place freq counter. Impossible.

I am not a tech either; just a hobbyist. I'd recommend to do it right, or pass it off to a shop and let them do it right. No such thing as 'half way done'. Otherwise, you won't be happy with the results!

 

[M0GVZ]

My radios were off frequency

Based on what? I bet they sure as hell are now. Did you allow them an hour of being turned on in a room at ambient temperature to get up to operating temperature? They don't have a TXCO or OXCO so the frequency will drift as the radio warms up and cools down. If you're "calibrating" them just after turning them on you are likely to find that after an hour or so use they'll be out again.

 

[QRP 725]

Based on what? I bet they sure as hell are now. Did you allow them an hour of being turned on in a room at ambient temperature to get up to operating temperature? They don't have a TXCO or OXCO so the frequency will drift as the radio warms up and cools down. If you're "calibrating" them just after turning them on you are likely to find that after an hour or so use they'll be out again.

On LSB some of the locals told me I was off. Yes I waited an hour for it to warm up. I plan on just sending the radios to a repair shop to have it done properly. I was just hoping what I did would work. There are not any repair shops where I live. I will pack things up and mail them to a shop.

 

[QRP 725]

Sorry; It is not 'far better'. Has nothing to do with that. It is about accuracy and what the Service Manual calls out for accuracy. When the Service Manual states that a freq is to be held to +/-10hz, you simply cannot do that with a 7 place freq counter. Impossible.

I am not a tech either; just a hobbyist. I'd recommend to do it right, or pass it off to a shop and let them do it right. No such thing as 'half way done'. Otherwise, you won't be happy with the results!

I do agree it is best to have it done correctly. I had a feeling the way I was doing was wrong. It was to easy lol! Thank you for your help.

 

[KD8HMO]

This is why I snagged that nice Fluke 1920A. My old Heathkit IM-4100 bit the dust, so i opted for an older lab grade counter.

 

[M0GVZ]

On LSB some of the locals told me I was off. Yes I waited an hour for it to warm up. I plan on just sending the radios to a repair shop to have it done properly. I was just hoping what I did would work. There are not any repair shops where I live. I will pack things up and mail them to a shop.

Never trust reports from other people because there's no guarantee their gear is on the ball either. In the next town the golden screwdriver jockey has "tuned" most of the rigs in the town using a cheap CB frequency meter like yours and they're all off frequency but as far as they're concerned in that town the sun shines out of his arse the local rig doctor can do no wrong and its everyone else who is off frequency.

Nothing wrong with doing it yourself but you need the proper tools for the job. A decent frequency counter and RF signal generator will do the job and they don't need to be the latest and greatest either, kit like KD8MHOs is more than up to the job just as long as you check its accuracy first.

 

[Big Kahuna]

While I agree with everything above - but after your adjustments did everyone say you are now "on frequency"?

If after adjusting you hit the sweet spot and it worked I wouldn't worry about sending them off to have them adjusted again. Next time you want the right equipment but if something is working I wouldn't worry too much.

When adjusting many radios you'll often have test points on the board itself you connect to with leads to make the adjustments - it's not just a "is the frequency counter reading 27.38500" on your coax. You have to adjust one point to xxxx and then the next one etc. Example - http://www.rangerusa.com/align2950.html That's why having a good frequency counter is handy.

And at the end of it then you'll be on frequency in all modes. The first time you do a full alignment of a radio with proper test equipment, it's actually pretty amazing to see the result at the end.