Receiver alignment question

[Night Ranger]

I have a question for 2600, Sangbagger, and other experienced techs. In the CB service manuals and photofacts it shows instructions on peaking the receiver IF stages for maximum signal. In the case of a dual conversion radio it usually shows injecting a 455 kHz signal for the 2nd IF section, a 10.695, 11.275, or 7.8 MHz signal for the 1st IF section, and a 27 MHz signal for the front end and then peaking each section accordingly. Is there really any advantage over doing it this way versus just peaking the whole receiver chain by injecting a 27 MHz signal at the front end and working your way through the receiver chain? The received 27 MHz signal is going to be converted to the other IF frequencies as it moves down the receiver chain anyway, so what is the advantage of generating the individual IF frequencies for each section?

Night Ranger

[Sandbagger]

Dec 14, 2013 7:56:15 GMT -5 Night Ranger said:

I have a question for 2600, Sangbagger, and other experienced techs. In the CB service manuals and photofacts it shows instructions on peaking the receiver IF stages for maximum signal. In the case of a dual conversion radio it usually shows injecting a 455 kHz signal for the 2nd IF section, a 10.695, 11.275, or 7.8 MHz signal for the 1st IF section, and a 27 MHz signal for the front end and then peaking each section accordingly. Is there really any advantage over doing it this way versus just peaking the whole receiver chain by injecting a 27 MHz signal at the front end and working your way through the receiver chain? The received 27 MHz signal is going to be converted to the other IF frequencies as it moves down the receiver chain anyway, so what is the advantage of generating the individual IF frequencies for each section?

Night Ranger

The best I can figure is that you tune each IF stage for the exact frequency that it's supposed to tune to. But if the L.O. is a little off frequency, this will show up when you put the "real" 27 mhz freq in later. Then you can correct the L.O. frequency errors and everything SHOULD line up fine. In reality though, if your IF peaks at 456 Khz instead of 455, it's not that big of a deal.

In actuality though, IF cans should not simply be peaked for strongest carrier signal, they should be swept for a flat response across the whole 4-6 Khz of bandwidth, so that the fully modulated signal passes through undistorted. But most techs do not have a sweep setup, so we simply tune for max level with modulation and look at the sine wave out of the detector for minimum distortion. I usually use 90% modulation, as any distortion tends to show up more readily with higher modulation than it does at the 30% level that is commonly used.

[Timekeeper]

I assume you refer to the HR-2600? if so, there is only one i.f. frequency@10.695 MHz. , it's single conversion for AM and SSB, but dual conversion in the FM mode. I strongly advise that you do not peak the receive as you've described. You'll ruin the bandwidth of the radio, and you'll also increase front end overload, as well as compromise one of the best NB's ever.

But, if you insist, just follow the steps given in the service manual, and if you don't own a signal generator, then leave it to someone who does. He can do as described by Sandbagger, or he could use an FM generator and a SINADER. Even though FM isn't probably your preferred mode, the earlier single conversion stages will be likewise tuned for lowest noise and distortion with highest signal.

The truth about front end alignment is that tuning for the highest S meter readings is only going to add noise, not readable signal,
while at the same time causing a host of other receive issues, including front end overload, narrow band coverage, and a poor NB. That in itself is a deal breaker for me.

LEAVE the tuned 3 IF can assemblies alone! Those are pretuned when they are made, and anything you do will not improve the radio. They are there to clean up the receive. You CAN tune the preceding coil, but you'll only be peaking it for the specific band of frequencies you have your generator set for. IOW's you'll peak the 40 channel CB band, but in the process, detune higher and lower frequencies. Then why use a wide band radio at all? Lots of more narrow bandwidth radios will slightly out perform a wide bander, on a specific band. Not by much, and not necessarily in a more usable way.
The way true amateur radio's do this is by internally switching to a different tuned IF chain for each band, although the super compact rigs will often use only a few, opting for wide band coverage out those, to save space and cost.

HR-2600's are getting a little long in the tooth, so also have your tech. check for leaking capacitors, and check the crystal or two in there to make sure they havn't drifted out of spec. I replaced the caps. and ordered crystals online, and then re tuned my 2600. Wow! What an improvement in stability. It LOCKED on frequency, and stays there. SSB was much better even on transmit I was told.

Summarizing - Low noise, but the ability to hear and copy speech in all modes is the ticket, not noise, QRM, and a higher S meter reading on your neighbor. If 20 over S-9 isn't good enough for him, tell him to get a signal.

[Night Ranger]

Jan 10, 2014 18:45:21 GMT -5 Timekeeper said:

I assume you refer to the HR-2600?

2600 is the name of a knowledgeable user that frequents this board.

The Cobra 148 GTL re-issue I am working is deaf right out of the box. I have probably 50 radios and they all blow this radio away on receive. The radio was a half kHz off frequency on transmit right out of the box as well. So far re-aligning the receive only made a minor difference. When I get back to the radio I'm going to temporarily disconnect the AGC to see if the sensitivity comes up as well as check the front end overload diodes. From there I'll just have to walk through each section until I find the problem.

If you search Google you will find other users complaining about the new Cobra 148GTL radios lacking in sensitivity. Cobra has already had problems with counterfeit transistors showing up in the doomed Cobra 200, so I would not be surprised if I find counterfeit transistors in the new China made Cobra 148GTL as well. One user claimed his receive came up after he replaced the counterfeit parts with the genuine transistors.

As far as widebanding, I don't care about that as I have no plans to install extra channels. If I want to hit the HF band I'll just use one of my HF rigs.

Night Ranger

[BBB]

My Uniden 980 SSB says Made In Vietnam. I'll have to peek in there but I'm assuming there's China parts in there.