WEAK SIGNAL RADIO TUNING TIPS AND MODS

UPDATED 4-7-2000

Kenwood TS-790 SSB Power Increase

If you look at the specs on your new TS-790, you will notice that the SSB power output is about 10W lower than FM or CW. This is an easy fix.  On the IF board, on the bottom of the radio, near the MIC/RF POWER knobs, there is a small jumper/diode labeled "SSB PWR DOWN".  It is cut from the factory.  Reconnect it.  On the schematic, it is shown as a jumper but it looks like a diode. There was enough metal left on the leads on mine to reconnect it.  The PEP power will now be the same as FM/CW.  The PA for both 432 and 2m on this radio are rated at 60W so it shouldn't be a problem.  I havn't tried to raise the power up to 50W yet as my Commander likes about 45W+ if it is carefully tuned for around 1100W+ out.  With these PA's, 50W should be quite safe on both bands.  This mod should have no affect on 1296 as it is rated the same on SSB and FM.

NEW!!!! 2-15-2003

902/903 Mhz RX, 370 Mhz RX and MARS/CAP Mods for the TS-790A

This mod will open up the 790A's 900Mhz and 300Mhz RX and open up the TX for MARS/CAP use. The 900Mhz mod requires the installation of an external antenna jack on the back of the radio just under the 432 antenna port. There is a small black plug to be removed, just the right size for a BNC chassis connector. From there, a small piece of coax needs to be run from the BNC connector to TP 201 (center) and TP 202 (shield) on the 432 RF board. This is the 900 Mhz input to the radio. Don't close it yet! You will have to align the RX path as it is set up for 870 Mhz from the factory. The instructions are in the service manual, or email me and I will send them. It is very simple. Now on the CONTROL BOARD, the vertical board behind the front panel, there are several diodes labeled D23,24,29 and D30. These should all be cut. Most US models do not have D23 and D24 installed. D30 activates the 900 and 370 Mhz RX. D29 opens up the MARS/CAP TX. All these are accessable by removing only the top cover. To access the 900 Mhz input, you will have to "unfold" the 2m and 432 RF Modules and remove the cover on the 432 unit. This is much easier than it sounds. It is self explanatory when you look at it. You may want to consider installing a 900 Mhz preamp inside as the RX sensitivity is pretty numb....about 1uV for 12db SINAD on FM. The 370 Mhz seems to be fair at about .5uV. The 370Mhz RX uses the 432 antenna jack. Some other notes, the IF SHIFT should be left in the center as it will affect RX frequency in the 900 Mhz band only. I don't understand why, but it does. Also, the freq display on 900 Mhz does not have the SSB filter offset built in so the display will read 1.5Khz low on USB and 1.5Khz high on LSB. My VCO will lock from 300-386Mhz, 830-931Mhz, 420-470 TX&RX (power out above 460 very low), 135-173 TX&RX and 1200-1304RX, 1240-1304TX.

What about this "Active Bias" to improve the linearity of my amp?

Well.....Here's what I discovered about this system. It does indeed improve the linearity of an amp, but, how much and at what expense? Remember that the only "true linear" amp out there is a class A amplifier and about the only service out there that uses them is TV broadcasters and high speed digital services needing precise phase angles in their transmitters. The active bias circuits will rarely increase the output of your amp by more that 0.5db and increase the current consumption by as much as 20% on the 350W TE 2m brick due to the extra bias current. Hardly worth it for an extra 50 watts! The PA in your radio does NOT use this type of bias circuit so why should you put it in the amp that follows it? The nature of a class AB amp is that it has more gain at the low power levels, usually less that 10% rated power, and more efficiency at the high power levels with acceptable linearity for most services. If you get picky and look at the actual waveform coming out of an amp, before the filters, and an amp with the active bias circuit, there is very little difference as the lower half of the waveform is still flat. Just the transistor is still on for that portion of the waveform instead of being off in a standard amp. this may result in a slightly higher second harmonic but will do little to change the "in band" harmonics caused by our voices. The important thing is to make sure that the transistors are turned on during voice highs and lows. If you ever want to test this, turn your bias off and talk thru your amp and watch the harmonics within the band! YARG!!! This is because the transistors are being turned on and off at voice frequencies. If they are turned off and on at RF freqs, the spurs end up at double your RF freqs and higher. These are easier to filter out but do not interfere with signals "in band". All in all, if you want to do it, go ahead, as long as you have the extra power supply and heat dissipation to deal with it. Another danger is that if the regulating transistor ever shorts, it is almost instant death for your amp. With no limiting resistor in the bias, a shorted regulator will dump full current to the base of the transistors and vaporize them almost instantly. To keep your signals clean in band,...WATCH YOUR DRIVE LEVEL!

Better RX sensitivity on the SSB/CW portion of the bands.

Most of todays radios are designed to provide good sensitivity across the entire band they are designed for. For most of us, this is a waste as we mainly use only the lower portion of the bands. By tuning the front end of some of these radios for a peak at the SSB/CW freqs, a very noticeable improvement can be made. Example: I re aligned my Kenwood TS-711A for max sensitivity at 144.2 instead of spreading the receiver preselector over a full 4 mhz as instructed my the manual. This resulted in the radio's ability to recieve a copiable CW signal down to -145 dbm! Not bad for a stock radio with no preamp! It also, however, resulted in a reduction in sensitivity at 148 mhz of about 3-4 db......WHO CARES!!. I proceeded to do the same to my TS-811A for 432.1 and was able to achieve almost the same results.....about -143 dbm was still a copiable CW signal.

8-12-98

Earlier I had said that doing this to the IC-275 could not be done due to it's broadband nature......I WAS WRONG!...After a closer look at the schematic, I noticed that the 275 has two front ends! One is for the wide band freq's and one is for 2m. The wideband frontend comes in below 143mhz and above 148mhz! I then found the 2m section and proceeded to peak it for 144.2....the results were about 3-4 db increase in receive at 144.2! ...so....TUNE AWAY!!

I have done this to many different Kenwoods with very good results.

PASS BAND TUNING FOR THE ICOM 275

8-21-98

While aligning my 275H, I found that the radio has a PBT circuit in it. (Some of the early ones have this already in use instead of the Data Level). It is in the fixed center position and the control was designed to go where the DATA LEVEL control is. On the same board, there are three wires connected to 2, 4.7k resistors. It just so happens that the DATA LEVEL pot is a 10k pot! The 3 wires are for the Data Level and can be removed from the radio. You must then remove the two 4.7k resistors and add 3 jumpers to re rout the appropriate lead to the three connections on the pot. There are four connections that solder this little board to the main front board. The two in the center are GND and +8vdc. The connections on the two outer ends are for PBT and NOTCH. Both must pass thru their own 1k resistor to the center wiper of each pot. The NOTCH should already be set up like this. The two outer connections on the DATA pot must be connected to GND and 8vdc. It doesn't matter which goes where, although when you look at how to install the jumpers, it is easier one way than the other. After you are done with this, you MUST get a filter from Icom that was left out of the radio. It is part # FL-65 and the component # is FI3 on the main board. It costs around $50. There is a Murata replacement avail Part# MURATA CFJ 455 K5 or CFJ 455 K6. The filter goes in to a designated spot at the corner of the main board between IC-3 and IC-7. There is a small resistor (or CAP) there currently to bypass the filter that is missing. Removal of the front panel and circuit board is the hardest part of this mod so be careful! The picture below is what is required to do the mod! If you're not comfortable disassembling a radio this far, have someone help you. I installed mine and it works great!........and yes, that really is a 275H in the picture!

<---This is the Schematic for the DATA LEVEL circuit board. This is for the PBT----->

As you can see in the schematics, there are jumpers to swap the +8V and GND on opposite sides of the PBT POT so the direction of "UP and DOWN" are correct.

If you still wish to use the DATA input, you can connect AMO2 and AMO3 together and externally control level.

SPEECH PROCESSORS AND AUDIO LEVELS

This is a touchy subject. Many people sware by them......I despise them! Here's why.

Speech processors are supposed to increase your average power.....They do, but at the expense of clarity and dynamic range. Yes, they will make your power meter read higher , BUT, they will NOT increase your PEP power! This is what determines your readibility at the other end. The only way to increase your average power is to reduce your dynamic range...in other words, you must reduce your voice peaks and increase your vioce lows. It is the highs and lows that makes our voices understandable. Without any dynamic range, our voices would be nothing more than a series of changing tones......like CW! The last thing I want when there is a weak station trying to copy me is for my voice to be "compressed". How many times have you heard....."I can tell you're in there but I can't understand you". I have been on the weak end of several people in the same area running equivelant power and the hardest ones to copy are the ones that are running too much mic gain or running processors. "But they sound louder".......Well, when they are close to you with half their power output goung to background noise, It will sound louder because you can hear the fan in the refrigerator next door running just as strong as the operator......but believe me, they are not getting any more power out because of it. A lot of the reason these operators are getting further contacts is because they are running a lot of power to make up for their lack of clarity! The main point is to make sure that your rig is reaching it's maximum potential on voice peaks...regardless what your power meter says! I have yet to see a so-called "Peak Reading" wattmeter that gives an accurate representation of PEP power. I use a simple detector sampled off the output connected to a scope, turn the power to max in either CW or FM, read the wattmeter and mark the spot on the scope, then go to SSB. You will be surprised to find that most radios peak at about 20% higher than rated output. If you are not reaching rated output, THEN increase your mic gain until you do. If your rig has an ALC meter, check to see that it just begins to move when the output is just at rated power output. With a scope, you can also watch to see if you are hitting the compression point of the final amp in your rig. It will show up on a scope as a "flat top" on voice peaks. This means you are driving it too hard and will cause extreme splatter if you have an amp connected. Most people tend to blame this splatter on the amp when in fact many times it is the radio being driven too hard! I try to set my mic gain so I can easily achieve full ALC deflection or full output on voice peaks when I am talking in a loud, contesting voice and the mic is next to my mouth, and also so my power meter will just barely move with the same voice and the mic at arms length away.....I don't need anyone to hear my neighbor's refrigerator!

I know some may disagree with my views on processors and probably have many stories to try to prove their side and justify their use, but I have yet to have it proven to me in person...in other words, I have never been able to copy someone with their processor on and not copy them with it off if their mic gain is set properly. I have heard others say they sound louder with it on...but it still doesn't prove anything to me.....I'll leave mine off.

ZERO BEATING

I have heard too many people do this. They are listening to 144.2 and decide to call the station on CW. They switch to CW and all you hear is 'Key Clicks". Remember that if you want to get someone's attention with CW and the guy on the other end is in USB, you must move up about 800 hz for him to hear you! Otherwise, your CW signal is on the same frequency as his BFO and he won't hear you! When your radio is in the CW mode, your receiver is automatically shifted about 800 hz so you can copy a CW station on the same freq. If this doesn't make sense to you, throw away your key!

THE SAME OLD COMMENTS

Some serious, some humorous...you decide.

A speech processor will NOT increase your PEP...... Isn't PEP what SSB is all about?

If you want a stronger signal, bottom line, increase your power, not your audio.

Why give someone a "20 Over" signal report when your preamp is on?....Why is your preamp on?

A preamp in the shack will NOT make up for coax loss!.....Put it at the antenna!

Why do people call a station during a contest that they can't hear just because they heard you work them?

Why do people think they are doing everyone a favor by moving 2khz up from the calling frequency?

Before you accuse a station of being "wide", turn off your noise blanker!

Before you accuse a station of being wide.....get a 4th opinion first!

Why is it that after spending 20 min trying to work a station you can barely copy on CW, conditions peak for 15 seconds and your neighbor works him SSB with 10W and a dipole?

Why is it that 6m always dies shortly before the beginning of a contest and then comes back shortly after the end?

73 de Greg

kj6ko@innercite.com

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