Guys:

Who has any of these radios ?.....I ask 'cause I'm working on a project of adding more sensitivity on the AM band with an additional RF power stage built inside the radion with parts that are already in the radio and my project is almost finished and I wonder if there is anybody interested in reading about this.

I would be interested in reading about your endeavor. Go ahead and start a thread here about it.

I second the motion! Way cool idea!

I have an RF-2800 and a 7-2990A. I'm all "ears"

I have an RF-2600 and am all "eyes" to see your changes/additions/modifications.

Dave

Ok here is my report:

After many month of frustracion that my beloved Pana RF-2900 was not up to the level in AM sensitivity of my Panasonic RF-
2200 despite its good looks I decide to do something beyond a change from 6 to 12 inch ferrite antenna so I began studying the AM front RF stage and observed that this same circuit is used in many model like RF-2800, RF-2900, RF-2600 and the GE 7-2990 wich is also made by Panasonic and who knows what something else.

The RF stage on these models is the classical Emiter follower circuit whose sole function is to isolate the antenna from the mixer circuit but it DOESN'T AMPLIFY the signal coming from the antenna.

With minimun new parts I changed the circuit to a real RF amplifier stage, I only needed a new 82 Ohms 1/2 W resistence and an inductance (Metallic RF can ) salvaged from an old 20 Dls radio tape recorder which happen to have an inductance of 140 uH and of course some changes in the conections of some parts.

I provide 3 pictures, the first one is the original circuit taken from the service manual, the second is a simulation of this circuit working with the program Multisim where you can see that the gain from this stage is ZERO and last one is the simulation with the modifications where you now can observe that the stage has a gain up to 40 dB (with peak around 4 Mhz) and more than 20 dB of gain from 500 khz to 30 Mhz. The value of the new inductance affects where the peaks occurs. If you don't use an inductance the stage has gain of about 20 dB flat.

With those changes the RF-2900 now performs noticeable better than the RF-2200. In shortwave it performs like a real champ. To date I have modified a Pana RF-2600 and RF-2900 without any side effect.

If you want to make the changes here's how:

1. Desolder R146 put in its place a NEW resistence of 82 Ohm in series with a NEW inductance of around 100 to 140 uH.
2. Take this R146 and use it to Replace L118 and discard L118.
3. Desolder C168 and solder it in parallel to R146 to bypass it.
4. Desolder C169 in the end that connect the Drain of the FET and solder it in the Source lead of the FET (The other lead that is not the gate).
If you want to change the 2SK104 FET with a new modern one let me suggest the NTE312 which is a good performer, of course if you can suggest me a better one, please do it !

Original circuit taken from the Service Manual



SIMULATION of the original circuit
NOTICE THE ZERO GAIN OUTPUT



SIMULATION of the MODIFIED circuit
NOTICE THE GAIN FROM 500 kHZ TO 30 MHZ

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Very interesting....congratulations! I wonder how the dynamic range of the input stage is affected. Do you notice any overload from strong, local signals?

Dave

Dave Doughty:
There are 8 local AM station near here (less than 8 miles away)with power that varies from 10 Kw to 1 Kw) and 4 of them overload the radio (they are within 1 mile ) but it's not big problem remember the radio has a gain control which can lower the voltaje feeded to RF FET and it can easily remove the overload, besides the overload only occurs when you tune that station 'cause the selectivity is better than before (razor sharp)

Hi.

I am new to this forum and found it by accident when serching for a service manual for my Panasonic DR-28 (European RF2800)
I have been serching for mods for ages ever since I first purchased the radio as dead from ebay.

My mods so far are

1) get the radio working ( this comprised of replacing two resistores and a power swich, power swich was compleatly burned, all other swiches slightly dirty)
2) replaced the blown pilot light with an ultra bright white LED with step down resistor

I like the idea of your mod and want to do it to my DR-28, However I have a question.

Q: You mention a 140uH metalic RF can (I cant find one any where and have no clue how to determing if any in my junk PCB's are 140uH) Can a 100 or 150uH RF choke be used instead.

Also what 2900 service manual are you using my 2900 service manual is compleatly diferent to yours please see your original image and mine




Thanks in advance

nay27uk



Yes you can use 100 or 150 or NO inductance at all, with higher L values you are moving the peak gain toward the low frecuency of the band while lesser values move the peak gain to higher frecuencies. If you opt for NO inductance then the gain is flat over the whole RF frecuency band BUT the gain is much lower.

To measure the value of inductance I use this multimeter:

http://www.tequipment.net/Wavetek27XT.html



My diagram is from RF-2800 which is almost identical only the part numbers change whereas the values remain the same.



Don't forget to change the ferrite rod with a longer one

Any ideas where a longer ferrite rod can be gotten?

I've wondered if there was a mod like this that would work for the RF1170 (PSB, no SW). They show up for cheap quite often, and look nice.

equant

The longest ferrite I've seen is 8 inches and the one in the picture is made of 2 ferrites 6 inches long, they are glued togheter and inside a thin carbox to hold them tight.



Avoid at all costs the RF-1170...it has NO RF Amp...the antenna output goes straight to the mixer and this make it very weak in AM reception.

Thanks My radio is also an DR28 although I can not find a service manual online for a RF2800 or DR28 so had to download the 2900 one so thanks for ansewrinbg my question. So I can use an RF choke instead of a can

Guys:

I know that none of you care about this mod being made to this fine radio but I write this because someday somebody could experiment a little and this could be of any help, so here´s my report to the history.


After many months of having done the mod to my Panasonic RF-2800 radio I´ve been observing that it has become very sensitive radio on all bands (except FM) but also it´s a little less selective than an unmodded one (I´ve got 3 of them) not too much but a little, so I was thinking that Panasonic designed the radio to have a front end with high impedance input that´s the reason to use the FET as a Source Follower which can provide high impedance but not voltage amplification leaving that part to the integrated circuits in there. Now when I changed the role of this FET going from Source(Emitter) Follower to Common Source (Emitter) it now provides voltage amplification at the expense of lower input impedance and this make feel the radio very sensitive but in SW it´s not that peaky as it used to be, so I did made a little change:

I just added a capacitor in serie between the input of the RF transitor and the selector switch which connects the different antennas or tank circuits, the value of the cap was found to be in the range of 100,000 to 50,000 pF. I´ve been testing the radio with this new modification and it seems it is more selective than before the mod and I also notice that it has even more amplification with frecuencies from 550 to 7,000 Khz. I also added a 9.8 Meg resistor between the input gate and earth because the radio was becoming inestable and sometimes the radio went mute for no apparent reason but with this resistor everything is back to normal.

Up to today the radio is extremly sharp with high levels of amplification and I´m talking with signals picked off the whip, with externals antennas it performs like a dream.

Quite the contrary;

I've certainly enjoyed reading your progress, and am doubtless not alone.

fifties

Thank you for your kind words if you or anyone have any question please don´t hesitate to ask.

Meanwhile I think I´m going to experiment with different bandwidth for the SSB frecuencies but I don´t know of any good source for 455 khz crystals if you or anybody know please let me know where to get them.

I certainly enjoy reading anything about transistor radios.... so add me to the list.
Crystal filters? I can only think of ceramic filters for 455 khz. Murata is the defacto standard.

~ Mitch ~

Yes 455 Khz that´s what I´m looking for, but my question was about, Can anybody recommend me some places where to buy them (I need different bandwiths) in small quantities ?

You could try here:
http://www.surplussales.com/filters/filters-1.html
455 Khz ceramic resonators and crystal filters are at the bottom of the page.

~ Mitch ~