PDA

View Full Version : Icom R-75 DRM modification


LeonKoning
29-12-2007, 14:56
http://www.stoepplernet.de/drm/IcomR75DRMmixer.zip

The zip containes 7 jpg's which discribes the ready built mini-mixer from Sat Service Schneider and the build-in in the Icom R-75 receiver

If you need more support then please send a e-mail to
LeonKoning@Hotmail.com

Icom R-75:
Use AM Wide, AGC OFF, ANF OFF, NR OFF
(keep AGC key pressed until it says AGC Off).
Use manual RF gain knob for best signal quality on Input Spectrum scope of DREAM (turn knob slowly).

DREAM:
select "Flip Input Spectrum"

Thanks to Simone

Grtz Leon

jabi
13-01-2008, 10:22
Dear Leon:
N't is possible adquiry the 450 KHz signal from the unused connector Optional Xtal filter BPF?
Salu2, de Jabi, EA2ARU.

LeonKoning
17-01-2008, 08:25
Dear Jabi,

It'doesn't harm to try but i think that the 455kc signal is to low at that point. I didn't try'd this point because i'm planning to buy the filter options in the near future. After the amplifiers Q891, Q912 and Q1031 the signal is stronger and the AGC and Manual RF gain works better at this point.

If you want the Service Manual (pdf) then plz ask me via e-mail.

Grtz, Leon

KD7YUF
06-11-2009, 10:13
I am going to modify an R75 myself but using the ON6MU 455 KHz to 12 KHz down mixer based on the Philips SA602 IC given that I can actually build the thing myself and that the SA602 seems to offer good performance as shown in the audio samples and also the spectrum samples from the Dream software. The optional IF filter connectors are attractive to me as well given that standard solid core hookup wire seems to fit into the socket quite nicely and stays in place. I will of course switch to something else if there turns out to be interference problems caused by the wires acting as antennas. Even if this point for IF extraction does not work its still worth a try and if it does then the R75 might be one of the few receivers that can be modified without any solder points to the radio's main board.

KD7YUF
16-11-2009, 05:11
Sorry to double post but I finally got a down mixer built! It was made around the Philips SA602 mixer/up/down converter IC and I unfortunately have not had a chance to test it because I might have an IF transformer/inductor in the parallel L/C circuit which tunes the SA602 local oscillator for 462 KHz hooked up wrong. I have one capacitor which is .001 pF connected to one leg on one side along with two ceramic capacitors hooked up in parallel to (hopefully) result in a capacitance of .002 pF from pin 2 on the IC to a leg on another side of the inductor and a second leg on the opposite side going to ground. This is my first RF circuit ever and I am just trying to make sure I have this part hooked up correctly. If needed I think I can take a picture of my board layout.

edit: I am in SERIOUS need of help troubleshooting the board shown in the attached schematic. I am not sure exactly what is wrong but the board is getting power and does not seem to work at all. No AF output and it seems to act like a variable attenuator when wired into the R75's optional 455 KHz filter socket. I suspect the 467 KHz local oscillator might be non-functional as well even though everything is wired up correctly and all but one capacitor in the L/C circuit is a fixed one of the correct value.

LeonKoning
02-12-2009, 09:22
Hi KD7YUF

Try changing all C and R vallues as used in my schematic.
One C is missing in your circuit, from one end of the coil to ground.

If you have a signal generator (455kc) then connect it to the input to test the circuit and to allign the coil.

Never the less i believe that the signal at the input is to low. If you can't get it to work then try using the point that i described.

Btw: AGC OFF and use of the manual RF gain knob isn't nessesary if you put in enough signal with the input potmeter (discoverd after writing in 2007).

Regards, Leon

mvs sarma
02-12-2009, 16:24
while other mod may be done as planned, can this circuit be given a try as it has no much complication and not many adjustments like that of the the coil.
http://users.belgacom.net/hamradio/schemas/DRM_455kHz_to_LF_Converter_FRG-100.htm

another circuit element that comes to mind is from elektor. this element is said to be working well.

KD7YUF
06-12-2009, 05:49
I can't build the ON3MU one because it uses a crystal or ceramic resonator that is hard to find in the US. A 465 KHz crystal or resonator is difficult to find so a tuned L/C circuit is my only option unless I can get one from someone in my area as I already have the SA602 IC.

mvs sarma
06-12-2009, 18:31
I can't build the ON3MU one because it uses a crystal or ceramic resonator that is hard to find in the US. A 465 KHz crystal or resonator is difficult to find so a tuned L/C circuit is my only option unless I can get one from someone in my area as I already have the SA602 IC.
Perhaps you may salvage an IF transformer from any cheap Chinese scrapped radios. 470MHz resonators are many times found in IR remotes that could also be salvaged.

KD7YUF
06-12-2009, 18:47
I never thought of remote controls using that type of resonator before but I do have a CB radio that I scrapped recently which has quite a few IF transformers and that is what I am trying now since I determined that a .001 pF ceramic capacitor with a 116 uH inductance will resonate at 467 KHz and the schematic shows a parallel LC circuit for tuning rather than a crystal or resonator. The IF transformer might be wired up wrong so that is a possibility as well but I do have a general wiring diagram for them in an electronics catalog which I can use for help if needed.

KD7YUF
08-12-2009, 22:15
I did some rewiring of the IF transformer and its STILL not working even though the circuit is complete as per the schematic diagram and all the appropriate components are hooked up as shown and grounded off where they should be. I think I am going to go bald over this thing and I am way too young to do so.

mvs sarma
09-12-2009, 03:23
I did some rewiring of the IF transformer and its STILL not working even though the circuit is complete as per the schematic diagram and all the appropriate components are hooked up as shown and grounded off where they should be. I think I am going to go bald over this thing and I am way too young to do so.
Little more patience needed. Perhaps it might be working and you don't have a way to check!!
generally they use the outer 2 pins on the three pins side of the IF transformer. that is the primary winding and it has the necessary capacitor inside the transformer itself. so, mostly external capacitor not needed.

Directly it tunes to 455KHz generally or as specified in your CB radio. Also please check whether you did not use any 10.7 MHz IF coil? as I Googled, I understand AM CB radios use 10.&MHz IF transformers. What we need is 455KHz to be tuned to 470 by us.For this, you need to take the core a turn outside by rotating it anti-clockwise.

KD7YUF
09-12-2009, 05:38
The CB did have a 10.24 MHz stage and filter for it but this IF transformer I am 100% certain is for 455 KHz as it was adjacent to a Murata 455 KHz IF filter and connected directly to it as shown on the track side of the PC board. Maybe I should have learned a bit more about IF transformers before starting this since I have never wired one up before in a circuit but have wired an IC with the same package as the SA602 which was the NE555 and it also has 8 pins with four on each side. The SA602 its self is fine and functional, no power going where its not supposed to or I would have known by now because of the little chip releasing the "magic smoke" which is never good. I also found another issue and this is at the dummies level, there is an open in the circuit to the 455 KHz IF level control pot but it looks like the local oscillator is working now on the board and just needs tuning in order for the thing to come to life.