Modification of Sony ICF-SW77 for DRM Software receiver

Date: 19-11-02

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Table of Contents

1 INTRODUCTION..... 3 1.1 1.3 PURPOSE OF THIS DOCUMENT.... 3 ORGANIZATION OF THIS DOCUMENT.... 3
SYSTEM OVERVIEW.... 3 CHANGE OF 455 KHZ IF FILTER.... 3 3.1 3.2 3.3 SCHEMATIC..... 3 FILTER BANDWIDTH.... 5 IF FILTER TYPES.... 6
CONNECTION OF DRM MINIATURE MIXER UNIT... 6 4.1 4.2 POWER SUPPLY..... 7 IF CONNECTION..... 7

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1 Introduction
1.1 Purpose of this document This document describes how to modify a Sony ICF-SW77 short wave receiver to be able to receive a DRM signal. Therefore the DRM Miniature Mixer Unit from Sat-Service Schneider is used, to mix down the 455 kHz IF of the receiver to a 12 kHz IF. This 12 kHz IF is feed to a PC sound card. Please note that the manufacturer's warranty will be discarded if you do this modification. 1.2 Organization of this document Chapter 1 provides an introduction to the document. Chapter 2 provides an overview of the system. Chapter 3 describes how the IF filter of the ICF-SW77 has to be changed. Chapter 4 describes how the DRM Miniature Mixer Unit has to be connected to the receiver.

2 System overview

The 2. IF (455 kHz) of the ICF-SW77 receiver is feed to the miniature mixer unit and down converted to a 12 kHz IF. This IF signal is then connected to the input of a PC sound card. This PC decodes the DRM signal with a decoder software.

ICF-SW77

455 kHz mixer

12 kHz

3 Change of 455 kHz IF filter
The bandwidth of the ICF-SW77 receiver is too small to receive a DRM signal. Therefore the IF filter has to be changed. 3.1 Schematic The following diagram shows the schematic of the SW 77 IF section.

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wide band filter

narrow band filter

The IF section has two IF filters, a narrow band filter and a wide band filter. One of these filters has to be replaced by a wider filter with appr. 10 kHz bandwidth. The filter W is active if the receiver is in wide mode, the filter N is active if the receiver is in narrow mode.

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3.2 Filter bandwidth The with the receiver supplied wide band IF filter has a bandwidth ob appr. +/- 3 kHz. The bandwidth is shown in following diagram:
The recommended replacement filter is a Murata CFWM455GY, which has the same footprint as the original filter, but a bandwidth of appr. +/- 4,5 kHz. The bandwidth is shown in following diagram:

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3.3 IF Filter types The following Murata filter types are fitting into the IF filter footprint in the SW77 receiver: Part Number CFWM455G CFWM455F CFWM455E 6 dB bandwidth +/- 4.5 kHz +/- 6.0 kHz +/- 7.5 kHz
4 Connection of DRM Miniature Mixer Unit
The IF frequency is 455 kHz in the SW77 receiver. Therefore the mixer unit with a 467,0 kHz oscillator is needed. It is recommended to use the mixer unit with the crystal oscillator.

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4.1 Power supply The mixer unit needs a power supply with 7-20 V DC. Unfortunately the receiver supplies a maximum voltage of 6 V. Test showed that it is possible to use the 6V instead of the 7-20 V to run the mixer unit. An other possibility is to remove the voltage regulator on the mixer unit and replace it with a diode (e.g. 1N4007). The 6V power lines of the mixer unit can be connected to the connector at the right side of the PCB (back view). The +6V line is the most top solder point. The ground can be connected to the last solder point of the connector. It is recommended to use the external power supply and not the batteries when the receiver is used as a DRM front end.

4.2 IF connection The hot line of the IF input should be connected to pin 14 of IC301. The ground connection of the IF input should be connected to pin 12 of IC301.

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Modification of Sony ICF-SW 7600GR for DRM
This receiver (like the old model SW7600G) has a 2nd IF of 455kHz, so the SATSchneider crystal mixer can be used for downconverting the IF to 12 kHz. As there is not enough space inside the receiver, I put the mixer board, together with a 9V battery and a filter CFW455F in an extra case. I tapped the signal before it enters the IF filter on the board (CF201, see picture below) and connected it to the receivers line out jack (for listening to AM you can still use the earphone jack).
Pic 1: schematics showing CF201
Pic 2: showing CF201 on the PCB For the connection use a shielded wire and connect the shield to ground next to the signal. The other end of the wire can be connected to the line out after removing the capacitors C248 and C250.
Pic 3: showing the line out connection
In the extra box put the filter CFW 455F in font of the mixer board. All you would need for this is: 1 filter CFW 455F 1 crystal mixer board (both available from SAT-Service Schneider) a piece of coax cable and a 9V battery Its a simple modification but it has been working great, even with the telescopic antenna, on some transmissions I achieved up to 27 dB SNR and I especially like it for receiving DRM on medium wave with the built-in ferrite antenna. For the old model SW7600G it is nearly the same, except the point for tapping the IF is CF401 instead of CF201, and the capacitors that need to be removed are C 421 and C423.
If you are interested in the complete service manual of the SW7600GR as a pdf file or for further questions, send me an email to: stoeppler@yahoo.com
Simone Stoeppler stoeppler@yahoo.com www.stoepplernet.de/DRM/

Feb 2004