Sony Receiver ICF 7600D ICF 7600DS Operating Instructions

							Dietmar Adolf ([email protected])      
DRM Reception With   
SONY ICF-7600D/DS (ICF-2002/2003) 
  
v1.02  2004-02-09  first released version  
v1.10  2004-02-14  external antenna connector, EMI suppression, AC harmonics suppression 
v2.00  2004-02-17  compact version with lowres pics   
This report is intended to help others, but no abso lutely no warranty is granted, if any damages resul t from following these steps. 
  
This popular vintage receiver from 1983 should be m odified to support DRM radio transmissions 
while all other operation should remain unchanged.  
 
  
The receiver has a PLL synthesized tuning basis, bu t largely relies on analog circuitry for AM 
reception, using double conversion with IF1 = 55.84 5 MHz and IF2 = 450 kHz. We want to 
integrate the widely used DRM down mixer (manufactu red and sold by Sat Schneider 
www.sat-
schneider.de). 
  
A first inspection of the receiver reveals the chal lenge: the case is packed, but it is still possible to 
install the DRM mixer circuit, hook up an output ja ck and install an external antenna connector: 
 
 
    
						

							The 12 kHz DRM output signal will be hooked to the “line out” jack socket labelled with a tape 
symbol. The original audio output signal will be di sconnected, as it is not of much use anyway. The 
signal is far below specs for “line out”.    
This is how to proceed:  
   
						

							  
IF Tapping              
 
Looking at the circuit diagram, we identify two pos sible IF tap points for the DRM down mixer 
input:  
a)  Pin 14 of IC1: This is the input pin of the AM I F amplifier. Here, the signal has passed the 
450 kHz ceramic filter CF3, then has been amplified  by transistor Q12 and DC-decoupled 
by C69. As the IF filter is reported to be a CFW450 H, we expect an AM bandwidth of +/- 3 
kHz only. If the specs are strictly met, this will  not be enough for DRM reception. 
 
b)  Between T9 and R68: This is just after the IF LC -oscillator and prior to the 450 kHz ceramic 
filter. SONY uses a very similar circuit setup to t he Sangean ATS-803A, described by David 
M Pratt. We may expect a IF signal with bandwidth o f at least 10..11 kHz 
  
The DRM down mixer gain is adjustable: we should be  able to account for the differring signal 
levels of both options.    
Removing the front cover and the control board,  
 
  
we can inspect the IF components actually installed :     
						

							   
 
    
The ceramic filter SFR450I is different than what w e expected and has a destinctively small 
bandwidth of only +/- 2 kHz and rather steep edges.  While yielding an very good selectivity for AM 
baseband signals, this means, that option a) would  fail for DRM, as we need a bandwidth 8 kHz or 
so. Yet, this also means that we cannot use the pre -amplified IF. 
  
Instead, we need to identify T9 (circled red) and R 68 on the soldering side and connect the down 
mixer IF input to them. The mixer gain needs to be  set to the maximum. 
  
After carefully removing the HF shield, the correct  tap can be identified as the secondary side of 
T9, that is not connected to ground.   
    
						

							 
 
  
As it turns out, the correct pin is about the only  pin of T9, that is accessible without removing the 
HF shield:   
 
      
DC supply  for the DRM mixer 
  
For its AC-Adaptor, SONY used a transformer with a  simple half-bridge and load capacity. Ripples 
with 5 % of the nominal voltage (6 V) remain. Regul ator transistors within the receiver further 
stabilize the DC supply voltage: Q30 supplies 5.2 V  for the AM circuit. The DRM circuit easily draws 
its power from this tap. Also, we can bypass the 78 05 voltage regulator on the mixer board, as the 
receiver ICF-7600D has a maximum voltage 6 V DC and  the SA612A operates at nominal 6 V    
						

							(range 5.4 .. 8 V) and has an integrated voltage regulator. We found that the mixer SA612A still 
operates at around 4 V.    
To achieve DRM reception with low batteries, we  need to tap the mixer supply voltage from 
the emitter of Q30 . The tap is readily accessible: 
 
 
  
The  DRM mixer  board can be placed near the load speaker. To shiel d interferences with the ferrite 
antenna, it was wrapped with paper and  aluminum foil. 
   
 
        
						

							HF Noise Suppression / External Antenna   
HF noise picked up by the cable linking the DRM IF out jack and the sound card impaired the 
overall reception of regular SW signals while the D RM output was connected. 
 
With the DRM down mixer, the 12 kHz DRM output sign al passes a simple band filter favoring the 
range 1..20 kHz and suppressing HF noise. The imped ances of the DRM output (around 2 kOhms) 
and the sound card “line in” (around 5 kOhms) are m atched fair enough not to expect further 
signal degradation.  
By some experimenting, it was found that the built- in telescope antenna was the culprit for 
unwanted EMI/RFI pickup, as it placed very near the  DRM down-mixer and is using an unshielded 
cable.   
Installing an  SMA connector for a 50 Ohm coax connection to an ex ternal antenna really 
made the difference . Disconnecting the internal telescope antenna and  using the newly installed 
shielded link to the antenna dramatically reduced t he EMI/RFI interference. The antenna leads 
were soldered directly into the provided socket spa ce on the antenna board: 
 
 
  
The SMA connector fits perfectly into the gap, when  the receiver case. After these steps, plugging 
or unplugging the DRM output did no longer affect t he normal short-wave reception. 
       
Possible Further Improvement   
Add a ceramic filter CFW450F (+/- 6 kHz) [or CFW450 E (+/- 7.5 kHz)] prior to the DRM mixer input 
for better selectivity.  
prepared „pins“ 
1 (left) : shield  
3: core   
						

							  
Results   - The behavior of the AM IF handling is only slightly  degraded. The AM 2
nd mixer is only 
offset about a few kHz with the new device installe d. This may be corrected by fine tuning. 
 
-  Adjusting the the LC oscillator of the DRM down mix er is critical and needs some very 
careful adjustments.  
-  The total power consumption of the receiver is incr eased by 5 mA to a maximum of 55 mA 
in AM mode (The DRM mixer is powerless in FM mode u sing 45 mA). 
 
-  Typically the DRM signal was only a few dB above no ise level with faint signals. The 
resulting SNR of around or below 10 dB could not be  decoded while the external power 
supplies of the receiver and the notebook used were  connected. 
  
2004-02-07  22:34 UTC  1296 kHz (Orfordness, UK. 70  kW beam to NL (96°)). Receiver in 
Berlin, DE. Built-in ferrite antenna and telescope  antenna. 
 
 The spectrum shows a neighboring AM station (we hav e no 450 kHz IF filter!) below the DRM center frequency. Also, we 
suffer from a lot of noise below 1 kHz due to bad,  noisy SONY DC power supply. 
 
- Cheap power supplies without HF suppression  insert transient distortions into the IF 
signal, that  severely impair the signal decoding , mostly rendering it impossible. The SNR 
indicator will never be stable in this case.   
-  On battery power, even a very faint signal from NRN W at 21.780 MHz transmitted with 10 
kW from Bonaire on the Antilles could be decoded in  Berlin, Germany.