Grundig Yacht Boy 400 Service Manual 2

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Itmust betaken intoaccount thatthesignal levelatthis point isnot affected by the AGC (Automatic GainControl), soitwill notbevery powerful. Weareworking it out bynow but,forthe moment, anaudio amplifier atthe output doesthework. This will be explained later. Thisfrequency isset tobe 455 KHz although itcould beshipped witha centering error.Wehave found itto be asbig as300 Hzinone model. Thiscanbe solved just by adjusting a resonator blue marked in figure 2. Another important factabout thissignal bandwidth isthat itis only limited by the RF and IF1(55.85 MHz)filters, whichisquite wide(a50 KHz BWwasmeasured). The effects ofthis greater bandwidth areanoise increment andtheappearance ofthe image frequency (currenttransmissions areisolated enoughtoavoid thislastone). A simple filterdesign willbepresented soon,forthe moment anunfiltered signalisstill suitable. Thelastfact relating tothis filter isthat itmay have notbeen correctly tunedin the manufacturing process,causinganasymmetric response.Thisdoes nothave a visible effect in the base band AM signal (aside from a Signal to Noise Ratio reduction), but ithas forthe 12KHz DRM output. Thiseffect canbesolved justbytuning acouple of resonators whilemonitoring itin aspectrum analyzer.Thesetwopoints arered marked in the following figure: Figure 2. IF filter tuning. The second step,thedown-conversion toa12 KHz IF,has been solved usingthe 467 KHz mixer usedinthe other receivers modifications presentedinthis site. This output wouldsuffice forthe line input ofthe sound card,butwithout theAGC working an extra powering is needed.

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Inorder toachieve thisextra powering aCEBEK (PM-4)audiolineamplifier has been used. Wehave removed theinternal radiospeaker tomake roomforthis module as shown in figure 3. Theneed forthis amplification inthe last stage, at12 KHz, isdetermined bythe digital natureofthe sound card.Thiscard, aPC standard one,hasanADC afterthe anti-aliasing filter.TheADC ideally shouldbeexcited withanormalized signaltoits dynamic margin,otherwise thequantification errorwillincrease about6dB ofextra noise foreach unused bitofthe digital generated word.Although theS/N inthe 12KHz output maybegood enough, thiseffect couldderive inasmaller S/Ninthe digitalized 12 KHz signal. Experimentally, withouttheBB amplifier andgood RFconditions theprogram synchronisation ispossible andaudio isavailable atthe PC speakers. Theongoing work in filters andAGC use,tonormalize thesignal atthe PCinput, willminimize theRF conditions effect. Results Thecomplete modification withthe455 KHz down-converter andtheaudio amplifier is shown in figure 3. Figure 3. Complete DRM modification to the Grundig YB400 receiver.