ORBAN 8200_Section_5 电路图.pdf

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1、Section 5 Troubleshooting pagecontents 5-2Problems and Possible Causes 5-2RFI, Hum, Clicks, Or Buzzes 5-2Poor Peak Modulation Control 5-2Audible Distortion On-Air 5-3Audible Noise on Air 5-4Whistle on Air, Perhaps Only in Stereo Reception 5-4Shrill, Harsh Sound 5-4Dull Sound 5-5System Will Not Pass

2、Line-Up Tones at 100% Modulation 5-5System Will Not Pass Emergency Broadcast System (“EBS” USA Standard) Tones at the Legally-Required Modulation Level 5-5Interference from Stereo Into Subcarriers 5-619kHz Frequency Out-of-Tolerance 5-6LR (Stereo Difference Channel) Will Not Null With Monophonic Inp

3、ut 5-6General Dissatisfaction With Subjective Sound Quality 5-7Security Pascode Lost (When Unit is Locked Out) 5-8Troubleshooting IC Opamps 5-8Technical Support 5-9Factory Service 5-9Shipping Instructions CAUTION The installation and servicing instructions in this manual are for use by qualified per

4、sonnel only. To avoid electric shock do not perform any servicing other than that contained in the Operating Instructions unless you are qualified to do so. Refer all servicing to qualified service personnel. OPTIMOD-FM 8200TROUBLESHOOTING 5-1 RadioFans.CN 收音机爱 好者资料库 page Problems and Possible Cause

5、s Always verify that the problem is not the source material being fed to the 8200, or in other parts of the system. RFI, Hum, Clicks, Or Buzzes A grounding problem is likely. Review the information on grounding on page 2-19. The 8200 has been designed with very substantial RFI suppression on its ana

6、log and digital input and output ports, and on the AC line input. It will almost always operate adjacent to high-powered transmitters without difficulty. In the most unusual circumstances, it may be necessary to reposition the unit to reduce RF interference, and/or to reposition its input and output

7、 cables to reduce RF pickup on their shields. Particularly if you are using a long run of coaxial cable between the 8200 and the exciter, a ground loop or other problem may inject noise into the exciters composite input particularly if the exciters input is unbalanced. This problem can almost always

8、 be cured by the Orban CIT25 Composite Isolation Transformer (see page 2-18). Poor Peak Modulation Control The 8200 ordinarily controls peak modulation to an accuracy of 1%. This accuracy will be destroyed if the signal path following the 8200 has poor transient response. Almost any link can cause p

9、roblems. Even the FM exciter can have insufficient flatness of response and phase linearity (particularly at low frequencies) to disturb peak levels. Section 1 of this manual contains a complete discussion of the various things that can go wrong. Even if the transmission system is operating properly

10、, the FM modulation monitor or reference receiver can falsely indicate peak program modulation higher than that actually being transmitted if the monitor overshoots at high or low frequencies. Many commercial monitors have this problem, but most of these problem units can be modified to indicate pea

11、k levels accurately. Audible Distortion On-Air Make sure that the problem can be observed on more than one receiver and at several locations. Multipath distortion at the monitoring site can be mistaken for real distortion. Verify that the source material at the 8200s audio inputs is clean. Heavy pro

12、cessing can exaggerate even slightly distorted material, pushing it over the edge into unacceptability. Refer to Orbans publication Audio Quality in the FM Plant (included with your 8200) for hints on how to achieve the cleanest source quality. The subjective adjustments available to the user have e

13、nough range to cause audible distortion at their extreme settings. Advancing the CLIPPING and/or FINAL CLIPPER DRIVE controls too far will inevitably cause distortion. Setting the LESS-MORE control beyond “9” 5-2 TROUBLESHOOTING OPTIMOD-FM 8200 RadioFans.CN 收音机爱 好者资料库 will cause audible distortion o

14、f some program material with all but the Protection Limiter structure. If you are using the 8200s analog inputs, the headroom of the 8200s analog-to-digital (A/D) converter must be correctly matched to the peak audio levels expected in your system (using the “SETUP” screen). If your peak program lev

15、el exceeds the peak level you have specified on setup, the 8200s A/D converter will clip and distort. (See page 2-47). If you are using an external processor ahead of the 8200, be sure that it is not causing problems. For example, if a “stereo enhancer” is used, be sure that it does not significantl

16、y increase the average level of the stereo difference channel (LR). This will almost certainly exaggerate multipath distortion. The Orban 222A Stereo Enhancer is fully compatible with the 8200 and will not cause this problem. Amplitude modulation of the carrier that is synchronous with the program (

17、“synchronous AM”) can cause subtle distortion, and can exaggerate existing multipath distortion. Syn- chronous AM should be better than 35dB below 100% modulation as measured on a synchronous AM detector with standard FM de-emphasis (50s or 75s). The “incidental AM” position on most modulation or st

18、ereo monitors is insufficiently wideband to provide an accurate reading of synchronous AM such metering was designed to indicate non-synchronous AM like hum and noise. Audible Noise on Air (See also RFI, Hums, Clicks, or Buzzes on page 5-2.) Excessive compression will always exaggerate noise in the

19、source material. The 8200 has two systems that fight this problem. The compressor gate freezes the gain of the AGC and compressor systems whenever the input noise drops below a level set by the GATE THRESH- OLD control, preventing noise below this level from being further increased. In the Multi- Ba

20、nd structure, dynamic single-ended noise reduction can be used to reduce the level of the noise below the level at which it appears in the input. Both of these features are explained in Section 3 of this manual. If you are using the 8200s analog input, the overall noise performance of the system is

21、usually limited by the overload-to-noise ratio of the analog-to-digital converter used by the 8200 to digitize the input. (This ratio is slightly better than 90dB.) It is important to correctly specify the A-I CLIP LVL (dBu) in the SYSTEM SETUP screen to optimize the noise performance available from

22、 the analog-to-digital converter. You should specify the INPUT PEAK OVERLOAD as the highest peak level that will be presented to the 8200 under normal operation. If, in an attempt to build in a “safety factor” or increase headroom, you specify a higher level than this, every 1dB of extra headroom th

23、at you gain will be accompanied by a 1dB increase in the 8200s noise floor. The 8200s optional AES/EBU input is capable of receiving words of up to 20 bits. A 20 bit word has a dynamic range of approximately 120dB. The 8200s digital input will thus rarely limit the units noise performance even with

24、very high amounts of compression. OPTIMOD-FM 8200TROUBLESHOOTING 5-3 RadioFans.CN 收音机爱 好者资料库 If a studio-to-transmitter link (STL) is used to pass unprocessed audio to the 8200, the STLs noise level can severely limit the overall noise performance of the system because compres- sion in the 8200 can

25、exaggerate the STL noise. For example, the overload-to-noise ratio of a typical analog microwave STL may only be 7075dB. In this case, it is wise to use the Co-Operator to perform the AGC function prior to the STL transmitter and to control the STLs peak modulation. This will optimize the signal-to-

26、noise ratio of the entire transmis- sion system. Section 1 of this manual has a more detailed discussion. Composite STL systems with marginal paths or co-channel interference can cause noise (hiss) which will be most apparent when listening in stereo. Whistle on Air, Perhaps Only in Stereo Reception

27、 This could be caused by a number of problems, any of which could present a spurious tone (perhaps supersonic) to the input of the stereo encoder. In any such case, the first thing to do is to examine the left and right analog outputs with a spectrum analyzer to see if any spurious tones are visible

28、. If the whistle is 6kHz, there could be a beat between the 8200s 32kHz sampling frequency and the 38kHz stereo subcarrier. In this case, the most probable cause is a failure of the 8200s digital-to-analog converter system (on the Analog I/O Card), including a failure of the reconstruction filters f

29、ollowing the converter. A whistle at another frequency might be associated with power supply oscillation, oscilla- tion of any opamp in the digital-to-analog converter system, or composite STL problems. Shrill, Harsh Sound If you are using the Multi-Band structure, this problem can be caused by exce

30、ssively high settings of the BRILLIANCE and/or PRESENCE controls. If you are driving an external stereo encoder with built-in pre-emphasis, you must set the 8200s output to FLAT on the SYSTEM SETUP screen to prevent double pre-emphasis, which will cause very shrill sound. You will always achieve bet

31、ter peak control by defeating the pre-emphasis and input filters of an external stereo encoder, permitting the 8200 to perform these functions without overshoot. Section 1 of this manual contains a detailed explanation of these, and other, system design considerations. Dull Sound If the Protection o

32、r Two-Band structures are used, dull-sounding source material will sound dull on the air. The Multi-Band structure will automatically re-equalize such dull-sounding source material to make its spectral balance more consistent with other program material. If the 8200s output is set to FLAT on the SYS

33、TEM SETUP screen, there will be no pre-emphasis unless it is supplied elsewhere in the system. This will cause very dull sound. 5-4 TROUBLESHOOTING OPTIMOD-FM 8200 System Will Not Pass Line-Up Tones at 100% Modulation This is normal. Sine waves have a very low peak-to-average ratio by comparison to

34、program material. The processing thus automatically reduces their peak level to bring their average level close to that of program material, promoting a more consistent and well-balanced sound quality. The 8200 can generate its own test tones, and these can be triggered from the 8200s optically-isol

35、ated remote control terminals, or by recalling a tone preset from the front panel, or by a remote computer connected to the RS-232 or RS-422 serial ports. Page 1-20 provides a further explanation. Recall the BY BYPASS preset to pass line-up tones transparently. System Will Not Pass Emergency Broadca

36、st System (“EBS” USA Standard) Tones at the Legally-Required Modulation Level See System Will Not Pass Line-Up Tones at 100% Modulation (directly above) for an explanation. The 8200 itself can generate the standard EBS tones at the correct modulation level. See page 3-64. Interference from Stereo In

37、to Subcarriers A properly-operating 8200 generates an immaculately clean baseband, with program-corre- lated noise below 85dB above 57kHz. If the 8200 and the rest of the transmission system is operating correctly, subcarriers should experience no interference. Interference from the stereo into a su

38、bcarrier is best diagnosed with a spectrum analyzer. First examine the spectrum of the 8200s composite output. If the 8200 is operating properly, program-correlated noise should be below 85dB above 57kHz. Even the slightest amount of composite clipping will degrade this protection dramatically. Such

39、 composite clipping may be intentional (in a composite clipper), or unintentional (you could be over-driving a composite link between the 8200s composite output and the exciters input). If the exciter is non-linear, this can cause crosstalk. In general, a properly-operating exciter should have less

40、than 0.1% THD at high frequencies to achieve correct operation with subcarriers. To prevent truncation of the higher-order Bessel sidebands of the FM modulation, the RF system following the exciter must be wideband (better than 500kHz) and must have symmetrical group delay around the carrier frequen

41、cy. An incorrectly-tuned transmitter can exhibit an asymmetrical passband which will greatly increase crosstalk into subcarriers. Amplitude modulation of the carrier that is synchronous with the program (“synchronous AM”) can cause program-related crosstalk into subcarriers. Synchronous AM should be

42、 better than 35dB below 100% modulation as measured on a synchronous AM detector with standard FM de-emphasis (50s or 75s). OPTIMOD-FM 8200TROUBLESHOOTING 5-5 The subcarrier receiver itself must receive a multipath-free signal, and must have a wide and symmetrical IF passband and a linear, low-disto

43、rtion FM demodulator to prevent program- related crosstalk into subcarriers. 19kHz Frequency Out-of-Tolerance First, verify that a problem really exists by using a second frequency-measuring device and/or verifying the problem with your monitoring service. If the problem is real, you must replace cr

44、ystal Y1 (on Card #2); there is no frequency trim available. LR (Stereo Difference Channel) Will Not Null With Monophonic Input This is often caused by relative phase shifts between the left and right channels prior to the 8200s input. This will cause innocuous linear crosstalk between the stereo ma

45、in and subchannels (and vice versa). Step 5-F on page 4-12 presents a complete discussion. General Dissatisfaction With Subjective Sound Quality The 8200 is a complex processor which can be adjusted for many different tastes. For most users, the gamut offered by the LESS-MORE control is sufficient t

46、o find a satisfactory “sound.” However, some users will not be satisfied until they have accessed the FULL CONTROL screens and have adjusted the subjective setup controls in detail to their satisfaction. Such users must fully understand the material in Section 3 of this manual to achieve the best re

47、sults from this exercise. By comparison to competitive processors, the 8200 offers a uniquely favorable set of trade-offs between loudness, brightness, distortion, and buildup of program density. If your radio station does not seem to be competitive with others in your market, the cause is usually p

48、roblems with the source material, overshoot in the transmission link (including the FM exciter) following the 8200, or an inaccurate modulation monitor that is causing you to under-modulate the carrier. A station may suffer from any combination of these problems, and they can have a remarkable effec

49、t upon the overall competitiveness of a stations sound. Section 1 of this manual provides a thorough discussion of system engineering considera- tions, particularly with regard to minimizing overshoot and noise. Orbans publication Audio Quality in the FM Plant (included with the 8200) provides many suggestions for maximizing source quality. 5-6 TROUBLESHOOTING OPTIMOD-FM 8200 Security Pascode Lost (When Unit is Locked Out) If the unit is locked out, and you do not know the pascode number to access SYSTEM SETUP, you can unlock the unit and reassign your pascode by completing the foll