Scott-LT-10-Assembly-Manual 电路图 维修手册.pdf

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1、RadioFans.CN 收音机爱 好者资料库 Important Firsts . BY H. H. SCOTT First high fidelity AM-FM Stereo tuner using wide range AM design. First to successfully use wide-band circuitry in high-fidelity FM tuners. First to market The Stereo-Daptor, a stereo control unit that prevents ob- solescence. First to provi

2、de center channel ouput on Stereo amplifiers for added realism in playback. RadioFans.CN 收音机爱 好者资料库 NOTES ON H. H. SCOTT TUNERS What IS function the of a tuner? A tuner takes a radio signal from the air e.nd converts it to an audio signal of sufficient streng-th to drive a power amplifier. FllONT EN

3、D How is this done? IF1 LIMITER DETECTOR AUDIO The station you want is selected in the front end and amplified by the RF amplifier. At this point, however, the signal is still not strong enough and must be further amplified. It is a principle of engineering that better performance will result if an

4、amplifier is designed to operate over a very narrow band of radio frequencies and does not have to be tuned every time the fre- quency is changed. It is also easier to build an amplifier to handle frequencies lower than the FM band. Because of these facts, the tuner takes the radio signal, no matter

5、 what station it is set to, and converts that signal to a new, lower fre- quency which remains the same regardless of the station you are hearing. This process is called conversion and is handled in the tuners converter. This new fre- quency is called the intermediate frequency, or IF. The amplifier

6、s that are tuned to that IF frequency are called JF amplifiers. Here is how the signal for the radio station to which you are tuned is converted to the inter- mediate frequency: It is a phenomenon of nature that when two frequencies are mixed together, resulu:int fre- quencies are created that are t

7、he difference and sum of the original frequencies. This principle is used jn producing the IF frequency in the tuner. The IF frequency is the difference frequency. What actually happens is that the incoming radio signal is mixed with a signal generated in the tuner itself. The section of the tuner w

8、hich pr0- duces its signal is called the local oscillator, which is designed so that its frequency is always a fixed amount different than the incoming sta- tion. To do this, the frequency of the oscillator is changed every time the station is changed. The difference frequency produced in the mixer

9、-is ln turn amplified in the IF amplifiers. The next step in an FM tuner is called limit- ing. Here, any undesired disturbances which affect the amplitude of the signal are removed in order to make the tuner a true FM tuner. In this way, the tuner will be sensitive only to changes in frequency, not

10、amplitude. Obviously, an AM tun.er does not contain a limiter. After limiting, we are ready to remove the audio signal from its carrier. This is done in the detector. There are two basic types of detectors -Foster-Seeley and the ratio detector. The ratio detector has the added advantage of providing

11、 extra limiting, and it is this type of detector which is used in H. H. Scotts FM tuners. The final stage of the FM tuner is one of equali- zation and amplification of the audio signal. Just as on a record, the transmitted signal is equalized differently from the original signal to improve the signa

12、l-to-noise ratio. Equalization is not necessary in AM tuners, as the signal is trans- mitted flat. One of the most important overall criteria of the tuner is its sensitivity, which is a measure of its ability to receive weak signals. Sensitivity can be expressed as the amount of signal required on t

13、he antenna terminals to give a certain ratio of audio output to background noise. The sensi- tivity of a typical H. H. Scott tuner could be stated as 0.75 microvolts for 20 db of quieting. This means that the background noise wm be at 1 RadioFans.CN 收音机爱 好者资料库 2 least 20 db below the a ulio signal.

14、This definition of sensitivity does not take into consideration the quality of the audio signal. Therefore, it is not a really accurate guide to the performance of a tuner. After all, what is the use of hearing a weak signal when it is so badly distorted that it is virtually unlistenable. The Instit

15、ute of High Fidelity Manufacturers (IHFM) has devised a new method of measuring sensi- tivity whicn is known as Uble Sensitivity. This indicates that the amount of signal required on the antenna terminals t-0 obtain Jess than 3o/o of total hum, noise, and distortion ( -30db). This is a much more sev

16、ere criterion. Measure- ments made by this strict standard assure you that H. H. Scott tuners like the LT-10 are the most sensitive on the market. Now that we have discussed the basic theory of tuner operation, we have to again look at the individual parts of the tuner to see what charac- teristics

17、separate a good tuner from a bad one- in particular, we will discuss H. H. Scott design concepts for each slage of the tuner as compared with conventional tuners. Front End The front end consists of the RF amplifier, local oscillator, and the mixer. It is the part of the tuner that is actua!ly tuned

18、 when you select a station. There are several import.ant reasons why the H. H. Scott front ends give superior results: 1. Use of carefully selected low noise RF tubes. 2. Sliver-plating on the front end and its components reduces circuit losses to a mini- mum and provides for maximum gain. 3. Preali

19、gnment of the front end subassembly which assures optimum uniformity in per- formance from each front end. One of the basic problems inherent in front end design is drift. FM works on such high frequen- cies that even the slightest change of value of one of the electronic components can cause the tu

20、ner to go off station. The conventional way of keeping a tuner on station is to provide an auxiliary circuit called Automatic Frequency Control or AFC. AFC has certain inherent draw- backs: 1. The AFC circuit it.self usually adds distor- tion ui the received signal. 2. AFC attenuates low frequency r

21、esponse. 3. AFC tends to pull to strong stations-this is a disadvantage when you want to receive a weak station next to a strong station. Of course, if you defeat the AFC, you will re- ceive the weak station, but the tuner will drift without the AFC. To overcome these limitations of AFC, the H. H. S

22、cott engineering department designed a completely new front end that did not require AFC. Part of the new design was careful atten- tion to the local oscillator so that as the tuner heated up, changes in the values of the electronic components could be compensated for by special temperature compensa

23、ted components. Another important characteristic of the front end is its ability to reduce interference known as cross-modulation. Cross-modulation simply means that you get a station at many different places on the dial in addition to the one place it should be. This is, of course, an undesirable c

24、haracteristic because strong stations can appear at a point along the dial where a weaker station is desired. In the design of H. H. Scott tuners, this cross-modulation characteristic is practically eliminated by our special design. A finnl point about our front ends is that they are calefully shiel

25、ded on both top and bottom, in conformity with FCC specific.ations, in order that the local oscillator will not interfere with nearby television sets. This shielding also reduces the cross-modulation just discussed. IF Stage (n your LT-10, this includes Tl, T2, T3, the two GA U6s, and associated cir

26、cuitry). The ideal IF amp lifter would: 1. Amplify only the station to which you are tuned and completely reject any other signals that might be passed by the tuners front end to the IF amplifiers: This is referred to as Selec- tivity. Even the best designed front end will sometimes pass stations ot

27、her than the one the tuner is set to, so the IFs must be able to reject the undesired stations. z. It will amplify, without distortion, soft and loud signals. 3. If the station is not exactly tuned in, the IF will still pass the desired station without dis- tortion. The selectivity of a tuner is def

28、ined as its ability to separate stations that a.re very close together. It is determined primarily by the shape of the IF amplifier characteristic. The more nearly perpendicular the IF curve sides, the more perfectly the tuner c.an separate adjacent signals. If the sides are at an angle rather than

29、perpen- dicular, they will not separate nearby stations nearly as well. H. H. Scott has the most selective IF charo.cteristic.s in the entire tuner industry. On H. H. Scott tuners, the desired signal, whether weak or strong, will be amplified with virtually no distortion and nearby stations are virt

30、ually totally discarded. The amount of distortion in receiving a very weak signal is determined primarily by the shape of the top of the IF characteristic. The more rectangular the curve over the bandwidth of the station, the lower the distortion. It is often a trick of less expensive tuner manufact

31、urers to give tremendous amplification to the signal in the IF amplifiers -the result, however, is very high distortion and an IF characteristic that is very pointed. In comparison with competing tuners which often have peaked characteristics, the H. H. Scott tuners have far f11.tter characteristics

32、. The wider IF respmse of H. H. Scott tuners is part of its pioneering wide-band FM circuit de- sign, with all its inherent advantages. Because of the Wide-Band IF characteristics exclusive to H. H. Scott tuners you are able to align your LT-10 using a much simplified Ez-A- Lign system. This would n

33、ot work with conven- tional narrow band tuners. The Limiter (One half of your 6U8 and agsociated circuitry does the limiting in your LT-10) The ideal limiter is able to remove amplitude or height changes in the signal very rapidly. The faster it can do this, the more likelihood that all such variati

34、ons will be removed. H. H. Scott limiters are wide-band . 2 MC. The wider the band the faster the limiting action, and the H. H. Scott tuners have the widest band limiters in use today . therefore, the fastest. The result is superior rejection of any man-made or electrical interference in H. H. Scot

35、t tuners. The Detector (The detector stage of your LT-10 constitutes T4, the two diodes, and associated components.) As stated before, the function of the detector is to remove the audio signal from the radio fre- quency carrier. The perfect detector would not disturb the audio signal in nny way -it

36、 would be removed from the carrier intact. A measure of the ability of the tuners detector to do this perfectly is the frequency bandwidth of the detector -the wider and more linear this bandwidth, the more nearly perfect job it can do. Normal detectors are ma.de with frequency bandwidths ranging fr

37、om 250 to 600 kilocycles. H. H. Scott utilizes a 2 meg- acycle detector, four times the width of most other tuners. H. H. Scott uses ratio detectors which have the advantage of providing additional limiting. The wide-band detector is the heart of H. H. Scotts wide-band design. To a great extent, thi

38、s wide- band detect-Or accounts for the low-distortion per- formance of our tuners on weak signals, the freedom from drift, and the ease of tuning. The H. H. Scott wide-band detector also gives the most favorable capture ratio in the industry today. It is characteristic of FM reception that if there

39、 are two signals on the same frequency, the wenker signal will be completely rejected as long as it is a specified amount weaker than the other signal. This specified amount is known as the capture ratio. Since it is desirable to suppress as much interference as possible, the ideal tuner would have

40、a c.aptu1e ratio such that interference, only slightly weaker than the signal itself, would be completely rejected. The wide-band detector and wide IFs improve the capture ratio of H. H. Scott tuners. After the audio signal has been removed from its carrier in the detector, it is equalized and am- p

41、lified in the audio section of the FM tuner. At this point, the quality of the audio signal in H. H. Scotts tuners is far superior to that of ordinary narrow band tuners. One of the reasons that H. H. Scott tuners offer high sensitivity with low distortion is that the circuit losses on H. H. Scott t

42、uners are less. The front-end losses are less because the front end is silver-plated. You will notice that the rest of the chassis uses copper bonded to aluminum. Copper has the best oonductivity of any met.al except silver. Other tuners usually use cadmium plated steel and cadmium is a very poor co

43、nductor, also it is considerably less expensive. In dealing with RF frequencies, the conductivity of the chassis surface becomes all important. Poor conductors mean high circuit losses. As you can see, H. H. Scott utilizes many excit- ing new principles in our design. We spare no expense in pads or

44、construction. We feel that there can never be any compromise with quality. When you buy an H. H. Scott product we want you to know it is the finesL 3 LT-10 Chassis PreMounted Front End, Lineeord, Power Transformer 1 314-M6 Bottom Cover 1 LT-10-M3 Control Shield 1 RCV-600-P (Pll Potentiometer 1 RCV-5

45、00K-P (P2 Pontiometer I SPR-11 (PS) Power Supply Swiuh l CEC-4x40/260 (Cl) Electrolytic 2 TRV-10.7 IF (T2-T3) IF Tranaforrner 1 TRV-10.7 D (T4) Ratio Detector Transformer 3 AGL-IF Brass IF WI/ =.-. t1 B-J-.J. Connect a 4 1N red wire from pin 5, Cl to pin 6, S6. FIGURE 9 8-3-5. 8-3-6. Connect 41/l bl

46、ack wire from pin 3, V 4 ( 2) to ground lug A on S:3 (ground lug A is the small hole on the side of S3 -notice pictorial above). Push wire through thi:- hole, and bend it down. Connect a 71/4 grey wire from pin 4, V4 to pin 3, V2. Connect one end of a 7 grey wire to pin 1, Pl (S2) and one end o( a 6

47、1/4 brown wire to pin 3, Pl (S2). Twist these two wires together mid connect the other end of the grey wire to pin 5, V3 (S1). Connect the other end of the brown wire to pin 4, V3 (SI). (Check all wire.-, connections, and soldered joints before continuiug. Make sure your layout looks exactly like th

48、e diagram above.) I c:J (QJ c:, i .-,Jra) l!J O 0 0 (Note: There is no card of wires for this page. One end of the colored leads discussed below is already connected to the prea sembled front end. All other connections are made with the short pieces of un- insulated stiff buss wire that are provided

49、. Please observe that the buss wire has not been cut to length. It can easily be cut by superimposing a piece of buss wire over the pictorial diagram. As the diagrams are all full scale, you can trim the buss wire on this basis. Just allow extra on each end for a good mechanical joint. Please note that uninsulated buss wires should be as short as possible. Do not permit them to touch the chassis or other bare wires and pins accidentally, as this will cause b ort circuits. Use the spaghettivhe