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下面是引用 花祭 于 2005-07-12 12:57 发表的:
谢谢spwan兄指点! [s:21]
可惜俺英文很烂,不知对它评价如何?
原厂机器和L-02T比较，音场扁平，低音紧，久听比较平淡，但没有衰声。声音可以做得更好。
FMTUNERINFO评价KT-7500是摩机的好材料，音频输出部分摩机后声音非常好，更换窄带陶瓷滤波器以后可以在远程接收方面打败很多原厂机器。（*没有仪器和熟练经验本人不主张更换滤波器，音频部分摩机值得一试）
附FMTUNERINFO上KT-7500音频摩机指南：
Kenwood KT-7500/7550
"The KT-7500 and 7550 are particularly suitable for modification because there's easy access above and below the chassis, lots of room inside, a good design, and they can be bought at reasonable cost in the used market. Their weaknesses can be improved as described below, and we hope technically oriented readers will write in with even more suggestions.
(1) The first thing I saw inside that needed changing was 120-volt AC on the same circuit board as the RF circuitry. I moved the fuse to the back panel, lifted ALL AC wires with 120-volt potential off the circuit board, and placed them on standoffs next to the power transformer. On some KT-7500s that I've worked on, the power transformer is switchable between 120 and 240 volts. As far as I know, all KT-7550s came this way. If you have one of these dual-voltage models, several wires for 220-240 volts can be tied together when only 120 volts is required. Tie the yellow and white wires together for the return and tie the red and blue wires together for the hot. This configuration usually leaves the least standing voltage on the chassis. Measure both directions before permanently hooking down your polarized cord.
(2) I added John Camille's "snubber" circuit after the fuse and before the transformer. The snubber circuit consists of two 10-Ohm, 5-watt resistors in series with the AC wires. One resistor is in series with the hot and one with the neutral. On each side of the resistors and strapped between them are two .01 µF 400-volt or higher disc capacitors. Don't use some generic film cap because of its unknown time constant. Next raise the power transformer 1/2 inch off the chassis on INSULATED standoffs so the transformer is not grounded to the chassis. I also replaced the power cord with a heavier gauge polarized cord. I oriented the cord for the least AC leakage, measured from the chassis to ground.
(3) Tracing the signal path shows the audio out op-amps go through a relay, switching, a long signal trace around the circuit board, a parallel run alongside the AC(!), then routed to the RCAs out. By tapping off the negative side of C49 and C50, which are the 47 µF 10V op-amp coupling caps, and running with good wire direct to the fixed output, the sound is infinitely improved. Be sure to cut the old path away at the fixed output but not the variable. You lose muting and AM but keep them on the variable out - it's a small price to pay for better sound. I now mute by turning my volume down. With a low DC offset op-amp like the OPA2604, you can bypass these coupling caps. More on that below.
(4) Remove the old audio op-amp at IC6, NJM45580, install an eight-legged socket. I now use an op-amp socket with long legs. Under the circuit board these longer legs can be used to tie the new de-coupling caps very close to the op-amp's negative and positive supply points. These are not part of the original circuit. Using good caps for decoupling at V- and V+ to ground will yield improvements. I now use two 470 µF caps for decoupling. Things are made easier if you unsolder standoffs marked 27, 28 and 30. Standoff 30 is a ground point and a good place to ground the - and + decoupling caps now used at the op-amp. Remember your polarization negative and positive directions when wiring these two caps. Standoffs 27 and 30 are a good point to tap the stereo signal to wire to the RCA output jacks. I add two resistors here before going to the RCA jacks. 100 to 1000 Ohm should be fine. Using exotic wire here is fine but can be a pain tapping into the old RCA outs. One simple solution is to reuse the yellow and blue wires already wired to the output jacks. Cut these as close as possible at the point they leave the circuit board, points marked J11 Y and J10 B, pull these two wires out of the wiring harness and tie them to the resistors you've tied to standoffs 27 and 28 (cut to length). Now you can plug in and play your favorite audio op-amp. My favorite is the BB OPA2604.
(5) You can go a step further and run the op-amp in class A, learned here: http://www.audioasylum.com/audio/tweaks/messages/31076.html. I have this configuration running in class A in my KT-7550 and it sounds great! In the beginning, I used polypropylene caps along the circuit path but they are a major pain to work with. I have since experimented with Black Gates, Sanyo and Nichicon high-quality electrolytic caps with copper leads, all nice-sounding. I've also been able to pull out deeper bass from the KT-7500 with the following changes in cap values along the audio path. Watch your polarities.
(1) C31 - from 47 µF 10V to 100 µF 16V Black Gate
(2) C33 - from 4.7 µF 25V to 22 µF 16V Black Gate
(3) C41 - from 4.7 µF 25V to 22 µF 16V Black Gate
(4) C42 - from 4.7 µF 25V to 22 µF 16V Black Gate
(5) C45 - from .15 disc type? to .47 polypropylene
(6) C46 - from .15 disc type? to .47 polypropylene
(6) DXing performance (selectivity) can be improved by replacing the filters at CF5 and CF4, and maybe CF3 and CF2 also, since all 4 of these filters are engaged when the tuner is in Narrow mode. Leave CF1 intact for best sound quality in Wide mode. I recommend low-loss Murata 150 kHz filters for the replacements (used to be Part No. SFE-10.7MJA-10A, but check the cross references on Murata's site for the current part number). Soldering in 3-legged sockets here makes it easy to swap different filters in and out. Go to Bob's Filter Corner for more on filter mods.
(7) On the KT-7500 you can hardwire the 75 microsecond de-emphasis directly on the board with 2 one-inch pieces of wire instead of the switch on the back panel. This eliminates about 20 or more inches of signal travel off the board, through the switch and back to the board. This will be for the U.S. and other areas that require 75 µS instead of 50 µS de-emphasis. For 75 µS add a wire between standoffs 28 and 29 and between standoffs 26 and 27. For 50 µS, no wires are required.
(8) Since my first KT-7500 mod I've tried many experiments. One is adding ferrite beads before and after the power supply. Another is adding damping material around the chassis. And don't forget to tighten those four screws that hold the circuit board to the chassis to insure ground return path integrity. I use a wet sponge to keep excess solder off my iron's tip for best results. Also, I like to use a toothpick to clear the hole after removing any old part and before trying to insert a new part. Heat solder at the hole, push the toothpick in and wait a few seconds for the heated solder to 'dry'. Always be quick in and out when heating things up on the board, especially around the IC's. jim..."
KT-7500 Power Supply Upgrade
Jim offers this simple power supply upgrade for the Kenwood KT-7500, which may work equally well with similar Kenwood models:
"Feel free to disagree with the values I chose. I was being conservative with these changes. Also, choice of brands and types of parts are a matter of personal opinion and can be discussed in our "FMtuners" Yahoo group.
To start, unplug the tuner, remove the top and bottom covers, and stand the tuner on one end.
(1) Remove C78 (220 µF 16V) and C79 (220 µF 25V) and replace with 470 µF 35V to 50V caps. 50V caps won't fit very flat on the board. Be sure to orient the caps for the same polarity as the old ones. Remember we are working with negative and positive voltages in this power supply.
(2) Remove C81 (100 µF 16V) and C82 (100 µF 25V) and replace with 220 µF 35V caps. 50V caps might be a little too tight to fit. Again watch polarity. These two should be marked on the circuit board, probably because they are in the negative voltage section of the power supply.
(3) Remove C80 (1000 µF 25V) and replace with 3300 µF 35-50V caps. A larger cap won't fit under the dial string so buy a cap with - and + leads on the same end of the cap and extend the - lead for the connection below the dial string. Watch your hands and that soldering iron around the dial string! There is some really nasty, sticky glue holding, at least, C78, C79 and C80. Be very careful removing these caps so you don't break anything including traces on the bottom of the board. Did I mention that old glue?
(4) For added reliability and better performance, I now replace all large capacitors around the board including C44 for the signal meter. The total now includes C82, 81, 80, 79, 78, 75, 74, 67, 44, 40, 15 and 14.
(5) I suggest replacing the 4 diodes (D13, 14, 15 and 16) with the Bob-recommended 100V Schottkys. All changes were, IMO, conservative double or triple values, and I changed the bridge diodes just in case the old ones weren't chosen by Kenwood for the higher capacitance charging value of the new caps. These changes gave each tuner a richer, fuller sound from top to bottom.
(6) If you remember section (1) of the first KT-7500/7550 mod (see above), I talked about lowering the stray AC voltage on the chassis. I noticed when working on the third KT-7500 that there is a 2.2 meg Ohm (two point two) resistor designated R1. I must have removed it from my 7550 because it wasn't there. I lifted the end of this resistor closest to the front of the chassis so it stands vertical to the circuit board and hooked the open air end of this resistor to the hot side of AC at a point after the snubber circuit. This was done by extending a small insulated wire to the resistor from the AC. The end of the resistor on the circuit board is ground potential. This changed the voltage potential on the chassis from about 30 volts to 7 volts on the two 7500s and to 4.5 volts on my 7550. With the AC plug polarized you get this 7 volts but about 100 volts potential if the AC cord is plugged in "backwards." I stood the resistor vertical, again in order to keep the 120 volt potential off the main circuit board." |
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狗仔卡
发表于 2005-6-22 13:47
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