Discussion in 'Tuners' started by bully, May 22, 2003.
How is that working out for you?
About the two Kenwood KT-7500 tuners, as long as I keep them set for narrow bandwidth they both deliver consistent good sound and signal level from the desired weak station. They typically come in with full quieting and a great sound stage. Each example initially had problems. One had a bad RF Amp transistor and the other one had problems with the multi-gang tuning capacitor. Once each problem had been corrected and the IF aligned each unit performed exactly the same on the station I prefer. I used a BF998 DGMOSFET to replace the original shorted 3SK45. I had to use an adapter board to mount the surface mount BF998 device. The BF998 has just slightly lower noise figure, so it equals or betters the OEM device performance. Another person recently informed me that the BF998 is no longer available, so if I had to do the repair again another part search would be required.
I recently restored a Fisher 202-R AM/FM tuner which has 6 IF stages plus the golden cascode RF stage using a 6DJ8 followed by a dual triode 6AQ8. It delivers full quieting on the desired station, but is mono. The detector in the 202-R is a more narrow bandwidth than Fisher's later tube type tuners such as the FM-200-B. It is recommended to change the ratio detector of the 202-R to the later type if going for a WX multiplex decoder to get the best results, however I suspect the wider bandwidth ratio detector is going to allow the alternate channel frequency to edge in and interfere. It does so on other wide bandwidth tube type tuners that I have regardless of brand. The earlier FM tuners and radios had a more narrow IF selectivity and detector selectivity than the later ones made for multiplex reception. The only other Fisher tube type tuner with 6 IF stages was the FM-1000 unit which also had a 4-gang RF front end using a 6DJ8 and two RCA Nuvistor triodes for mixer and oscillator circuits. Unfortunately the FM-1000 is said to not have a very good sound stage and just does not do justice to the sound compared to the Fisher FM-200-B - go figure. I plan to connect a Fisher WX decoder to the 202-R and see how that sounds and performs.
The MPX-65 shown is being modified to be a WX decoder using 12AT7 tubes instead of 12AX7 tubes. The last two images are of my FM-200-B tuner which has an excellent sound stage.
Modern solid state tuners with ceramic IF filters have a definite advantage over tube type tuners for selectivity in difficult signal conditions. The best units can even deal with strong adjacent as well as alternate channel stations. It will be interesting to learn how well a KT-5020 would do with the situation I have. I plan to keep watching for an example to acquire and test it.
Thanks for the info good stuff brother... I also have a Scott LT 110-B tuner that works very well and sounds really good, selectivity as you say can be an issue at times just depends on the station. Since you have one I always wanted to try the Fisher 200B. Can you tell a little about it? Have you ever compared it with other vintage tube tuners?
The FM-200-B is an excellent tuner for the majority of situations. As long as a strong alternate or adjacent channel frequency is not right next to the desired station. It usually brings in stations with full quieting and I have received stations quite well that were 100 to 200 miles away, but that really depends upon atmospheric conditions. The 200-B has 5 IF stages so it delivers full quieting on most all stations that are received unless the signal level falls to about 2.5 or less. Most stations come in at 4 or better on the signal level meter. If local stations do not reach that level or better there is a problem in the front end or the IF strip. The separation of decoded multiplex signals is very good being around 35dB or so. If it is less consistently, there is likely a weak tube in the multiplex decoder circuitry. The mistake most people make is messing with the separation control. Without a proper multiplex stereo generator to inject a left or right channel only signal it is nearly impossible to set the separation control by ear. The sound stage is very clear and 3 dimensional on most decent broadcasts. Keep in mind there are a lot of stations that broadcast signals which are quite distorted and compressed these days.
Fisher's FM-100-B is quite similar to the FM-200-B but has a different RF front end. Its multiplex circuit is identical and it also has a great sound stage. The front end uses the 6DJ8 golden cascode circuit followed by the 6AQ8 dual triode as mixer and oscillator similar to the 202-R and some other Fisher tuners. It has the 5 IF stages like the FM-200-B and also includes the muting circuit but does not have AFC (Fisher's Microtune/Autotune circuitry).
Many people say that the soundstage of the Dynaco/Dynakit FM-3 can't be beat. It has a dynamic presentation of performances that many find to be the most accurate and pleasing to listen to of most any tuner. I have one myself that I put together from a kit in 1964. I always kept a Rotron fan running behind it everytime it was turned on. Those babies get quite hot due to the large number of tubes in such a small package. I can't use it where I am now because of the hot alternate channel station edging into the weak distant station I want. Its station tuning is very stable from cold to hot without any AFC. The front end Oscillator temperature compensation is excellent. The FM-3 RF Amp circuitry is not its strong point. It is not as sensitive as Fisher's better tuners, so it is not good for long distance reception.
I also have a Dynakit FM-5 that I put together from a kit back in the early 1970s. It is an early model that had 7 3-pin ceramic IF filters. Its selectivity is pretty decent but even it does not do well with the situation I deal with here. It seems to go into RF Intermod in its simple JFET front end. It also requires some modifications in the multiplex circuit to get the audio up to par. The coupling caps are electrolytics and they need to be replaced with modern Nichicon Muse or Panasonic high quality audio type electrolytics with some lower value polypropylene caps across them to get the very best audio out of them. The Dynaco AF-6 has a better front end design with better cross-modulation performance than the FM-5 so it may be an acceptable performer for many people. The same comments apply to the AF-6 multiplex circuit as that of the FM-5.
Some older radios perform quite well. I have a Scott Radio Laboratories 800-B that was made in 1946 which performs quite well and is able to reject the strong alternate channel local station next to the desired distant station without interference. It has a separate Pentode RF Amp stage, Mixer and Oscillator stages plus a 4-IF stages and discriminator circuit. E H Scott used high Q FM IF transformers they made themselves. I also had a 1946 Silvertone 8127D model that was made for Sears by Continental Electronics of New York which later went into mostly military radio equipment. It had a 4-gang FM RF tuning capacitor which was not seen again until Fisher introduced their FM-200-B and H-K their Citation III FM tuners. Both the Scott and the Silvertone received the difficult station I deal with here without problems. The Scott was a $1295 dollar radio in 1946 and the Silvertone was a $500 dollar unit. The Scott radio cost as much as a new car in those days so most of them were sold to wealthy people. Even the Silvertone unit was quite expensive in its day and included a wire recorder plus a 79rpm record changer and the AM/SW/FM radio in one cabinet with a programmable GE Telechron clock to turn the unit on and record a radio program while the customer was gone and then shut it off.
Sherwood also made some great FM tube type tuners. The S-3000 series had a good reputation and later versions included multiplex demodulation. Their audio was also reported to be quite good, although I never had a multiplex equipped model.
Sherwood made their own IF transformers for years wound on ceramic coil forms for better quality and higher Q leading to good selectivity.
H H Scott tuners for many years had silver plated FM front end circuitry. This provided the highest Q possible in the RF circuits and was one of their selling points. Some of their multiplex decoder circuits are similar to those of the Fisher products and perform about the same. However. the H H Scott multiplex circuits seem to have their own characteristic sound which some like and some don't.
Thanks for the info on the Fisher's I appreciate it, I may look for one here in the near future just to compare it's sound.
Well sherod I got the tuner and have listened to it for a short time only, it is great on reception drags in all the stations easily. I haven't even moved my antenna at all so far I don't need to, all the stations are pegging the meter!!! Sound quality is very good and crystal clear, I am hear more inner detail than with my tubed tuner. More time is needed but for sure it's a keeper...
You are spot on about the KT 5020 really nice tuner excellent sound and drags in everything!!!
I have 4 stereo tuners at present:
Magnum Dynalab MD 90t
Keep in mind on the tube type tuners that they are all quite old now and can benefit from new parts. One area that is especially important is coupling capacitors in the multiplex and audio path. Replacing ceramic capacitors with polypropylene caps can improve the sound. In the case of Fisher and H H Scott tuners that use electrolytics as a coupling capacitor for the multiplex composite signal, any of that type should be replaced with a new part and have at least a .1uF polypropylene or polyester (Mylar) cap bridged across it for best results. In cases that use germanium diodes in bridge circuits to detect the left and right channels, the diodes should be checked for aging resistance imbalance and replaced as necessary. The manufacturers took steps to try to assure equal conduction of the diodes, but best results are obtained by choosing replacements that have matched forward resistance using ohm meters to test select them. Some friends and I took the time to do this by selecting from a lot of 100 new germanium diodes and/or schottky diodes and were rewarded with better sound.
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