With a little free time between projects, I was finally able to get my own FM-100B out on the bench for the restoration its never had. It's a pretty little thing that I got from Audiodon well over a year ago, but have never had the time to get to.
The 100B has all the guts and glory of the fabled FM-200B, save the latter's Microtune/AFC feature, front panel level control, and unbalanced 72 ohm antenna option. Oh, and two front panel jewels to indicate when the muting and Microtune feature are activated. But beyond that however, the 100B has an identically performing conventional miniature tube version of the 200B's nuvistor populated Golden Cascode front end, a 5 tube IF strip utilizing 2 amplifier stages and 3 limiter stages just as the 200B does, the same MPX sub-chassis, the same muting feature (sans indicator lamp), and in the later 100B's, the same matrix based FM Automatic switching feature. It even shares the same dial glass with the FM-200B.
Within the Fisher line then, the FM-100B is the Buick, the 200B the Oldsmobile, and the FM-1000, the Cadillac. But when it comes to the basics of receiving the station and decoding FM stereo, the 200B and 1000 can't do it any better than the FM-100B can. They just have more bells and whistles to make for a prettier package. Inside where it counts however, they are all just about the same.
My particular unit had issues from the get-go:
1. The power supply was only producing about 125 vdc for the tuner circuits to operate off of, which was 40 volts low, in spite of a 121 vac line voltage. This was solely due to the selenium rectifier, which was sagging badly. New silicon and 100 ohm dropping resistor resolved that issue. A new AC line bypass cap was also installed as well.
2. The muting feature either didn't work, or was very intermittent. This was primarily due to broken pins on the muting side of the 6DJ8 tube socket that serves both the muting and auto-stereo functions. New pins to replace the broken ones in the existing socket resolved that issue. But there was still occasionally intermittent operation that was not due to the tube socket. This final issue was traced to a lose connection from the factory. It was one of those connections that appeared OK, but when you looked underneath the connection, no solder ever made it to the connection that was lose.
3. The decoupling caps in the matrix switching circuit were shot, making for a very lazy switching between mono and stereo functions. The electrolytic cap associated with the ratio detector was also physically leaking, so all of these components got replaced.
4. The MPX sub-chassis would trigger the stereo lamp, but that's about all it would do. Replacing the usual suspects on that chassis resolved much of that issue. But the worbling and general lack of sensitivity indicated a full alignment was needed. A couple of new tubes and the alignment returned a red hot 1.25 uV for 20 db of quieting at 98 mHz, and 38 db of stereo channel separation. That's about as good as any premium Fisher tuner can get.
5. De-emphasis. This topic has had its fair share of discussion over time, with various reasons offered for the apparent discrepancy of the networks installed, versus the standard for de-emphasis (75 ms).
On the stereo channels, Fisher's original value for R222 and R223 in the MPX section was 22K, which was used in both the FM-100B and 200B. This value provides a time constant of 48 ms within the networks of R222/C225 and R223/C226, which would normally cause an overly bright presentation. But it must be remembered that the 38 kHz sub-carrier filter augments these networks, as does the chassis wiring in conjunction with R224 and R225.
The best way to find out just how effective these networks are, is to simply measure the actual frequency response produced by these networks, without any pre-emphasis added at the transmitter. From a theoretical standpoint, the corner frequency of the ideal 75 ms de-emphasis network is 2122 Hz. Since simple de-emphasis networks produce an attenuation of 6 db per octave, that implies that accurate de-emphasis should produce a response that is down 18 db at 16,976 Hz -- or for all practical purposes, 17 kHz. In measuring the response of my 100B, the left and right channels measured down 21 and 22 db respectively at 17 kHz, which frankly isn't to bad, but does establish that some additional roll off is present.
In researching how others have addressed this, I've found everything from a famous Fisher restoration name recommending to remove the 38 kHz sub-carrier filters and reduce R222/R223 to 16 k (which is simply NUTS), to adding an active de-emphasis section, which is overkill.
The most rational modification I found was from our own Drew Bolce who recommended simply replacing R222/R223 with a value of 18K. This seems quite rational, moving the response in the direction needed, without unduly introducing anymore 38 kHz hash, which the de-emphasis network also helps to remove. In addition to his recommendation, it would also seem reasonable to install high accuracy components at C225/C226 to ensure matched de-emphasis in both channels. I plan to install 18K resistors at R222/R223, and retest the stereo frequency response.
Fisher played with these networks, moving R222/R223 to 27K in their receivers and consoles, which acts to further roll off response over that created by their original value of 22K in that position. This certainly helps to present a smooth FM stereo presentation, which may have been a marketing driven decision. Such a move however required the monophonic de-emphasis network to be adjusted as well, which usually ended up being a 47K/1800 pF network, producing a time constant of 85 ms, and notable roll off as well. Against this, Drew recommends lowering the 47K resistor to 39K, producing a time constant of 70 ms, with circuit wiring adding the remaining roll off. This too is a very reasonable value to use on the mono side if the the stereo networks are modified as he suggests.
In the 100B however, Fisher went temporarily off the reservation with the mono network using a 47K/2200 pF combination, producing a time constant of over 103 ms. In the 200B, they completely lost it with a mono network of 47K/2700 pF for 127 ms! I have no idea what they were thinking.
In any event, I have modified the mono network in my FM-100B by changing the 2200 pF cap to 1500 pF. In conjunction with the original 47K resistor, this produces a time constant of just over 70 ms, allowing for wiring capacitance to take care of the remaining roll off.
With this restoration work, my FM-100B can now hold its own just fine regarding it's reception and stereo decoding performance against the best units Fisher offered. It really is the best FM bang for your Fisher buck!
Pics include:
1. Underside after the work discussed was performed. The original can cap remains at this point, measuring fine with no leakage (current draw), and running cool.
2. Old parts removed.
3. Understated Fisher elegance. It's a sharp performer now!
With all the performance that the 100B brings to the table, I really couldn't want for anything more.
Dave
The 100B has all the guts and glory of the fabled FM-200B, save the latter's Microtune/AFC feature, front panel level control, and unbalanced 72 ohm antenna option. Oh, and two front panel jewels to indicate when the muting and Microtune feature are activated. But beyond that however, the 100B has an identically performing conventional miniature tube version of the 200B's nuvistor populated Golden Cascode front end, a 5 tube IF strip utilizing 2 amplifier stages and 3 limiter stages just as the 200B does, the same MPX sub-chassis, the same muting feature (sans indicator lamp), and in the later 100B's, the same matrix based FM Automatic switching feature. It even shares the same dial glass with the FM-200B.
Within the Fisher line then, the FM-100B is the Buick, the 200B the Oldsmobile, and the FM-1000, the Cadillac. But when it comes to the basics of receiving the station and decoding FM stereo, the 200B and 1000 can't do it any better than the FM-100B can. They just have more bells and whistles to make for a prettier package. Inside where it counts however, they are all just about the same.
My particular unit had issues from the get-go:
1. The power supply was only producing about 125 vdc for the tuner circuits to operate off of, which was 40 volts low, in spite of a 121 vac line voltage. This was solely due to the selenium rectifier, which was sagging badly. New silicon and 100 ohm dropping resistor resolved that issue. A new AC line bypass cap was also installed as well.
2. The muting feature either didn't work, or was very intermittent. This was primarily due to broken pins on the muting side of the 6DJ8 tube socket that serves both the muting and auto-stereo functions. New pins to replace the broken ones in the existing socket resolved that issue. But there was still occasionally intermittent operation that was not due to the tube socket. This final issue was traced to a lose connection from the factory. It was one of those connections that appeared OK, but when you looked underneath the connection, no solder ever made it to the connection that was lose.
3. The decoupling caps in the matrix switching circuit were shot, making for a very lazy switching between mono and stereo functions. The electrolytic cap associated with the ratio detector was also physically leaking, so all of these components got replaced.
4. The MPX sub-chassis would trigger the stereo lamp, but that's about all it would do. Replacing the usual suspects on that chassis resolved much of that issue. But the worbling and general lack of sensitivity indicated a full alignment was needed. A couple of new tubes and the alignment returned a red hot 1.25 uV for 20 db of quieting at 98 mHz, and 38 db of stereo channel separation. That's about as good as any premium Fisher tuner can get.
5. De-emphasis. This topic has had its fair share of discussion over time, with various reasons offered for the apparent discrepancy of the networks installed, versus the standard for de-emphasis (75 ms).
On the stereo channels, Fisher's original value for R222 and R223 in the MPX section was 22K, which was used in both the FM-100B and 200B. This value provides a time constant of 48 ms within the networks of R222/C225 and R223/C226, which would normally cause an overly bright presentation. But it must be remembered that the 38 kHz sub-carrier filter augments these networks, as does the chassis wiring in conjunction with R224 and R225.
The best way to find out just how effective these networks are, is to simply measure the actual frequency response produced by these networks, without any pre-emphasis added at the transmitter. From a theoretical standpoint, the corner frequency of the ideal 75 ms de-emphasis network is 2122 Hz. Since simple de-emphasis networks produce an attenuation of 6 db per octave, that implies that accurate de-emphasis should produce a response that is down 18 db at 16,976 Hz -- or for all practical purposes, 17 kHz. In measuring the response of my 100B, the left and right channels measured down 21 and 22 db respectively at 17 kHz, which frankly isn't to bad, but does establish that some additional roll off is present.
In researching how others have addressed this, I've found everything from a famous Fisher restoration name recommending to remove the 38 kHz sub-carrier filters and reduce R222/R223 to 16 k (which is simply NUTS), to adding an active de-emphasis section, which is overkill.
The most rational modification I found was from our own Drew Bolce who recommended simply replacing R222/R223 with a value of 18K. This seems quite rational, moving the response in the direction needed, without unduly introducing anymore 38 kHz hash, which the de-emphasis network also helps to remove. In addition to his recommendation, it would also seem reasonable to install high accuracy components at C225/C226 to ensure matched de-emphasis in both channels. I plan to install 18K resistors at R222/R223, and retest the stereo frequency response.
Fisher played with these networks, moving R222/R223 to 27K in their receivers and consoles, which acts to further roll off response over that created by their original value of 22K in that position. This certainly helps to present a smooth FM stereo presentation, which may have been a marketing driven decision. Such a move however required the monophonic de-emphasis network to be adjusted as well, which usually ended up being a 47K/1800 pF network, producing a time constant of 85 ms, and notable roll off as well. Against this, Drew recommends lowering the 47K resistor to 39K, producing a time constant of 70 ms, with circuit wiring adding the remaining roll off. This too is a very reasonable value to use on the mono side if the the stereo networks are modified as he suggests.
In the 100B however, Fisher went temporarily off the reservation with the mono network using a 47K/2200 pF combination, producing a time constant of over 103 ms. In the 200B, they completely lost it with a mono network of 47K/2700 pF for 127 ms! I have no idea what they were thinking.
In any event, I have modified the mono network in my FM-100B by changing the 2200 pF cap to 1500 pF. In conjunction with the original 47K resistor, this produces a time constant of just over 70 ms, allowing for wiring capacitance to take care of the remaining roll off.
With this restoration work, my FM-100B can now hold its own just fine regarding it's reception and stereo decoding performance against the best units Fisher offered. It really is the best FM bang for your Fisher buck!
Pics include:
1. Underside after the work discussed was performed. The original can cap remains at this point, measuring fine with no leakage (current draw), and running cool.
2. Old parts removed.
3. Understated Fisher elegance. It's a sharp performer now!
With all the performance that the 100B brings to the table, I really couldn't want for anything more.
Dave
