THE FISHER MPX-50 SUB-CHASSIS
A big thanks to AKer Vendo81 for sending me the (truly) rarely seen MPX-50 sub-chassis that came with his 100-T tuner/preamp for analysis! This is the unit that was without a doubt specifically designed -- even if it was at the 11th hour -- to be retro-fitted (retro-fitted because the units themselves were never designed for this type of sub-chassis) into the Crosby designed Fisher units that were initially offered with a blank plate, awaiting the final FCC decision as to format and commencement date. In all of my years, this is the only MPX-50 unit I've ever seen even in pics, let alone physically on the bench. They must truly be one of, if not the rarest of Fisher items produced.
Like all of Fisher's MPX sub-chassis units, the MPX-50 also operates on the time division principle. And while the design of the unit is very similar to the more common sub-chassis units, it is also notably different in one important aspect.
At issue is how Fisher engineered the sub-chassis to revert to true mono operation (L+R signals out of both channels of the adapter), since once the adapter was installed, Channel A was then permanently receiving the Channel A output of the adapter for any FM setting of any type on the Input Selector switch. Being able to receive any FM station in mono mode is important both back in the day and today, since all stations did not immediately converted to FM Stereo MPX operation once the decision was handed down, and being able to listen to noisy MPX broadcasts in mono is desirable when the noise becomes excessive. As well, there are still a number of news/sports/talk stations today that broadcast in mono to increase their reach.
Allowing FM mono stations to play through the typical Fisher MPX sub-chassis allows the ring demodulators to try and work backwards and become modulators during that time, which produces distortion in the output. The answer Fisher adopted for this is to select the adaptor for stereo stations only, while selecting the output of the mono de-emphasis network that is sourced right off the ratio detector transformer to listen to mono stations. This is how all Fisher Wide-Band units are designed. Sometimes the switching is done automatically, sometimes it must be done manually, but the capability is there for that reason. In the Crosby based tuner/preamps however, there's no automatic function, and the selector switch doesn't allow for such switching, so the adapter itself much be electronically switched between stereo and mono operation -- and done so with no more than the grounding (or not) of a single switch terminal on the Input Selector switch, based on its setting. Since I indicated that there was just no good way to do that with the typical Fisher sub-chassis, it was of interest to find out just how in fact they dealt with this issue. If there was some easy way to modify the typical adapters to achieve the required switching that I wasn't aware of, without the need for (at least) an audio control relay, I was certainly interested in finding out what it was.
First up then, are the similarities between the MPX-50, and the other Fisher sub-chassis units:
1.The MPX-50 employs two 12AT7 and one 12AX7 tubes, just like the WX sub-chassis does.
2. The MPX-50 employs the same 19 kHz and 38 kHz transformers as to Fisher's other sub-chassis units.
3. The tubes in the MPX-50 all perform the same basic functions as they do in the other sub-chassis units, with only very minor alterations in some of the parts values associated with the tubes.
4. The alignment instructions would be the same between the MPX-50, and the other Fisher sub-chassis units.
5. The MPX-50 is the same physical size and structure as the other sub-chassis units.
So what are the differences?
1. There is no indicator output on the MPX-50, which is as would be expected, since there is no provisions for a stereo indicator on either of the stereo tuner/preamp offerings.
2. The MPX-50 requires only one source of B+, about 185 vdc. The other source of the commonly seen dual B+ requirement for the other sub-chassis units is not used in this unit.
3. The de-emphasis tailoring is split up into two separate parts of the signal path, with one network before the matrix output stage, and the customary networks similar to those seen after the matrix stage in the other sub-chassis units.
4. The big one is -- relative to the question of this discussion -- that the beauty and symmetry of the dual balanced 38 kHz ring demodulators -- as used in Fisher's other sub-chassis units -- is scrapped for two single diodes -- one to demodulate the L-R, and the other the -L+R signals accordingly. Additionally, since no coupling caps are used coming of off the 38 kHz oscillator, these two diodes (after appropriate isolating resistors) see B+ voltage levels applied to their anodes, and therefore have their cathodes returned to B+ as well for AC ground, rather than actual ground as is typical of the other sub-chassis units. The result of this configuration is that if a 470K resistor is connected between the output side of the 1 uF composite signal coupling cap and ground, then the two diodes become back biased and cease to conduct. Without any conduction there can be no demodulation of any 38 kHz signals present, meaning that the outputs will only contain L+R information or FM mono signals, regardless of the type of broadcast received. And because the voltage source is great enough that produces the back bias for the diodes, the L+R audio signal that is present is not strong enough to cause the demodulators to attempt modulation, so the mono audio output is clean. It's definitely a neat little solution -- but of course one that required a complete redesign of the demodulator section.
So which approach is best? I plan to power up this unit to give Vendo a status report on it so he'll know how to proceed with it when restoration time comes. Typically however, the best separation numbers that most dual diode designs can approach is about 30 db, which is short of what the elegant balanced approach of the other sub-chassis can produce. To be fair however, whether you can hear that difference or not is certainly debatable. That would suggest that if you have an MPX-50 for a Crosby unit, it should certainly perform well. On the other hand, if you don't have an MPX-50, or you want the best possible performance, then it's doubtful that an MPX-50 -- even if you could find one -- will match the excellent performance that the WX chassis can produce in a tuner setting.
That leaves two last points to touch on:
1. The design of the MPX-50 is so similar to the other sub-chassis units when it comes to the design's frequency and phase response, that no doubt an MPX-50 would also benefit from the installation of the later ratio detector transformer. However, this has not been tested. I do not have any units here using the older RD transformer, but the MPX-50 can certainly be tested on a unit that has the newer transformer installed, and therefore compared to the performance of a WX sub-chassis that way. I hope to get to that in the next few days.
2. Fisher clearly decided to punt on the idea of modifying the eye tube circuit so that it could also act as a stereo indicator, so the action of the eye tube does not change in either of these units when a MPX-50 is installed.
More as the info is generated. No pics this time, since Vendo has already provided pics of the unit he sent.
Dave
A big thanks to AKer Vendo81 for sending me the (truly) rarely seen MPX-50 sub-chassis that came with his 100-T tuner/preamp for analysis! This is the unit that was without a doubt specifically designed -- even if it was at the 11th hour -- to be retro-fitted (retro-fitted because the units themselves were never designed for this type of sub-chassis) into the Crosby designed Fisher units that were initially offered with a blank plate, awaiting the final FCC decision as to format and commencement date. In all of my years, this is the only MPX-50 unit I've ever seen even in pics, let alone physically on the bench. They must truly be one of, if not the rarest of Fisher items produced.
Like all of Fisher's MPX sub-chassis units, the MPX-50 also operates on the time division principle. And while the design of the unit is very similar to the more common sub-chassis units, it is also notably different in one important aspect.
At issue is how Fisher engineered the sub-chassis to revert to true mono operation (L+R signals out of both channels of the adapter), since once the adapter was installed, Channel A was then permanently receiving the Channel A output of the adapter for any FM setting of any type on the Input Selector switch. Being able to receive any FM station in mono mode is important both back in the day and today, since all stations did not immediately converted to FM Stereo MPX operation once the decision was handed down, and being able to listen to noisy MPX broadcasts in mono is desirable when the noise becomes excessive. As well, there are still a number of news/sports/talk stations today that broadcast in mono to increase their reach.
Allowing FM mono stations to play through the typical Fisher MPX sub-chassis allows the ring demodulators to try and work backwards and become modulators during that time, which produces distortion in the output. The answer Fisher adopted for this is to select the adaptor for stereo stations only, while selecting the output of the mono de-emphasis network that is sourced right off the ratio detector transformer to listen to mono stations. This is how all Fisher Wide-Band units are designed. Sometimes the switching is done automatically, sometimes it must be done manually, but the capability is there for that reason. In the Crosby based tuner/preamps however, there's no automatic function, and the selector switch doesn't allow for such switching, so the adapter itself much be electronically switched between stereo and mono operation -- and done so with no more than the grounding (or not) of a single switch terminal on the Input Selector switch, based on its setting. Since I indicated that there was just no good way to do that with the typical Fisher sub-chassis, it was of interest to find out just how in fact they dealt with this issue. If there was some easy way to modify the typical adapters to achieve the required switching that I wasn't aware of, without the need for (at least) an audio control relay, I was certainly interested in finding out what it was.
First up then, are the similarities between the MPX-50, and the other Fisher sub-chassis units:
1.The MPX-50 employs two 12AT7 and one 12AX7 tubes, just like the WX sub-chassis does.
2. The MPX-50 employs the same 19 kHz and 38 kHz transformers as to Fisher's other sub-chassis units.
3. The tubes in the MPX-50 all perform the same basic functions as they do in the other sub-chassis units, with only very minor alterations in some of the parts values associated with the tubes.
4. The alignment instructions would be the same between the MPX-50, and the other Fisher sub-chassis units.
5. The MPX-50 is the same physical size and structure as the other sub-chassis units.
So what are the differences?
1. There is no indicator output on the MPX-50, which is as would be expected, since there is no provisions for a stereo indicator on either of the stereo tuner/preamp offerings.
2. The MPX-50 requires only one source of B+, about 185 vdc. The other source of the commonly seen dual B+ requirement for the other sub-chassis units is not used in this unit.
3. The de-emphasis tailoring is split up into two separate parts of the signal path, with one network before the matrix output stage, and the customary networks similar to those seen after the matrix stage in the other sub-chassis units.
4. The big one is -- relative to the question of this discussion -- that the beauty and symmetry of the dual balanced 38 kHz ring demodulators -- as used in Fisher's other sub-chassis units -- is scrapped for two single diodes -- one to demodulate the L-R, and the other the -L+R signals accordingly. Additionally, since no coupling caps are used coming of off the 38 kHz oscillator, these two diodes (after appropriate isolating resistors) see B+ voltage levels applied to their anodes, and therefore have their cathodes returned to B+ as well for AC ground, rather than actual ground as is typical of the other sub-chassis units. The result of this configuration is that if a 470K resistor is connected between the output side of the 1 uF composite signal coupling cap and ground, then the two diodes become back biased and cease to conduct. Without any conduction there can be no demodulation of any 38 kHz signals present, meaning that the outputs will only contain L+R information or FM mono signals, regardless of the type of broadcast received. And because the voltage source is great enough that produces the back bias for the diodes, the L+R audio signal that is present is not strong enough to cause the demodulators to attempt modulation, so the mono audio output is clean. It's definitely a neat little solution -- but of course one that required a complete redesign of the demodulator section.
So which approach is best? I plan to power up this unit to give Vendo a status report on it so he'll know how to proceed with it when restoration time comes. Typically however, the best separation numbers that most dual diode designs can approach is about 30 db, which is short of what the elegant balanced approach of the other sub-chassis can produce. To be fair however, whether you can hear that difference or not is certainly debatable. That would suggest that if you have an MPX-50 for a Crosby unit, it should certainly perform well. On the other hand, if you don't have an MPX-50, or you want the best possible performance, then it's doubtful that an MPX-50 -- even if you could find one -- will match the excellent performance that the WX chassis can produce in a tuner setting.
That leaves two last points to touch on:
1. The design of the MPX-50 is so similar to the other sub-chassis units when it comes to the design's frequency and phase response, that no doubt an MPX-50 would also benefit from the installation of the later ratio detector transformer. However, this has not been tested. I do not have any units here using the older RD transformer, but the MPX-50 can certainly be tested on a unit that has the newer transformer installed, and therefore compared to the performance of a WX sub-chassis that way. I hope to get to that in the next few days.
2. Fisher clearly decided to punt on the idea of modifying the eye tube circuit so that it could also act as a stereo indicator, so the action of the eye tube does not change in either of these units when a MPX-50 is installed.
More as the info is generated. No pics this time, since Vendo has already provided pics of the unit he sent.
Dave
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