Odds are that Fisher also used silicon devices in this location as well --
Dave
Dave
500C Acquisition
- Thread starter audmod01
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audmod01
Super Member
I think I will take this unit's transformer out and remove the cover just to find out just what they really used. For the most part others I have seen stayed with germanium diodes because they have a lower on threshold compared to silicon types. It is worth checking. I will report what I find.
Joe
Joe
Agreed -- particularly for AM detector applications. With FM ratio detectors however, the normal 75 kHz deviation of 100% modulation produces significant output from the detector. I have seen both used in this application. Scott stuck with germanium diodes for the ring demodulators in their MPX decoders almost until the end.
Dave
Dave
audmod01
Super Member
Today I removed the Ratio Detector transformer, Z5 from the chassis. Fortunately Fisher made removal easy. The connecting component leads under the chassis only have to be heated and pulled away using small needle nose pliers or tweezers. They are not wrapped around the transformer solder lugs. The diodes inside are 1N541 germanium type diodes. The 27pF capacitor shown on the schematic is inside near the top of the transformer soldered to the two small lugs where the two diodes connect. The 27pF disc ceramic is a N080 temperature compensated type. There are various sources for data sheets.
The primary of the transformer does have an embedded silver mica sheet in the bottom of it. I will see if I can get a reading on its capacitance.
Joe
The primary of the transformer does have an embedded silver mica sheet in the bottom of it. I will see if I can get a reading on its capacitance.
Joe
audmod01
Super Member
After un-wrapping the two primary leads of the transformer, I was only able to measure about 6/10ths of a pFd between the solder lugs, so I concluded that there is no mica wafer in the bottom of the transformer assembly to deal with. The very low value I measured is consistent with the capacitance of the wires I used to make the connections within reason. The manufacturer of the transformer must have been relying upon the coil loop-to-loop winding capacitance to develop the 10.7mHz resonance of the primary winding.
I need to get some 1N541 diodes on order. Measuring forward resistance of the diodes with the external components disconnected shows they do not measure equally on either R20K or R2K ranges of my Fluke 8600A digital meter. I plan to test select from a collection of 100 if I can get a good buy on the diodes.
Joe
I need to get some 1N541 diodes on order. Measuring forward resistance of the diodes with the external components disconnected shows they do not measure equally on either R20K or R2K ranges of my Fluke 8600A digital meter. I plan to test select from a collection of 100 if I can get a good buy on the diodes.
Joe
Joe --Thanks for sorting all that out. I've had to work on about every other type of IF transformer for Fisher gear, but I don't think I've ever had to tear into one of their ratio detector transformers. Great information!
Dave
Dave
CORYK
Active Member
Dave, I installed the LM353 op-amp into my 800-C. Compared with my KX-200 both the high and low frequencies are nowhere close to the 200. Could the coupling caps have any effect on this?
Very doubtful -- but I will tell you this: For some reason, the Tone Control PC in your 800C is the same PC that Fisher used in virtually every other passive tone control application, EXCEPT the KX-200/X-200 models. For some reason, the tone control network values are distinctly different in those two models, which I bet you will find at the heart of the difference. Again, I would strongly bet that if you simply bypass the buffer in your 800-C, that you'll find it is sonically about the most transparent circuit you've ever heard -- or not heard depending on your perspective!
Dave
Dave
audmod01
Super Member
Latest on the 500-C:
Yesterday I received my order of 100 of the 1N34A diodes (102 were actually sent). I took time to cut some Size A paper into two equal strips lengthwise and taped all 100 diodes with their cathodes on the left and anodes on the right. I used painter's blue masking tape to secure them to the paper, numbering each one. Then I used my Fluke 8600A to measure their forward resistances using the R2K ohms scale. I found forward resistances ranging from 296 ohms to as high as 550 ohms. Most of them were from 300 to 330 ohms. I wrote the measured resistances along side each one. I let them rest while I ate lunch. Then I remeasured them all, again writing down the measured resistances. I then averaged the readings to allow for some variance in measuring. I then selected two of them that were measuring about 302 ohms each and carefully removed the old diodes from the ratio detector transformer. I soldered in the new diodes being careful to heat sink each lead as it was soldered into the circuit.
I reinstalled the transformer into the 500-C chassis and soldered each component carefully making sure I had no solder splashes to short anything out. Then I turned it on and had no sound!! I looked again and saw that I had taken the last limiter tube out of its socket. I turned the unit off and reinstalled the tube. After a few seconds of warmp-up I was rewarded with sweet non-distorted sound from my favorite station! Apparently the ratio detector diodes were the problem all along. If the MPX-65 gets a decent composite audio signal from the ratio detector circuit it deliveres excellent results. Now this Fisher rivals the Kenwood KT-7500/Dynaco system I have for quality of audio. There appear to be some subtle differences, which I expected, but I am now very pleased!
I conclude that having the detector diodes in the ratio detector as close as possible to each other in forward resistance is important to deliver undistorted sound. The key was that I was also hearing distorted sound even in monophonic mode. It was not quite as obvious, but it was there. The old diodes appeared to have some sort of sealing compound on one end and I suspect that over time moisture got inside and had changed something about their performance. The new diodes are entirely hermetically sealed glass enclosed parts.
I have not even attempted any changes to the RF/IF MPX alignment that I had already done. I might recheck the ratio detector alignment, but it is working so good right now I doubt it is off enough to matter.
Joe
Yesterday I received my order of 100 of the 1N34A diodes (102 were actually sent). I took time to cut some Size A paper into two equal strips lengthwise and taped all 100 diodes with their cathodes on the left and anodes on the right. I used painter's blue masking tape to secure them to the paper, numbering each one. Then I used my Fluke 8600A to measure their forward resistances using the R2K ohms scale. I found forward resistances ranging from 296 ohms to as high as 550 ohms. Most of them were from 300 to 330 ohms. I wrote the measured resistances along side each one. I let them rest while I ate lunch. Then I remeasured them all, again writing down the measured resistances. I then averaged the readings to allow for some variance in measuring. I then selected two of them that were measuring about 302 ohms each and carefully removed the old diodes from the ratio detector transformer. I soldered in the new diodes being careful to heat sink each lead as it was soldered into the circuit.
I reinstalled the transformer into the 500-C chassis and soldered each component carefully making sure I had no solder splashes to short anything out. Then I turned it on and had no sound!! I looked again and saw that I had taken the last limiter tube out of its socket. I turned the unit off and reinstalled the tube. After a few seconds of warmp-up I was rewarded with sweet non-distorted sound from my favorite station! Apparently the ratio detector diodes were the problem all along. If the MPX-65 gets a decent composite audio signal from the ratio detector circuit it deliveres excellent results. Now this Fisher rivals the Kenwood KT-7500/Dynaco system I have for quality of audio. There appear to be some subtle differences, which I expected, but I am now very pleased!
I conclude that having the detector diodes in the ratio detector as close as possible to each other in forward resistance is important to deliver undistorted sound. The key was that I was also hearing distorted sound even in monophonic mode. It was not quite as obvious, but it was there. The old diodes appeared to have some sort of sealing compound on one end and I suspect that over time moisture got inside and had changed something about their performance. The new diodes are entirely hermetically sealed glass enclosed parts.
I have not even attempted any changes to the RF/IF MPX alignment that I had already done. I might recheck the ratio detector alignment, but it is working so good right now I doubt it is off enough to matter.
Joe
Great ending Joe! Did you measure any of the old diodes characteristics to know if one was actually defective or that they were poorly matched?
Dave
Dave
audmod01
Super Member
Indications were that the diodes were poorly matched. I decided it was worth a try. The transformer is not really that difficult to remove from the chassis or to work on. Thanks to Fisher's assembly methods for simple insertion of the under chassis components through the open window of the transformer solder lugs installation and/or removal is quite simple.
Joe
Joe
audmod01
Super Member
Today I decided to look at the MPX-65 alignment again to settle what lissajous figure coincided with the correct L and R channels. I found that if you begin with the adjustment core close to the top of the transformer shield and advance down through the 1st and 2nd Lissojous patterns that the correct Lissajous lock is the top or 1st phase lock rather than the second. The second lock makes the L channel swap with the R channel. Dave, is that what you have seen with your MPX alignments? Also the best sound and freedom from interference is obtained when the pattern on the scope screen is at its most symmetrical shape on the scope screen.
I also found that one of the 6AU6 tubes I had in the IF strip was somewhat weak even though it was sold to me as being a NOS tube. I had received a new order of all GE NOS tubes from a Florida source and the GE tubes proved to be better than one Sylvania tube that I had from a previous order. The GE tube in the 3rd IF position brought up the received signal level on the signal strength meter compared to the Sylvania tube. That is not to say that all Sylvania 6AU6 tubes are low gain, just that the specific tube I had in the 3rd. IF socket was not up to the task. In any case, the signal level with the new 6AU6 is higher and the stereo signal tuning is less touchy on center of channel position plus there is less background noise (full quieting). I think it is a good idea to have more tubes on hand than just the amount to fill all the sockets in case of difficulties.
Joe
I also found that one of the 6AU6 tubes I had in the IF strip was somewhat weak even though it was sold to me as being a NOS tube. I had received a new order of all GE NOS tubes from a Florida source and the GE tubes proved to be better than one Sylvania tube that I had from a previous order. The GE tube in the 3rd IF position brought up the received signal level on the signal strength meter compared to the Sylvania tube. That is not to say that all Sylvania 6AU6 tubes are low gain, just that the specific tube I had in the 3rd. IF socket was not up to the task. In any case, the signal level with the new 6AU6 is higher and the stereo signal tuning is less touchy on center of channel position plus there is less background noise (full quieting). I think it is a good idea to have more tubes on hand than just the amount to fill all the sockets in case of difficulties.
Joe
bhamham
Super Member
Joe, I know what you're saying about keeping a stock of tubes of hand. Not long ago a seller on the Antique Radio Forum sold me a box of 50 or so 6AU6s for not a lot of money. I've gone through most of them with my TV-7 and swapped out the weak ones on my Fisher receivers. It made quite a difference.
Good on you to post your results on FM MPX alignment. There's not a lot of information anywhere to be found and your thread is a tremendous resource. Keep up the good work!
Good on you to post your results on FM MPX alignment. There's not a lot of information anywhere to be found and your thread is a tremendous resource. Keep up the good work!
Joe -- That's exactly right. It is possible to align the decoder to deliver wonderful stereo -- with the channels perfectly reversed! Thanks for sharing your experience with this!
Dave
Dave
audmod01
Super Member
One thing that I noticed different about the original MPX-65 versus the one now in the unit is that the original unit had discrete diodes in the 4-diode ring matrices with the 1M ohm balancing resistors. The diodes seem to be clipped into a compression slot on solder lugs which have some other components attached to them. The newer MPX-65 has diode ring matrices that are enclosed in sealed cases and with leads soldered to the external connection points. The newer MPX unit likely took less time to assemble and therefore cost less to produce. The new unit did not need the 1M ohm balancing resistors. I am going to recheck the old MPX-65 to see how the diodes measure individually. It is possible that one or two of these may not be making a good connection to the rest of the circuitry - maybe a high resistance connection due to corrosion.
The new MPX-65 is currently installed and working so well that I hesitate to ever put the old one back in the receiver.
The new MPX-65 is currently installed and working so well that I hesitate to ever put the old one back in the receiver.
audmod01
Super Member
After considerable listening to the 500-C over time, one thing I noticed is that there is very little or no frequency drift when tuned to a local or weak station from a cold start. This speaks well of Fisher engineers. The local oscillator in the front end is well temperature compensated and the ratio detector has no feedback to the front end oscillator. There is an AGC loop, but that has no effect on tuning - just on received signal levels to some extent. The better FM tuners and receivers of the period did not use AFC or at least made it switchable if desired. My Dynakit FM-3 never needed AFC either from cold to hot it was always right on station from where I had left it the night before.
Joe
Joe
Joe; Over the years in various radios, and tube receivers, I've found that the GE 6AU6 is about the best one out there, beating RCA, Sylvania, etc. even when weak. Their 6BA6 and 6BE6 are also very good. The Sylvania's tend to sound sloppy, while the RCA's are inconsistant, although somewhat better overall than the Sylvania's. The JAN or JHS Sylvania's and RCA's are a different story tho.
My 400 came with JAN Sylvania 6BA6 & 6au6WBs along with a worn out 6HS6. The 6BA6 and 6AU6's tested high in emissions and all over double the min gm, so left them in. I did replace the 6HS6 with a 6AH6 and it perked right up. I replaced all fo them (except for the 6AH6) with GE's and there was a slight improvement in overall selectivity, but not enough that it would matter, so the Sylvie's went back in, and the GE's back on shelf for spares. Overall I find the JAN / JHS RF tubes to be highly consistant in quality and damned near bulletproof no matter who made them. But for Non JAN/JHS RF tubes I'll take GE's all day long. Definate SLEEPERS. Everyone else wants RCA's and Sylvies, so the GE's tend to be in better shape in the used / old stock market. Same thing with my 800-C in the Executive. It had a mismash of RF tubes in it and the reception was horrid. Put in new GE's, and it was like I had bought a new tuner. slight tweaking and everything is good. Absolutely no drift from cold with the GE's where with RCA's or other brands (non JAN/JHS tubes) it would drift about .5mHz within 10 minutes, then stable. This was the 6AQ8 tube in the 800c. I've switched to 6DT8's and find they drift less (if at all ) than the 6AQ8's.
My 400 came with JAN Sylvania 6BA6 & 6au6WBs along with a worn out 6HS6. The 6BA6 and 6AU6's tested high in emissions and all over double the min gm, so left them in. I did replace the 6HS6 with a 6AH6 and it perked right up. I replaced all fo them (except for the 6AH6) with GE's and there was a slight improvement in overall selectivity, but not enough that it would matter, so the Sylvie's went back in, and the GE's back on shelf for spares. Overall I find the JAN / JHS RF tubes to be highly consistant in quality and damned near bulletproof no matter who made them. But for Non JAN/JHS RF tubes I'll take GE's all day long. Definate SLEEPERS. Everyone else wants RCA's and Sylvies, so the GE's tend to be in better shape in the used / old stock market. Same thing with my 800-C in the Executive. It had a mismash of RF tubes in it and the reception was horrid. Put in new GE's, and it was like I had bought a new tuner. slight tweaking and everything is good. Absolutely no drift from cold with the GE's where with RCA's or other brands (non JAN/JHS tubes) it would drift about .5mHz within 10 minutes, then stable. This was the 6AQ8 tube in the 800c. I've switched to 6DT8's and find they drift less (if at all ) than the 6AQ8's.
audmod01
Super Member
Larry;
Your observations on the various brands agrees with mine. I have had more consistent results with GE RF/IF tubes than the others. I have not had that much experience using JAN/JHS types though, so your comments about them are a good reference point to keep in mind. I might get a few 6DT8s to keep on hand for the front end. Much of my previous experience was in dealing with analog color televisions. I saw the same results there - the GE tubes held up better over the long haul than the other brands. I have had some experience with JAN/JHS types used in communications receivers (amateur radio equipment and some modified surplus military radios) in which the JAN/JHS tubes were quite reliable and seldom gave trouble.
Joe
Your observations on the various brands agrees with mine. I have had more consistent results with GE RF/IF tubes than the others. I have not had that much experience using JAN/JHS types though, so your comments about them are a good reference point to keep in mind. I might get a few 6DT8s to keep on hand for the front end. Much of my previous experience was in dealing with analog color televisions. I saw the same results there - the GE tubes held up better over the long haul than the other brands. I have had some experience with JAN/JHS types used in communications receivers (amateur radio equipment and some modified surplus military radios) in which the JAN/JHS tubes were quite reliable and seldom gave trouble.
Joe
audmod01
Super Member
To our Fisher Group;
Dave found a schematic error which originated with the factory released schematic on the 500-C serial numbers 10001 to 19999 regarding R77 which should connect to the junction of R64 and R71, not to the junction of R71 and R75. Accordingly, I have modified the schematic I did for adding the EFB, IBAM, Buffer Op Amp to this series and you can find Rev 11 of the document at:
http://s1213.photobucket.com/user/r...0C/fisher 500c EFB mod11.jpg.html?sort=6&o=34
Joe
Dave found a schematic error which originated with the factory released schematic on the 500-C serial numbers 10001 to 19999 regarding R77 which should connect to the junction of R64 and R71, not to the junction of R71 and R75. Accordingly, I have modified the schematic I did for adding the EFB, IBAM, Buffer Op Amp to this series and you can find Rev 11 of the document at:
http://s1213.photobucket.com/user/r...0C/fisher 500c EFB mod11.jpg.html?sort=6&o=34
Joe
