202-R Arrived

I measured the height of the 202-R chassis at the rear and see that it is 1 7/8 inches high. The Bud Box that I bought is 3" high so i will be removing 1 1/8 inches from the box to make it the same height as the 202-R chassis. I also measured the MPX-65/WX multiplex chassis and they are about 4 1/2 X 3 1/4 inches. The top of the MPX sub-chassis needs to overlap the chassis it mounts to by some amount to allow for sheet metal screws to secure it to the mating chassis. The best I can determine the cut-out in the Bud Box will need to be 2 1/2" W X 4 1/2" long. That allows for the sub-chassis to overlap the mating Bud Box chassis and space for the folded metal tabs on either end of the long dimension of the MPX sub-chassis. These MPX sub-chassis are a little tight to fit the mating chassis even on units that came with a removable plate where the MPX sub-chassis is to mount. The 202-R had no room left over for mounting a MPX sub-chassis, hence the need for a separate box to hold the MPX unit.

I marked off the parts of the Bud Box that will need to be removed. After I get the box height shortened, I will measure and cut out the hole to receive the MPX sub-chassis. I have a metal nibbling tool that I can use for that purpose. I hope to get a start on this soon. I am curious to learn how the 202-R will do when connected to a MPX unit. This box will also serve to locate the transformers for filaments and B+ needed by the MPX sub-chassis.

Joe
 
It should run the mpx nicely if you've changed the Ratio Detector coil to the wide band one. I can't remember if you did or not.
 
I plan to try it first with the existing ratio detector just to see what happens. I will report the result here. At this time I do not have a later ratio detector without buying a donor chassis. Everything I have on hand is functional and restored except the 610-ST chassis I have and it has the older type ratio detector too. I know from discussions on the FM Tuners group of Yahoo that the minimum IF and detector bandwidth that is acceptable by most who have researched the issue is 150kHz. Wider is generally better, with reduced distortion etc. in the final recovered audio.

Joe
 
You reminded me of something. I still have to make a template of the ST-610 faceplate, or at least get the size and center to center dimensions for you. I'll see if I can get all the rigs off the top and open it up and get them for ya this week. Been busier than a long tailed cat in a room full of rocking chairs lately.
 
Larry;

Please do not bother with this. After I decided to use this chassis as a parts donor, I no longer want to pursue a faceplate. Thanks for remembering though. You have enough on your plate right now.

We hope and pray that Dee is getting better.

Joe
 
One interesting thing that I noticed about the ratio detectors of the 202-R versus the later wide-band ratio detectors is that they both use identical component values in the secondary of the transformer. That leads me to think that the main difference between the early and late ratio detectors has to do with the degree of coupling between the primary and the secondary. It could be the spacing between the two (over coupled versus nominal coupling) or perhaps a different number of turns in the coil section connected to the center tap of the secondary. Another possibility would be different AWG size of wire to change the coil's effective Q. Any of these parameters could be involved.

Ratio Detector Data 01.jpg
This was a drawing I made of the internal parts of the ratio detector that was in my SN 10001-19999 early 500-C model. It may be helpful to others that want to replace the diodes inside the transformer.

Joe
 
Absolutely correct, Joe. That's why the transformer itself had to be changed to achieve the wider bandwidth -- it wasn't just a matter of changing a couple of components external to the original transformer. When you get this project finished, there's really something special about having Fisher's older, non - GE/Zenith based receivers operate in full Wide Band MPX stereo mode.

Have you given any thought to a possible stereo indicator lamp or how you might implement that feature? You could always work out something on the MPX chassis, but it if it hidden away behind the tuner, it does little good save during servicing. You also hate to re-purpose any of the existing indicator lamps, as that changes the original features of the unit. One possibility for consideration might be to have the lamp illuminating the FM signal strength meter change color when tuned to a stereo station (dual color LED). Just a thought. Or, don't have any indicator at all. Just pondering........

Dave
 
Dave;

Today I went ahead and cut down the size of the Bud Box I bought to house the modified MPX-65 sub-chassis. I mounted two power transformers on it. One handles the filament requirements with amp capacity to spare and the other handles the B+ needs also with some mA capacity to spare.

You read my mind on the FM signal strength meter going through a color change when receiving a stereo signal. I can use LEDs and handle the switching with some transistors. That involves no change to the face of the tuner. No extra holes would be needed. I have a jewel from another tuner that is red which I could use, but I remember our discussion about how Fisher used green to indicate stereo reception on units with a front panel jewel stereo indicator, so I agree that changing from white meter illumination of the FM signal strength meter to green meter illumination would serve quite well.

I still need to wire the power supply, add an AC cord, add audio jacks for the composite signal in and L & R audio out, work out how to switch the audio. I am leaning toward Fisher's method used in the 500-C using a switching diode matrix. By plugging its AC cord into the accessory socket on the back of the tuner it will turn on & off MPX power with the tuner on-off switch.

Another issue to decide is whether to use the existing audio output jacks of the tuner and thereby maintain level control on the chassis rear, or just use the normal output of the MPX sub-chassis (simplest method). If the tuner's audio output level adjustment is to be used, a way to switch the AM connection from the AM tuner section to the second channel of audio from the FM section and feed it through the AM audio output gain stage would have to be designed.

Here are a couple of pictures of the unit so far.
Outboard MPX web 01.jpg Outboard MPX web 02.jpg
Joe
 
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Joe

Looks great! :thumbsup: So please refresh my memory...are you planning to mount that inside the cabinet with your 202-R or use it as an external standalone unit?
 
Ron;

I bought some walnut boards that I can use to extend the cabinet to the rear and mount the MPX unit behind the tuner chassis. Some of my other audio equipment uses deeper cabinets (front to back measurement) and so it will not be that noticeable. If I use some ingenuity I might make the addition to the rear of the cabinet a removable section. Attachment by some metal brackets would probably work OK for that purpose.

Joe
 
Oh yes, I remember now. :oops: You're doing a great job with that 202-R (and your 440/490 project as well)!
 
Looks great Joe. Remember to add heater balancing resistors for the heater transformer, unless it's got a center tap to the winding.

Dave
 
Dave;

Yes, I intentionally bought a filament transformer with a center tap on the secondary. That will help eliminate hum from the filament circuitry.

Joe
 
Today it was too cool in the barn, so I worked on converting the MPX-65 to the WX configuration. I changed R205 to a 330K ohm 1/2 watt resistor. I added one of the 1112 diodes at the grid of V101 and the associated C212, R206, R207 and C213. I am going to have to order some .005uF disc ceramic capacitors to replace C216, C217, C218 and C220. C221 and C222 have already been changed to .047uF value. I put paper description tags on the MPX input lead and the two output leads. I pre-twisted the filament leads of the filament transformer secondary. I found and set aside some 1N4007 diodes for the B+ voltage doubler and have two 100uF@250VDC electrolytics plus two 40uF@350FDC for the power supply too and set them aside. I have some long terminal strips that have two ground lugs toward each end that may serve well to mount the electrolytics and B+ dropping resistors on. I will likely also need to order some resistors for the B+ that may have to be determined after I see how much drop is added by placing the tubes in circuit. The B+ wires from their tie points in the MPX circuit will be routed around one side of the sub-chassis while the filament leads will go on the other side of the shields as in typical receiver and tuner installations. So now I am in a waiting mode until more parts arrive.

Joe
 
Joe -- Use a 5.6K .5W resistor and decoupling cap (20 uF or so) to drop the the 160 volt supply for the pilot amplifier, trigger, and oscillator stages, down to the 135 volts needed to supply the input composite amplifier and matrix amplifier stages.

Dave
 
Dave;
Thanks for that extra information!
So far I am not having any luck finding .005uF or 5000pF radial lead disc ceramic capacitors of 500VDC rating! Every usual source I have been ordering from has practically eliminated disc ceramic capacitors with radial leads from their stock and no longer offer them. I do find some 1KV and higher rated units that are physically so large there is no way they will fit in the space available. I may just have to leave in place the .001uF disc ceramics that are in there now.

It seems that manufacturers of such parts just no longer want to make any of the disc ceramic capacitors of the values and voltages we need. Such is life!

Joe
 
I would think that 4700pf or .0047uf would be ok due to original tolerances. and at 600vdc or 630vdc. But then again I might be blowing smoke out of my anal orifice AGAIN!
 
Larry;

Yes a .0047 would be OK as the original parts were +/-20%. The problem is finding parts that will fit in the available space. The original parts were quite small, not any more than 1/4inch in diameter. I finally did find some on eBay that are 1/3 of an inch, so I will have to order those. This size of disc ceramic capacitors is simply no longer being made and those remaining in stock apparently have been captured by those who are profiting from those of us who have to have them. Even the film capacitors and silver mica capacitors of the correct value ranges are so large that fitting them into the space available is very difficult if not impossible. There are capacitors of the physical size needed, but their voltage ratings are not adequate for the application. There are some surface mount multi-layer ceramic capacitors that would work, but there is the issue of attaching leads and stabilizing them with some sort of encapsulation so that they would survive being soldered to the circuitry along with the stresses induced by bending the attached leads to the lugs of the coil and terminal board lugs as they are installed.

I will order enough to do this job, but in the future other solutions will have to be found for parts replacements and modifications of some circuits. If the original multiplex decoders had not been designed to fit into such a small space it would not be so difficult. The MPX-65 and WX plus related models of Fisher multiplex decoders were a very dense 3D packaging of the circuitry.

Joe
 
I did find some Panasonic polyester film capacitors of 4700pF@630VDC that appear to not be too large, so I ordered some. I also ordered the .005@1KV disc ceramic capacitors from eBay, so I will have some of both and if the Panasonic caps will fit I will use them as they are 5% tolerance, which can't hurt. I also ordered some 270ohm 3W resistors and some 5.6K ohm @1W resistors to get metal film types of 500V or better voltage rating. Some of the 1/2 watt resistors are not rated at voltages seen in these units so they can flash-over and fail.

Joe
 
The last few days I have been connecting both the filament transformer (which has 6.3VAC center tapped) to the filament circuit of the MPX-65 which is now a WX multiplex adapter after all the modifications. I also added a power transformer from Antique Electronic Supply, their P-T442 transformer which has a 125VAC HV secondary @15mA plus a 12.6VAC winding @300mA (not using that). I installed a bridge rectifier with a 100uF@350VDC at its output then connected a 5600 ohm 1W resistor to another 100uF@350VDC. Then the red B+ lead was connected to the output of the bridge rectifier, the orange B+ wire was connected to the second 100uF@350VDC capacitor. I installed the two 12AT7 tubes and one 12AX7 tube in their sockets, then added the 1/4 amp fuse to the AC input to both power transformers and applied power. I was getting a little over +160VDC at the output of the bridge rectifier then about 134.4VDC at the second 100uF@350VDC capacitor. I then added a 2.4K ohm resistor in series with the 5600 ohm resistor. That dropped the voltage to the orange lead to 128VDC. I plan to get an 8200 ohm 1W resistor to drop the voltage a tiny bit closer to 125VDC on the orange wire. 128VDC is of course perfectly OK, I just want to use a single resistor instead of two in series.


The next step is going to be making a short interconnect cable to run between the MPX out jack in the 202-R and the MPX input jack I added to the WX Multiplex Chassis I built. I used some gold plated RCA jacks that I had on hand for the MPX IN and the Left and Right output jacks. Then I should be able to connect it to the 202-R and see if I actually get stereo output.

I need to take a few pictures to show what the Multiplex Adapter Chassis looks like.

Joe
 
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