Discussion in 'Fisher' started by audmod01, Jun 27, 2017.
ok, fair enough. I've learned the hard way to check the easy things first.
No problem. I was glad you suggested this. The simple things are often overlooked. That is one reason why I find that when I get frustrated it is a good thing for me to set a unit aside for another look the next day. I can't count how many times over the years that I had overlooked something obvious upon a second look at it. Sometimes a casual observer can point out simple problems we overlook. I still intend to check parts values around this portion of the circuit just in case a resistor or capacitor has gone bad.
I spent this morning doing an installation of a new lavatory faucet and associated hoses and shut-off valves. I am through for the day. Lying on my back in crazy positions takes its toll on me these days. I need the rest now.
I did some checking in and around the FM level meter circuit late today. I found one 220K ohm resistor which was out of tolerance on the high side, but it made no real difference in the performance. I plan to start some checking of alignment tomorrow. I located the 5-45pFd variable trimmer and have it waiting to bridge across each IF transformer winding and see it that makes a difference too.
I installed two more LED #47 lamp substitutes in the 202-R. These two are the ones that illuminate the AM and FM level meters. The LEDs are cool white to my eyes although they are sold as warm white. In any case the light they put out on the meters is a bright white and makes the meters look like they are new compared to the incandescent lamps which make them look old and yellowed (which they actually are).
I am going into surgery on Tuesday and may not be active for a while. A small cancer was found and hopefully can be completely removed. According to CT scan and biopsy it is well contained, so I just want it out.
Hey Joe. Hope all goes well. I was an Independent Duty Navy Corpsman. Let me hone my KA-BAR knife and I can take care of that problem right away, complete with a .30cal bullet for you to bite on! Plus I stayed at a Holiday Inn the other night!
Seriously, good luck with the Surgery! And raise hell with the nurses!!
Good luck Joe, and prayers for a speedy recovery!
Dave & Larry;
Thanks guys! Just getting more decrepit as time goes along - typical for my age group. Best to all of you too!
Ain't we all. Most of our gear is in better shape than we are.
Just a quick note to let everyone know that the surgery was successful. The surgeon had a pathologist giving him immediate reports on material removed. It was surface cancer like the biopsy and CT scan showed.
Thanks for good wishes and prayers from everyone. I have some pain to deal with but, after a few days I should be back to normal activities. Remember, early detection is key to cancer survival.
Super news Joe! So glad they were able to get it all, and that your recovery time will be short!!
Wonderful news indeed, Joe, that is great! Correct, early detection is the key. Congratulations!
WAY TO GO, JOE! Long sleeve shirts, floppy hats over your ears (or a Nice Stetson) and Truckloads of SPF-100 are your best friends now!!! Along with a good Dermatologist!
Yeah, my family tells me that I look like an alien from outer space when I don my gear to mow the yard!
However, this surgery was for a sore place on the side of my tongue that would not heal. About 10 years ago I began to bite my tongue during my sleep and the cancer developed from that. Re-injuring an area of the body can lead to development of cancer over time. Boy is my tongue sore! I am so glad to be rid of the cancer though.
I am currently retouching a WX multiplex schematic so I can have a decent schematic to work from as I modify the MPX-65 that I have for working with the 202-R. Although the adapter will be outboard, I want it to be the Fisher design that was originally developed for use in their tuners rather than the receivers. That way I can obtain power for the adapter directly from the tuner. I see a way to add the necessary interconnect cabling with a pendant cable using some existing ventilation holes in the chassis. I might consider standing the MPX adapter on edge and fastening it to the back of the tuner if I can find a way to do it without making any extra holes in the chassis. I need to do some investigating.
My work on the WX Multiplex schematic (taken from a Fisher 800-B service manual) is now complete. I looked at both the WX and the MPX-65 schematics side by side and here is what I notice aside from the use of two 12AT7 tubes and one 12AX7 in the WX versus the three 12AX7 tubes in the MPX-65:
R205 in the WX = 330K; R205 in MPX-65 = 220K
WX adds CR101 detector diode cathode at pin 2 of V101 12AX7; WX R206 = 1M, MPX-65 R206 = 1.8M
WX C212 = .001uF; MPX- 65 C212 = .05uF
The WX has CR101 anode connected to junction of C212 (.001uF) & R206 (1M) & R207 (1.5M) and adds C213 (.05uF) to filter the CR101 detected voltage out to the MPX eye tube tuning indicator.
The rest of the components associated with V100 in both WX and MPX-65 are identical in value.
WX C216 and C217 are .005uF capacitors while the MPX-65 uses .001uF in both places.
WX C218 and C220 are .005uF capacitors while the MPX-65 uses .02uF in both places.
WX C221 and C220 are .02uF capacitors while the MPX-65 uses .047uF in both places.
WX R216 and R221 are 10M resistors while the MPX-65 uses 22M in both places.
Both WX and MPX-65 use the same value of components in the bias detector circuitry of CR100. Dave has already made comments about how to improve performance of this circuitry area.
Both WX and MPX-65 use the same resistor values associated with the diode bridges at the output of the 38kHz transformer.
I haven't read all the way through this, but why not find a later model MPX-100 (S/N 20000 or higher, I think)? It will match the faceplate style of your 202R, and since it's designed for standalone use, will be a lot easier to set up than a WX with a new power supply. The earliest version of the MPX-100 used a less desirable MPX decoder circuit, from what dcgillespie and some of the other Fisher gurus have said in the past. There was also a faceplate-less MPX-200 designed to be hidden inside a a console, but the MPX-100 seems to be substantially more common.
Joe -- Later versions of the WX sub-chassis used:
1. .001 uF at C216 and C217.
2. .02 uF at C218 and C220.
3. .047 uF at C221 and C222.
Earlier versions used the components you cited. This will make less work for you in not having to change out these components, other than recapping C221 and C222.
As for CR101, in the WX chassis, it is actually part of a half-wave voltage doubler circuit that helps to boost the indicator output. In the MPX-65, the Pilot Amplifier stage has more gain (using a 12AX7), making the voltage doubler circuit unnecessary.
Your comments agree with my thinking on C216/217, C218/220 and C221/222. My thoughts were that the use of larger value capacitance would extend the low end of the audio response some and therefore would be desirable. Space in these is tight enough as it is without going to excess in changing out parts that are good to begin with. Most of the capacitors I see here are disc ceramic types and I seldom have seen that type go bad. Most of the failures I have seen were due to shorts. In the applications here I do not see much in the way of voltage stress upon those parts.
I looked upon CR101 as being part of the ability of the MPX circuit to provide greater closure of an eye tube to indicate a tuner/receiver was tuned to a stereo signal. I think this is what you are describing? Of course in the 202-R there is no eye tube to deal with unless one were to be added (lots of issues there). I think a simple relay activated lamp or LED indicator would be sufficient for that purpose.
The use of CR101 (or not) in the indicator circuit is more about achieving (about) -10 vdc at the indicator output when a 1 volt composite signal is applied to the sub-chassis that contains a 10 % pilot level. It was primarily the tuners that used the eye tubes, which of course also used the WX sub-chassis that included CR101. But note that the 400 receiver also uses an eye tube -- but with an MPX-65 sub-chassis that does not include the diode in its indicator output. By using a 12AX7 in the V-100 position of the MPX-65, Fisher was able to generate enough indicator output voltage without the voltage doubling circuit to still (virtually) close the eye tube in the 400 receiver, and save a few bucks in the process.
Good points. I had noticed that the 400 used an eye tube, but had not paid much attention to the MPX sub-chassis used. The 400 continued to use an eye tube well beyond when it was dropped in other models. Late production 400 models added a 4th IF stage like the 500-C, 800-C and 1800.
The extra 202-R faceplate that I ordered from eBay arrived this afternoon in good condition. I need to contact Keith to let him know I am ready to send it to his friend to duplicate in aluminum with the engraved lettering. It turned out that his system does not have the power needed to engrave brass but works well with aluminum. The 202-R is a heavy tuner. Any reduction in weight is welcomed by me.
This morning I got around to looking at the FM IF alignment. I started by injecting a low level signal at the grid of the mixer using a 10.7mHz crystal oscillator output from my post-sweep marker generator. It is necessary to use the lowest signal level possible while still being able to see the level on the Signal Level meter in the tuner. I can see fairly good peaks at several stages in the IF strip ahead of the level meter circuit. There one glaring exception - the output of the Mixer. This circuit goes from pin 6 (plate) of the mixer straight to the primary coil of the 1st FM IF transformer. No matter where I adjust the primary, I never see a sharp pronounced peak. The secondary does. It appears the capacitor that tunes the primary may be open. It is a 24pFd@1KV N150ppm disc ceramic right at the 6AQ8 tube socket. Although I bought some 24pFd COG capacitors, it turned out I goofed and ordered 6KV caps of that type. While I might be able to install one, it is physically so much larger than the one in the tuner that I hesitate to do so.
I remembered that I have a Electra III 610-ST chassis that I bought for spare parts. I checked and it has the same part although of a physically different construction. It is a 24pFd@500VDC N150ppm but not a disc ceramic. Instead it is one of the tubular ceramic types with color stripes to indicate its value and ratings. I managed to remove it without damaging it and after I have lunch and relax a bit I will remove the old part from the 202-R Mixer plate circuit and install this one. This should maintain the frequency stability of the primary side of the 1st FM IF transformer perfectly. One thing I checked was that the IF transformer used in the 610-ST does have a different part number on it, ZZ630-114, has the same form factor as the one in the 202-R which uses part number ZZ612-117 and has the same type of tuning slugs - the slotted type. I am lucky that I have the correct type of tuning wand to adjust these. Both primary and secondary use the same slotted adjustment tool type.
When I first began to adjust the primary of the 1st FM IF transformer in the 202-R, it was difficult to turn. I finally managed to adjust it in a CCW direction, which raises the ferrite toward the top of the transformer can. I believe that ferrite was all the way down to even approach the resonant point of the coil and never gets there.
The part that I removed from the 610-ST is small enough to install and preserve the lead dress of the original disc ceramic capacitor. It is up close to the side of the enclosing wall of the FM RF compartment. In all cases of part replacement in a FM tuner front end or IF circuit it is critical to maintain proper lead dress in order to maintain original alignment. The FM IF is not as critical as other areas in the FM front end, but it is still important there too.
I will report on results after lunch.
Hope that restores the 1st IF transformer to proper operation. Let us know how it turns out!
One trick you can use to easily inject the IF signal into the IF strip without potentially un-tuning the circuit at the point of injection (or having to remove the RF can bottom cover) is to slide an UN-GROUNDED tube shield over the mixer tube. Then apply the IF signal between the chassis and the shield, allowing the signal to be injected through the tube itself. The tube shields that have a ribbed side and hug the tube tightly work best in this case. I have both a 7 and 9 pin tube shield of this type, and both have a short 4 inch flying lead soldered to the shield that allows me to quickly and easily inject an IF signal at the mixer tube with minimal disturbance of circuit operation.
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