Pulling in stereo radio on 500-C - very distorted

Discussion in 'Fisher' started by thornev, Jan 23, 2018.

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    What tubes do you have in the MPX? Old Stock, or New Russian Tubes? Brand? I've found quite by accident that tubes with sections that have pretty much equal readings (gm and emissions) will have less problems than those with a good section and a weak section in V100 and V101. This goes for both new Russian Tubes (not too likely unless from the factory that way) and old stock domestic and euro tubes. Like Dave said, Russian tubes are more likely to have problems in the MPX, especially V100 and V101.(Russian tubes also have problems in the Phase Inverter section so using a Domestic or European old stock tube is a good idea here. Make sure the sections read close to each other.) What I do is to put Domestic or Euro old stock tubes with fairly matched sections in V100 and V101, and a domestic (if I have any left over at that point) in V102 (which is the MPX Amplifier) or a highly matched, very good testing (gm and emission) Russian Tube.

    As an example of a somewhat similar problem; I have a Sansui 1000A which has the following tube line up in the MPX. 6AQ8, 6BL8, and a 12AU7. It's aligned as is the tuner. Now for the fun part. A 6DT8 is the sub for the 6AQ8. The 1000A uses 3 of the 6AQ8. 1 on the Front End of the tuner(Mixer/Osc), one as the Muting Amp, and one in the MPX (Mpx Amp and Indicator Amp). The 6BL8 is the 19kHz synchro and Doubler, while the 12au7 is the "DUAL, OUT & De-Emphasis). I put Brand New 6DT8's in all 3 positions and lost STEREO. So I put a crappy reading 6AQ8(both near minimums and mismatched by 2000mhos) back in the MPX and I got Stereo again. The other two posit's are fine with 6DT8's. Matter of fact I use the 6DT8 in most all places that call for a 6AQ8 (cost is $3-$4 for the 6DT8 vs. $15.-$25. for the 6AQ8) in all of my tuners, or wherever a 6aq8 is called for. I haven't had to replace one 6dt8 in 10 years on any of the units.
     

     

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  2. thornev

    thornev Active Member

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    The 12AX7 MPX tubes are:

    V102 = RCA USA ??C83
    V101 = RCA USA
    V100 = GE MH 188-5

    So it's a mish mosh with some of the text rubbed off. They look old. In an earlier post in this thread I tried 3 new, "matched and balanced" Tung-Sol tubes and there was no change. Each tube package has a sticker with "88" written on it. I thought Tung-Sol is an acknowledged good brand but maybe not for this Fisher. Maybe it's true that when new tubes are installed, a complete alignment is required.
     
    Last edited: Jan 27, 2018
  3. gadget73

    gadget73 junk junkie Subscriber

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    Thats what I run in all my Fisher MPX units, a mixed bag of old stuff that is nearby when I need them. I haven't had any of them act badly unless the tube was just bad.
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    The NEW Tung-SOL's are Russian re-Issues. Of all the various Russian 12ax7's these are probably near the top of the heap. I haven't tried the "MULLARD's from Russia. They(Tung-Sol) are on par with the EH12ax7, Sovtek 12ax7 (except for the 12ax7LPS which is a good low noise tube and should be used in Phono sections). In sensitive circuits the Russian tubes are inferior, as Dave has shown more than a few times (see the thread on Phase inverters ("400 Service Bulletin") in the Stickies Subforum for example, and the one on MPX (there are a couple)).
    The Tung-Sols are my 2nd choice in Phono circuits after the 12ax7LPS, and I use them in the Tone Amp positions. In the Phase Inverters, I can run the Tung-Sol tubes as long as the noose has been removed. This pretty much goes for all of the russian 12ax7's.

    Changing tubes doesn't change the alignment in most cases. At least not on the FISHER's, I've retubed. And I retube the tuners on all of my receivers. Generally GE or Sylvania old stock. I don't like RCA tubes in the tuners. I've found RCA tubes tend to go weak faster. I've got GE and Sylvania 6BA6, 6au6, & 6be6 tubes out of used tuners that still show very high in emissions and 4 to 5 times the minimum gm of my Philco 9100's (ReBadged Hickok 533A) chart.

    You can get GOOD old stock tubes from sellers like abcvacuumtubes.com in Fl. and other tube dealers. I use ABC tubes and haven't had one yet that tests bad or weak. Plus compared to a lot of dealers he's fairly inexpensive.

    For output tubes, use Jim McShane for The TUNG-SOL 7591 ReIssues, or the EH7591's. Both are decent tubes as Dave and Steve Lafferty have demonstrated here ( http://www.tronola.com/html/7591a_tubes.html ), but they still fall short in some places against old stock 7591's. Jim McShane tests these tubes in a rig that uses operating Hi-voltages and matches quad for tube draw to within 1ma. Most of my Tung-sols are 4 years old and the EH's are 8 or older and they are holding up well. Install an IBAM or IBBA circuit and it'll sound even better with the tubes matched for current draw. Plus you have the ability to adjust them hot or cold as you wish. Run them at now more than 80% of max dissipation and they'll run very well. 70% is the bottom end of the range. This will give you an amp that will cause smiles, and run the tubes easily.
     
    Last edited: Jan 27, 2018
  5. thornev

    thornev Active Member

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    larry - What is IBAM and IBBA? Where do I get them and where on the chassis do they get installed? I could google it, but you're in the know. Thorne

    UPDATE: OK, found this: "The IBAM (Independent Bias Adjustment Modification) removes the fixed bias circuit and allows the individual bias adjustment of each 7591 output respectively to 32mA draw."
     
    Last edited: Jan 27, 2018
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    You don't get them, you build them. Basically the simple version is they are Individual Bias Adjustment Mod and Individual Bias / Balance Adjustment. The IBAM Circuit is in the "COMMON PARTS FOR FISHER's" Thread in the Stickies subforum. The IBBA is an upgrade to the IBAM adding a balance ability to each pair of output tubes. It's also in the Stickies subforum, under "TECHNICAL THREADS" and is titled "Alternate BIAS Adjustment Modification". They aren't hard to build.
     

     

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  7. thornev

    thornev Active Member

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    OK, I don't get this. Just wanted to see if there was one MPX tube causing the distortion. One by one I replaced each 12AX7 with a new Tung-Sol. And then all 3 were replaced and no distortion??? Last night I put in the 3 tubes and had distortion.

    Thorne

    UPDATE: Just to see what would happen, I put the old MPX tubes back and the stereo distortion returned. So I put the new tubes back and that's that.
     
    Last edited: Jan 27, 2018
  8. thornev

    thornev Active Member

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    I've found that many components in my 500C have already been replaced. Of particular notice is the circuit board in the attached picture. Does anyone know what it's for? Thanks, Thorne
     

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  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    That is an aftermarket cap board that is designed to replace all the old electrolytic can caps. As a result, you will likely find that all the original can caps are still in place, but have had their terminals either snipped off, or the leads to them carefully removed and then attached to the new board. Either way, with a board like that installed, the whole power supply has effectively been recapped.

    Dave
     
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  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Take a couple of pictures of the whole underside of your 500c. It may be that it's already been mostly redone, and we're just rehashing what's been done (spinning wheels in water. I prefer bleach burnouts). With the pictures we can pinpoint areas that will possibly need additional work to allow it to work at it's best and safest.
     
  11. thornev

    thornev Active Member

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    OK larry d. Pictures attached. Almost everything in metalbone's 500C restoration kit was completed by a previous owner or by me. Note that what is NOT displayed in the attached pictures are these updates I did:

    1. piggy back 47K ohm 1/4W resisitor to R204 to lower stereo detection threshold.
    2. Replaced C210 W. Germany ROE lytic with 1uf 400v nichicon lytic.
    3. Cleaned terminals on stereo beacon relay with Deoxit.
    4. Replaced C221 with .047uF 400v capacitor.
    5. Cleaned all front face pots with Deoxit.

    There are still problems with stereo. I would guess a realignment is the only remaining task and I have neither the expertise nor tools to do it so crap-o-la. I hope you guys find something. Thanks in advance for looking. You guys have been great already. Thorne
     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OK. So let's dig into the FM a little bit. Perform this experiment:

    1. Connect your DVM between Test Point 3 and ground using clips on your meter leads (black meter lead to ground). This test point is located on the schematic near the upper right corner, and in the chassis, it is associated with IF Transformer Z5.

    2. Connect an antenna and BEING CAREFUL, turn the set on, and tune in a strong local station. Tune for maximum signal strength indication, and note the reading on your DVM, which will be a negative voltage. Record and post the voltage achieved.

    3. Now carefully rock the tuning control slightly to either side of the peak signal strength indication and watch the DVM as you do this. Does the DVM indication increase more negatively if you rock the tuning slightly to one side of the indicated signal strength peak? Or does the DVM indication peak at the same tuning point that the signal strength indication peaks at as well? Let us know!

    Dave
     
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  13. thornev

    thornev Active Member

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    Dave,

    At max signal strength (Step 2 in your post) the DVM displays -36v.

    When moving the tuner left and right of strongest signal (Step 3 in your post) the DVM displays less negative i.e. on both sides of the strong signal strength the DVM registers toward -30v.

    Sanity check... I'm not sure what Test Point 3 is. With the receiver sitting on its left side as you face its front, looking at the bottom of the receiver, the block that is IF transformer Z5, there are 3 posts on Z5 that face the receiver front and 2 that face the back. I put the "+" lead of the DVM on the front post closest to the lytic next to Z5.
     
    Last edited: Jan 28, 2018
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Thorn -- the correct post to connect to -- which is on the side of the transformer with 3 posts -- is the one that uniquely has a 1.5K resistor (brown-green-red-silver) connected to it as well as a small ceramic cap also. However, there is little doubt that you had your meter connected correctly to Test Point 3, as connecting it to the other terminals of the transformer would not have given you the indications you noted on your meter.

    The good news is that based on the observations you noted, the FM tuner section proper in your 500C is operating just fine, and is in excellent alignment. This is important, as if the FM tuner section is in poor alignment, it wouldn't matter how well the MPX section is operating -- the FM MPX Stereo performance would still be poor. So this exercise proves that the problem you are noting with FM MPX Stereo is in fact located in the MPX sub-chassis itself.

    If you have recapped and retubed the MPX section and are still having enough problems with it that you'd like to pursue it through to a proper resolve, PM me and I will discuss a unique option for resolving it.

    Dave
     
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  15. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Looks good so far. Couple of items you might want to do.
    1.) screen stability resistor mod.
    2.) replace that 1.2K 7W "Praying Mantis" Tan resistor and it's smaller 1.2K 1w neighbor next to it on the green block. (Most of the7W 1.2K I've found in FISHER's are Praying Mantis Green) These take a lot of abuse and are running at the ragged edge. We recommend upgrading to a 10W and a 2W resistor These are R139(small) and R143(Large)
    3.) Cathode Resistors.




    Replacement parts.
    1.) 1.2K 10W 5% Ohmite Part #588-40J1K2E $2.02ea
    https://www.mouser.com/ProductDetail/Ohmite/40J1K2E?qs=sGAEpiMZZMukHu%2bjC5l7YaQ4P8Kyhi8j1BxFnJWmcoU=

    2.) 1.2K 2W 5% Vishay Part #594-5083NW1K200J $0.34ea
    https://www.mouser.com/ProductDetail/Vishay/PR02000201201JR500?qs=sGAEpiMZZMsPqMdJzcrNwnDHAJTsQNMjOMsEVgilMQI=


    DAVE G. Take a look at the pic where the grid returns are normally found on a 400 and 800c. Areas in green rect. Then look at the red area's next to them. Am I seeing things or is that where FISHER put the Grid Return Resistors on the 500c? Am I going nuts here????

    thornev: What is the value stamped on the 4 yellow caps next to the green rectangles. Those are the output coupling caps. Original values were .047uf @ either 250V or 400V. Replacement caps are generally .1uf or somewhere in between the 2 values named. If the values have changed and those resistors are the grid returns which are 330K, those resistors need to be changed to 220K . They should be in the Metalbone kit

    500c grid returns.JPG
     
    Last edited: Jan 28, 2018
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  16. thornev

    thornev Active Member

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    larry - Thanks for the detailed post. If you could edit your post above and add schematic component numbers it would help. I'm a novice at reading schematics and electronics in general but am greatly interested in learning. I'm enjoying this project. It's a challenge for me to map schematic parts to the actual receiver internals as I'm just learning how to identify parts by their values in the parts lists. Adding pointers to images of components would be very helpful to me. When you list parts for me to purchase, links to online stores would help or at least detailed descriptions of the part values so I can be sure I'm ordering the correct parts. If it helps I've made a list of parts that I believe were altered before I bought the receiver. Those large green rectangular caps and all the yellow cylindrical caps were altered by a previous owner. Also that SDS Labs board in the power section was added by a previous owner. Let me know any info I can provide and I'll do my best.

    To answer your question, if you mean the 4 cylindrical yellow caps C77, C78, C79 and C80, their values are .047K 630v. They were installed by a previous owner. I do have the Metalbone kit and have completed almost all his instructions. I don't know what resistors you mean that are the grid returns. I believe "grid" means one of the points attached to the tube pins, but I don't know tube topology well enough to be able to tell you the values of those resistors. I'll guess that you're referring to the resistors connected to pin 6 on the tubes. They would be R126, R127, R129 and R130. They are 1K resistors. The only other resistors connected to the power tubes are the pin 5 circuit protectors which are 10 ohms. Those values are way off from the ones to which you refer so I guess I don't know which ones you mean.

    Thorne

    UPDATE: It just occurred to me you mean the resistors circled in green in your posted picture (the ones Dave in his post after this one says are light tan), they are 220K ohms. They are labeled R121, R122, R123 and R124.

    The grid return resistors you circled with red rectangle are 200 ohms and 2.2K ohms.
     
    Last edited: Jan 28, 2018

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Larry -- The resistors in the red box are the cathode resistor for the first stage of the driver tube, as well as the NFB resistor and cap. If you look closely in the green box, the grid return resistors are there. They look to be very light tan in color, and clearly replacement pieces.

    Dave
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Ok I couldn't see the grid returns (R121 thru 124) even with the pic blown up.

    Thorne. No need to edit the previous post. All info (part #'s from the schematic, etc.) is there. The links are direct links to the page @ mouser to order the 2 power resistors (R143 & R139).
     
  19. thornev

    thornev Active Member

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    larry - I'm having trouble identifying R139 and R143. Is one of them the green cylindrical part partly under the SDS Labs board? The other one is yellowish and hidden under the SDS board? If you look at the image I posted in post 28, you can sort of see the green one under the word "Labs" on the SDS Labs board.
     
  20. Tubologic

    Tubologic Well-Known Member

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    The working of the MPX adaptor in the 500C should not be so tube dependant, actually the replacement of these (3) tubes has very little effect on the MPX performance and doesn't significantly affect alignment (provided you use know good quality N.O.S tubes), a slight touch-up may be needed but that's all. From your description it looks that the MPX decoder is out of alignment and the 38KHz oscillator is on the verge of loosing sync with the 19KHz pilot signal. The cause is either an insufficient amplitude 19Kc pilot signal at TSP101 (Z100 alignment needed) or the free running frequency of the 38Kc oscillator is out of range (Z101 alignment needed), or both if someone has tempered with the coils. A careful re-alignment of the MPX decoder is in order and should resolve all your issues. Most preferably a good MPX stereo generator and oscilloscope are needed to proceed but there are other (less accurate) "emergency" methods based on the broadcasted received signal. There are very few know parts failures in the 500C multiplex units apart the 1µF (C210) reinsertion elcap you already replaced. Ony once I had a defective Z100 19KHz coil but it should be a very rare occurence.
     

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