Magnavox CR-192 tuner problem

Discussion in 'Tuners' started by Tube Radio, May 6, 2017.

  1. Tube Radio

    Tube Radio Super Member

    Messages:
    3,334
    I recently got a Magnavox CR-192 FM tuner that had been restored.

    I did an alignment, but didn't have a way to do the discriminator properly.

    Took it to work and did the discriminator alignment, but while trying to get it just right I suddenly lost the ability to adjust it right and found when I connected anything to the grid of the mixer tube I got a 10.7 MHz oscillation. I could also connect to a couple other pins and get the same oscillation.

    Now I did have to fix the discriminator transformer when I got it as one nut holding the trimmer cap had come loose. Had to resolder a wire inside as it had come loose when I removed the transformer cover. I doubt an issue with that transformer could be causing the oscillation.

    I can remove any if tube or the mixer tube and the oscillation stops.

    There is one original electrolytic cap still in use which I will replace Monday.

    Any idea what the problem could be?


    Here's the schematic

    http://www.nostalgiaair.org/Resources/327/M0010327.htm

    I won't be able to do further troubleshooting until Monday.

    Tuner had worked well, but had some distortion which according to the alignment instructions was to be expected if the discriminator wasn't adjusted properly.
     
    Last edited: May 6, 2017
  2. Tube Radio

    Tube Radio Super Member

    Messages:
    3,334
    Here's what I know.

    I replaced some out of tolerance resistors.

    I also replaced the one remaining original electrolytic cap which is 10uF with a 30uF cap as that's all I had.

    Still get the oscillation.

    Even got it to sustain itself once after removing the lead connected to the grid terminal of the mixer tube and pulled the mixer tube and it didn't stop until I removed an IF tube.

    Sounds like to me I have an IF section on the verge of oscillation and the lead connected is enough to make it oscillate.

    Thought I found the issue when I noticed pins 1 and 5 of the 6SG7 were connected to ground through a .01uF cap until I realized pin 5 is the second cathode connection.

    Doubt it's an issue on the B+ line but I will put my scope on it and see if I get any sinewave showing. Will also do the same to any bypass caps in the IF section and other areas that could cause the IF to oscillate.

    EDIT:

    I checked the B+ line and no signals present on it.

    I then proceeded to connect a .0047 uF cap across each bypass cap one at a time and no change.

    Realized that I hadn't tested the tubes so am bringing them home today to test them.

    If it isn't a tube issue then I don't rightly know what the issue could be.

    Now if this was an AM, SW radio or audio amp I'd have already had it fixed, but I haven't done much troubleshooting work to tube FM tuners.

    Now the problem happened while I was adjusting the discriminator, but don't know how any problem with it could cause the oscillation.

    Its as though I connected a capacitor, resistor or wire between the IF output and mixer grid.

    :dunno:



    EDIT 2:

    Took the tubes home and tested them.

    The 6H6 has one section that was weak which may explain the slight distortion I had in the audio mostly noticeable in the bass. I have a good 6H6 and am going to use that.

    The 6SB7 has high grid leakage and I happened to have one in my stash that tests good with no grid leakage.

    Could that have caused the oscillation and it just so happened to be at 10.7MHz due to the IF stages?

    The rest checked ok, although I did have one 6SH7 that tested ok, but had another one that tested a little better. I'll take them both and see which one works best in the circuit.
     
    Last edited: May 8, 2017
  3. Punker X

    Punker X AK Subscriber Subscriber

    Messages:
    3,833
    Location:
    Panic in Detroit
    A tube may test good, but not work or oscillate when operating the the frequency required. A tube tester on statically tests the tube.
     
  4. Tube Radio

    Tube Radio Super Member

    Messages:
    3,334
    Agreed.

    Remember reading once in an rca tube manual that the best test for a tube is in the circuit it is used in.

    EDIT:

    I finally solved the problem.

    I figured out that maybe something was amiss not allowing the unwanted oscillation when I initially tried to align the tuner, but cleared itself thusfore allowing the oscillation.

    I tried something different with the connections and got no unwanted oscillation after trying the good 6SB7 with the previous connection methods and getting oscillation.

    I aligned the IF transformers properly then did the discriminator.

    Both went good.

    When I aligned the RF section I found out something the instructions don't tell you. The trimmer cap across the coil in the plate circuit of the 6AG5 affects the discriminator.

    I wound up playing with it and the oscillator trimmer cap until I got maximum amplitude of the IF signal then adjusted the RF amp trimmer cap for maximum.

    I then used a 50Hz audio frequency (chosen because I had distortion in the bass) and fed it to the frequency modulation input on the RF generator.

    I adjusted the tuning until I got a symmetrical sinewave then removed the signal and proceeded to adjust the trimmer cap across the coil in the plate circuit of the 6AG5 until I got to 0Vdc and rechecked it then repeated until I got as symmetrical a sinewave as I could with the modulation on and the discriminator voltage returning to 0Vdc when the modulation is turned off.

    I will test it this afternoon and see how good it is.

    Audio out of tuner before de-emphasis filter

    20Hz 1.93 Vrms.
    1K 1.91 Vrms
    10K 1.54 Vrms
    20K 1.45 Vrms
    30K 1.24 Vrms
    40K 1.07 Vrms

    So it should work with an MPX decoder quite nicely.

    The RF level fed into the tuner through a 300 ohm resistor with one antenna terminal grounded is -16 dBm which is the point where the audio signal no longer increases in amplitude which I figure simulated a strong FM station.



    EDIT 2:


    Well...........I tried the tuner at home and it only picks up the strong stations. Tuning between stations produces no static at all and strong stations seem to come in real strong, whereas the weaker stations I can turn the volume up on my stereo and barely hear them clearly as there is none of the typical static that should normally be heard with weaker stations.

    It's as though the gain is fixed and won't respond to whatever controls the gain.

    I don't have the eye tube connected, but maybe connecting it will show where the problem is as if it isn't working right then the voltage that controls the gain isn't present or just isn't getting to where it needs to be.

    Think I understand now. That problem could be what caused the oscillation.

    Thinking about it when I had the RF generator loosely coupled to the tuner I should have gotten increasing static on the scope as I reduced the RF level, but I didn't get that.



    EDIT 3:

    Found the problem.

    I installed a 1.2 meg resistor in place of a 1.5 meg resistor because the second band was faded to where it looked red.

    Only discovered that when I looked at the schematic at that resistor and saw it was 1.5 megs.

    Didn't have a 1.5 meg resistor so given I had used a 1 meg and 200K for the 1.2 meg resistor I added a 220K and 82K in series which gives me 1.502 meg.

    I did have to touch up the alignment, but I left the discriminator alone and when all was said and done the discriminator didn't need any adjusting.
     
    Last edited: May 9, 2017

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