No Fm or AM tuner action on 1800 receiver

Discussion in 'Fisher' started by rufleruf, May 10, 2018.

  1. rufleruf

    rufleruf AK Subscriber Subscriber

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    I bought a restored 1800 from RSsteve recently and it shipped with tuner working and showed up with FM and AM tuners dead - not the slightest peep out of either. I haven't messed with the tuner sections in an 800c, so I don't really know where to start except that I tested all the tubes in the tuner section and found them adequate. Is there a single point of working vs not working for the two tuner sections that may have been disturbed?

    On the bright side I'm using my 202-T and Scott 208 now for the first time... time for a temp change of signature
     

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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    try wiggling the tubes first. I've had a lot of dead tuners come back to life that way.

    A quick glance at the schematic doesn't seem to indicate that the two tuners share any parts, so it may be a voltage supply problem. Looks like it all feeds off the last filter cap section, so if something has failed to cause no voltage there, the tuner would be dead.
     
  3. notdigital

    notdigital AK Subscriber Subscriber

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    Make sure all your slide switches are correctly positioned, particularly the Tape Mon switch.
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    If FM AND AM are dead, and you've wiggled the tubes, then it's gotta be a power supply problem like gadget alluded to. 800c manual serial 51501 and up will cover the 1800. Power comes off C91A. Check R121 and R122 for opens 1st. These are on C91 itself btwn D&C and B&A sections. Look for cold solder joints on the cap sections too.
     
  5. rufleruf

    rufleruf AK Subscriber Subscriber

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    I have listened to it for several hours, so the rest of it functions as expected. The tuner section tube heaters all light up.

    I'll do as you recommend this evening Larry, maybe something came loose in shipping.
     
  6. RS Steve

    RS Steve Tube Junkie Subscriber

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    Hope it's an easy fix Matt, can't help but worry having it travel so far in the hands of misfits. I've had a few cold solder joints show up over time and have moved on to a much better iron. Just know I will take care of you if it costs anything to fix. :thumbsup:
     

     

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  7. rufleruf

    rufleruf AK Subscriber Subscriber

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    Thanks Steve. I think the Fishermen here can help me sort it out. I don't know anyone in my area that works on Fishers besides me, so I doubt it will cost anything.
     
  8. Tim D

    Tim D AK Subscriber Subscriber

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    Matt,

    I'm with Larry. Check the B+ on the tubes in the tuners. I haven't seen the diagram, but the filaments for the tuner tubes would be on a different power circuit than the B+ for the tuner tubes. I.e. "The light is on, but nobody's home" kinda thing.
     
  9. fred soop

    fred soop Super Member

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    It does appear that V3 (first IF) is shared by AM and FM.
     
  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Fred's right. 6HR6. I replaced 6HR6's in my 800c's with a 6AH6 with no problems. The 6AH6 is a Sharp cutoff Pentode vs. the 6HR6 which is a Semi Sharp cutoff Pentode. You could try a 6AU6 in there but will exhibit decreased performance. The 6AH6 is your best bet as it's readily available and it's cost is under $6.00 where the 6HR6 and 6HS6 are going for almost $50 a tube.
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    A little late to the party, but wanted to add if I may, that both the first and second IF stages of the 800C/1800 receivers are shared by both the AM and FM sections. In this case, V3 is the 1st FM IF stage and only AM IF stage as Fred indicated, while V4 is the 2nd FM IF stage, but also serves as the AM Detector as well. If either of these two stages is not functioning correctly, then neither AM nor FM will operate in the 800C/1800 receivers.

    Now please note that when AM is selected, the normal B+ voltages seen during FM operation will be removed from V4 as in its AM function, the tube does not need any B+ because it is not amplifying anything. But a good tube still needs to be in place with its heater lit at the V4 position, so that the cathode and control grid elements of the tube can do their thing in acting as a diode to perform the AM detector function.

    I hope this helps!

    Dave
     

     

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  12. rufleruf

    rufleruf AK Subscriber Subscriber

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    I will pull it out of the wood cabinet and go for it tomorrow when I get up. Crossing my fingers it's easy.
     
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Matt; AK'er Ferninando is near San Jose on the west side (IIRC). He's a Retired H-P guy, so he might be able to help you if you get in a pinch.
     
  14. rufleruf

    rufleruf AK Subscriber Subscriber

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    Thanks Larry - I don't think I'll need help, but good to know it can be had.

    I went down to work on it yesterday morning and there was a dang 800-C I picked up locally last week sitting in front of it that was one of those "started to restore it and lost the parts" deals. Anyway, I decided to rebuild the powersupply on it to get in the mood, and next thing I knew I had done a bunch of work on it. THEN I went to a mothersday brunch and came home with a nice Mac 1700 from a family friend... needless to say, I ended up not touching the 1800 yesterday.

    I'll take it from the top tonight when I get home unless the universe has other plans for me.
     
  15. Tim D

    Tim D AK Subscriber Subscriber

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    Ok. I want to hang out with Matt so I can get his cast-offs.
     
  16. fred soop

    fred soop Super Member

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    I'm missing something here. It appears that AM out is taken from Z4 through C57, so how is V4 involved? Or is it acting as a rectifier (cathode to control grid) for the path through Z3 and Z4? Also, would M1 show variations with modulation?
     

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Hi Fred -- The audio is in fact taken from Z4 as you say. But the energy appearing in the secondary of Z4 is a modulated 455 kHz envelop, with the audio modulation appearing on both the positive and negative excursions of this 455 kHz envelop "carrier" (it takes an audio signal representing 50% of the RF Carrier Wave signal to modulate the carrier 100%. Because it is modulating both sides of the carrier, a 50% audio signal effectively then causes the carrier wave to swing between a value of 0, and 2X the carrier signal alone at 100% modulation). Since the modulation signal appearing then on the positive excursion of this carrier is 180 degrees out of phase with the modulation signal appearing on the negative excursion, they cancel each other out, so that if applied to a speaker, no audio would be heard -- unless -- one side of the envelop is "chopped off". That would allow just one side of the modulated envelop to appear at the output of Z4, with audio that could then be heard. That's the job of the detector stage: by connecting one side of the Z4 secondary to ground through a diode (in this case, the cathode/grid elements of V4), and the other side of the secondary to ground through a capacitance that only reacts to the 455 kHz energy (C58), one side of the envelope is then effectively ignored when the carrier swings in the direction that reverse biases the diode elements of V4, relative to the charge on C58. When the carrier swings in the other direction, V4 conducts, completing the circuit, so that the audio modulation on that side of the envelop -- and only the modulation on that side -- is then "recovered", and heard as the original audio modulation signal. Think of it as an audio signal that is recovered at a 455 kHz sampling rate.

    The detector diode could be connected either way, to recover either side of the 455 kHz modulated envelop. But it is most often (and conveniently) connected with its cathode end grounded. This then causes the voltage appearing at terminal 2 of Z4 to not only contain the recovered audio, but also a negative DC voltage that is proportional to the strength of 455 kHz energy appearing in the secondary of Z4. This negative voltage is often used to provide AVC bias voltage (Automatic Volume Control) back to the RF and Converter stages in the front end to help prevent "blasting" on strong signals. In this case however, the AVC voltage is developed elsewhere in the circuit (by CR1), so that the negative voltage at terminal 2 of Z4 is only used to drive the tuning indicator (M1 in this case) to indicate relative signal strength. Because the meter is a DC device, it responds only to the negative DC voltage developed at terminal 2, rather than the AC voltage representing the audio modulation. Therefore, it's indications are almost entirely independent of the audio modulation present, and indicate only the DC component, representing the strength of the 455 kHz carrier -- itself, a product of the strength of the RF signal received at the antenna.

    In this way then, V4 is a vital component of the AM tuner in an 800C or 1800 receiver. Yank the tube, and the AM section will go dead as then, the secondary circuit of Z4 is permanently opened. The fact that the secondary of Z3 is in series with the secondary of Z4 is insignificant. Because the secondary of Z3 is tuned to 10.7 mHz, it appears as a dead short to a 455 kHz signal, so that its presence in the circuit when operating in AM mode is invisible.

    I hope this helps!

    Dave
     
  18. fred soop

    fred soop Super Member

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    Thanks, Dave.
     
  19. fred soop

    fred soop Super Member

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    Some years ago, there was information regarding improved AM detectors. One of the problems mentioned was the forward voltage drop of the detector diode. At or close to 100% modulation, the minimum peak would be below the diode forward drop and cause distortion. Of course, the amount of distortion and the point where it begins would be a function of the RF envelope voltage available relative to the diode drop. What sort of voltage drop would V4 have in this application?
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Likely no more than that of any high-perveance tube that was either designed for detector service (think 6AL5), or had elements intended for that service included in the package (think 6AV6). The 6HR6 is a high Gm tube, which dictates close spacing of the cathode and control grid elements, and why I would expect the voltage drop to be very similar to a purpose built detector tube. As for the actual voltage drop, I don't know. It was be an interesting experiment to determine the amount seen in service.

    Dave
     

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