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Sherwood S7700-III good stereo on fm but neon won't light

Discussion in 'Tube Audio' started by Patrice B, Jul 10, 2017.

  1. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,243
    Location:
    Montréal, Canada
    Hello all,

    Going through first restoration steps on this Sherwood:

    - two voltage doublers (120uf) replaced with 150 uf
    - can cap of 120 uf and 40 uf replaced with discrete components mounted on perf board
    - 4 output capacitors couplers replaced
    - a couple of other caps replaced, I left the blue ones for now, especially in the FM section and in the tone control loudness part (practically unreachable)

    What remains is replacing double 50uf 500v can with a JJ (for now, I use two 47 uf/450v caps with alligator clips for testing, I know not enough voltage ratings), replacing quad can (30-30-20-60) with a JJ 32-32 and discrete components underneath. Also, adding 10 ohms sensing resistors from cathodes to ground (I saw a bit of redplating yesterday, maybe the old Fisher's 7868's are on their last legs...?).

    So, I placed an order today for the two can caps, one 80uf 500v to replace the 60uf part and a quad of EH 7868's.

    Now, stereo fm is operating with outstanding separation (even if some tubes in the fm and mpx string test low on my Jackson). How do I know: when doing the sound test, a Beatle song was playing panned hard right and left and there was no trace of Paul's voice in the left channel.

    Here's the bug: the neon light doesn't respond to stereo reception. It light up sometimes but between stations with a lot of hiss...! It also lighted up yesterday randomly when selector was placed at phono position???

    Is it possible that the neon is just shot? Something more touchy with bad alignment?

    Anyway, this thing is a beast, heavy and cluttered inside. Quite dirty too...

    Thanks,

    Patrice

    Link to schematic: https://1drv.ms/b/s!AsCftN_jdRMFr1W8Ry9ppj8CsgMy
     

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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    Possible the neon is bad, also possible that whatever drives it is bad. Appears that it drives off the 12AT7. I'd probably swap the 12AT7 in the multiplexer first just to see if that sorts it out. Would also be worth checking the 150K resistor across the neon bulb. If that is high in value, the neon will be prone to coming on when it should not.
     
  3. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
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    Location:
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    One of the first thing I've tried is swapping tubes in the MPX section with known good ones but the neon never worked correctly...

    Thanks anyway Gadget.

    I have a spare neon I could try but I have some more work to do in the output section before returning in the MPX fine-tuning. Just getting ideas from experts here about what's going on with the stereo indicator, knowing that stereo reception looks ok...
     
    Last edited: Jul 10, 2017
  4. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    This appears to work off a 19kc pilot tone detector. Output from the pilot is positive, which causes more current to flow in the 12at7 which turns on the neon. If there is something up with the detector or the bias circuitry of that tube it will might make it act up. Alignment will too of course but since it seems to be working well, thats probably not it. If you didn't have good 19kc pilot signal it wouldn't sound proper.

    Worth checking X11, X10, R241, R240, R243, R244. I'd be especially leery about that 6.8M resistor, those real high value ones seem to go open for whatever reason.

    Generally with neons, when they start going funny they get dim or require more voltage or current to light. This one lighting at the wrong time just feels more like a problem with the circuit than the bulb.
     
  5. Patrice B

    Patrice B AK Subscriber Subscriber

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    Location:
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    Noob question Gadget: how to test a diode (x10 and 11) on circuit? Is the diode test mode on a DMM enough?

    As for the mentioned resistors, am I correct to assume that replacing one (or several) of them should have no impact on the present alignment? This part of the circuit (half of the 12AT7) appears to be related only to the functionning of the neon indicator? Am I correct or completely wrong?

    Thanks,

    Patrice
     
  6. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    diode check function should be fine.

    Pretty sure the resistors will not alter alignment. It looks like it just controls the bias around that tube section, which looks to only drive the lamp. You can probably check them in circuit just to see if there is anything way off. The 6.8M may need one end unhooked though.
     

     

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  7. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
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    Thanks again. Will report back about my findings.

    In the meantime, juste a piece of nonsense I read when searching for "sherwood stereo neon " on Google: the author replaced a bad neon on a Sherwood tube tuner:

    "All set! The indicator light correctly responds to stations transmitting in stereo, and there’s about 10 dB of separation between channels. Much more often just isn’t possible on a tube MPX design. All in all, it sounds great!"

    10db!!! Fisher, Scott, Marantz, Sherwood had MPX circuits designed to achieve 30 dB or better...
     
  8. gadget73

    gadget73 junk junkie Subscriber

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    yeah, you should get easily more than 20 if its working right. There are some garbage MPX adapters out there though, usually low end stuff out of consoles or whatever. I've got a couple that you only know are in stereo because its got more static.
     
  9. Patrice B

    Patrice B AK Subscriber Subscriber

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    I have a GE and a couple of console addons that could fit the cheap mpx description (Clairtone, Electrohome).
     
  10. gadget73

    gadget73 junk junkie Subscriber

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    I have a Madison Fielding one that just makes everything sound hollow, and the other is a Curtis-Mathes that makes everything noisy. The Fisher WX units and my Sherwood S3MX sound good. The Pilot MPX-100 is so-so. That one might be alignment related though. It had been meddled with before I got it, and I attempted an alignment as best I could without the right gear.
     
  11. Patrice B

    Patrice B AK Subscriber Subscriber

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    It's my problem too: I don't have the skills or the right gear to attempt alignments. I usually just tweak the MPX with a cheap FM transmitter coupled to an MP3 player generating a left 1kHz tone. Then, I connect the tuner output to my soundcard and fiddle with the 19 kc and 38 kc until there's best separation as seen on the graph.
     

     

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  12. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
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    Well, 6.8 meg is 6.7 something and 150k is 161k. I'll replace them anyway and put a new neon too.

    I received my parts and tubes today but unfortunately broke a 33 ohms ceramic in the power supply tonight. I'll have to wait until tomorrow morning to buy a few spare parts.
     
  13. Patrice B

    Patrice B AK Subscriber Subscriber

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    Diodes are ok. Replaced neon and now, it lights up on all inputs (tape head, aux and phono) when selector is on norm stereo or reverse stereo. Not on "mix" or fm mono obviously. However, it refuses to work in fm stereo mode...

    I've added 10 ohms 1/4w resistors from all cathodes to ground and... measured 55-60 ma!!!

    With pots at minimum, the lowest I could achieve was 40 ma... Then, I made a mistake and installed the new EH's: unable to bias lower than 40 ma too... So, the originals were good, albeit not really matched but now, I won't be able to reinsert them as I stretched the sockets with the fat EH's. Oh well, I tweaked the 6.8 k in the bias supply (replaced with 3.9 k) and now, I can set them at 32 to 35 ma.

    I also replaced all electrolytics, almost all the ceramic resistors and a couple of small ones.

    What now remains: about ten caps in the circuit and fixing this damn neon circuit...

    Thanks for listening.
     
  14. gadget73

    gadget73 junk junkie Subscriber

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    hm, what sort of voltage do you read at the grid of the 12AT7 when the neon is lit vs when its not? Seems like something is just arse backwards if the only time it doesn't light is in the mode it ought to light in.
     
  15. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
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    I'll measure tomorrow, enough for today... Anyway, it lights up between stations when hush circuit is completely off: looks to me an alignment is in order!

    Stereo separation is best when meter is detuned off center and sound is also louder and clearer off center. There's also another point really off center where the station could be heard loudly and it's usually at this point that the neon flickers. FM detector is probably way off...

    As for the neon, maybe it's the way it works: lit when stereo is selected for any of the inputs and lit also when receiving correctly 19 Kc. Any owner of Sherwood receiver could confirm? I thinks that once the fm strip will be correctly aligned, it'll be easier to tune the MPX.

    Another thing: the 6v bulbs lights up only in fm mode, thus maybe the neon indicates stereo selection in the other modes?

    Thanks Gadget,

    Patrice
     
    Last edited: Jul 13, 2017
  16. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,243
    Location:
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    Tried spares 6GH8A's in the detector position:

    - a Sylvania: meter is more accurate but more distortion;
    - two "Canada" made (RCA I would guess): one give the same behavior as the Sylvania and the other barely move the meter and audio is weaker;
    - a GE Japan: same behavior as the actual Raytheon.

    For now, I noticed that even if the audio is louder off center (and separation, to my ears, seems slightly better), there's more distortion.

    I PM'ed member Arts and he does alignments here in Canada (there's not a lot of people here doing this sort of things).

    Thanks for listening.

    Patrice
     

     

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  17. Patrice B

    Patrice B AK Subscriber Subscriber

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    I guess pin 7: 4mv D.C. on and 0v off... Schematic says 0.1 vdc, hmmm!

    Pin 8: 2 vdc on, 4 off.
     
  18. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
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    Location:
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    Arts, Gadget or even Mr Gillespie, please help me!!!

    Anyway, bias is steady at 32 to 34 ma for the four 7868's. No problems here.

    Neon:

    Measured in circuit (no resistors lifted):

    - R240: 17.3k, supposed to be 68k;
    - R241: 7.02M, varied to 5M when hot, supposed to be 6.8M;
    - R242: 160.6k, supposed to be 150k;
    - R243: 1.75k, supposed to be 1.8k;
    - R244: 13.9k, supposed to be 68k.

    No R245 and 246 in mine...

    Something that looks wrong to you?

    Thanks,

    Patrice
     
  19. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
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    Progress: swapped for another 12AT7 (strong Zenith) and I get some lighting with the neon but stereo separation suffers. Half of this tube is probably implied with the mpx 19 kc.
     
  20. Patrice B

    Patrice B AK Subscriber Subscriber

    Messages:
    2,243
    Location:
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    Well, only the combination of the GE Japan (6GH8 detector) and Zenith (12AT7 amp light and 19kc) works (sometimes).
    Add to this that neon lights up better when meter is off center to the right 2 notches...

    Alignment is messy: Arts, where are you???
     

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