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 Post subject: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Wed 06, 2011 11:18 pm 
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Joined: May Mon 17, 2010 2:13 am
Posts: 13
Location: Sherrills Ford, NC
Gents,
I have a Scott 340B (early version) which I recently recapped. I replaced all major electrolytics and coupling caps and selenium rectifier. The unit sounds fabulous except for one problem. It no longer switches from mono to stereo fm automatically or manually. I replaced all tubes in the fm section with new ones but no change. When I remove tube {V504B (12AU7) on the schematic}, the unit plays loud and clear in fm stereo.
Here is the schematic:
http://hhscott.com/pdf/340B_EARLY.pdf
Any ideas on what I should check out under the chassis?

John


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Thu 07, 2011 4:58 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Utica, NY 13502 (USA)
Changing caps in the stereo decoder section often means it will need realignment. That 12AU7 (V504) forces the receiver to mono if it doesn't see a 19 khz pilot signal. Changing caps in the 19 khz filter section (V501) will upset the critical tuned circuits associated with that section.

Dave


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Thu 07, 2011 1:37 pm 
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Location: Baltimore, MD, USA
I have a couple of 340B receivers and one of them was a stinker with the same problem. I did not get into the MPX section for the reasons above. I am in unfamiliar territory when it comes to RF and for that matter, it usually works anyway. If you have other tubes, you may want to do some swapping. Even NOS tubes may not get it going. I did manage to swap some tubes and it worked OK but I suspect there still is a problem.

I think that the 340B had two different versions and I don't remember if it was in the tuner or the amplifier section.
Sorry that I can't offer any better info. Now if you were after the volume control, that would be a different story!

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Don


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Thu 07, 2011 9:29 pm 
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Joined: Jan Tue 11, 2011 5:36 pm
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Location: Boston
If it were my unit I would leave the tube out and switch between stereo and mono manually....hey I actually have to do that anyway with my 340A receiver since it doesn't have the convenient auto circuitry that yours has....

In my neck of the woods all of the stereo stations that are worth listening to come in weak so I usually listen in mono anyway.


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Thu 07, 2011 11:19 pm 
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Location: Baltimore, MD, USA
I hate to admit it but I almost never listen to FM Stereo because it is so processed, bass enhanced, compressed that it doesn't sound natural. It is a shame. When I was a teenager and just got into HiFi Audio (I am 63 now), I could not get enough of it. I am somewhat embarrassed to admit that I have a Sirius Satellite receiver in the car and home. It pains me to not be able to listen to the great HH Scott tuners that I have (Lots of them :oops: ). But I will not get rid of them because they are part of history.

I don't know if you can "force" stereo on the 340B since I don't have one connected now. I am also somewhat tongue in cheek to say that the Sansui 1000A that I have is the cat's arse! But it will not sway me to part with my "Scotts"...

Please enjoy your Scott 340B. It is great!

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Don


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Fri 08, 2011 2:30 am 
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Joined: May Mon 17, 2010 2:13 am
Posts: 13
Location: Sherrills Ford, NC
Thanks for the replies. I did try different tubes in the switching circuit and can get the receiver to switch to stereo on strong stations but after about 10 minutes when the receiver fully warms the stereo signal fades to mono again.
I think Dave is correct. This receiver needs an alignment. Problem is I don’t know of anyone in the Charlotte, NC area who is capable and willing to align the unit. And I don’t want to pack it up and ship it off. Too many horror stories about what the Post Office and UPS has done to equipment like this.
I think I’ll play it without that one 12AU7. Does anyone see a problem running the unit this way?
John


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Fri 08, 2011 2:56 am 
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Joined: Aug Tue 24, 2010 8:56 pm
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Location: Florida
Nope.

But IMO, the real star of that show isn't the tuner anyways, it's the amp :)

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William


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Fri 08, 2011 4:27 am 
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Location: Utica, NY 13502 (USA)
I don't see a problem with running it without the 12AU7 but, because of the possible misalignment of the decoder, you may not be getting the best separation and lowest distortion in stereo.

There is no way to "force" stereo on any tuner. The 19 kHz pilot tone along with adequate detection of the 38 kHz double sideband supressed carrier L-R signal is needed for stereo to happen. Otherwise, you will just hear increased noise in a mono signal. In this case, it is the job of the 12AU7 to "force mono" if it thinks these signals aren't strong enough for decent, noise-free stereo. The nice feature of the 340B is that you can manually bridge the channels to mono with the stereo selector switch. The noise components in each channel which are 180 degrees out-of-phase thus cancel each other out so it's the same as having the 12AU7 circuit do this for you. Mono FM has a 22 db noise advantage over stereo FM. This is significant when listening to distant or low power stations.

Dave


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Sun 10, 2011 5:26 am 
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Location: Yardley, Pennsylvania
Quote:
I am somewhat embarrassed to admit that I have a Sirius Satellite receiver in the car and home. It pains me to not be able to listen to the great HH Scott tuners that I have (Lots of them :oops: ). But I will not get rid of them because they are part of history.


Hey Don. I got an XM receiver 6 months ago and love the programming. Soon after I bought and assembled a Ramsey FM30B to listen to the XM where ever I wanted to. Now I turn on the XM and Ramsey and if I'm in the kitchen I turn on the Fisher 500TX, in the living room an H. H. Scott, in the garage it's an old Sony receiver but it sounds good. Bathroom, bedroom, basement, wherever I'm at I can turn on a receiver and listen to good music. I will be picking up a Kenwood 4140 soon and will be putting it in my office.


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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Sun 10, 2011 1:45 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10634
Location: Baltimore, MD, USA
PaulC wrote:
...

Soon after I bought and assembled a Ramsey FM30B to listen to the XM where ever I wanted to. Now I turn on the XM and Ramsey and if I'm in the kitchen I turn on the

...


Good to know Paul. I listen to my AM radios with a transmitter and watch TV with Blonder Tongue Agile Modulators. That would be a welcome addition. Thanks for the tip...

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 Post subject: Re: Need Help with HH Scott 340B FM problem!
PostPosted: Jul Fri 15, 2011 9:12 pm 
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Joined: Apr Sat 29, 2006 4:48 pm
Posts: 112
Location: Bel Air, Maryland
Interesting. I have a Scott 345 which is similiar to the 340B except it has a tuning eye instead of a meter. I also don't like to listen to FM Stereo nowadays because it sounds so processed compared to what I used to listen to when I was much younger (I'm 69 now). I use the amp section of my Scott mostly to listen to my old vinyls which sound like they were cut yesterday. Just for grins try listening to a CD of a certain piece through your tube stereo amp and compare what you hear from a stereo vinyl of the same exact piece. You will definitely notice the diffference especially in percussion sounds.


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