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Finalizing 800c...loose end questions.

marloubow

marloubow

Best Day Ever
Almost done with my 800c and have a few questions that I would like to get answers to before putting it all back together....in it's finished cabinet :thmbsp:

1) Is MPX just another name for a basic Stereo signal?

My tuning meter is 4.5 out of 5 on several stations. My beacon light is on and the signal is clear, but not impressive. Seems there should be more dynamics. IIRC, before my restore, the stereo signal would "lock in" and would just go WOW!
Or is it that I've come down from my "freebie high"? :D

2) I've heard mention in other posts about measuring voltage to the 12AX7 heaters. Why and where?

3) Which tubes are associated with which sections (sans output & MPX)?
V1-9 & V15-19.
thanks
martin
 
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1.) Multiplex is the Trade name for "STEREO" reception in the 60's and 70's. The mono signal is split, folded, and mutilated into two separate signals. FM-AM Simulcast and FM-FM Simulcast were the precursors to Multiplexing.

My multiplex signal isn't that much different than mono on my 800C. It better on my 400 but I think for me it a matter of alignment. Yours could be 1st blush, or could be changes resulting from cap replacement. A good alignment would probably help.

2.) Pin 4&5 on each 12AX7. Use AC setting except for the 4 on the BIAS circuit (V16-17-18-19) They are Series-paralleled in DC. Measuring the voltages are another indicator of circuit health and correct voltages. Voltages between 10.5 and 11.0 are good and will help with tube life. 12V is acceptable but tubes run hot and life suffers.


3.) The schematic tells all!

Larry
 
Just a technical note, "multiplex" isn't a trade name, it's a functional description of how the current (finalized in April 1962) FM stereo system works.
 
Martin - attached is the tube layout for the 800C.

Also - have you had the 800C aligned?

If not you may want to play with the small adjustment knob on the side of the MPX (I believe it's called the MPX seperation control) - it can make quite a difference in Stereo seperation/quality.
 

Attachments

Thanks guys...
I got the "multiplex" issue resolved. Seeing how its just a strong stereo signal, the obvious answer is a better antenna. So I built a "Modified Cubic Quad" antenna.

http://audiokarma.org/forums/showthread.php?t=215209
http://audiokarma.org/forums/showthread.php?p=2594850

Just finished mounting it in the attic to the rafters and have strong stereo signals up and down the dial. Clean and clear. Where I live it's absolutly flat. No obstructions. On a clear you can see a building 20 mi. in the distance...in any direction!
So I feel that issue is resolved.

Visman...I'll still play around with the MPX seperation control.
Thanks for taking the time to upload the tube layout.
I guess what I really need to know is where I should put the "premium" tubes. Maybe some tube rolling at a later date.

Larry...I took a shot at measuring the 12AX7's last night and I didn't get anything close to the 10.5-11V. Your instructions were good, so I'll give it another shot tonight. Thanks

Sam...thanks...I found this...
http://transmitters.tripod.com/stereo.htm

Martin
 
Mark that mpx section control before you move it. You shouldn't have to move it at all. It's factory set for alignment. If everything works right, don't turn it.
 
Sam Cogley said:
Just a technical note, "multiplex" isn't a trade name, it's a functional description of how the current (finalized in April 1962) FM stereo system works.
Click to expand...

I understood what he meant, and he's correct with the observation. Everyone from Llyod's, Sound Design to McIntosh plastered "FM Multiplex" all over everything they made, right up until about 1980, long after "simulcast" was long gone. You want to tell me they all skinned the cat the same way as far as execution?

Ubiquitous marketing, almost like Clorox or Kleenex in the sense being discussed.

http://en.wikipedia.org/wiki/Stereophonic_sound
 
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Actually Sam is correct. I used "Trade name", due to a lack of better term at the time. The Multiplex circuit that everyone had to use after FCC approval, was originally designed and built by one Co.(GE or Scott I think)

I never differentiated Multiplex and Simulcast really. Never knew about simulcast until a couple years ago. It was Stereo or Mono to me. And yes, it's kind of like Clorox or Kleenex as a generic term for a certain product. I have a Simulcast Console and one of the stations here has a FM and AM station that play the exact same programming at the same time. I've listened to it on the DUMONT, and really can't see what everyone saw in it. Rather have MONO.

Larry
 
Clean&Tight said:
I understood what he meant, and he's correct with the observation. Everyone from Llyod's, Sound Design to McIntosh plastered "FM Multiplex" all over everything they made, right up until about 1980, log after "simulcast" was long gone. You want to tell me they all skinned the cat the same way as far as execution?

Ubiquitous marketing, almost like Clorox or Kleenex in the sense being discussed.

http://en.wikipedia.org/wiki/Stereophonic_sound
Click to expand...

You're more than welcome to be wrong, but I was entirely accurate in my statement. The circuit may be implemented in different ways, but they all have to do the same thing in order to decode the standardized FM stereo signal - which is a MULTIPLEX signal. The only non-MPX FM stereo broadcasts were the old simulcast broadcasts that ended sometime before 1965.

http://en.wikipedia.org/wiki/FM_broadcasting#FM_stereo
 
I should have added as if it were a special feature above and beyond the ability to capture and reproduce standardized stereo FM signals as you have pointed out.
All this in response to the OP's original question.

"1) Is MPX just another name for a basic Stereo signal?"

This term certainly was used in a way by marketers to imply some special ability or feature, for a LONG time after standardized FM Stereo signals were a matter of fact. Still is actually. So much so that even today it is part of the lexicon. I can't keep track of all the listings I see where they point out the fact that this or that model has an FM Multiplex, rather tham simply say this model has an FM Stereo tuner etc. Well of course it does! Of course it's "multiplexing"! But it's use even had the OP wondering.

As if blowing your nose in a Kleenex is a preferred, or different experience to doing the same in a plain old facial tissue. The gratuitous use of the term Multiplex for all these years still conveys a sense of technical superiority to the average consumer's mind. That's all I meant by my comment. They've gotten a lot of mileage out of that one.
 
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audiodon said:
Mark that mpx section control before you move it. You shouldn't have to move it at all. It's factory set for alignment. If everything works right, don't turn it.
Click to expand...

That's good advise - However, I see no harm in playing with the setting (after it's been marked). In my case it made a significant improvement.

As far as 12AX7 tube rolling - if you use the phono a lot V18 and V19 are the ones. V12 and V15 are the 12AX7 Left and Right Channel Driver tubes and I would consider them important.

As far as FM try rolling V1 (6GK5) and V2 (6AQ8) - made a difference in my 800C.

Remember to mark where each tube is located before removing :yes:
 
Note to those rolling V2. A 6DT8 is a direct sub for the 6AQ8, and it's about 60% the cost of a 6AQ8. The only place i've found where it doesn't work well is in a Sansui 1000A in the Multiplex board.

Larry
 
larryderouin said:
Note to those rolling V2. A 6DT8 is a direct sub for the 6AQ8, and it's about 60% the cost of a 6AQ8. The only place i've found where it doesn't work well is in a Sansui 1000A in the Multiplex board.

Larry
Click to expand...

x2 on this, I've used the 6DT8 in a couple Fisher receivers with great results as a cheap effective sub.

Kory
 
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