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 Post subject: HH SCOTT LT 112B Question -
PostPosted: Oct Mon 03, 2011 5:32 am 
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Does anyone here have a schematic or service manual for this that they can share?

I recently bought one and it works well, in fact it is the most sensitive and selective FM receiver I have seen in a long time.

The only problem it has is that the Multiplex light is not bright enough. At first it had a bad #49 bulb in there (it is the correct type) so I replaced with a new #49. It lights but it's dim and only has about 1.2 volts.

I don't know what the voltage is supposed to be when the discriminator sees a Stereo signal.

All transistors are original and the construction is outstanding.

Thanks!

J.

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Thu 06, 2011 11:42 pm 
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J,

I built an LT-112B-1 in 1967. Most likely very similar if not the same. I still have it (somewhere) but should have the assembly manual. I am away until Nov 4 of this year so I can't help you right now. So, if no one comes forward before then, I will be delighted to help. Just shoot me a PM after that date to "wake up" the brain cells and I will get it scanned and off to you.

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 07, 2011 12:33 am 
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Thanks Don. Will do!

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 07, 2011 3:48 am 
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Unusualdesigner wrote:
Thanks Don. Will do!


No worries Mate!

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 07, 2011 8:05 am 
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Hi J,

PM sent.

Pat

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Tue 11, 2011 4:48 pm 
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Location: Marietta/Moultrie GA
If I might ask... if you find out what's causing this problem on the LT112B, please let us know here. I have a Scott 344B receiver, which I think has just about the same tuning section. My multiplex light just doesn't work at all, no matter what bulb is in it. I suspect it may be a more advanced case of the same condition?

Regards,
Gordon.

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Tue 11, 2011 10:01 pm 
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Hi Gordon.
Sure, as soon as I find out what's going on I'll post it. The bulb has to be a #44 and open circuit it has about 3.4 V on it but drops to about a little over a volt once a good bulb is in. It lights up and you can see the jewel glow but only in a very dim room.

At this point, unless I get the schematic (I can buy it but I'm too cheap <g>) I'm not going to go poking in there. I suppose I could reverse engineer the circuit but it's not that much of a priority; my ears can tell if I have tuned a MPX station ;)

Best,

J

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Thu 13, 2011 4:37 pm 
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Same here. Mine is very obviously working in stereo, on a stereo station, so I'm not overly motivated to rip it apart.

But, it would be nice to have it ALL working, someday...

Regards,
Gordon.

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Thu 13, 2011 7:19 pm 
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If you post the part of the schematic with the mpx stage it would help. The thing
to check is the transistor that turns on the bulb. Is it a germanium type? When
the lamp is on, and off, what's the EC voltage of the transistor? The bulb circuit
looks for the pilot so the question is working in full stereo with lots of
seperation?
Did they want you to test that amp with a dim bulb after you built the kit?

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Thu 13, 2011 8:14 pm 
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radiotechnician wrote:
If you post the part of the schematic with the mpx stage it would help. The thing
to check is the transistor that turns on the bulb. Is it a germanium type? When
the lamp is on, and off, what's the EC voltage of the transistor? The bulb circuit
looks for the pilot so the question is working in full stereo with lots of
seperation?
Did they want you to test that amp with a dim bulb after you built the kit?


Aye, there's the rub.... I don't have a schematic. I didn't build the kit, I bought it from the guy who originally built back in the 60s. The guy is an engineer and his wiring is beautiful, better than most "factory" wiring, solder joints are damnd near perfect.

This particular tuner is extremely sensitive and selective, the IF appears to be perfectly aligned so I don't want to go twiddling on anything unless I know what I am twiddling.... I can follow the circuit to the bulb driver and see what's up; maybe a couple of out of spec resistors?

Going by sound alone, the separation appears to be spot on. As you tune in and out of a stereo station you can see the bulb come on and off, sharply. There is no gradual brightening or dimming; it's either on or off - just not full brilliance.

When I first got it I brought up the AC using the variac and a current meter while monitoring the DC voltage at the filter cap. All was well.

I considered selling the tuner and the associated amplifier on fleabay but after seeing how tight the selectivity is I decided that I am going to keep them.

As long as we're using it at home it doesn't really matter if the Mpx light is bright or not.

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Thu 13, 2011 10:20 pm 
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Location: Powell River BC Canada
If you have a scope probe around the ferrite coils in that stage if you get a
big signal feeding the diodes after the 38Khz one, when you tune in, it is
probably just the bulb transistor or power circuit to it.

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Thu 13, 2011 11:30 pm 
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Location: Utica, NY 13502 (USA)
Unusualdesigner wrote:
The bulb has to be a #44...

The bulb should be a #49 (not #44). This is a low voltage, low current bulb which works in conjunction with a switching circuit that detects the presence of a clean, 19 khz pilot tone. The bulb gets fed 6.3 volts AC but a resistor and a diode drop the voltage to a safe value for the bulb and supply (pulsating) DC to the switching transistor. I have one of these tuners and bought the expensive manual so I can diagnose a problem with the tuning meter. This may help with the understanding of what is going on with the stereo indicator circuit.

Dave


Attachments:
LT-112B MPX BD Parts List.jpg
LT-112B MPX BD Parts List.jpg [ 39.57 KiB | Viewed 4419 times ]
LT-112B MPX BD Parts Layout.jpg
LT-112B MPX BD Parts Layout.jpg [ 50.17 KiB | Viewed 4419 times ]
LT-112B MPX BD Schematic.jpg
LT-112B MPX BD Schematic.jpg [ 42.65 KiB | Viewed 4419 times ]
LT-112B MPX BD Connections.jpg
LT-112B MPX BD Connections.jpg [ 24.79 KiB | Viewed 4419 times ]
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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 14, 2011 6:43 am 
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thanks very much for the information, that makes sense. I am away from the shop so I can't go and look right now but I suspect that you might be right about the bulb #. I might have made a typo. In the back it says 2 #47 and 1 #(44/49?)... I'll look at it tomorrow. I know that I had the right bulb to put in.
I'll check the drive to the transistor and see what pilot level I'm getting. As I said, I don't want to screw with the IF alignment as it appears to have the right bandwidth and slope. Sensitivity and selectivity are excellent so as long as it's working....

The one bulb I need to get is a 1819 for the amp's meter lighting. Why they put a 28 V bulb in there? I can get them at one place but it's across town and I won't be getting there for about a week at least.

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 14, 2011 12:21 pm 
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There is a separate 6.3 volt winding on the power transformer for all bulbs. The (2) dial light bulbs and (1) meter bulb are all shown as being #1847 (long lasting #47). The meter bulb has a 10 ohm resistor in series to reduce it's brightness.

Dave


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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 14, 2011 3:18 pm 
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Dave Doughty wrote:
There is a separate 6.3 volt winding on the power transformer for all bulbs. The (2) dial light bulbs and (1) meter bulb are all shown as being #1847 (long lasting #47). The meter bulb has a 10 ohm resistor in series to reduce it's brightness.

Dave


Not in this one.... Clearly marked in the back "three # 1847, one #49" ..... and that is what I have in the Mpx socket, a #49 -just checked again now.

Anyway, thanks for all the ideas. Later today I'll check the voltages around the driver.

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 14, 2011 3:42 pm 
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Unusualdesigner wrote:
Dave Doughty wrote:
There is a separate 6.3 volt winding on the power transformer for all bulbs. The (2) dial light bulbs and (1) meter bulb are all shown as being #1847 (long lasting #47). The meter bulb has a 10 ohm resistor in series to reduce it's brightness.

Dave


Not in this one.... Clearly marked in the back "three # 1847, one #49" ..... and that is what I have in the Mpx socket, a #49 -just checked again now.

Anyway, thanks for all the ideas. Later today I'll check the voltages around the driver.


Isn't that what I said earlier?
"The (mpx) bulb should be a #49 "
"The (2) dial light bulbs and (1) meter bulb are all shown as being #1847"

Dave


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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 14, 2011 3:56 pm 
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Dave Doughty wrote:
Isn't that what I said earlier:
"The (mpx) bulb should be a #49 "
"The (2) dial light bulbs and (1) meter bulb are all shown as being #1847"Dave


Right, and the CORRECT bulb is installed and still dim.....

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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Oct Fri 14, 2011 4:01 pm 
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I guess I was confused when you quoted my statement about which bulbs to use and said "Not in this one". Good luck with your tuner.

Dave


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 Post subject: Re: HH SCOTT LT 112B Question -
PostPosted: Feb Wed 14, 2018 4:36 am 
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he guys nice site here! i'm working on ascott lt112b now, love it and plan on keeping it, sounds great. I had a couple questions, i just replaced the power supply diodes and te cap can w/ Panasonic fc, replaced the two electrolytic and all the light blue 0.022 caps with solen polyprop sounds great sofar... wondering about replacing the two black 50uf 10v caps in/near the output section.. use some nichion muse bp caps or some polyester? any suggestions. ? and what are the red barrels? resistiors?


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