But I still don't understand why they put the electrolytics in the preamp and run AC up the coax. Could it have anything to do with trying to inhibit corrosion with a bipolar voltage?

Are the electrolytic caps in the preamp wired to make a non-polarized electrolytic cap ?



With the DC power supplies, considering their low current draw, and where they are to be mounted, are perfect candidates for solar power, coupled with some low cost NiMH batteries.

Are the electrolytic caps in the preamp wired to make a non-polarized electrolytic cap ?

From the picture and scope trace it is rectifying the negative
part of the AC wave -- you can see the cap charging during
the relative flat of the sine tip at -13~16V minus the diode drop.
The first cap looks to have a fair amount of charging ramp which
presumably the cascaded 39-ohm resistor and second cap
were intended to reduce.

The corrosion avoidance theory was in the case there was
either a moisture bridge across the coax or a loose/intermittent
coax connector bridged by moisture. Who knows?

The corrosion avoidance theory was in the case there was
either a moisture bridge across the coax or a loose/intermittent
coax connector bridged by moisture. Who knows?

My current theory is that powering with AC allowed Channel Master to manufacture preamps using NPN transistors needing a positive voltage and other models using PNPs that require negative and ship the same power supply / injector with each.

Brian

As mentioned above, I believe they used AC in early Preamps due to use of 300-ohm twin-lead,
which the installer would have to take care to connect with correct polarity if it were DC.

Locating the electrolytic in the Preamp also has significant performance benefits,
rather than being located many ohms and lamdas away from the active circuits.
And cost benefit if a SECOND electrolytic has to be used in Preamp anyway.....

I believe they used AC in early Preamps due to use of 300-ohm twin-lead, which the installer would have to take care to connect with correct polarity if it were DC.

Good point, it would have been very easy to screw up.

I fixed a few glitches and uploaded a revised version
which may be found here:

http://freepdfhosting.com/cd7267a855.pdf

The pdf hosting site deletes files after 30 days so this
will turn into a pumpkin at that time.

I forgot to thank you for all the work you put in tracing out that circuit. It's interesting and nice to have.

I was just at Fry's a little while ago. They had a cheaper CM preamp ($30) I hadn't seen before. It has a gain of 13 dB, if I recall correctly, and a noise figure of 3.5 dB. The single 300-ohm input is via a thick, heavy-duty, captive balanced line a couple feet long that terminates in large spade lugs. The injector is a small satellite type with a cable shrink-wrapped at the power socket. The cable runs to a wall wart.

The more familiar CM preamp (I forget the number now but it's been discussed here) was $60.

Brian

I was having some trouble with that PDF for some weird reason so I PM'ed uhmgawa - the result is that I saved the image and scaled it bigger, so its now available here:

http://www.user.dccnet.com/jonleblanc/images/channel-master-0265dsb.png

They had a cheaper CM preamp ($30) I hadn't seen before. It has a gain of 13 dB, if I recall correctly, and a noise figure of 3.5 dB.
Yeah, they have that one over at summitsource and solidsignal too. Its still better than the Radio Shack preamps.

I fixed a few glitches and uploaded a revised version
which may be found here:

http://freepdfhosting.com/cd7267a855.pdf

The pdf hosting site deletes files after 30 days so this
will turn into a pumpkin at that time.
Are those typos in the resistor values, e.g. "4K7 ohm"???

Stampeder: Your image is blurry and nearly illegible....

Are those typos in the resistor values, e.g. "4K7 ohm"???

Heh, actually Ive seen some resistors labeled like that, and I went WTH ???

Are those typos in the resistor values, e.g. "4K7 ohm"???

No that is as intended. It is a fairly common convention
of replacing the decimal point with a (less likely to get
lost in the noise) units/multiplier. So in the above example
4K7 -> 4.7K, 3V3 -> 3.3V., etc..

Stampeder: Your image is blurry and nearly illegible....Is the original PDF working properly for you, and is it clear enough?

Uhmgawa's original comes through just fine....

Three locations exist where SMD resistors are stacked. The values
of the obscured resistors were calculated via series current, and
were within 5% of EIA standard values. Updated schematic is here:

http://freepdfhosting.com/1e7ee1e7dc.pdf

Note: this pdf hosting site deletes files after 30 days so this
will turn into a pumpkin at that time.

Mirrored here, where there is no deletion deadline:

http://www.user.dccnet.com/jonleblanc/Canada_TV_Stations/CM-0265SDB-schematic.pdf

mlord:

I completed my DYI b4b antenna, and recive most HD channels, however I'm looking to squeeze more gain, should I attempt the DYI 1/2 wavelength balun, or just buy a balun.


I believe the RS one I bought is about 5 db. No real info on it..

I believe the RS one I bought is about 5 db. No real info on it..
Yeah, pretty much overpriced crap, heh.

300ohm:

What about you, you suggest I make my own, is it better?

For outdoor use, a Channel Master balun at $1.99 (U.S) is a good bet.