CM-7777 / 7778: "New & Improved"? - Page 3

Somehow yesterday my attic installed Ability preamp stopped working. Don't know if it was due to a power surge during a thunderstorm, or due to overload from abnormally strong signals due to the weather conditions. The power supply still works, but I had to replace the preamp piece directly connected to the antenna balun. Fortunately, I had previously ordered a 2nd Ability preamp for future use, so I was able to determine and replace the part that was not working. The power supply is still good. Also, an original CM-7778 connected to another attic antenna and TV was not damaged. What do you think may have happened? I use the low noise Ability amp for weak distant signals, but perhaps it was overloaded and shorted out from strong local signals. Does that sound reasonable?

No. Strong local broadcast signals won't cause amplifier electronics to fail.

Surges from nearby lightning events, OTOH, is a common reason for amps to fail.

The preamps that have the lowest noise figures use transistors in the front-end that are more sensitive to static and impulse (EMP) damage; that's the trade-off.

There are three types of preamp overload, in order of increasing signal strength:

1. The strong signals cause enough IM distortion to interfere with the reception of weak desired signals. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.

2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen.

3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong.

Edited by rabbit73 - 6/25/12 at 4:51pm

I think a power surge during a thunderstorm may have toasted the preamp. The signal was split to 2 TVs and one of them had no surge protector.

Overall, I have had best luck with the original CM-7777 and 7778. But somehow, only the Ability preamp would allow me to decode a weak, distant translator signal from about 65-70 miles. I suppose it is the low noise figure and phemt circuitry that makes the difference. But I removed the splitter, and have the new Ability amp only connected to one TV with a good surge protector. Hopefully this will lessen future power surges and reduce the impact of potential damage in the future.

My antenna is in the attic, so my main concern is with electrical power surges that produce occaissonal pixelation and audio dropouts. Don't want the power surges to short out the preamp again. The power surges/electrical interference is noticeable on a weak UHF channel, and one VHF channel, usually between 5pm 6:30 pm most days and during thunderstorms. Trying to figure out if there is some type of whole house surge regulator that maintains stable power levels when the electrical currents enter the house. Going to Home Depot tomorrow.

If you're experiencing dropouts, it's probably from impulse noise. If it occurs at a certain time of day, it may be from some electrical device a neighbor is using at that time, or you may have a power company problem with a nearby transformer, etc. Ferrite chokes that clamp on the coax where it connects to the TV tuner may offer some help. But you won't be able to eliminate VHF dropouts from lightning in the region; unless the station moves to a decent (UHF) channel and broadcasts at an acceptable power level. In lightning-prone areas, VHF is a very poor choice for a DTV channel to be on.

Any should work. Like these, for example:
http://www.avsforum.com/avs-vb/showt...9#post21795489

They also sell them at Radioshack, but not sure about price. And they are normally an included accessory when you purchase a plasma TV.

If it reduced interference; yes.



It would depend on the type of noise, how bad it is and whether or not the ferrite choke at the tuner input eliminated it or not.



It's certainly not going to do this. That claim is total BS.

Reminds me of Black Friday 2003 when the guy at Best Buy said i needed that $200 Monster power strip with my new 52" RP RCA set, otherwise, it would get all kinds of bad images from dirty power.

No, but they are cheap and worth a try. Also, check your breakers to make sure the micro and TV tuner, preamp, etc. are not on the same circuit. Unlikely, though.

Glad it solved your pesky VHF reception/interference problem.

I replaced the fried Ability preamp with the other one I had ordered for future use. It is now connected to one TV again with a good surge protector, no splits. All is working well there. And the orig 7778 was also connected to a separate TV and antenna, also with a surge protector, so it was not damaged. I just wonder if splitting the first Ability amp caused some sort of uneven power flow, and that is what caused the damage.

Is there a link to a post where they recommend these devices? I am not an electrician nor engineer, so hopefully there is a somewhat simple, nontechnical solution. What is the best way to ground an attic antenna and protect the preamp?