Did I damage my CM7777 ?? Also - a coax question

[bhelms]

Well, this past weekend was spent trying to turn my temporary new OTA antenna set-up (CM4228/CM7777 about 33' in the air) into a permanent installation (maybe up to 35' high), adding the CM9521A rotator, proper grounding, guy wires, etc. The results of the temporary set-up were encouraging - I could receive 3 co-located digital signals (ch 24) from 25+ miles away that were previously blocked by a ridge and trees. (Ch 23 analog = ch 24 digital.) I was hoping the rotator would help me fine-tune enough to receive at least one other digital channel in the same location, and two others from 55 miles in a slightly different direction.

In summary - it didn't all go as planned! I made a number of changes along the way adding several variables, thus it's hard to pin-point the real cause(s) of the latest problem. The initial result was *zero* signal.

In a real "Do'h" moment - I realized that I connected the coax from the house into the "VHF/Separate" input on the CM7777 instead of the "To TV" output. The system was powered-up that way for at least 30 minutes while I was trouble-shooting. Once I found that stupid mistake I corrected it, but the analog ch 23 signal is quite fuzzy (maybe worse than with the temporary set-up?) and I still can't get anything on digital ch 24. I still do receive one set of digital subchannels (ch 15) very well that I had been getting all along with the temporary system (and also with my old antenna system), so perhaps all is not lost. Regardless, something has happened to degrade my signal enough from where I was with the temporary set-up that I can no longer lock the digital ch 24 signals.

So my real question: Is there any chance I damaged the CM7777 with that screw-up? Any series capacitance on that VHF input would certainly block the DC power and prevent any damage, but I don't have a schematic to confirm. Since I can get ch 15 well, I think the answer is no damage, but I guess I want to confirm that...

Also - I am a believer in quad shield cable in OTA antenna systems where electrical interference or multipath can be an issue, and I have had both at various times based on my historic analog results. The temporary set-up used "Andrew" (Channel Master) dual-shield cable, their type A660-BVV-3, a full 100' between the pre-amp and the PS that was then located at the HD set. For the permanent installation I switched to Belden 1189A quad, about 70' from the preamp to the PS, then it's into some existing Tandy quad cable for about another 40' to the HD set. I did a spec. comparison between the Andrew and Belden types and in terms of RF loss through 1GHz they are comparable with the Belden slightly better. But in the DC resistance category, the Belden is superior at about 33 ohms/1000' (center cond. plus shield) vs. 41 ohms ("loop") for the Andrew. So I went with the Belden thinking it would be possibly superior in performance and have the better shielding. With either type the final installation will include that 40' of Tandy coax that's in conduit and can't be replaced (packed too tight!) I did manage to test the analog ch 23 at the PS using another analog TV (thus eliminating that 40' Tandy coax run) and it's equally fuzzy compared with the HD TV. Is my analysis of the Belden QS vs. Andrew DS coax valid, i.e., that the Belden should perform equally well? I'm using compression connectors throughout.

Any thoughts would be appreciated...!

TIA...!

 

[NightHawk]

In most RF amplifiers designed for minimum noise figure there will indeed be a capacitively coupled input with both a series coupling capacitor and parallel filter capacitor, possibly preventing the DC voltage/current from damaging the input transistor. It is also quite possible the power supply voltage exceeded the survival voltage rating of the input capacitor(s) and got through anyway. It's impossible to say if any damage occurred without bench testing. Low-noise amplifiers like this can be damaged and still function somewhat, albeit with a poor noise figure.

As far as the coax is concerned, if the amplifier is working properly it will provide 20 dB or so gain at the antenna. What this means is the signal levels are elevated by a factor of 100 and pretty much makes small differences in cable quality and shielding insignificant. QS is completely unnecessary in this case and any simple RG6 (or even RG59) with reasonable quality will suffice up to several hundred feet. In any case you can't draw any conclusions regarding the quality of an RF cable with a DC ohmmeter unless it's completely open.

 

[Rick0725]

if you are concerned of damaging your amp. call channel master technical and ask them.

they would know.

 

[bhelms]

if you are concerned of damaging your amp. call channel master technical and ask them.

they would know.

Tks, Rick - I e-mailed them for that exact reason. Here is their (partial) response:

"....if the 7777 is still working then chances are you did not do any major damage, however some damage may have occurred...." Kinda a non-commital answer, however I agree with Nighthawk that it might still be amplifying but at a higher noise level. It would be tough to tell unless I put it on the bench. I might just have the possibility of doing that!

If the input is capacitively coupled then it would most likely be via ceramic chip caps, and those typically have BDV ratings of 100 volts or more, depending on the circuit impedance (more specifically, source resistance and its affect on the voltage spike rise time). The power supply was putting out about 22 VDC unloaded. I don't think that would destroy any ceramic cap. I asked for a schematic so I could confirm the circuitry at the inputs, but they won't give me one.

I'll yank it back down this weekend and maybe do some more investigating. Tks for your replies...

EDIT - In fairness to CM - they did offer to replace this amp even tho' it's a few days past its 90-day warranty and even tho' any problem was admittedly my fault.

 

[Rick0725]

if they offered to replace...take them up on it.

 

[bhelms]

I will likely do that even tho' I caused the problem. (The amp was definitely working well in the temporary setup.) I was hoping to "baseline" it on the bench first and if the gain and noise levels looked OK then I wouldn't need the swap. If either was suspect then I'd have a basis for comparison when a new unit arrives. Now the only question is whether or not I can get it tested here at work (I'll need help with that!) in a reasonable amount of time!

Tks again for your interest...

 

[NightHawk]

You would need a noise figure analyzer and calibrated noise source. Otherwise you can only do a gain check.