72 ohm coax and connector used in UHF converter

[Jonathan]

Hello,

I have a RCA U70 UHF converter that I just finished fixing up. Instead of trying to reform the 1953 multicap electrolytic, I removed it and replaced it with some modern ones. The electrolytic capacitors used for the power supply filter were 30uF, 30uF, and 50uF that I replaced with 3 Xicon 47uF 450V electrolytics. I changed some of the cloth covered wire, although the only wire in bad shape was the cloth covered wire coming from the power transformer. If bent, the inner insulation would crack and the outher cloth would frey. The interconnecting cloth covered wire would bend fine and hold up perfectly, which I thought was odd. I powered it up and the tubes lit up, the capacitors I just put in did not get warm or pop, and no smoke was released.

Anyway, the UHF input was accomplished with an SMA-looking 72 ohm coax connector, which was connected to the UHF RF section with 72 ohm cable. RCA servicing data tells me the connector and coax is 72 ohm.

When I was fixing it up, I got a piece of 75 ohm RG-59 coax, stripped off the outer sheath to expose the copper shield braid with the inner conductor surrounded by a thick sheath of plastic, like you normally see in a RG-59 cable. I unsoldered the inner conductor of the 72 ohm coax from the inductor tap (the center tap from the main inductor of part of the UHF section was a loop that went around a plastic tube with a makeshift HF inductor, the center tap point was on the bottom of the loop where a shaft was connected to it to support that HF inductor--refer to servicing data as my description is crap and soldered the inner conductor of the 75 ohm coax to the HF inductor center tap and routed the coax where the old coax was, closing the loops and soldering the braid to the loops like the old coax was soldered, and put a standard F connector on the mount that I drilled out the mounting hole to fit it, and connected everything like it was previosly connected before. I know my description is completely impossible to understand without actually working with this converter, please do your best.

Anyway, the coil tap, I'm guessing it's input impedance is 72 ohms, in theory, or doesn't it have any defined input impedance? 75 and 72 ohms is not much of a big difference, but I want to do this the proper way. Also, since these UHF converters have no RF amp, I suppose using a signal booster box/UHF amp or a modern amplified antenna is probably the best thing to do to get any decent performance out of these?

And do you think they'll work if you connect them to a modern TV that has no fine tuning, or do most modern tv's have AFT? I really want to see how well this unit will work, so I won't have to go through the headache of realigning.

Thanks again.

Jonathan

[OvenMaster]

Jonathan, I know absolutely zip about most of what you said above (I'm sorry!) but any TV set made today with the pushbutton "up/down" and direct access tuning is indeed fully automatic and will home in on the carriers, meaning yes, they're AFT. As long as the converter's output is within a couple of MHz of a standard broadcast channel, the TV set will follow it and lock onto it.

At least one question answered for you.

Tom

[Chad Hauris]

You can use heat shrink tubing to repair old brittle wires from power transformers...if it looks really bad you may want to take the covers off the trans and make sure the tubing runs over the wire all the way to the core, but most times if you can get the tubing just up in side the metal opening of the trans where the wires come out this will be OK.