The Channel Plus 5445 four-channel analog modulator with H838 distribution amplifier (DA) includes infrared (IR) remote control of our cable DVR from various locations around the house. I've had the system, running continuously, for about 15 years. The modulator powers the DA over the coax and the same cable returns IR from devices at the TVs, through "IR blaster" ports on the modulator, to an IR emitter at the DVR. Over the last couple of years, the DA has occasionally shut off spontaneously. After I unplugged the system for a while, it would work okay. Finally, in Fall 2015, it stopped altogether. I powered the DA with a separate supply, but lost IR control of the DVR. Since this system's IR targets run on 5 volts, and nothing made today works with them, I was looking at a complete system replacement for more than $600. I considered upgrading to high definition distribution, but it just didn't seem worth the substantial cost for a kitchen or basement TV.

I couldn't find any schematics online, and the Channel Plus company has been through many name changes and owners. Their customer service number rings busy all the time, and currently, they are owned by a distribution company that has no customer support line at all. I was going to have to fix it by myself, or buy it all over again.

I traced through it and made hand-drawn sketches of the whole thing, except for the delicate cable input filter on the H838. That probably works fine, but it's not in use.

The circuit I've drawn here is the over-current limit circuit, in the modulator power supply going to the DA, to protect the supply from shorts. The function is needed because if you forget to put a DC blocker on a DA output that doesn't have an IR target in-line, the TV can draw a lot of current on that cable.

The specific problem is the 10 µF electrolytic capacitor on pin 9 of an inverter.

The large blue cylinder is the 5.1 ohm sensing resistor. Below it are the contact pads for the pass transistor. The first time I tried to diagnose this, I broke that transistor while trying to unsolder it. It was an On Semi J31C and after this photo, I installed a TIP29. The circled parts are the ones in my schematic. X marks the bad 10 µF capacitor. (Plus I've labeled the transistor that drives the IR outputs from the modulator.)

The circuit measures the voltage across the sense resistor. The MC14069 inverter chip is supplied with the same 15 volts as the rest of the circuit, so its thresholds are related to its supply. As long as the sense voltage drop is minimal, the 11/10 inverter holds pin 9 low, so pin 8 stays high and the pass transistor stays turned on. The 10 µF capacitor holds pin 9 low during initial startup, when a brief current surge through the system pulls the sense voltage down, pushing pin 10 high. This 10 µF capacitor was partially shorted, so it was acting as a resistor in parallel with 510KΩ, rather than as a current sink to keep pin 9 from rising. Instead, at startup, pin 9 jumped up and shut down the pass transistor, and that was that. A replacement capacitor, worth about 30 cents, solved it completely and it's been running fine for over a month.

Modulator fixed