Intermittent Upstream Timeouts in a 48-Unit MDU
You are the third technician sent to unit 214 in a 48-unit garden-style MDU. The customer reports internet drops most evenings between 19:00 and 22:00 and during HVAC cycles. At the outlet, downstream 256QAM at 573 MHz measures +1.5 dBmV with MER 38.5 dB; upstream at 36.4 MHz transmits at 50.5 dBmV, close to the 51 dBmV modem ceiling. Upstream SNR is 34 dB at 14:00, then falls to 23 dB with T3/T4 timeouts when the building's air handler starts. The drop is 180 ft of RG-6 from a 6-port tap at +16 dBmV; the tap has one open port with no terminator. A wall-plate barrel connector is pitted. The building has new LTE equipment on the roof. The customer is frustrated and has already missed two appointments.
1. Rank the likely causes of the upstream degradation and explain why the downstream MER remains acceptable while the return path fails.
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Ranking: (1) intermittent HVAC/arcing noise entering through the open tap port and pitted barrel; (2) shield or grounding problem on the drop; (3) common-path distortion from a marginal drop; (4) rooftop LTE equipment, likely a red herring at 36 MHz. Downstream 573 MHz MER of 38.5 dB is healthy because the ingress is strongest at low frequencies where the HVAC motor and loose connections radiate, and because downstream levels at the outlet are high enough to keep the carrier-to-noise ratio favorable. Upstream is different: the modem's 36.4 MHz signal arrives at the node very weak after the drop and tap, while ingress adds at the same frequency. The open port is an antenna, and the pitted barrel is a nonlinear point. When the air handler starts, broadband noise raises the upstream noise floor from 34 to 23 dB SNR, causing T3/T4 timeouts. LTE at 600+ MHz cannot directly explain a 36 MHz impairment, so do not start by replacing the modem.
2. Using the measured levels, assess whether the drop loss is within expectations and list the tests and repairs you would perform, in order.
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Expected 573 MHz loss in 180 ft of RG-6 is roughly 5.4 dB/100 ft x 1.8 = 9.7 dB; add about 1.5 dB for connectors and wall plate, so tap +16 dBmV should yield about +4.8 dBmV at the outlet. The measured +1.5 dBmV is about 3.3 dB low, suggesting a damaged drop, bad barrel, or extra splitter. At 36 MHz the drop loss is much lower, so the upstream TX of 50.5 dBmV is not explained by normal drop loss. Sequence: (1) replace the pitted barrel and terminate the open tap port; (2) inspect the ground block and drop for shield damage; (3) run a return-path spectrum sweep at the tap and at the ground block while the HVAC cycles; (4) if ingress persists, replace the drop; (5) verify upstream SNR and TX level over a 24-hour monitor.
3. What do you tell the customer, and how do you handle the two missed appointments?
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Acknowledge the two missed appointments, apologize for the inconvenience without blaming prior techs, and explain that the problem is intermittent and shared with the building's electrical equipment. Tell them you found a likely entry point and are repairing it, but that you need to observe a full HVAC cycle before closing. Give a two-hour arrival window, call before arrival, and follow your company's missed-appointment credit policy. Avoid promising a specific root cause until the repair is verified; offer a follow-up call after 24 hours. If the customer is upset, ask what outcome matters most, such as reliable video calls or work-from-home, and set expectations around that.