442.275 Failed and Repaired

The club’s 442.275 repeater began acting up this past week. At times it worked normally and at times the transmitter would suddenly drop out for a second or so, then come back to full power. KI8B did some diagnosing and narrowed the problem down to the repeater itself rather than the power amplifier or duplexer/antenna system.

TKR-820 Power Amplifier Module

Things came together for a site visit on Saturday 3/9/2019 to find the problem. The root cause turned out to be that the power amp in the Kenwood TKR-820 failed. Normally the repeater’s output is between 15 and 20 watts which is then fed into the 100 Watt amplifier. In it’s failed state, the repeater would output 5 watts or so, and then fade down below 4, then 3 watts, then come back to 4w or 5w, and so on.

N8XPK disassembling the repeater

N8JDM, N8XPK and N8CD did some diagnosing then tore into the repeater at the site and figured out it was the power amp module. Marty reached into his parts cabinet and pulled out two replacement modules and said “here, try one of these”.

First – a little background on the power amplifier that is on the 442.275 repeater. Most power amplifiers scale output based on input so that 10 watts in give 50 watts out, and 20 watts in gives about 100 watts out and so on. The new Crescend power amp that was donated to the club is the 20-50 watt input model with 100 watts out, and has a power control circuit in it that makes it maintain 100 watts out no matter what the input is as long as it can. Basically the amp puts out 100 watts or no watts. When the input drive gets too low, the amp flips off and attenuates, and that low threshold is apparently somewhere around 3-5 watts. Our amp also does not have a dropout relay, which makes sense for a commercial amp.

Back to the repair – About 38 screws and 2 hours later, the repeater’s amp module was replaced and the repeater was back to full power and driving the power amplifier to its proper 100 watts output.

Transmit & Receive audio taps

One casualty of repairing the repeater in a less-than-ideal work environment was that a wire to the controller was pulled loose in the process. So in the end the repeater went back to N8CD’s workbench to find the loose wire. The repeater has a couple of soldered-on wires to get the best quality audio possible in and out of the transmit/receive board.

TKR-820 with its new fan

While on the bench, the repeater had a fan and thermal switch added to the power module to improve its chance of survival in the future. The controller was upgraded from a Raspberry Pi 2 to a Raspberry Pi 3 B+ to improve the audio processing, it received a new SD card to replace the nearly 3 year old one, and a new interface cable was fabricated to a shortened and sturdier version.

After all that, the repeater was put back into service on Sunday morning. The SARA Technical committee is taking this as an early warning that replacement of the 28 year old UHF repeater needs to be planned. SARA should expect a recommendation in the coming weeks with a plan to modernize the UHF repeater system. Thanks to N8XPK for donation of the replacement power module, and KI8B, N8JDM and N8XPK for the diagnosis and repair work.


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