Adding WiFi Remote Control To Home Electronics? Be Prepared To Troubleshoot

January 19, 2022 by Donald Papp 32 Comments

[Alex] recently gave a Marantz audio amplifier the ability to be remotely-controlled via WiFi by interfacing an ESP32 board to a handy port, but the process highlights how interfacing to existing hardware often runs into little, unforeseeable problems that can sink the project unless solved.

At its core, the project uses an ESP32 and the ESPAsyncWebServer project to create a handy web interface that is accessible over WiFi. Then, to actually control the amplifier, [Alex] decoded the IR-based remote signals by watching the unit’s REMOTE ports, which are intended as a pass-through and repeater for IR signals to other Marantz units. This functionality can be exploited; by sending the right signals to the REMOTE IN port, the unit can be controlled by the ESP32. With the ESP32 itself accessible by just about any WiFi device, [Alex] gains the freedom to control his amplifier with much greater flexibility than just the IR remote would offer.

Sounds fairly straightforward, but as usual when interfacing to an existing piece of electronics, there were a few glitches. The first was that high and inconsistent latency (from 10 ms to 100 ms) made controlling the amplifier a sometimes frustrating experience, but that was solved by disabling power saving on the WiFi interface. Another issue was that sending signals by connecting a GPIO pin to the REMOTE IN port of the amplifier worked, but had the side effect of causing the amplifier to no longer listen to the IR remote. Apparently, current flowing from the REMOTE port to the ESP32’s GPIO pin was to blame, because adding a diode in between fixed the problem.

The GitHub repository holds the design files and code. This kind of project can be pretty complex, because the existing hardware doesn’t always play nice, and useful boards like a modern ESP32 aren’t always available. Adding a wireless interface to vintage audio equipment has in the past involved etching circuit boards and considerably more parts.

Classic Amplifier Reborn With Modern Transistors

May 15, 2016 by Jenny List 26 Comments

Someone brought a dead Marantz amplifier to [Lansing]’s attention, a rather nice model from the 1980s with one channel entirely dead and the other very quiet. His account of its repair is straightforward, but provides some insights should you find yourself with a similar item on your bench.

Opening up the box, he was presented with 35 years of accumulated dust. It’s the annoying side of cracking open classic kit, we all have our dusty horror stories. His first task was routine: to replace all the unit’s capacitors. The mains voltage in France has gone up by 10 volts from 220V to 230V as part of EU harmonization in the years since the amp was built, so he used capacitors with an appropriately higher rating to compensate. We might have waited until the rest of the amp was proven fixed before splashing the cash on caps, but maybe we’re more thrifty.

The quiet channel fix turned out to be from a muting circuit designed to keep the amp quiet during the turn-on phase and suppress that annoying “thump”. A dead transistor replaced, and all was well. The dead channel though had a whole slew of dead transistors in it, which turned the problem from one of repair to one of transistor equivalence. Quite a few of the 1980s parts were no longer available, so modern replacements had to be found.

It is tempting to think of particularly all small-signal transistors as functionally equivalent. You will get away with this in logic and switching circuits in which the device is either On or Off and never in between, but in an audio amplifier like the Marantz things are not so simple. A lot of effort will have been made by the designers to calculate resistances for the current passing through them to deliver the right DC bias points without sending the circuit into wild oscillation. An important part of that calculation comes from the current gain of the transistors involved. [Lansing] had to carefully select his transistors for equivalence, though it some cases he had to do a bit of creative lead-bending to fit a different pin-out.

So, all dead transistors replaced with appropriate equivalents, and the amp was reborn. Success, and very much worth the effort!

We’ve covered a lot of amplifiers here in the past. Some were dead, like this little amp with blown capacitors or this smokin’ subwoofer. Others are more esoteric, like this ion wind 1KV tube creation.