Monoprice 30W Powered Desktop Speakers (605300)

Encouraged by my resurrected Insignia powered subwoofer, I dug up another item from my to-do list. These are Monoprice Pro Audio Series 30W Powered Portable Speakers, item #605300. (No product link as this item has long since been discontinued, though their Powered Desktop Speakers category is still alive and well.) I had bought it for use as my computer desktop speakers and they worked well for a few years before falling silent. Then they sat for many more years in the teardown/repair pile until now.

The two speakers are not symmetrical. One of them have all the equipment and the other is a simple box with drivers. The fancier box (wired up to be the right channel but shown to the left in above picture) has a volume knob and two audio jacks. One jack is an auxiliary input to temporarily replace signals coming in from rear main audio input, and the other a headphone jack we can plug in to temporarily listen to something privately. This latter jack still works: I could hear the audio signal through headphones plugged into this jack, and I can hear loudness changing as I turn the volume knob.

The asymmetry is very visible when looking at the rear of both speakers. One has the power plug and switch, plus the aforementioned main audio input. A slider switch for “Bass Boost” On/Off (I never noticed much of a difference either way) and speaker level output to drive the other speaker.

The volume knob is surrounded by a ring of plastic that glows blue when it is powered on. This light still illuminates, so I don’t think the problem is as simple as a blown fuse.

Looking inside the simpler box first, it’s hard to see very much through the small opening. The electronic bits we could see is probably an audio crossover circuit.

Moving on to the other speaker, we see a lot more and thankfully they’re more accessible as well. AC power enters the enclosure to an in-line fuse. (I didn’t think the fuse was the problem, but I checked anyway and there is indeed electrical continuity.) Power then flows to a transformer which steps ~120V AC down to ~14V AC. This stepped-down voltage connects to the circuit board, adjacent to a large four-pin package that looks like a rectifier.

Four sets of wires lead from this board into the speaker enclosure. The smallest and thinnest pair of wires go to the smaller speaker driver for higher frequencies, and the thicker pair goes to the larger driver. Two gray bundles lead to front-panel controls, one for the volume knob/power LED and the other for the auxiliary/headphone jacks.

Examining the circuit board, I see discoloration underneath these two components. Labeled Z1 and Z2 with diode symbols, I infer these are Zener diodes. Z2 was held down by a white-colored compound of unknown nature. That stuff was tenacious and refused to peel off, but I could cut it with a knife allowing me to unsolder both Z1 and Z2. Once removed I could read diode markings as IN4742A, confirming they are Zener diodes. I don’t have any replacements on hand, but I could give these two a quick basic test. With my multimeter switched to diode test mode, they read ~0.72V the one way and nothing the other. These are expected values of a diode proving they have neither failed open nor failed short. Circuit board discoloration showed that they’ve been running hot, but that fact by itself is not necessarily a problem with Zener diodes. A full diode test is beyond my abilities at the moment, so I soldered them back into the board and tested the speaker again. I had a slim hope that heat stress damaged a solder joint and resoldering them would bring the speaker back to life. No such luck, but it was easy to check.

Next, I looked into the still-functioning headphone jack. The speakers would go silent when audio is going through the headphones. Perhaps the jack is stuck in the “we have headphones” configuration. This would keep the speakers silent even when there are no headphones present. Unfortunately, the audio jacks are mounted on this circuit board, glued to the enclosure. Breaking the board free may be destructive, so I put this off to later.

Looking for promising components to investigate, I settled on the audio amplifier chip. It is a big component with large pins that I could probe, and its markings are visible for easy identification. I found and downloaded the datasheet for ST Electronics TDA7265 (25+25W Stereo Amplifier with Mute & Stand-By) and got to work understanding how it was used here.

I printed out a picture of the circuit board (*) so I could take notes as I probed the board (with the power off) while comparing it to TDA7265 datasheet information. The first order of business was looking for pins 1 and 6, which the datasheet said were both negative side of input power. I found those two pins connected to the same copper trace on the board leading to one pin of the rectifier, giving me confidence that I’m looking at the right part and I am oriented in the correct direction. I noted the pins I wanted to check once I’ve powered on the board:

  • Pins labeled R+ and R- should be DC power rectified from the ~14V AC transformer output. If there’s no voltage, I may have a dead rectifier.
  • There are two inputs, each with their positive and negative pins. I’m not sure which is wired as left and which is right, but I can connect a stereo signal to both input jacks. I should see line level voltage if audio signal makes it to the amplifier chip. If not, I can backtrack from here.
  • If audio makes it to input, I will probe Outputs 1 and 2, which should have speaker level voltage relative to a shared ground.
  • If there is input signal but no output, I will probe pin 5 which controls mute & standby behavior. See what voltages I read, and compare behavior to what datasheet says.

With this plan in hand, I prepared my tools. My LRWave web app written earlier for Lissajous experiments will provide test input signal. For probing the circuit, I have my multimeter and I have my oscilloscope. As a quick test, with the power still off I probed the audio input jacks while LRWave was running full blast. I measured ~0.6V AC on those pins (in the above photo, labeled in the lower right as “R IN, R GND, L IN, and L GND”.) This is a great start. I then turned on the power strip (powering up the speaker) and was immediately blasted by the sound of LRWave’s 440Hz sine wave.

The speaker works now! That is great, but… why does it work now? The last hardware modification I deliberately made to the device was to resolder Zener diodes Z1 and Z2. I tested the speakers then, and it didn’t make any sound. I must have made another (non-deliberate) change to the hardware to bring it back to life. Was it reaching for the audio jacks and jiggling a loose cable connection? Was it something I did by accident while probing the amplifier chip circuit? I don’t know. The speaker works again, but this success was unsatisfying. I wouldn’t call it “repaired”, either, as I can’t explain how I fixed it. It could just as easily and mysteriously break again tomorrow. But if it does, at least I have a plan to investigate for Round 2.

(*) The lone surface mount IC visible on this side is a ST TL074 quad op-amp.

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