recently we bought a Hitachi TRK-612E boombox. there's not much info on these online other than listings of other people selling them, but suffice to say it's a pretty average budget boombox probably from the late 80s. it has tape and 4-band radio, but that's about it. you can record tapes from mic or from radio, and the one really impressive feature is that this has a fairly nice tape deck with an actual electromagnet for the erase head instead of a permanent magnet, so re-recording tapes should sound cleaner. it can work from mains (ours is a UK model hardwired for 230V 50Hz) or 6 D-cells at 9V. there's a switch in the back to select whether to power it from AC or batteries.
our one came from an ebay listing saying that everything worked but the tape... when we plugged it in we found that suddenly nothing in our house worked anymore, in fact. the mains plug was wired between live an earth (😬) instead of live and neutral, so the transformer tripped the RCD breaker. we figured this was probably wired that way since the 80s and the previous owners moved into a house with RCD, and thought the boombox no longer worked (it would work fine on a wiring without RCD protection)... once the mains plug was rewired correctly, surprise! everything worked perfect, even the tape! at fairly correct speeds without even needing belt replacements (by ear anyway)
so now that the boombox was working, we wanted to make it portable again, but without wasting a fortune on D-cell batteries. turns out, you can get off-the-shelf USB boost converters for 9V, which the boombox expects off DC input, and with that it was easy to power the boombox off a sufficiently beefy powerbank. the back of the boombox rates it at 8W, so at 5V that would be around 1.6A, meaning a 2A supply (which most modern powerbanks provide) is enough to keep everything running within spec. i've done some USB power conversions like these before and i worried about possible noise coming from the powerbank's circuitry bleeding into the amplifier input (since batteries don't produce line noise like boost power circuits do, and therefore battery-designed circuits don't generally foresee having to filter it), but it turns out this worked great even with some older powerbanks! that was easy!
the slightly more daunting mod i wanted however was to give this boombox an aux in. it wasn't uncommon for boomboxen from this era to have an aux in so you could use it as a speaker for a CD player for example, and record tapes from it, but cheaper models like this one seems to be usually omit this feature. the trick to modding a line input into an 80s boombox is to find a mode you can set it to so that the amplifier circuit is powered but whatever audio input it's amplifying can be replaced by your own external one. this one has two modes: "tape", which powers the amp only while the play head is engaged, and "radio" which powers the amp and the radio circuitry together. i had a while looking inside the boombox and i found that the radio side of the equation was actually quite simple: a single LA3361 chip was in charge of demodulating the radio signal and outputting the line audio that went into the amplifier.
from there, it was a matter of figuring out a way of disabling the chip and tapping into the same audio input it did for an external line instead. i did this in a fairly rudimentary way by lifting the VCC leg of the chip away and wiring it in series with a switch instead. this way, i can switch the boombox to "radio", and then switch power to the radio chip off. the amp will be powered, but there would be no audio signal coming into it. i tested that and sure enough as soon as it opened the switch, the sound was gone and there was the ever so faint hum of the amp still powered, so that was good news!
next, i simply wired a scrap bit of 3.5mm audio cable to the same ground, left and right audio pins of the LA3361. this way i'm exploiting the same circuitry that's already in place to take care of the incoming line audio from the radio chip to bridge my external input into the amplifier. this worked pretty well, the sound is perfectly in line with what the amp expects and this has made the boombox into a great portable speaker, but it's not the best way to do it. because i have no passives protecting the other end of the audio cable, for example my phone sometimes detects ghost presses on the audio jack (like a headphone with inline volume and play controls). also, you need to make sure that the radio chip is off before you plug an audio input, otherwise you might backfeed line voltage into your audio output (which might not be too harmful since it's the same voltage range anyway, but best to be safe)
considering all of this, i consider the mods i did a huge success! the boombox now can record and play external aux in, and it works from new power sources which means it's portable again. we also replaced the big old hard-wired mains lead with a figure 8 receptacle, so there isn't a huge cord you need to coil back inside the boombox every time you want to access the battery door (which is often since the new radio switch and aux in cable are housed there). an improvement i want to make some day is to use an aux jack with a switch that opens when a plug is inserted to automatically switch off the radio chip. one reason i haven't is because to use a jack rather than a built-in cable i would want to mount it somewhere and i'm not sure yet i want to physically modify the shell. plus, some passive circuitry to isolate the audio input and protect against the ghost switch presses on mobile phones. but for now, it's enough to enjoy some music