My TC-D5M has good belts and looks very clean inside but I just recorded a tape and it records, what seems to me, to be 1.5x speed on a brand new TDK SA60 tape. I am running it from batteries, and they are almost spent; could low batteries cause the device to record fast? I currently am snowed in, so can't venture to get new batteries or DC charger at the moment, but I sure hope its something fairly less problematic than a capacitor issue (though I am no stranger to replacing those). And apologies if this seems to be a rookie problem, I am still learning.
First of all, pay attention to the fact the TC-D5M uses a center negative plug for the DC in jack. So if you are to use an adaptor, either buy one that has a center negative plug or wire it yourself to be center negative. I'm stating this because there's no indication on the device itself that plug is center negative. In regard to batteries: motor governors are unable to keep the speed up below a certain voltage threshold, below which the speed will start to decrease with voltage. So the answer to your question is no, depleted batteries won't make the speed faster, rather it will make it slower. First thing to check is if the speed adjustment trimmer renders any variation in tape speed. Maybe the tape speed is just not adjusted properly, that would be a simple fix. If the trimmer does not produce any change, the next thing to look is the FG coil and if it's properly connected to the PCB. A missing FG singal will cause increased speed.
Perhaps it's worth noting that if the tape speed is slower when you record with low battery's, and if you playback your cassette on another device with correct speed it will speed up. Another possibility is that power consumption is higher during recording, again as a result playback alone consumes less , tape speed is higher. But this is hypothetical, I guess you got some new battery's now.
Battery consumption is higher during recording because of the bias oscillator, but that won't slow down the tape unless the batteries are depleted. What was described here definetely sounds like a problem at 1.5x faster, but it might be something as simple as incorrectly adjusted tape speed.
