I must restate my position on this. I believe what you're saying is incorrect, because I've done it...many times. In my R2R days, I owned about a dozen or so early half-track stereo prerecorded tapes that I played quite often on my quarter track TEACs of various models. There was NO "big level difference".
So, to summarize: I
strongly disagree with "it's not possible", although I recognize your caveat of "correctly". Maybe I didn't feel the same level of incorrectness that you do.
Well, since track layouts are set by standards, it is easy to go past the "agree/disagree" point and put actual figures on the level differences I was talking about.
A well-known good source for track widths is
this page on the IASA web site, and I will refer to this page for figures and reference data.
- Figure 2 shows that many standard layouts have existed for 2-track 1/4", so we'll concentrate of the one commonly used for consumer decks, including TEAC, namely NAB 1965: 6.3 mm total tape width, 2.1 mm channel width, 1.85 mm centre gap width. Taking the upper edge of the tape as zero, this means that the left channel extends between 0.125 and 2.225 mm, and that the right channel extends between 4.075 and 6.175 mm.
- Figure 3 shows the layout for 4-track, which is the same for all the decks. The left channel extends from 0.025 to 1.025 mm and the right channel extends from 3.525 to 4.525 mm.
From there, it is straightforward to determine the common part of both channels, and see that the left 2-track channel covers 90% of the 4-track playback head's channel ([1.025-0.125] mm out of [1.025-0.025] mm), ie. a 1dB level loss. OTOH the common part of the right channel only extends between 4.075 and 4.525, ie. 0.45 mm for a playback channel width of 1 mm. That's only 45% coverage, ie. a 7dB level loss. As a result, the right channel will be played at a level
6dB lower than the left channel - and it's what I consistently measure on my machines. This is a huge difference, and I don't know why you didn't notice it but it is definitely there.
I can only suggest that you were somehow compensating the output levels, or just not paying attention, or those tapes were not really 2-track but 2-channel in a 4-track layout (I've noticed that, especially in the early days, the prerecorded tape manufacturers were not always crystal-clear in their technical explanations), or one of the machines was (possibly voluntarily) misadjusted in such a way that the 2-track recorded channels covered the 4-track playback head.
Another possibility is that these 2-track tapes of yours were recorded on wide-track machines such as those used in broadcast radio: I've done the same calculations using
Studer's data and the level imbalance is an unnoticeable 0.25dB in this case. Of course, this is a rare case, and even if you were so lucky to only have wide-track tapes, you shouldn't take this for granted. Well, anyway, the important point here is that the 4-track PB head of a Teac A-7030GSL cannot correctly play a tape recorded on a Teac A-7030GSL (or any other 2-track Teac) due to the track layouts - and Teac never claimed it could do it, they just said it was "a fourth 4-track playback head for playing back pre-recorded 4-track stereo tapes".
Another thing to consider is that, even with a hypothetical 100% channel coverage, you would still be playing with a 1 mm wide head a signal that was recorded with a 2.1 mm wide head. By doing this you are losing 3dB in signal/noise ratio, so it still wouldn't be a very good way to play 2-track tapes.