I've completed the playback alignment (balance and EQ). I found the image in an old post of mine useful, in which there is a picture showing which pots are for which channel and purpose. In that post and another post the balance/EQ alignment process is outlined. In the old posts I had a crack at the playback balance & EQ adjustments. I had some luck but had some difficulties. I've now replaced the old VRs and redone the head alignment and things have worked out nicely.
One note regarding the azimuth head adjustment. The key, for me at least, was not worrying about getting the phase of L and R perfectly aligned at the beginning when doing the coarse adjustment (at 8 kHz on my test tape) and just focusing on getting the maximum signal from both channels - don't worry for now if the L and R levels are a bit different (if they are very different you may have to look into other head adjustment as per manual, but otherwise you can balance it up in balance/EQ stage later). I watch the two waveforms (L & R) on the scope. As you adjust the azimuth, the relative phase of L and R channels changes (a lot) and you want to find the maximum signal of both channels. What the manual doesn't tell you is how easy it is to have the phase between the signals 360, or 720 degrees off, especially if you just focus on fine adjustment with 16kHz test tone. Maybe not a big deal since everything is in phase, but anyway, the point is to get the heads aligned so I wanted to do my best. Anyway, so with 8kHz is it a bit easier to do the coarse adjustment, making sure you are near 0 degrees phase difference (and not 360, 720 or so off) and maximum signal, and then at 16kHz you can do the fine adjustment to get as close to 0 as and maximum as possible, once you have got the 'correct peaks' approximately lined up on your scope. You can of course use a relative phase 'x-y' function on your scope if you have it, but I guess I would just say take care you are not an integer*2*pi out of phase. Maybe silly ramblings to the experts here, but useful notes for a newbie like me I think.
Also, some steps of the balance (gain) and EQ setup are tricky since they affect each other, and it is easy to get obsessed with getting it perfect. Don't worry about getting all frequencies perfect at first. You just have to focus on getting the high frequency response point (10kHz) close to the reference point (500 or 1kHz in my case). Focusing on those points got me to the aligned setup quickly with just a few tweaks to get it straight out to 20kHz; whereas worrying about all frequencies at the beginning and trying to get it perfect is the path to madness - it all worked out nicely when i started just focusing on the two main reference points and then tweaked after. Also I noted that on my machine when I balanced gain at 0dB, it L & R would be a bit off at -10dB! It was driving me mad trying to get the balance right at both volume levels, but then at some point I realized it was well within spec and decided get on with my life!
The following graphs tell the story of before and after playback balance/EQ setup. The graphs explain perfectly what I heard in listening tests beforehand (forward directly image shifted to one side, reverse image centered and brighter tone). Note that voltage values in the graphs have more uncertainty at higher frequency (especially from 10kHz and up).
Here is the before and after or the reverse playback.
And here is the result for the forward playback.
Also, I made notes about how stable the amplitudes of the test tones were on playback, especially at high frequency when things start to jump around a bit. Useful info I think. For example my left channel on my reverse direction is a bit more unstable than other 3 channels; I've done my best with the head alignment and I think it is a limitation of the head wear but it's not bad and I have a spare, almost new, set of heads on standby for the future. The rest of the tape path looks good mechanically. Is it worth new rollers if the old ones 'look okay'?
And, and the final point. No matter what you do, if you get the EQ flat within say 100Hz - 20kHz, you have that increase bass below 100Hz. Will be curious to see if it is the same on my other 707s and thus by design... might explain why the 707s sound so pleasing on the bottom end?
Great post! This will be very helpful when I start lining mine up after the Head Block returns from re-lapping. The bass boost is interesting. I don't know if you have that data in terms of dBV or not? Which tape were you using to do the EQ and Balance adjustments? Again, thanks for the through post.
BTW - what tape did you use to set the bias?
