Reel to Reel tape question

1. Do you find this deficiency on both channels when you play the second side? The outer tracks, which is there the left channel lives, are more likely to show a loss of fidelity since they are at the edge of the tape and will be most prone to less than perfect alignment.

2. Good tape to head contact is essential. Even if azimuth and zenith alignments are correct, there may be a slight deviation from a perfectly vertical front-to back head angle. You might try this experiment: While the tape is playing, using a Q-tip, press the tape gently to the play head and see if the sound improves.
What is puzzling me is why you find that this loss of quality only appears on the reverse tracks.
What the head sees on your tape should be exactly the same from the forward and revers tracks, any alignment would be mirror-image, either both bad or both good. Same with height.

See if that Q-tip diagnostic tell you anything.

You know, I never thought about whether it is both channels that are affected equally. Good question. I will check on that tomorrow. My guess is yes - but only because I do not notice any skewing of the soundstage - I don't need to re-center the soundstage when I flip over the tape. But that may simply be volume, not clarity. Again, thanks, I'll check. 

And all of these are commercial,  pre-recorded tapes, usually from the 60's and 70's. Their age made me wonder if storage for 50-plus years has a greater negative effect on the first side, rather than the flip side. Doesn't sound logical, but just reaching. 

And I might call ReelAudio and talk to Jim, as you suggest, @jnovak.  He also sells calibration tapes, and would know exactly which ones my local bench folks would need.

Thanks again, everybody...

David

I did a lot of professional reel to reel and cassette tape duplication back in the day. Duplication facilities recorded all tracks at once in one pass. That means one side was playing and recording forward and the other backward simultaneously. Most duplication machines used heads with all four tracks in the same head. It was critical that we kept the duplicating equipment carefully aligned with professional alignment tapes like the ones from MRL (surprised they are still around) to prevent the very same problem you are describing. If you are having the same issue with tapes from various vendors and different time periods, I’m pretty sure your problem is head alignment which is easy to adjust with proper alignment tapes.

I agree with the consensus that it is most likely a head alignment issue. Although it could be many things. Worn heads. Misaligned tape guides. Tension arms out of adjustment. A worn pinch roller. Or a combination of all of the above and more. A rough way to test alignment is to simply listen for apparent crosstalk. In the silence between songs. If you turn up the volume can you hear the other tracks. They would be playing backwards of course. And is it worse on side one than side two? If you are hearing crosstalk of a significant level that is a sure thing that your tape heads are out of alignment. All tape suffers from instrument measurable crosstalk. It is a fact of nature. But if it is very apparent to your ear, then the tape heads are not in line with the tracks on the tape for what ever reason. Also, are there any individual tweak adjustments for the individual channel output from the tape heads? This could be a tape biasing problem. The MRL tapes will give you a reference to adjust by. The service manual gives some helpful direction. If you are into DIY. Just dive in. You can always correct a readjustment mistake that you might make. Another thing to consider is, if I understand you correctly, the unit has been modified quite drastically. The 1/4 /track 7-1/2 ips record head has been replaced with a 1/2 track/15 ips playback head. And in order to get it to run at 15 ips, was the motor swapped out? Or was different voltage supplied to the same motor? Was the replaced 1/2 track head from the same manufacturer? And did it have the same dimensions/mounting structure of the original recording head? Or was modifications necessary to mount it? Is it interfering with the tape contact surface on your 1/4 track playback head, in any way? If you aren't really up for a DIY project, and the associated learning curve that that may entail. I would take the advice of the people that are giving you the names of knowledgeable people to contact. Speak with the experts about the problem. I would also study the service manual in detail. So that you might be able to have a fruitful conversation and ask the correct questions.  There are many adjustments that can be made for that tape deck. As @stager  previously brought up. And it is a key question. Is your L/R channel outputs the same level in relationship to each other in both directions? Or is one lower than the other? And when you reverse direction, does the problem move to the opposite speaker? And finally, a simple thing to do, though not that accurate is to look at the your VU meters. Do they seem to be reading the same value in both directions. Or do both L/R both drop/gain level when reversing the tape?