OK, it is working now.
Once you record a square wave successfully, and it comes back square, play it in different decks to make sure they put out a mostly square wave. Do al l of this with the Dolby turned off, or with HX only. If oyu have a deck wi thHX and theothers without, use the HX one to record always, even oi the he ad is more worn it will make a better recording. Of course I recommend dubb ing everything that is non-replaceable, but that is your perogative.
The first thing I would recommend is to post the model numbers of all these things. We might be able to get the service manuals online from like HIFIe ngine or HIFImanuals. These will allow us to identify the parts that will a ge, like electrolytics and tantalum caps. the small poly ones and all that will either uisually be shorted, open or something, and it will be noticabl e and most likely in one channel.
Assuming tape head wear is not really bad, the channels should be equal. wh en the heads start losing it, usually the left goes first being the edge tr ack. Hopefull your decks have (working) pad pushers and use regulated tape tension to maintain head to tape contact.
I would record the square waves and see which ones record the best and whic h ones play back the best. Are any of these things three head ? Evne in the high end units, not all are. And hopefully they are not autoreverse.
There are mainly only so many alignments to do and not all exist on all mac hines. You have playback EQ, this may be separate for 70 and 120 uS, depend s. that's something we want to know. When it comes to record there may be a nother for metal tape if equipped.
Then comes Dolby calibration. Dolby calibrated at the factory is almost nev er right because they use the absolutely best type of tape for their unit o f course, to get the best specs. If you plan on dubbing the material out to another format like CD or FLAC on the computer, you might want to calibrat e the Dolby for the source tape used. The only other good way to ake the no ise reduction work right is to have an outboard Dolby decoder, though if th e deck has output level controls that PRECEDE the Dolby decoder, you're in luck and don''t have to go through all that.
This is why we need the prints for these things. There are different ways t o proceed, depending on not only which adjustments are provided, but also t he exact architechture of the machine.
We coid just wing it, but you indicated you want to be sure and not guess. To do that we need to establish a reference and to do that we need to know what COULD be wrong electronically.
As such I highly recommend we get the prints, in fact if you have them you could scan them and upload them to one of those cloud things like dropbox p or photobucket. In fact if they are clear scans they might be better than w hat is available out there. I have seen some terrible ones.
When you get into recording though, tha tis a whole new ballgame. Underbias will cause a loss of a certain range of frequncies, probably around 2Khz t o 5Khz or so. Overbias will lose the real high end, even worse. both condit ions cause odd order distortion. Too much bias will cause compression of th e peaks wheras too little bias will cause expansion of the peaks.. Either i s a nonlinearity and results in high harmonic distortion. This can only be removed with big buck digital sound equipment which I doubt any of us has.
Really, give me the model numbers. We can go from there after you run some square wave tests. If these decks have continuously variable bias, you migh t just be able to make a good enough test tape. The square wave will tell y ou the general response quite accurately, and don't be disappointed if it s hows what looks like ringing. that is characteristic of the system.
what causes that is that the response is not flat, and I mean not like anyt hning else that is unflat. It is not simply a rolloff usually. When they sa y +-3dB, it could mean +2Db at 2,100Hz, -2.5Db at 3,000Hz, 0Db at 5,000Hzz, +2,8Db at 8,000Hz, -2Db at 9,300Hz, and so forth. They ALL do that. (of co urse that's not really worse than most speakers)
If they don't you have some DAMN GOOD decks there.
The square wave tels all, except distortion. For that we will have to find a way to mix two frequencies, just like they to run an intermodulation dist ortion test. When it comes to tape recording, the cause for both THD and IM distortion are the same so the results are the same.
The reason a square wave test should work at least halfway decently is that it is extermely unlikely that faults could be in both the record and play and null out and resut in a good recording. the odds are astronomical again st. So that is one of the facts we use to try to determine some sort of ref erence.
We can do this, remember those signs that say :
:We the unwilling, Led by the unknowing, Have been doing so much, With so little, For so long, We are now qualified to do anything with nothing" ?
That's me. I can align the record section of a VCR without freq. counters o r any of that stuff. I'll reveal how on request, I remember a bunch of thos e early Funais came in with the controls all whacked out by the customer. I was the one to get them done without ordering a shit ton of equipment.
This is no different. In the meantime if we ruin into any real difficulty I think I might be able to pump some info or maybe even something resembling a test tape on one of the purely audio forums.
I knda like doing shit like this, ust because you "can't" haha.