My friend,...
I'd suspect the tape, first and foremost. You should start with a known good new reel of high quality tape, hopefully not used tape. NOS tape can often be good, but varying by brand, type and batch, getting sticky tape is more of a risk with NOS.
Along with the suspicion of the tape having undue friction from SSS (sticky shed syndrome), is the inherent layers of oxide you'll get, which can diminish the record/playback response. The head must be as clean as can be, normally with nearly a mirror finish. Even oxide on the head you can't see can diminish response. You have to clean the heads a lot and start with new/fresh tape.
(These days), You gotta think of the tape as the culprit, most of all,... (or at least I do).
Beyond that, if perhaps your deck has been sitting with original factory setup all these years, I have one word: calibration.
The issue of needing a calibration is a foregone conclusion if the deck has gone any number of years without a tune up. A few (3-5) db of dropoff audio response would be normal. It's not the end of the world. It can usually be adjusted back to 0/0.
As well, if you check the manual, there's a section for reel torque. (After verifying tape!!!!!),..... you might logically assume the reel torque should be checked and adjusted. If the reel torque is off, you might have undue tension on the tape, which would exacerbate any tape wear, drag or cleaning issue. The reel torque affects tape tension in play mode and FF/RW response, directly.
Check the tape to 100% certainty with multiple reels. If you can do that and still have the problem, it's tech time. Some DIY'ers or any worthy tech could do the rec/repro and reel tension adjustment. HINT: You need the manual, a calibration tape and a spring scale to do both.
You're in LA, so with a good number of audio electronic fixit shops to choose from, you should be alright. TEAC Service 'bout 30 minutes away from you, too. Learning to do it yourself is a lot cheaper in the long run, and it's technical but maybe not brain science.
For DIY cal you have to invest in a few tools right off. I've seen the effects of calibration on my own well kept 38 that have me convinced of the efficacy of a proper calibration as related to these two issues. I've had both issues. Adjusted it myself. Was surprised at the improvement when tension was corrected and e'thing was setup right.
[IMO] most home-kept personal studio 38's would tend to have low or medium wear and maybe have never been calibrated since factory setup. Things have to be checked & set back in spec again. If you clean and cal them up, 38's'll usually perform and sound great for many years to come.
So sorry to go on at length. I'll shut up now.
Tech DIY can be a learning experience.
