Me too...
Hey Whiny,
I'm not sure how much help this will be, but i have the EXACT same problem with my Tascam 38...
Ever since i got it, i've had problems with random channels going in and out, working intermittently, etc. I finally decided to pull the whole thing apart, and re-soldered all the points on the motherboard PCB. While i was in there, i re-capped the power supply and main control PCBs as well. Once i was finished and everything was back together, i was excited to find, that for the first time ever, i had 8 fully functioning channels working at the same time. Everything seemed fine with recording and playback, and then i discovered the erase issue.
When i re-record over existing tracks, the old tracks don't seem to be erasing completely.
I looked through the service manual, and it has some instructions on how to test the erase ratio. Basically, it says to record a 1kHz, 0dB tone for a minute or so and then re-record a no-signal section over the 1kHz tone that you just recorded. The difference between the tone and the erased signal at the output of the deck should be 65dB or more. Then the manual says...
"If the level difference is below the specification, check erase head output voltage for 60 - 70 V using an AC volt-meter"
...Which leaves me asking a couple of questions:
1. I'm not really sure where or how to check the erase head output voltage. Where do i take the voltage measurement? On one of the leads leading to the erase head? On the head itself? I dunno.
2. If i get that measurement and find that the erase head's output voltage is off, how do i adjust it? I was expecting there to be some sort of trim pot to adjust, but the manual doesn't mention any.
Hopefully somebody with a bit more experience calibrating tape machines, can chime in and help us both out. Until then, i'll let you know if i have any luck or make any progress in getting stuff to erase.
-Tom.