MCI JH110c tape machine erase depth issues

[Beneficial]

I'm having erase depth issues... I'll record a 1khz tone for 10 minutes... rewind the tape, and record over it then watch the output on a spectral analyzer. The first minute or so of recording things are good... the 1khz tone is erased, but then it starts creeping in... and starts fluctuating a bit... it will drop down to almost fully erased... then jump up 20db, then drop down again etc.

Any ideas what could cause something like this? It's a 2 track machine and it happens the same way on both channels so it seems to be a global issue. I've adjusted the erase capacitor trims to try to hit peak erase voltage, and I think I'm in the ballpark but it's hard to nail it with the fluctuation. I have a fluke 8060a multimeter that can read frequency and I tried to measure the erase osc to make sure it's at 120khz but I can't get a stable reading... I think I'm doing something wrong.

Anyway, 90% of the problems I've had with this machine have been due to bad molex connections. I'm basically trying to track down which board/pins to unseat/clean/reseat to try to get a more constant erase voltage running. Or maybe the main osc capacitor is going bad? I cleaned all the pins coming off of the power supply connections already, and have cleaned pins/reseated a lot of the other boards in the past.

[dirtyragamuffin]

Sorry for the late reply, I just joined.

When I had erase depth problems it ended up being an incorrect erase peaking trim job on my part...but I didn't have the problem of the erase depth fluctuating. I'd suspect the oscillator. I don't know how trustworthy your multimeter is at 120khz but does it seem to be fluctuating in time with the erase depth fluctuations? This could be a clue rather than the meter, but I haven't used that particular meter. If you have an oscilloscope, you could cross reference readings that way. They're a good thing to have with a JH110 anyway; I got a fine Tektronix off eBay for about 70 bucks and a set of probes for 15.

I agree with you on most JH110C probs being Molex related...I'm forever cleaning and resoldering in the audio drawers particularly. Have you pulled the drawer cards out and resoldered the motherboard and strip board Molexes?

[Beneficial]

Hey, thanks for the reply. I just have the software that came with my rme interface which has a spectral analyzer and phase scope... doesn't go up to 120khz though unfortunately. I could be using the fluke meter wrong too but the readings are all over the place... The tape recorder manual says to touch one point on the peak trim cap lead to a freq counter... so I was using the red lead off my fluke and doing that... then just touching the black lead to a random ground point on a completely different part of the circuit... not sure if that's right or not.

I've cleaned and reseated the cards in the audio drawer before, is that what you mean? In this case it is impcting both channels though. I suspect something is up with the oscillator too because when the erase depth issue started happening I noticed the high frequency record phase got out of wack too when I recorded some test tones... left and right channels became pretty badly out of phase like what would happen if there were bias issues... and of course the erase osc is tid to bias. Wish I could acccurately measure the osc frequency.

[dirtyragamuffin]

Yeah, without having a reliable way to check the oscillator frequency, you're stabbing in the dark as far as whether it's the culprit. Side question, are you having any issues biasing, or with your bias settings drifting?


I wasn't talking about the daughtercards (that is, the repro, rec, bias and i/o cards) but the motherboard, which is the board they attach to (those pins sticking up have solder joints on the bottom of the motherboard, those need resoldering sometimes...that's unlikely to be your problem, just saying), and the strip board, which is the board at the back of the drawer that the oscillator is on. You could trace around the strip board and see if there's intermittent continuity but...I don't know, I'd be suspicious of the the oscillator and its components. Hope it's not a faux pas to say, but the MCI forum could be a good place to check out for more answers.