Weird Sony JH24 issue (imagine that!)

[jdurango]

While trying to adjust idle tension, the take-up reel is spinning @ full speed (tach @ TP1 PPL board measures around 30hz even though it's spinning much faster than 30 revs/sec).

Oddly, the supply reel behaves just fine, but if I grab it to stop it from spinning, the take-up reel slowly winds down until it starts spinning at a more manageable speed (although tach is still reading well below actual speed). At this point, the idle speed can be adjusted with the trim pot.

I'm a veteran if MCI tape machines. This is a later model Sony with machine tooled IC sockets. Cleaned and wiggled all molexes, reseated cards a million times, checked voltages, etc. Seems like an issue with the tach or some feedback loop between the supply and take up reels. Any ideas? Thanks fellas!

 

[Brian Roth]

I'm also I'm a veteran of MCI tape machines.....the feedback from the reel tachs should be a reasonably clean varying DC voltage...if you are seeing any AC crap from the tachs, you have a problem....as in worn brushes in the tach.  But, just remembering possible failures.

 

[David Kulka]

Hard to believe that the reel is really spinning faster than 30 revs/second (1800 RPM!) but it doesn't really matter.

As Brian advised, check the tach signal again. It should be DC with a little bit of brush noise. Where are you measuring it? Maybe your scope ground was hooked up wrong.  If you're not sure, look at the tach on the good reel as a reference.

Does play tension work right? If so,  the tach is probably ok.

If the tach is ok, the issue might be a bad opamp in the analog torque board.

 

[Bill Wilson]

Was service mgr. for Nashville MCI dlr., The first suspect is definitely the tac.

 

[jdurango]

Hey fellas, thanks for the help!

I was measuring hz @ TP1 on the PLL board. Replaced a bunch of IC's in and around that circuit on the PLL and ATB,  I just realized that it was tensioning fine as of just a few days ago, but I switched the jumper on the PLL from HI speed (I believe 15b& 30 IPS) to LOW speed (7.5 & 15)....a few days later (now) I realized I never reset tension after doing this. That's when I noticed the issue.

Obviously I'll try switching back to HI speed. Does the LO speed option use a different crystal or buffer amp or something? Don't have the schems in front of me now, but I'll check when I get into the studio.

 

[jdurango]

Everything was good on voltage and freq coming from the board, into the molex pins going to the tachs. Pulled the take-up tach lead molex and the connection on the other end (at the tach) was popped off. Not sure if it was hanging on by a strand or completely disconnected or what, but soldered it back on and was able to continue troubleshooting. Voltage to capstan even with gain pot on PLL cranked was too low (around 19v) to move tape. Replaced Q3 and Q4 on PLL and that fixed it. Tape is moving nicely now.

Thanks fellas! Very much appreciated!