MCI JH-16 power supply question

[clemensknieper]

Hey guys!


I recently acquired a mci JH-16 with a ALIII from 1979. While trying to figure out why the take up reel motor wasn't spinning at all I fried one of the 5A fuses on the back of the machine right next to where the AC power plug goes (it's labelled "deck 5A")... The way I did that was by stupidly unplugging the connector to the Motor Poer supply in the bottom of the machine while the machine was on, and then plugging it back in but accidentally connecting the wrong pins to each other for just a second. Now I put a new fuse in there, and then it immediately started smoking and the capacitor (15UF/20volt) that comes before the LM309 5V regulator burned up. I disconnected that burned up component and measured the voltage going to there... it should be 8V but for some reasons its reading 13V... I am not sure what to check first and there aren't many components between the power transformer and that 5V regulator where it could be getting messed up. I am hesitating putting another capacitor in there and trying it again before I make sure its getting the 8V that it wants and not 13V.

I hope this makes sense...

 

[JohnRoberts]

voltage may rise under no-load...

JR

 

[Brian Roth]

As John mentioned, that "8 Volt" rail is typically higher than the schematic shows, even under load.  It's been awhile since I actually took a measurement, but 13 V seems plausible.

As for the bad cap, MCI uften used dipped solid tantalums which have a bad habit of randomly failing as a short circuit after decades have passed.  Regardless of what type of caps are there, replace both of those caps around the 3-terminal regulator with decent low impedance/low ESR aluminum lytics with a 105 degree C rating.  Use 25 VDC rated caps since that is a standard/"modern" voltage rating.

Bri

 

[clemensknieper]

Great, kind thought that might be the case... I will replace the caps like you said Brian (thanks a lot for the quick reply here and over on GS) and to you too John.