Bad 12V regulator in 1541 drive?

[+OLD CS1]

On my desk is a 1541 with PCB 1540050. Neither the drive motor nor the head stepper will run. I did some testing per Ray Carlson's page and found the these values:

 

Orange: 13.7VAC
Blue: 8.9VAC

 

CR4: + 5.13V
- 9.67V

CR2: + 0.61V
- 15.02V

 

Compared to a good 1541 (white body, did not record the PCB number):

 

Orange: 15.3VAC
Blue: 8.7VAC

 

CR4: + 5.06V
- 9.12V

CR2: + 12.10V
- 19.65V

 

Given these values it looks like the 7812 12V regulator may have failed, which I can believe as the heat sink on the regulators becomes scalding hot -- as in, left a mark on my hand. I read the 2V difference in the power input from the transformer and 4V difference from the bridge rectifier as a potential result of a bad regulator.

 

I do not recall ever having to troubleshoot the power in one of these, so I ask those who have if they would concur with my assessment? If so, I am considering this unit to be the guinea pig for replacing the old power supply with a new switching supply.

[norm8332]

There is the possibility of a shorted tantalum capacitor on the board. I had a drive with the same issue and it turned out to be that the short was dragging down the 12V and overheating the regulator. It was one of the "blue bead" capacitors in layman terms. See the very end of this document. http://personalpages.tds.net/~rcarlsen/cbm/1541/fix1541.txt exerpt: "TANTALUM CAPACITOR C15 3 can burn up or short and drop 12V power"

[+OLD CS1]

I will check that cap later this week and update. I have actually already completely disassembled the power circuit of the board in preparation for replacement by a switching PSU (the guts of an external hard drive power supply.)