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HH Scott A-457 with blown power fuses

Hi all. I'm having trouble locating a problem with with my HH Scott A-457. Both of the power fuses on the power board keep blowing. I checked the rectifying diodes and found them to be shorted. after replacing them the fuses still blow. I wonder if anyone has any ideas of what may be drawing the current.



HH Scott A-457 with blown power fuses

output transistors shorted or very leaky. also check for burned or open bias resistors and or current limiters and coupling caps. recheck bias after replacing the new transistors.

HH Scott A-457 with blown power fuses

got an update. i checked the power board and the driver board but did not find anything else shorted other than the diodes i had originally found. thanks for the tips. i noticed that the service manual calls for normal blow fuses. i was using fast burning ones. i'm putting this thing back together and running down to radio shack for a handfull more fuses.

HH Scott A-457 with blown power fuses

Normal blow and fast blow fuses are really the same. Slow blow are different in that they delay blowing. If someone previous to you put in fuses a lot higher in value, with the shorted rectifiers, the power transformer primary winding could be shorted. (if the fuse that is blowing is the main power fuse).

HH Scott A-457 with blown power fuses

shorted transformers usually have a burnt smell by the holes where the wires enter.
As a test, you can remove all secondary fuses (if your unit has them) or disconnect all secondary windings. If the primary fuse still blows, your transformer is shorted.

If the transformer is good, check for DC short at main filters (one lead should be short to ground, the other should be 10-15 ohms 1 direction, open other direction) If a short is found disconnect the (+ or -) leads one at a time to see which one is creating the short. follow it to the bad board. Could be a short on the regulator board, or short with output stage. (don't forget to check output collector to ground for a short caused by a bad insulator.)

Tom

HH Scott A-457 with blown power fuses

Just looked at the schematic
If the fuses that are blowing are "secondary fuses" on the regulator board, the the problem is not the transformer, and is most probably in the outputs and drivers. Shorts as well as leaky conditions can blow fuses.
Tom

HH Scott A-457 with blown power fuses

thanks guys. the output transistor are bad. i had to remove them electrically from the circuit to get a good reading. i'm stopping here for now until i can get some replacements in.

HH Scott A-457 with blown power fuses

check the ceramic caps near the bridge & the main filter cap .check across the filter caps on rail/s (after discharging) to see if there is a short on the rail/s if there is check the outputs or disconnect them & check again across the caps again.
also make sure you use the correct fuse slow or fast.they might pop if using a fast instead of time lag fuse & the amp could be o.k. -if the fuse is really going off then keep looking for a short .

HH Scott A-457 with blown power fuses

Check for shorted output transistors. If found to be shorted, check all components on the driver section after replacing bad outputs before powering back on. Advise you to use a variable AC supply to prevent further damage
Tom