xtr 275 amp repair

[allaroundsound]

this is my first post. I have a Xtr 275 revision C that is in need of repair. It has a blown fen16c rectifier and some issues with a resistor off the back of the power supply caps. I have a photo that shows the resistor i speak of. Thanks for the help. Would love to work through a full repair and add to my collection. HCCA 250, xtr2150, mbr70, xtr 275? If this is not the right section let me know...

thanks for the response Perry.

[allaroundsound]

ps thank you for the photo that i picked up off this blog also.

[Perry Babin]

The problem would seem to indicate that there is a leaking or shorted transistor. Have you checked all of the transistors in that channel carefully?

[allaroundsound]

which transistors should i focus on? 6488's or the 6491's i believe..will i need to check it on or off the board?

[Perry Babin]

All transistors in that channel. Shorted transistors can be found in-circuit. If you suspect that one is leaking, you may have to remove it for definitive results.

[allaroundsound]

if the fep16ct and fen16ct chips are removed off of the board the resistor does not burn up, however the torroid makes a very high pitched hiss. What job do these to transistors do? Thanks
Also I have replaced the stock 3300mf 35 v caps with 4700mf 50v's out of a rockford. I noticed that only one of the two terminals on both caps have 12 volts. How is this possible.

[allaroundsound]

delete post regarding caps. I must not be feeling to well today. Wow.

[Perry Babin]

The FEN/FEP components are the rectifiers. Without those, you won't have rail voltage.

Did you check the transistors?

[allaroundsound]

Amps torn down on the work bench at the shop. doubt ill have time to get back to it till Monday. And yes Perry, i have checked all the fets. Perhaps my technique is wrong. they all show some form of resistance to ground. however when i get them off the amp(ive only down one channel) they check out. I have read the bcae1.com write up for checking leaky/ shorted fets.

[Perry Babin]

You don't check transistors to ground. You check them between the legs of each individual transistor. You can't check for leakage in the circuit (unless it's leaking badly). In-circuit testing is typically only useful to find shorted transistors.

Pull the MPSA06/56 drivers and check them out of the circuit. Don't pull the outputs yet.

Did the amp draw excessive current when it was burning the resistor?