xtr 275 amp repair

allaroundsound · 2011-11-21 12:35 am

yes the resistor still burns with indicated resistor removed.

Perry Babin · 2011-11-21 12:37 am

Did you re-check the drivers after you found the shorted output?

allaroundsound · 2011-11-21 12:42 am

which ones are the drivers?

Perry Babin · 2011-11-21 12:44 am

The 4 small ones you removed and checked for shorts/leakage/open junctions previously.

allaroundsound · 2011-11-21 12:49 am

ok. if testing in circuit should i have power applied and do they test the same way as the output npn and pnp transitors. what is an expected reading for a good mpsa056 or 06 in circuit?

Perry Babin · 2011-11-21 1:15 am

You can't check them definitively in the board. You need to remove them.

The attached image is for an A06. All other probe/transistor-lead combinations should read OL. Reverse the probe orientations for the A56s.

allaroundsound · 2011-11-21 2:02 am

found a obvious problem. i had a mpsa06 and 56 reversed. i have replace the inverted transitors, burned resistors and have audio. with a shorted 2n6491 of the board still is it ok to test with some gain?

allaroundsound · 2011-11-21 2:29 am

id say i got it. Need to order a new 6491, but audio is clean and i am very happy to have learned a new aspect of my field. Have much to learn, but still satified. Thanks, props to Perry!

Perry Babin · 2011-11-21 3:43 am

You can't replace a single 6491 if it's being used as an output because there are others in parallel with it. You need to replace all 6491s in that channel.

allaroundsound · 2011-11-21 3:24 pm

since they are mixed around from being of the board i'll replace both channels.

allaroundsound · 2011-11-23 1:48 am

there seems to be a few choices from mouser, which model/manufacture do you recommend for the 2n6491 replacement? Thanks

Perry Babin · 2011-11-23 2:48 am

The ones manufactured by On-Semi.

allaroundsound · 2011-12-15 1:49 am

well im back....the 275 operated fine for a an hour or so, then came a loud pop. i opened the amp up and have it powered up with 10 amp fuse and remote. 8.3 volts at at all emmitter resistors for the 2n6491g, but no power at the emmitters for 2n6488. this applies for both bjt's on both sides. what do you suggest?

allaroundsound · 2011-12-15 2:05 am

the amp light is on and there is very very low output. It is distorted. If anybody has any insight it would be appreciated.

Perry Babin · 2011-12-15 2:26 am

Was the pop through the speakers or from the inside of the amp?

With no power applied, what's the resistance from the emitter of the 6488 to the RCA shield?

allaroundsound · 2011-12-15 2:37 am

pop was from the speakers. 178.5 ohms at all 6488 emitters

Perry Babin · 2011-12-15 3:02 am

Was it a constant resistance or did it 'count' (up or down) for a while before settling on ~180 ohms?

What's the resistance from the emitter of the 6488 to the heatsink?

allaroundsound · 2011-12-15 3:08 am

it counted up to 178.5 ohms. took about 3 or 4 secs. the resistance to aluminum clamp is 3.5ohms

Perry Babin · 2011-12-15 3:24 am

Disconnect everything from the amp and check it again.

If it still reads that low, lift the negative rectifier from the sink to see if the resistance increases.

allaroundsound · 2011-12-15 3:35 am

pulled rectifier part # u1620rg that was recently replaced and impedence to sink is gone. i checked for shorts on the trans. and it appears fine.

xtr 275 amp repair

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