Orion 2250 Gen II output help

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I got this amp from a local collector in non-working condition. On one of the channels I found 2 output transistors shorted so I clipped them off the board. Once removed, the amp powered up and was allowed to play. The good channel is fine; nothing wrong with the output from what I can tell. The channel which had the bad outputs still has an issue where when a speaker is connected the signal gets halved. Obviously I think another output transistor is shorted but how to tell is the problem.

Here is the bad channel on the scope with no speaker connected:

Here is the bad channel on the scope WITH a speaker connected:
All emmitter resistors in both channels seem to be within tolerance. The resistors are all 0.27 Ohms and my meter picks them up as 0.4 Ohms.

The good channel stays working just fine while the bag channel collapses.

I replaced the two output transistors I clipped off last night. One was a 6488 and the other was a 6491 but the problem remains.

This amp is using 6488 and 6491 for outputs. Which of these is for the 'lower' or 'negative' side of the signal? Should help with pinpointing and troubleshooting. I have some spare bulks of these transistors and a parts Orion 2150 to assist in getting this 2250 working.

Once I find out if this channel is repairable I'm going to replace all the outputs and their emmitter resistors on the bad channel so hopfully it will be matching.
That was a close call. Indeed the 100 ohm reisstor near the driver transistor is open. I replaced it with a spare, but unfortunately the new one couldnt take the heat. Upon powering the amp the new resistor burnt up.

I also noticed another resistor closeby that has failed. I'm not sure if it failed because of the 100ohm I just replaced or if it was failed before; I honestly didnt notice because its tucked behind a capacitor. Its a 10 ohm resistor and its still measuring ot OK. The resistor I just replaced is the 100 Ohm tan one in the photo. I only had 1/2 watt resistor available.


EDIT: I checked all the small resistors and transistors in and around the area and they seem to check fine with the dvm.
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Yup all the 1N4148 diodes check out. I dont see any solder bridges atm but will keep looking. Traces all look good, theres one cut near the opAmps that looks like factory modification.

I know which transistors are the outputs. Which are the driver and pre-driver transistors? MPSA06 and MPSU07? Which is what?
I think I'm getting very close to fixing this amp. After the 10A fuse popped I noticed on the bad channel that half the output transistors get slightly warm to the touch. From last check, they all seem to be testing out OK but the resistance between legs is not 100% matching the legs of the good channel. I'll probably try removing 2/3 the legs on the questionable outputs to see if I can track it down that way.

Anothing thing I'm thinking is perhaps another driver transistor that was previously out of circuit may be causing the channel to be driven when powered up. I should have a fresh set of eyes and time tonight.
I pulled the MPSU57 which I had installed in place of the burnt unit last night. The amp now powers up; not sure if it plays. I'll keep poking around this area for a bit, maybe try pulling the other MPSU57 transistors for testing plus other circuits.

With this MPSU57 removed, the output is still collapsed.



OK well I just found another dead MPSU57 - open and no conections from within itself. On the PCB is measured somewhat 'ok', but on the bench it measured no continuity at all. Replacing it now!

EDIT: Ahh shoot.. I was wrong. its an MPSU07 and its fine. DanG!
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THis is confusing me though. On the good channel, there are two MPSU57 and two MPSU07 transistors. On this bad channel, there are three MPSU57 and only one MPSU07 transistor. i'm starting to think someone played around with trying to fix this amp before me.

Does anybody know what parts and their proper location are for these MPSUxx transistors?

On the good channel pictured below; 07's are in top right and bottom left, while 57's are in bottom right and top left.

OK Just fixed it! As it turned out, it did indeed look like this amp had three MPSU57 and one MPSU07 installed in the bad channel. Referencing a parts Orion 2150, I was able to put two and two together to get this Orion 2250 working. The bad channel is now a good channel.

Now, time for me to replace some brown resistors and butten her up!

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.