Orion 2250 Gen II output help

[Dr Zeus]

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:

[Perry Babin]

Are all of the emitter resistors in the defective channel within tolerance?

Does the signal collapse on the good channel when the speaker is connected to the defective channel?

[Dr Zeus]

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.

[Dr Zeus]

Can I lift say the center leg of an output (one at a time) to see if one is causing the issue? its strange because under no speaker load the channel reads fine on the scope. One might be leaking I guess.

[Perry Babin]

Check the resistors near the driver transistors. The 100 ohm (could be 75 ohm) resistors that feed the drivers often burn open but don't always look burned.

[Dr Zeus]

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.

[Perry Babin]

Did you check the driver transistors?

[Dr Zeus]

MPSA06 are the little ones; one per channel. Both are measuring out identically with the dvm. The 8 medium sized MPSU07 seem to be measuring ok as well.

[Perry Babin]

Maybe one of the drivers is leaky?

Generally, those resistors only burn when there's a defective transistor (driver, pre-driver or output). Another possibility is a solder bridge but that's not likely since you cut the transistors out.

Are all 4 of the 1N4148 diodes (the ones grouped closely together) OK?

[Dr Zeus]

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?