2275 clipping in one channel

[Ziricote1]

I have a nice 2275 here that I just re-capped. Bias and dc offset adjusted easily without issue. Sounds good until I get to about 11:30-12:00 on the volume control and then the right channel starts popping. I tried 3 sets of speakers, same result. Also tried feeding a separate preamp into the main-in on the 2275 and same result, so it has to be the right amp board. I went through and shotgunned all the transistors on that board, along with h715 and h716 zeners, and new MJ21193/94 outputs. Still clips in the right channel only. I deoxited the molex connectors also and no change. Anything else I should be looking for? The only thing I can see on the board that is questionable is one of the 5w cement resistors has been replaced by a previous tech.

 

[dave404]

I have a nice 2275 here that I just re-capped. Bias and dc offset adjusted easily without issue. Sounds good until I get to about 11:30-12:00 on the volume control and then the right channel starts popping. I tried 3 sets of speakers, same result. Also tried feeding a separate preamp into the main-in on the 2275 and same result, so it has to be the right amp board. I went through and shotgunned all the transistors on that board, along with h715 and h716 zeners, and new MJ21193/94 outputs. Still clips in the right channel only. I deoxited the molex connectors also and no change. Anything else I should be looking for? The only thing I can see on the board that is questionable is one of the 5w cement resistors has been replaced by a previous tech.

Hi Zircote
Would you please post what part numbers you used for the replacement transistors and zeners? Were they from Mouser or DigiKey?
Thanks

 

[bryans12v]

The emitter that was replaced is the wrong value. They should be a pair of 0.2 ohm resistors. Looks like a 0.82 ohm in there. Not sure if this would cause you issue though but I would make it right regardless.

 

[wlhd1610]

Looks like the drivers were also replaced
With what?

Bob

 

[Hipocrates]

With what?

Seems like KSC2073/KSA940

 

[dave404]

Looking at the heat sinks of H711 and H712 it looks like they were replaced also. I would also agree that setting R740 to the correct value would be helpful.. that's 4X the resistance.. low value for sure so it's probably pretty sensitive.

 

[Ziricote1]

Yes, I used KSC2073 and KSA940 for the drivers. The clipping is present with these and also with the originals. Interestingly, the 2 amp boards are slightly different so someone may have swapped one out at some point. The right channel board had glass diodes at H720 and H721 while the left channel had the VD1122s. As far as the emitter resistors, I thought the left channel ones were also 0.82 ohm but I'll have to double check.

 

[Ziricote1]

Thanks for the help guys. So it turns out the left channel emmiter resistors are in fact 0.2 ohm. So that may be the issue, I'll have to swap the 0.8s out and report back.

Dave, here are the parts I used for the P700 amp board:

H701 - KSC1845
H702 - KSC1845
H703 - KSA992
H704 - KSC1845
H705 - KSC945C
H706 - KSA733
H707 - KSC945C
H708 - KSA733
H711 - KSA1220
H712 - KSC2690A
H713 - KSC2073
H714 - KSA940

H715, H716 - BZX85B15-TR
H720, H721 - 1N4148

 

[bryans12v]

I would adjust the bias for that channel once you get that new 0.2 in there.

If you had the idle current set at at 10mv across that pair (0.2×082) would be yielded an actual idle current of about 9mv instead of 25mv, where it should be.

Until the resistor arrives, you may be able to drive the trimmer up across that pair to 26mv to have an actual quiescent of 25mv assuming ohms law here. For testing purposes of course.

What the manual states:

10mv across the series pair 0.2+0.2 (0.4)
0.010/0.4= 25mv quiescent or idle current.

What's in the unit 0.2+0.82 (1.02)
0.010/1.02= 0.009 or 9mv

To obtain 0.025mv:
0.026/1.02= 0.025 or 25mv.


Maybe others can chime in on that before doing so. I'm no expert for sure!

 

[dave404]

Thanks for the parts list! This will go a long way in helping others!
Cheers

 

[Ziricote1]

That sounds right Bryan, only the resistors are both 0.82 ohm so the idle current setting is even lower than you calculated. I think I'll just wait for parts to come in before messing with it.

 

[Ziricote1]

Just to update, I got the correct 0.2 ohm resistors in and installed them in the right amp board. Dialed bias back in to 10mv. Everything is working great now, no more distortion in the right channel.

 

[Dingman]

Kudos for finishing the thread with solution.