2270 Intermittent noise issue

[preacherJohn]

Hey guys, I have an intermittent noise issue in my 2270 that has been driving me crazy for a couple of months. I noticed it when I play soft music and all of a sudden it pops or cracks really loud. Enough where as it causes me concern with the Wharfedales hooked to it. The problem is it doesn't happen very often, once or maybe twice but not always in a couple of hours listening session. I've had it apart a few times, and thought I solved the problem; and then after getting it all set back up in the system, out of no where it does it again. Two days ago I swapped in a 7025 main amp and used the 2270 preamp side. The last two days I haven't heard anything, so it has to be coming from one of the amps. I have recapped this unit, and replaced a lot of the old transistors except a few that tested OK. The ones in the amp that hasn't been replaced are H756, 757, 758, and 759. Have you noticed if these are troublesome? They did check out when I tested them, but like I said it doesn't happen very often. I don't think it's a solder joint as I went thru them pretty good, but who knows, I might have missed one. I'm into a 3200 preamp pretty good right now, and I'm thinking out loud this morning before church. I have the 104 tuner, and the 140 amp to do next and then I'll get back to the 2270. Those can transistors I can replace but I'm not sure about the others. Thoughts?

 

[kwayde]

Does it happen in both channels or only one of them?

 

[preacherJohn]

Does it happen in both channels or only one of them?

I really don't know; it's so sudden when it does there's no catching which one it comes from. I'm leaning toward the right channel; but that's just a guess. Everything looks fine on the scope too. It reminds me of a car that won't act up for a mechanic when he looks at it.

 

[jd3rdcoast]

Can you run your 3200 pre-amp into the 2270 amp sections? Then you should be able to ascertain if the problem is indeed in one of the 2270 amp modules. You replaced the differential pairs H751/H752?

The 2SC458LG's and 2SC1000's in P700 and P400 are known to go noisy. Replace with KSC1815 or KSC1845.

I have seen many times a transistor passes basic testing but fail (noisy/popping) under load.

 

[preacherJohn]

Can you run your 3200 pre-amp into the 2270 amp sections? Then you should be able to ascertain if the problem is indeed in one of the 2270 amp modules. You replaced the differential pairs H751/H752?

The 2SC458LG's and 2SC1000's in P700 and P400 are known to go noisy. Replace with KSC1815 or KSC1845.

I have seen many times a transistor passes basic testing but fail (noisy/popping) under load.

I could swap in the 3200 if I didn't have it a part sitting on my bench chasing down why the left channel isn't playing. I should find it by the end of this week. The 458s and 1000s Qs were replaced at the restoration last year on all boards. I know the problem is in the amp though as it hasn't acted up at all with it's preamp going thru the 7025 amps now. I'm curious about the TO66s on the amp boards and what I'm going to replace them with. They tested OK, but that's with the little Q testers, and not under much of a load.