Kenwood KA-9100 popping

[scottonnob]

My 9100 started randomly popping this morning. Nothing has changed in the placement or what is hooked up. The pops are quite loud, causing full excursion of the speaker drivers. I've never experienced this sort of thing before. The only component hooked to the amp is an Aune T1 DAC. The amp is stock. No alterations, replacements, or upgrades have been made. The unit can go as much as an hour without popping, or it will produce two or three fairly close together. Any thoughts? Occurrance is unpredictable.

 

[slimecity]

Chances are that its the sound of a small-signal transistor dying. This is fairly common in vintage gear. Don't use it anymore as you could blow something. Are you confident about working on this amplifier? If so, find and download the manual. Also do a search under the Kenwood forum on this site as someone else has probably had the same issue.

 

[scottonnob]

Okay, thanks. I knew it would eventually need an overhaul. I guess the time is now. I had always planned on doing the overhaul, I just didn't think it would be this soon. But since it's, most likely, an internal circuit problem, it's time to shut it down and put it on the bench. However, I have a Phase Linear 400 that's almost finished as a complete White Oak upgrade, and then a second Phase Linear 3300 pre I'm going to rebuild, mostly so I can update and upgrade the phono pre. Again, thanks. Scott

 

[scottonnob]

So I popped off the covers, in preparation for diagnosis of the problem, and I immediately saw something I'm not clear about. A couple of caps on the board are coated in some sort of goo. It looks like flux, but I've never seen flux present in this way. My first thought is that it's bad-cap-goo, but I'm not sure. Any thoughts? Thanks.

 

[whoaru99]

Looks like glue to me.

 

[scottonnob]

Yeah, that's likely. It certainly doesn't resemble any leaking cap I've ever seen. I'm just trying to decide how to approach this problem. I'm inclined to track down the specific issue and put it back together — for now. Otherwise, it would be a complete rebuild, which I'm not really up for right now.

 

[EchoWars]

Separate the amp and preamp with the back panel switch and see what happens re: the pop.

 

[scottonnob]

Really good idea. I need to unhook one of my amps and connect the Kenwood as a pre, and see what happens. But I suspect that the problem is in the driver board, since it was popping through both channels. If there's no pop as a pre, wouldn't that confirm my suspicion?

 

[CohibaJoe]

Yeah, that's likely. It certainly doesn't resemble any leaking cap I've ever seen. I'm just trying to decide how to approach this problem. I'm inclined to track down the specific issue and put it back together — for now. Otherwise, it would be a complete rebuild, which I'm not really up for right now.

It's Glue...Cannot tell from the Pictures....Is the glue RED..

 

[scottonnob]

The glue(?) is redish/brown. However, pardon my ignorance, but I didn't know they used glue on a board when the component is also soldered. Is that standard? And, 'Inductor', I'm not certain what your response means. Thanks for your contribution, but could you elaborate? Thanks.

 

[Nashou66]

Are there any slider switches in there? They might need a good cleaning.

Nashou

 

[scottonnob]

If I understand the question, I don't see any slider switches inside. There's one on the back, for 'Pre' or 'Separate' use.

 

[Binkman]

glue was used in many many amps. assembly and shipping... I.e; large caps would vibrate and that is why you see mains caps connected buy metal hardware screw harnesses. gorilla airport proof even back then.

 

[scottonnob]

I'm really hesitant to tear into this thing too far. Every post I've read on the topic suggests that this amp is a bear to work on and reassemble. I'd love to identify the source of the popping and just fix that, for now. Unfortunately, my diagnostic skills aren't that great. If it is a small signal transistor, neither the schematic, nor the Service Manual offer much help tracking this down. Plus, as far as I know, there isn't any way to test a transistor in circuit. But I may be wrong on that. So I'm a little stumped on how to proceed. I'm getting to the point of putting the thing on the shelf, until I have time to fully restore it (which doesn't really require exceptional diagnostic skills). But I love this amp, and it is a bulldozer of power. I'd hate to have to pull it out of service. I use it for recording with my reel to reel/turntable set-up. I really want to try and fix it. I have a Sencore PA 81 amplifier analyzer that a friend's wife gave me after he passed. But I haven't yet tried using it on anything. It connects easily to my analog HP scope, but I just haven't had time to familiarize myself with it. I'm not sure it was ever used. It looks brand new, and came with all the cables and connectors.

 

[scottonnob]

Thanks Binkman. More information for my brain-file. So I'm going to ignore that area and presume it isn't, in any way, the source of my problem.

 

[CohibaJoe]

I'm really hesitant to tear into this thing too far. Every post I've read on the topic suggests that this amp is a bear to work on and reassemble. I'd love to identify the source of the popping and just fix that, for now. Unfortunately, my diagnostic skills aren't that great. If it is a small signal transistor, neither the schematic, nor the Service Manual offer much help tracking this down. Plus, as far as I know, there isn't any way to test a transistor in circuit. But I may be wrong on that. So I'm a little stumped on how to proceed. I'm getting to the point of putting the thing on the shelf, until I have time to fully restore it (which doesn't really require exceptional diagnostic skills). But I love this amp, and it is a bulldozer of power. I'd hate to have to pull it out of service. I use it for recording with my reel to reel/turntable set-up. I really want to try and fix it. I have a Sencore PA 81 amplifier analyzer that a friend's wife gave me after he passed. But I haven't yet tried using it on anything. It connects easily to my analog HP scope, but I just haven't had time to familiarize myself with it. I'm not sure it was ever used. It looks brand new, and came with all the cables and connectors.

The Red Glue is used in some other Brands.....I call it "The Blob". As it EATS thru metal (corrodes). I have replaced Caps, Diodes and jumpers with the Legs eaten off.
If you do tackle this Recap....Glue must be scape off well. As far as working on the 9100...YES...front panel is packed and I wound up not getting into this Kenwood.
There is a Posting of a recent Recap.

 

[scottonnob]

Okay, I pulled this unit off of my 'Later' shelf to see about rebuilding it. I looked around inside, in an attempt to figure out why it keeps popping. I found a couple suspicious connections for the meter lights, with dried out electrician's tape I thought might be able to short out. I cleaned all that up and then turned the amp on for one more attempt to diagnose the problem. The thing ran perfectly for three days without a hitch. I thought, counter to my thinking, that maybe those joints were arcing, and that I'd fixed the problem. Then, on the fourth day of running well into each evening, it started popping again. Almost four full days with no problems, then 'Pop, pop, pop', just like before. It sounds a lot like the famous Phase Linear thump those amps generate when shut down. But it comes while running and occurs in weird spurts. Before I turned the Kenwood back on, I went over everything, literally, with a magnifying glass and flashlight. I was looking for anything out of the ordinary. I found nothing. No cold solder joints. No components that had visibly failed. No burn marks anywhere. And I re-cleaned all the switches and pots. I turned it on at night, over in a dark corner of my shop, because one of you suggested that whatever was happening was most likely the result of something arcing. So I put it somewhere that might make it possible to see where the arc was coming from. But I never saw anything, and the thumps yesterday came in the afternoon. Why would it run so long without any issues, then start again? That doesn't coincide with anything I can think of. What the hell takes fours days to show up? I'm really stumped. Can any of you suggest some sort of diagnostic approach, so as to finally locate the problems? I really don't know where to start. And I'd like to have some idea what's going on before I even think about rebuilding the thing. I have a lot of test equipment, but not really a notion of where to start, especially in that the problem is an example of 'The Great Electronics Curse' — an intermittent problem. I'd appreciate any input that would help me figure out what's going on.

 

[Blue Shadow]

Intermittent is hard to find. Since yours just needs time to show up, on time at that, maybe it is heat related. Cold no problem as the unit finally gets fully warmed up the popping begins. In some cases a cold spray can help diagnose the issue. Don't know if that will work here.

To find a consistent problem, you can poke and prod with a non-conductive tool to see if something is loose, found a solder joint in an NAD that way, thanks to Gadget73.

Another way is to follow the signal through the machine. When the sine wave looks great on the scope all is good when you get past or near the problem child, the scope will show the problem. That narrows it down a bit. signal generator and scope, but you said you had some test equipment. Trace forward or backward. Did the separating the unit, pre/power show which part has the problem?

 

[tarior]

Popping in the amp section of a KA-9100 is usually due to Q5, Q7(L-ch) or Q6,Q8(R-ch) (2SA912) going intermittent. Replace them with KSA1220AY. It won't hurt to replace Q1-4 (2SC1775) with KSC1845 or 2SC2240. Don't buy transistors on ebay.

 

[gort69]

If it's making the popping noise simultaneously in both channels as stated in post #8 I'd suspect a failing component - likely a semiconductor - on the power supply, which will almost certainly pass any test you can do with a DMM. If it was mine, I'd rebuild that board and see if it fixes the problem. If it doesn't, well, at least that part of the rebuild will be done.

Fortunately the power supply is pretty easy to access on the 9100. It's the upright board at the rear. I'd replace all four transistors and all 6 diodes.Might as well recap it and replace the relay while you have it apart.. Hopjohn lists the transistor, diode and cap replacements in his excellent ka-9100 rebuild thread here (post #71): http://www.audiokarma.org/forums/in...-9100-restoration.783947/page-4#post-10879335 The correct relay is Omron MY4-02-DC24