KA-5500, left channel noise, need help troubleshooting.

[mightyhorse]

Hey first off thanks to all for this community, this is my first post but I have done a lot of research here as I decided to get into a vintage stereo setup.

I'm using a KA-5500 which is my first integrated amp. I have what I now recognize to have been a slowly deteriorating problem over the past couple of weeks. This amp is new to me, have had it about a month and have really started to enjoy it. Supposedly it had been at least "looked over" or moderately serviced by an electrician not too long before I received it, but I have no records of that.

A couple of weeks ago I noticed a very intermittent (heard twice over ~8hrs) very short "fuzz" while listening at casual volume levels. Almost just a stutter, a little line noise. I knew I needed to DeOx the volume knob at some point so didn't think much of it.

A couple of nights ago I got a new clip lamp to light the turntable station, and was messing around with the light dimming features with a Grateful Dead record on very low volume, and the left channel (the one I was near) started producing a lot of noise...crackle...but it would come and go. I thought for a second it could be related to power flux dimming this little LED lamp but there was no way. I thought it could have possibly been a record in need of cleaning but also saw no evidence of what that could be...figured I would save that mystery for another day as I assumed it was related to the turntable or phono input itself.

Well last night while listening to music off of my iPad it hit the fan. It had been running really nicely for a couple of hours, anywhere from moderate to on the louder side (but nothing crazy)...I was listening to it on a lower volume for a while when all of the sudden the left channel started really "fuzzing", as if you were taking a hot line cable in and out of the input or something and creating these erratic buzzes. It would still come and go a little. I shut the amp down for a bit and ran it again for a little while, hit something similar.

As of this morning, when I turn it on there is now what I would describe as when you are in between tracks on a record...just this noisy vinyl crackle, but now it's there in the left channel full time, no longer intermittent. The right channel is fine. When music is played, both speakers still "sound" good though there is the underlying noise on the left. The noise is consistent at any volume level, comes on as soon as the amp is fired up.

This is happening regardless of it is A or B speakers (I tried switching), or which input is selected. So it seems there is something that has failed or introduced noise along my LEFT channel bath in the output stage. Is that likely a capacitor that needs replacing? I guess I am going to be diving into this head on now...but virtually no clue what I am doing. I've got my volt meters, solder station, precision tools, deoxit kit and worktable all ready to go...ANY help in determining what this issue might be and how I could start testing the fail point would be greatly appreciated!

I just turned it on again to take another look/listen. The left VU meter is jumping out of whack, and this is mostlyin line with when the crackles get louder to more of a full distortion and less off a "hiss". The right channel VU meter is normal (barely moving), but the left needle is acting more wobbly and then intermittently spiking.

 

[eizner23]

I'm a 5500 owner sorry to hear the bad news. The symptoms you describe would lead me to look at failing signal transistor in left channel.

Without getting too deep the easiest way to perhaps find it is spraying suspect transistors on left channel with freeze spray when the issue has surfaced. That may identify the bad device.

I don't recall where the preamp board is located, I'd start there, but grab service manual off hifiengine.com to help you locate left channel circuit.

 

[mbz]

As suggested, try the freeze spray on transistors Qd5 and Qd6

 

[Watthour]

While this could be due to deteriorating 2SA763s in the equalization amp, if this is also occurring when using a TAPE input (which can be done with the line level signal from an iPod) the phono preamp is out of the picture (for now). Q1/Q2 in the control amp (X11-1290-01 board) are presumed to be 2SC1345 devices, reputed to be static generators. They can be replaced with KSC1845Fs or equivalents. There are also 2SA763s used as differntial pairs at the front end of the main power amp. They are Q1-Q4 on the X07-1430-01 board. These are also reputed to be noise generators in some of their series, and can be replaced with KSA992Fs or equivalents.

In short, there are several commonly known potential trouble-makers in the unit, and those would be the prime suspects considering the symptoms you are reporting.

 

[markallen]

I've got a KA-5500 that has a low crackle in the L channel - you probably wouldn't hear it playing most music, but when the amp is powered up with no input, you can hear it in a quiet room, maybe every five or ten minutes.

I know it's not in the control amp; changed out those two 2SC1345 (and one WAS seriously noisy). It's also nothing to do with the phono amp, as it does this on any input.

Based on the advice in the preceding entry, I'm thinking of changing out the (potentially noisy) 2SA733s in the main amp. I've got a bag of KSA992FBU transistors, but how do I ensure they meet the definition of "differential pair"? I seem to recall it includes matching the hFE measurements, something my DMM can do. If this is true, 1) what is an acceptable measurement "spread" between the two transistors and 2) is a higher number better in a "matched" pair, or doesn't it matter as long as there is a match (or however close they can get)?

Thanks!

 

[Watthour]

The transistors in this application can be measured for forward voltage drop of the E-B junction. Using a DMM with a diode test range ( ─►│─ ) most commonly causes the display to indicate the forward voltage drop, usually in mV.

If you find pairs which are within a few mV of each other, those pairs can be further tested for current gain (hFE). Pairs within 2-3% are going to be acceptable for this use.

 

[markallen]

Wow, that was quick - Thank you! I'll give a try.