KA-7100, ridiculously high DC offset
- Thread starter rjsilva
- Start date
Measurements:
Ri66-preamp side: 38.5V
Ri66: 21.8V
Ri65-preamp side: 0
Ri65: 0.76V
Qi10/9 collector: 16.7V for both
Blue wire: 264mV, 2.5mV tone defeat on, 0.3mV subsonic on
Yellow wire: 280mV, 3.0mV tone defeat on, 0.2mV subsonic on
Two important things:
1. Di2 looks like it might be toast (it and Ri66 get really hot really fast).
2. R76, the burnt resistor I just replaced a few days ago, is becoming increasingly discoloured. I measured it and it's getting 43VDC which seems within spec. Not sure what's going on there as the amp has only been on about 20 minutes since replacing the resistor...
Something else:
I really don't want to complicate this any further, but just so all known information is out there. The DC offset issue appears to be new, however the amp came with another problem. It produces no output except when the volume knob is turned up to 70% or more, and then it only produces bursts of static. I only bring this up in case one or more of the readings above is not related to the DC offset issue.
Ri66-preamp side: 38.5V
Ri66: 21.8V
Ri65-preamp side: 0
Ri65: 0.76V
Qi10/9 collector: 16.7V for both
Blue wire: 264mV, 2.5mV tone defeat on, 0.3mV subsonic on
Yellow wire: 280mV, 3.0mV tone defeat on, 0.2mV subsonic on
Two important things:
1. Di2 looks like it might be toast (it and Ri66 get really hot really fast).
2. R76, the burnt resistor I just replaced a few days ago, is becoming increasingly discoloured. I measured it and it's getting 43VDC which seems within spec. Not sure what's going on there as the amp has only been on about 20 minutes since replacing the resistor...
Something else:
I really don't want to complicate this any further, but just so all known information is out there. The DC offset issue appears to be new, however the amp came with another problem. It produces no output except when the volume knob is turned up to 70% or more, and then it only produces bursts of static. I only bring this up in case one or more of the readings above is not related to the DC offset issue.
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I had assumed that since you replaced R76, that you checked and confirmed that the +/-28V regulators were now functional. Since neither one is working properly, all bets are off. Do not power up again until both of these regulators have been made functional. New transistors, zeners, capacitors, and resistors.
Thanks for persevering! Are you able to provide more specific details about what needs to be replaced?
In my ignorance I'm a bit confused why this would show up after replacing the caps. Assuming no error on my part, is it unusual for a recap to reveal or cause problems with other ageing components?
On a side note, I just did a quick look at Mouser for a WZ182 replacement, is the following acceptable?
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtQ8nqTKtFS/D313Kx94AdFIWgdFvKWGEM=
In my ignorance I'm a bit confused why this would show up after replacing the caps. Assuming no error on my part, is it unusual for a recap to reveal or cause problems with other ageing components?
On a side note, I just did a quick look at Mouser for a WZ182 replacement, is the following acceptable?
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtQ8nqTKtFS/D313Kx94AdFIWgdFvKWGEM=
Please allow me to make some very uneducated guesses:
- Since there is +/-41V +-5% going into Re76/75, everything before those resistors is probably ok.
- Since De17/18 both report +-35V or more, there may be a problem with them or Qe31/32 (though what's the likeliness of both failing in the same way at the same time??).
- Since pin 7 on the power amp board is reporting 0V, this reinforces the idea that there is a (catastrophic) problem with De18 or Qe32.
- Alternatively, there may be a problem with Ce25/26/27/28 ... but, I had just replaced those prior to the problem occurring, and they don't appear to be in backwards. I should recheck for the millionth time.
So, I'm wildly guessing that replacing De17/18 and Qe31/32 is a good place to start? Can anyone suggest replacement parts for them? Qe31/32 are C1419 and A755, and De17/18 are EQA01-28R (in the service manual).
- Since there is +/-41V +-5% going into Re76/75, everything before those resistors is probably ok.
- Since De17/18 both report +-35V or more, there may be a problem with them or Qe31/32 (though what's the likeliness of both failing in the same way at the same time??).
- Since pin 7 on the power amp board is reporting 0V, this reinforces the idea that there is a (catastrophic) problem with De18 or Qe32.
- Alternatively, there may be a problem with Ce25/26/27/28 ... but, I had just replaced those prior to the problem occurring, and they don't appear to be in backwards. I should recheck for the millionth time.
So, I'm wildly guessing that replacing De17/18 and Qe31/32 is a good place to start? Can anyone suggest replacement parts for them? Qe31/32 are C1419 and A755, and De17/18 are EQA01-28R (in the service manual).
If you shove enough current into a zener, it'll show a voltage considerably above that for which it is rated (but it'll get hot and eventually burn up). You can ponder exactly what might be the problem, but I'd simply replace Re75, Re76, Re77, Re78, De17, De18, Qe31, and Qe32.
I'd have wanted to replace De17 and De18 with 1W 28V zeners, but they aren't made, so I'd use two 14V 1/2W zeners and connect them in series (twist two ends together and solder...cathode facing the same way! Like this: http://www.electronicrepairguide.com/diodes-in-series-and-parallel.html Pic #2, and ignore everything else on that page). A 14V 1/2W zener is a 1N5244B. Replace the two transistors with On-Semi MJE15032G and MJE15033G.
I'd also replace Di1, Di2, Ri65, and Ri66. For the two zeners here, use a 19V 1/2W zener. A 1N5249B is fine.
I'd have wanted to replace De17 and De18 with 1W 28V zeners, but they aren't made, so I'd use two 14V 1/2W zeners and connect them in series (twist two ends together and solder...cathode facing the same way! Like this: http://www.electronicrepairguide.com/diodes-in-series-and-parallel.html Pic #2, and ignore everything else on that page). A 14V 1/2W zener is a 1N5244B. Replace the two transistors with On-Semi MJE15032G and MJE15033G.
I'd also replace Di1, Di2, Ri65, and Ri66. For the two zeners here, use a 19V 1/2W zener. A 1N5249B is fine.
It works! 
I replaced Re75-78, De17/18, Qe31/32 and Di1/2. When I was replacing the parts I did a little testing and De17/18 both had issues - one of them wasn't passing current at all and the other was passing in both directions.
Thanks so much for your help EchoWars! I've actually learned quite a bit during this which has inspired me to really put time into learning this stuff. Perhaps I'll eventually be able to contribute more intelligently to the forums.
And ... the static-no-audio issue is gone. Fabulous.
I replaced Re75-78, De17/18, Qe31/32 and Di1/2. When I was replacing the parts I did a little testing and De17/18 both had issues - one of them wasn't passing current at all and the other was passing in both directions.
Thanks so much for your help EchoWars! I've actually learned quite a bit during this which has inspired me to really put time into learning this stuff. Perhaps I'll eventually be able to contribute more intelligently to the forums.
And ... the static-no-audio issue is gone. Fabulous.
Sorry, I kinda fell out of posting to this thread as I didn't subscribe to it. Glad to hear you got your KA-7100 going, they're really nice amps. I actually managed to get mind almost hot the other day, driving a pair of old KLH model 331 speakers. I doubt it it's been driven over a watt since I recapped it and did some other work.
Lee.
Lee.
Thanks for being interested!
I do like the sound of the 7100, and I love the feel of the volume knob. The faceplate looks nice as well. Though I'm missing my Yamaha CA-1000 (I'm having a hum issue that is beyond my ken) which is better for the music I listen to. The KA-7100 is not as "transparent" as the CA-1000. But the volume knob on the CA-1000 sucks in comparison, which does matter to me for some reason
I use Klipsch RB75s (sensitivity is 97dB) and there's no issue driving them. The 7100 barely gets warm even when the music is quite loud!
I do like the sound of the 7100, and I love the feel of the volume knob. The faceplate looks nice as well. Though I'm missing my Yamaha CA-1000 (I'm having a hum issue that is beyond my ken) which is better for the music I listen to. The KA-7100 is not as "transparent" as the CA-1000. But the volume knob on the CA-1000 sucks in comparison, which does matter to me for some reason
I use Klipsch RB75s (sensitivity is 97dB) and there's no issue driving them. The 7100 barely gets warm even when the music is quite loud!
I wonder how the other components used in the Kenwood influence the sound? I rebuilt a NAD 3020 using metal film resistors and thought it sounded really sweet. If you do the work yourself (and ignore the cost of your time) then you just have the cost of the resistors which won't be much.
Did you use low leakage caps where there were orange Elnas originally? I tend to look at the schematic and replace with Nichicon KLs wherever I see electrolytics in the signal path. Of course, you could replace with stacked film, but there is usually the physical size to deal with. I'm doing another 3020 now, and have used stacked film 4.7uF in place of the electrolytics in the signal path in the phono stage. It took some nifty lead bending and a resistor relocation to squeeze them in. There is one higher value electrolytic (47uF) left in the preamp, but I wasn't even going to try replacing that with a film cap, it would be HUGE!
Lee.
Did you use low leakage caps where there were orange Elnas originally? I tend to look at the schematic and replace with Nichicon KLs wherever I see electrolytics in the signal path. Of course, you could replace with stacked film, but there is usually the physical size to deal with. I'm doing another 3020 now, and have used stacked film 4.7uF in place of the electrolytics in the signal path in the phono stage. It took some nifty lead bending and a resistor relocation to squeeze them in. There is one higher value electrolytic (47uF) left in the preamp, but I wasn't even going to try replacing that with a film cap, it would be HUGE!
Lee.
Replacing all the resistors sounds like a time consuming project! Did you use the same wattage? I wonder if the "sweet" sound you mentioned is simply less distortion due to the higher tolerance of the metal film resistors? If the 7100 were my only amp I would be very tempted, but the CA-1000 sounds so great. I wish I could just change the volume pot on it!
This KA-7100 seems to have a sort of small rise (relative to the CA-1000), or possibly (but not likely) distortion, in the vicinity of high mid-range/low-treble - at the top-end of the brass timbre but lower than cymbals. It prevents the 7100 from really sounding transparent to me. It does sound very good otherwise - nice bass and smooth midrange and treble. But I get a similar sense as I do with the A-S700, that I'm more aware I'm listening to a recording, in comparison to the CA-1000.
Did you use the same wattage? I wonder if the "sweet" sound you mentioned is simply less distortion due to the higher tolerance of the metal film resistors? If the 7100 were my only amp I would be very tempted, but the CA-1000 sounds so great. I wish I could just change the volume pot on it!This KA-7100 seems to have a sort of small rise (relative to the CA-1000), or possibly (but not likely) distortion, in the vicinity of high mid-range/low-treble - at the top-end of the brass timbre but lower than cymbals. It prevents the 7100 from really sounding transparent to me. It does sound very good otherwise - nice bass and smooth midrange and treble. But I get a similar sense as I do with the A-S700, that I'm more aware I'm listening to a recording, in comparison to the CA-1000.
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Since my CA-1000 is still not fixed I've continued to use the KA-7100. The frequency "rise" in the vicinity of high mid-range/low-treble I mentioned in my last post I'm beginning to believe is actually distortion...
The amp seems smoother in that frequency range when the input level is lower. When the level of the music rises it seems to get harsher. I wouldn't have the foggiest idea where to begin troubleshooting something like that. Any ideas?
The amp seems smoother in that frequency range when the input level is lower. When the level of the music rises it seems to get harsher. I wouldn't have the foggiest idea where to begin troubleshooting something like that. Any ideas?