Stack
Active Member
Hi,
For those people wanting to attempt to calibrate their KA-801 or similar Kenwood machine after a recapping project, I wanted to document the troubles I faced and ask for advice on a few key points.
A side note: for this project, you will need a multimeter. I used an early 2010s Radio Shack model, and it was sufficient. If you want to calibrate VU meters, you'll also need a dummy load for the speaker output (I didn't bother with this adjustment -- they were close enough.)
Excluding the VU meters, there are three main adjustments for this amp outlined in the spec sheet: Preamp offset, power amp offset, and Bias adjustment. I highly reccomend you download this spec sheet from hifiengine for the accompanying diagrams.
Preamp offset adjustment:
According to the spec: 1) Decline rear panel 2) Connect DC voltmeter to points 7 and 8 (left) or 9 and 10 (right) of the preamp board assembly. 3) Adjust trimming pot 1 (left) or 2 (right) for 0V reading on your meter.
In practice: Where to begin? This was by far the most difficult adjustment.
First of all, descending the rear panel doesn't really work if you want access to the pins. You have to completely unhook it, and remove the bottom panel of the amp. Get the amp onto an elevated surface of about 2 inches and hang the back off by 4-5 inches, so you can get your multimeter in there. Take care none of the pins on the preamp board bend while you do this!
When you take the reading, the reading on your meter will swing wildly for at least 10 minutes. Don't even try to get a reading while it's doing this -- wait a bare minimum of 10 minutes for the amp to warm up before doing anything. This means every single time you power it on, even if it was on a minute ago and still feels warm.
After this wild swaying (which can be up to half a volt or more!) the reading will settle into swaying around by about 20-30 mv or so. Now comes the time to make your adjustment. Use a plastic handled flat head eyeglass screwdriver to trim the pot through the tiny hole provided in the circuit board.
Now you're in for problem two. This is by far the most sensitive trimming pot I've ever encountered in my life. To get a fine tune ajdustment, don't even attempt to put any force on the screw driver. You're better off kind of shimmying it in place with very slight pressure in the direction you want to trim the pot. Do this until you can get the mv reading to fluctuate between 15 and -15mv on your multimeter.
Finally, the third problem I encountered came after doing all of this again for the second pot. It seems that adjusting one side can possibly affect the other side. Do yourself a favor before reassembling and double check that first reading again before putting your amp back together. My first reading had drifted so far out that when I got everything back together, the right channel was audibly clipping at medium volumes. It took many back and forth rounds of adjustment to get both sides in spec.
This was so frustrating that I'd almost recommend buying a second multimeter if you're going to attemp this adjustment, so you can do both at once. In fact, if your amp sounds good, don't even bother trying to adjust the preamp offset.
Power amp offset adjustment
According to the spec: 1) Remove the front panel 2) Connect DC voltmeter between positive and negative speaker terminals 3)Adjust trimming pot VR5 (left) or VR6 (right) for a 0V reading on the meter.
In practice: This one was much easier. There's no real tricks here except that, once again, the trimming pots are hyper sensitive. You'll just have to be patient trimming these. You'll also have to wait for the amp to level out again, but at least here you won't have to readjust over and over to get things in spec.
Bias current adjustment
According to the spec: 1) Turn the volume control knob fully counterclockwise. 2) Connect the DC voltmenter between adjusting points 18 and 20 (left) or 27 and 29 (right) of the power amp board assembly. 3) Adkust the bias current trimming pot VR1 (left) or VR2 (right) for an 18mv reading on the voltmeter.
In practice: This one should be very easy, because the power board is accessible simply by removing the top cover of the amp, but I faced a huge problem: it was impossible to get an 18mv reading on the meter! The left meter would read 35mv at its lowest setting while the right channel would read 23mv.
I've attempted this adjustment in the past with similar results, but chalked it up to the caps being out of spec. Turns out other things have drifted in here too. For the time being, I have the pots set as low as possible, but they probably need to be replaced.The spec sheet lists them as being 500 ohm potentiometers, so doubling that seems like a good place to start.
These post also look noticeably cheaper than the others. Their internals are open to the air. I tried to spray them out with deoxit, but it doesn't seem to change anything.
Finally, as always, you have to wait ten minutes or so for an accurate reading.
--------
So, that's everything with adjusting this amp. I figured I'd share in case anybody had anything to add that might've made my life easier (like maybe there's something I could've done to get the preamp voltages to stop swaying around), wanted to call anything that I've concluded as blatantly wrong for my own education, or maybe as a guide for people wanting to attempt fine tuning this amp.
I'll say that for all the effort, I barely notice any improvement. In fact, that preamp offset seems like it was on the money before I fiddled with it. The only place I might see improvement is replacing those bias pots so I can actually get the amp to read correctly.
For those people wanting to attempt to calibrate their KA-801 or similar Kenwood machine after a recapping project, I wanted to document the troubles I faced and ask for advice on a few key points.
A side note: for this project, you will need a multimeter. I used an early 2010s Radio Shack model, and it was sufficient. If you want to calibrate VU meters, you'll also need a dummy load for the speaker output (I didn't bother with this adjustment -- they were close enough.)
Excluding the VU meters, there are three main adjustments for this amp outlined in the spec sheet: Preamp offset, power amp offset, and Bias adjustment. I highly reccomend you download this spec sheet from hifiengine for the accompanying diagrams.
Preamp offset adjustment:
According to the spec: 1) Decline rear panel 2) Connect DC voltmeter to points 7 and 8 (left) or 9 and 10 (right) of the preamp board assembly. 3) Adjust trimming pot 1 (left) or 2 (right) for 0V reading on your meter.
In practice: Where to begin? This was by far the most difficult adjustment.
First of all, descending the rear panel doesn't really work if you want access to the pins. You have to completely unhook it, and remove the bottom panel of the amp. Get the amp onto an elevated surface of about 2 inches and hang the back off by 4-5 inches, so you can get your multimeter in there. Take care none of the pins on the preamp board bend while you do this!
When you take the reading, the reading on your meter will swing wildly for at least 10 minutes. Don't even try to get a reading while it's doing this -- wait a bare minimum of 10 minutes for the amp to warm up before doing anything. This means every single time you power it on, even if it was on a minute ago and still feels warm.
After this wild swaying (which can be up to half a volt or more!) the reading will settle into swaying around by about 20-30 mv or so. Now comes the time to make your adjustment. Use a plastic handled flat head eyeglass screwdriver to trim the pot through the tiny hole provided in the circuit board.
Now you're in for problem two. This is by far the most sensitive trimming pot I've ever encountered in my life. To get a fine tune ajdustment, don't even attempt to put any force on the screw driver. You're better off kind of shimmying it in place with very slight pressure in the direction you want to trim the pot. Do this until you can get the mv reading to fluctuate between 15 and -15mv on your multimeter.
Finally, the third problem I encountered came after doing all of this again for the second pot. It seems that adjusting one side can possibly affect the other side. Do yourself a favor before reassembling and double check that first reading again before putting your amp back together. My first reading had drifted so far out that when I got everything back together, the right channel was audibly clipping at medium volumes. It took many back and forth rounds of adjustment to get both sides in spec.
This was so frustrating that I'd almost recommend buying a second multimeter if you're going to attemp this adjustment, so you can do both at once. In fact, if your amp sounds good, don't even bother trying to adjust the preamp offset.
Power amp offset adjustment
According to the spec: 1) Remove the front panel 2) Connect DC voltmeter between positive and negative speaker terminals 3)Adjust trimming pot VR5 (left) or VR6 (right) for a 0V reading on the meter.
In practice: This one was much easier. There's no real tricks here except that, once again, the trimming pots are hyper sensitive. You'll just have to be patient trimming these. You'll also have to wait for the amp to level out again, but at least here you won't have to readjust over and over to get things in spec.
Bias current adjustment
According to the spec: 1) Turn the volume control knob fully counterclockwise. 2) Connect the DC voltmenter between adjusting points 18 and 20 (left) or 27 and 29 (right) of the power amp board assembly. 3) Adkust the bias current trimming pot VR1 (left) or VR2 (right) for an 18mv reading on the voltmeter.
In practice: This one should be very easy, because the power board is accessible simply by removing the top cover of the amp, but I faced a huge problem: it was impossible to get an 18mv reading on the meter! The left meter would read 35mv at its lowest setting while the right channel would read 23mv.
I've attempted this adjustment in the past with similar results, but chalked it up to the caps being out of spec. Turns out other things have drifted in here too. For the time being, I have the pots set as low as possible, but they probably need to be replaced.The spec sheet lists them as being 500 ohm potentiometers, so doubling that seems like a good place to start.
These post also look noticeably cheaper than the others. Their internals are open to the air. I tried to spray them out with deoxit, but it doesn't seem to change anything.
Finally, as always, you have to wait ten minutes or so for an accurate reading.
--------
So, that's everything with adjusting this amp. I figured I'd share in case anybody had anything to add that might've made my life easier (like maybe there's something I could've done to get the preamp voltages to stop swaying around), wanted to call anything that I've concluded as blatantly wrong for my own education, or maybe as a guide for people wanting to attempt fine tuning this amp.
I'll say that for all the effort, I barely notice any improvement. In fact, that preamp offset seems like it was on the money before I fiddled with it. The only place I might see improvement is replacing those bias pots so I can actually get the amp to read correctly.
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