You can easily determine if excessive loading is an issue. Excessive bias should show up as high heat sink temps. The heatsinks on those B&K models weren't all that large so it wouldn't take much excess bias to really heat them up. Either way get online and buy a Kil A Watt meter they're cheap and a great tool to have around the house. I've had mine for about 13 years now and use it all the time. You can use the Kil A Watt to see how much power your amp is drawing from the wall.
Based on the age of the amp my guess is its a mechanical problem, in other words loose windings.
Based on the age of the amp my guess is its a mechanical problem, in other words loose windings.
Thanks KC.
How will this meter show excess bias? My DMM has a temp wire to measure heat, but I don't have info on what the heat should be.
At this point, I would agree (in my limited knowledge) that the loose windings diagnosis is most likely ... since the heat sinks do not feel hot, but warm, and not at a level above what I'm used to with other amps of the era.
Is there a test for loose winding?
How will this meter show excess bias? My DMM has a temp wire to measure heat, but I don't have info on what the heat should be.
At this point, I would agree (in my limited knowledge) that the loose windings diagnosis is most likely ... since the heat sinks do not feel hot, but warm, and not at a level above what I'm used to with other amps of the era.
Is there a test for loose winding?
The current draw will be quite high.
When we're talking about excess bias being an issue its not +10% we're referring to. It takes a lot of extra bias to load a large amp transformer to the point it begins to hum, I would say like 40 - 50% more or double or higher. This shows up in the power draw from the wall which the killawatt can show you. Based on the heatsink size I'd be surprised if the AC power draw at idle exceeds 50 - 60 watts without overheating the sinks on that amp.
You also do not need a temp probe to check heatsink temps. Your hand will suffice. I know its not that scientific but it works. These heatsinks are not huge and do not have a lot of excess capacity so theyll heat up fast. I owned a pair of M200 monos for years that I bought brand new in 93. Without a signal their heatsinks were barely above room temp. I also owned an ST140 for a period of time. Once again barely warm to the touch when idling with no signal. If your heatsinks are getting to the point they are uncomfortably hot when the amp is just idling then your bias certainly is too high.
When we're talking about excess bias being an issue its not +10% we're referring to. It takes a lot of extra bias to load a large amp transformer to the point it begins to hum, I would say like 40 - 50% more or double or higher. This shows up in the power draw from the wall which the killawatt can show you. Based on the heatsink size I'd be surprised if the AC power draw at idle exceeds 50 - 60 watts without overheating the sinks on that amp.
You also do not need a temp probe to check heatsink temps. Your hand will suffice. I know its not that scientific but it works. These heatsinks are not huge and do not have a lot of excess capacity so theyll heat up fast. I owned a pair of M200 monos for years that I bought brand new in 93. Without a signal their heatsinks were barely above room temp. I also owned an ST140 for a period of time. Once again barely warm to the touch when idling with no signal. If your heatsinks are getting to the point they are uncomfortably hot when the amp is just idling then your bias certainly is too high.
Now that you've warmed it up good turn the music (input signal) off and leaving the the amp powered on let it sit with no signal for an hour then go see how hot the sinks are.
Its not uncommon for them to heat up playing music at a reasonably high output level and then cool down while powered on with no signal.
But yeah that seems like the bias may be too high.
Its not uncommon for them to heat up playing music at a reasonably high output level and then cool down while powered on with no signal.
But yeah that seems like the bias may be too high.
Ok. So after a little while, no signal, heat sinks are warm. I can hold my hand on them indefinitely.
DC offset is as close as I can get to spec (±5mVDC) ... under ±4mVDC ... but as I go to set each channel, I'm getting some readings that swing up into the teens. The hum didn't change from what I could hear.
DC Bias, I need to find someone local who might be setup for checking that ... I don't feel comfortable with the process outlined, and know all it takes is one mis-step to cause damage.
DC offset is as close as I can get to spec (±5mVDC) ... under ±4mVDC ... but as I go to set each channel, I'm getting some readings that swing up into the teens. The hum didn't change from what I could hear.
DC Bias, I need to find someone local who might be setup for checking that ... I don't feel comfortable with the process outlined, and know all it takes is one mis-step to cause damage.
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This diagnosis is supported by lack of hum thru variac, because that's acting as an isolation transformer, right?
I just want to confirm ... using my DMM set on VAC, can I clip the probes on the fuse holder to act as the resistor? I'm guessing "dots" mean the probe tips?
The B&K instruction sheet mentions connecting an 8 ohm load to each channel ... is that just connecting a preamp to the input? Is this without speakers hooked up?
I ask since I have to power up to a variac to do this, and I may put my dim bulb tester in between just in case.
Pics below show my probes clipped to fuse holder ... and meter set to VAC. Ignore the reading. The amp isn't plugged in.
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B&K instruction pdf says:
1. Plug the amplifier into a variac. Monitor the (-) negative rail of each channel by removing the fuse and placing a current meter across the fuse clips.
2. Connect an 8 Ω load to each channel. Supply an input signal of 100 mV RMS @ 20 Khz. I don't have equipment for this part of #2.
3. Slowly turn the variac up to operating voltage while monitoring the rails for excessive current draw. The meters should not read more than 400 mA DC. If one or both read more than 400 mA, turn the bias pots (P2) to minimum and retry.
4. Set bias of the amplifier by adjusting the bias pot (P2) mounted on the solder side of the board to achieve a reading of 200 mA on each channel for models ST 140, ST 140M, ST 202, and ST 202+. The bias for the Pro 600, EX 442 and M 200 amplifiers is 250 mA. Setting the bias higher than stated only causes the amplifier to run hotter and burn out faster. It does nothing for the sound quality of the amplifier.
5. Set the DC out of the amplifier by monitoring across the outputs. Adjust the DC pot (P1) for 0 VDC ± 5 mVDC.
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You need a source that can provide the 100mV, 20KHz signal to the input RCA's of the amp. While not ideal, an ipod might work for this. You will have to measure if it can produce 100mV. Then an 8 ohm, 5 to 20 Watt resistor accross the speaker jacks, not a speaker.
A laptop or a nearby computer will work for the tone generator. You'll need a headphone to RCA adapter. Then download one of many freeware programs to generate the 20kHz tone, there's even a few websites I found a few years ago that will allow you to set and activate a specific freq.
Also if you have a DAC in your stereo system that you can connect your computer up to you could generate the 20kHz tone that way as well.
Also if you have a DAC in your stereo system that you can connect your computer up to you could generate the 20kHz tone that way as well.
What an idiot I am 
I just realized I have the Tone Gen Pro app on my iPhone ... which I used another time I needed to have a tone source. I can set frequency, but don't see any setting to see if it can provide the 100mV ... but it goes up to 20kHz.
Found my Y cable (mini to rca stereo) ....
jhofland, so VDC and not the mA DC in the instructions from BK?
I just realized I have the Tone Gen Pro app on my iPhone ... which I used another time I needed to have a tone source. I can set frequency, but don't see any setting to see if it can provide the 100mV ... but it goes up to 20kHz.
Found my Y cable (mini to rca stereo) ....
jhofland, so VDC and not the mA DC in the instructions from BK?
What an idiot I am
I just realized I have the Tone Gen Pro app on my iPhone ... which I used another time I needed to have a tone source. I can set frequency, but don't see any setting to see if it can provide the 100mV ... but it goes up to 20kHz.
Found my Y cable (mini to rca stereo) ....
jhofland, so VDC and not the mA DC in the instructions from BK?
No, the mA DC is correct. I was responding to the designation of VAC, I believe it was.
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