I’ve restored an HK930 and HK 730, both of which have been working great. I clean them, replace the Electrolytic caps, and repot the power transistors. The HK 670 sounds great, and I was getting ready to button it up, when I noticed that the left heat sink is hot to the touch, while the right heat sink is at room temperature. I did a visual check of the components and nothing seems damaged. I checked the idle voltage at the speaker outputs. They are both high at -0.350 volts. The Service Manual says the should be +-20 millivolts. One of the things I found odd about this receiver is that there’s a diode inside of a spade connector that attaches to the heat sink. I have no idea what it does, but the left channel’s one was broken, so I replaced it with a spare from another HK670. Any suggestions would be appreciated. I’m a mechanical engineer and not very good at troubleshooting. I am pretty good at soldering, and can use a volt-ohm meter.
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Working on HK 670 Receiver and Left Channel Heat Sink Gets Hot
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Idle current can not be measured at the speaker output. DC offset/balance however can. Need to download the service manual and check and properly adjust the idle current.
Power amplifier idle adjustment = idle current or bias
Output DC voltage zero adjustment = DC offset
Page 2 in the service manual.
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Thermally tracks the output transistors heat and automatically dials down the bias to prevent thermal run away.
You can't. Its impossible.
Your confusing idle current/bias and DC offset.
Idle current is adjusted by measuring and adjusting the voltage across two of the output transistor emitter resistors.Each emitter resistor is .22Ω... 33mv is your target voltage adjustment.
Ohms Law E/R=I .. 33mv/.44Ω = 75ma 75ma is your actual idle current.
Ohms Law E/R=I .. 33mv/.44Ω = 75ma 75ma is your actual idle current.
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No..Should be 0.00vdc +/- 20mv. Zero volts being the target adjustment .35volts or 350mv is way to high.
As a Mechanical Engineer I took a semester of Basic Electrical Engineering and I know Ohm’s law.
The Technical Manual for the HK 670 receiver, under Alignment Preocedures, Output DC Voltage Zero Adjustment, it says Function Selector to AUX, Volume Control to minimum, and press in Speaker 1 push button to ON. Step 2 is measure left channel at the left channel speaker output terminals. It should be 0 volts +-20 millivolts. It was high so I adjusted it down to what the right terminal which measures -350 millivolts.
I’ll connect an 8 ohm 50 watt resistor to the speaker terminal and measure the Power Amplifier Idling Adjustment voltage tomorrow. It should be 33 millivolts +-4 millivolts.
My whole point in starting this thread is that I don’t know enough to design audio components. I would like to know, based on the above schematic what components control the idling voltage on the left power amp PC board, and yes I know that VR402 and VR 404 adjust it, but what components would make it go high. On a previous thread, someone had identical issues with an HK 560, which is a nearly identical lower powered receiver. The problem was a cracked resistor.
The Technical Manual for the HK 670 receiver, under Alignment Preocedures, Output DC Voltage Zero Adjustment, it says Function Selector to AUX, Volume Control to minimum, and press in Speaker 1 push button to ON. Step 2 is measure left channel at the left channel speaker output terminals. It should be 0 volts +-20 millivolts. It was high so I adjusted it down to what the right terminal which measures -350 millivolts.
I’ll connect an 8 ohm 50 watt resistor to the speaker terminal and measure the Power Amplifier Idling Adjustment voltage tomorrow. It should be 33 millivolts +-4 millivolts.
My whole point in starting this thread is that I don’t know enough to design audio components. I would like to know, based on the above schematic what components control the idling voltage on the left power amp PC board, and yes I know that VR402 and VR 404 adjust it, but what components would make it go high. On a previous thread, someone had identical issues with an HK 560, which is a nearly identical lower powered receiver. The problem was a cracked resistor.
Components controlling the base voltage of the output transistors.
These components of the voltage amplifier and thermal tracking transistor (TR416)
TR420 driver
TR418 driver
TR412
TR414
TR416
D420
D422
And the associated resistors and capacitor around these components.Including the adjustment trimmer.
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Outstanding.. Didn't mean to step on any toes.
As you can see there are quite a few components involved.
Thanks for the inputs. I’ll take a look at those components.