Harmon Kardon HK3490 Protect Mode - diyAudio
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Old 6th August 2012, 04:03 PM   #1
Stryngz is offline Stryngz  United States
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Default Harmon Kardon HK3490 Protect Mode

I have a 4-year old HK3490 that randomly goes into protect mode. When this occurs, the display reads "Voltage Protect Check DSP". The only info I've been able to find on the internet is "send back to Harmon Kardon for repair".

I have checked all the speaker connections and etc and found no issues. If I unplug the unit for several days and reconnect to AC power it will sometimes run for days without going into protect mode.

For these types of integrated receivers, is there any hope of a DIY'er, with electronics background, repairing this type of problem? Where to find a schematic or troubleshooting guide? I really like the unit, but could probably buy something else for what it would cost to send to HK for repair.
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Old 7th August 2012, 02:34 PM   #2
Stryngz is offline Stryngz  United States
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Found a service manual (SM) with schematic online.

There is a reset procedure in the SM that I tried. Still going to Protect mode thereafter.

Sometimes it seems the receiver runs longer without going into "protect mode" if I reseat the ribbon cables. Perhaps the reseating is "fixing" a bad connection on the PCB, for a short time? Sooner or later I end up back in Protect mode. I've been running the receiver with the top cover off for a while.

From the schematic, it appears that each "Protect" signal is derived from the DC power supply voltages for that circuit section. Each DC voltage is applied across a resistor divider, then the mid points of all the dividers are wire-ANDed and clamped with a zener to become the "Protect" signal. Going to check the PS voltages tonight.
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Old 7th August 2012, 02:55 PM   #3
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Most of those protection circuits are triggered when there is DC offset voltage on the speaker outputs of the power amplifier.
Ideally there only should be the AC voltage audio signal on these outputs. A significant amount of DC voltage indicates a device is failing in the amplifier and throwing it out of DC balance.
The bad device could be in the power supply, but usually it is one of the output power transistors failing when it warms up.
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