Harman Kardon AVR 460 DC Offset

[bluebear]

Hi

Background:
This unit suffered from high voltage, 27 V, on the surround Left channel. This triggered the protect circuit, as its supposed to, after start up.

After disassembly I checked the Power transistors Sanken B1560 and D 2390 and measured all channels. Found one B1560 broken.
Also the corresponding large dual ceramic resistor 0.27 Ohm had one resistor shut.

Problem:
Even though the 27 V offset value has been reduced I still have DC Offset on nearly all, including the one repaired. Occasionally the protect mode hits in.

Here is my measured values:
Center 2.7 V
Surr L 2.7 V
Surr R 1.3 V
SBL 0.117 V
SBR 0.0286 V
Front L 0.5 V
Front R 1.297 V

Service manual do not cover DC offset and I cant find any obvious reason why the values diverge this much. Other than having to change all transistors, is that really necessary?

[petehall347]

Re: Harman Kardon AVR 460 DC Offset

check all power supply voltages .
also check any fuse resistors

[bluebear]

Re: Harman Kardon AVR 460 DC Offset

Hi

I checked the mains from transformer, though semi-complete. Missing HDMI-board but powered on.

Measured BN96 from the main transformer board, see attachment.
Service manual is not quiet clear, two values are stated on different boards.

Pin Measured Should be
1 - 26.05 V -17, -20
3 25.23 V 17, 20
9 17.43 V 15 V

[bluebear]

Re: Harman Kardon AVR 460 DC Offset

Measured again on channel leads and got a different result (sloppy measuring probably):
Center 2.93 V
Surr L -2.968 V
Surr R 1.38 V
SBL -0.12 V
SBR -0.024 V
Front L -0.5 V
Front R -1.297 V

Also measured the Collector on the output transistors to see if they were off the corresponding voltage as measured above.
Center collector pins: -46,62 and 46,50
Surr L collector pins: -46,46 and 46,29

So it does not seem that they are out of balance like measured on the channel leads.