Re: HK 930 receiver blows right channel speaker fuse.



john jack wrote:

maxhifi wrote:
check the DC offset at the output of the amp

I found out how to check DC offset.

I got about 110 millivolts on the left channel, slightly less on the right.


john jack wrote:

It seems that the link to the tech manual doesn't display properly.

If you would like to reach it, this link works:

http://manuals.harman.com/HK/Service%20Manual/HK930%20sm.pdf

john jack wrote:
Can anyone shed some light on this problem for me?

I resurrected this receiver from a totally non-functioning state due to
some heavy-handed component butchery by persons unknown.

After getting it to play again, the right channel speaker fuse (3 amp)
on this unit blows periodically. I haven't yet caught it in action but
from time to time I will notice that the music is flat somehow and then
discover that the right channel is out due to a blown fuse. Replacing
the fuse brings this right channel back to life, but this only lasts for
a while, days or weeks or hours of real time, probably within hours of
actual play time. The volume knob is always well below half-full when
this happens.

The fuse is in parallel with a 2200 ohm resistor, directly in line with
the positive speaker output.

This unit has two separate power supplies, one for each channel.

The tech manual for this unit is in PDF form here:

http://manuals.harman.com/hk/Service Manual/HK930 sm.pdf

This manual has the schematics.

I am an "as-needed" junior techy. I have fixed various receivers and
amps, read schematics, know how to use a multimeter, have a basic
grounding in electronic amp circuitry, but I don't do this for a living
or on a regular basis. I am hoping to get a heads up on what to
investigate given the symptoms.

I expect the problem is somewhere in the power amp section.

My first guess would be the power output transistors, but I am wondering
if these can be working correctly but overdriven to the point of blowing
the speaker fuse due to incorrect voltage being supplied to the base of
one or both output transistors (they are two per channel).

I am unsure if this kind of problem be caused by a fault in the power
supply.

Also, this is a 60 watt amp. What size and wattage of resistor would
work as a dummy load?

Any help gratefully accepted,
Jack


I thought if there was appreciable DC offset it could be blowing fuses. Given
the age of this receiver, I'd change all the electrolytic capacitors in the
power amplifier section before proceeding any further with troubleshooting. If
the transistors were shot, I think it would simply not work at all.

I used to have one of these - very nice sound


.

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