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john jack john jack is offline
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Default HK930 receiver--tuner good-amp good-preamp bad

1. When the HK930 is connected via the "tape out" jacks to the aux input
jacks of another receiver and with FM selected on the HK930, the tuner
of this HK930 will drive the other receiver and it sounds fine.

2. When the HK930 is connected via the "preamp out" jacks to the "power
amp in" jacks of the other receiver and with FM selected on the HK930,
the preamp of the HK930 fails to drive the power amp of the other
receiver above a very low volume and one channel is even lower in volume
than the other. This same behavior is exists when the HK930 preamp
drives its own amp.

3. When the other receiver is connected via the preamp out jacks to the
"main amp in" jacks of the HK930 and the tuner is selected on the other
receiver, the main amp of the HK930 is successfully driven by the other
receiver.

Probable cause:

Each channel on this unit has its own separate power supply. Everything
was working fine until I accidentally soldered together the two B+ lines
of the two separate power supplies.

Here is a link to the tech manual for this unit:

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

It is minimal but it has the schematics. The phono stage is referred to
as the preamp. The preamp is referred to as the "tone control board".
The main amp is referred to as the predriver.

I replaced all the electrolytic caps on this unit and the input
differential pairs of transistors in the main amp section. After this, I
attempted to switch the power supplies in an effort to isolate another
minor problem that could have arisen from the power supply, the preamp
or the main amp. That's when I accidentally soldered the two B+ lines
together. A very very thin filament of soldered bridged the very narrow
space between the two soldering posts.

I checked all the transistors and electrolytic caps in the preamp for
shorts and found none. The schematics do not have any voltages on them
except those of the B+ and B- lines and they check out okay.

Does anyone have any hints or suggestions on how I might isolate this
problem? Each channel in the preamp has a pair of 2SC644's at the input
stage and a pair of 2SC644's and a pair of 2SC828's at the output stage
with the 2SC828's being the final pair. The voltage into the preamp is
exactly half of the voltage between B+ and ground. The section of the
power supply that powers the preamp also powers the FM tuner and the FM
tuner appears to be working okay so I am going on the assumption that
the voltage to the preamp is okay.

I don't know exactly what to infer from the fact that I am getting at
least some output from each channel of the preamp output, with one
channel being weaker than the other. My guess is that even though none
of the transistors are shorted across any of the three leads in any
direction, the problem lies with the transistors. I am considering
borrowing the 2SC644 transistors from the phono stage to use in the
preamp. I guess my question would be which end of the preamp would the
faulty transistors most likely be located, at the input of the output?

Thanks,
Jack
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Posted to rec.audio.tech
john jack john jack is offline
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Posts: 10
Default HK930 receiver -- preamp fixed!

The 2SC828 preamp output transistors got fried.

I replaced them with NTE128's and everything is back to where it was.

:-)



john jack wrote:
1. When the HK930 is connected via the "tape out" jacks to the aux input
jacks of another receiver and with FM selected on the HK930, the tuner
of this HK930 will drive the other receiver and it sounds fine.

2. When the HK930 is connected via the "preamp out" jacks to the "power
amp in" jacks of the other receiver and with FM selected on the HK930,
the preamp of the HK930 fails to drive the power amp of the other
receiver above a very low volume and one channel is even lower in volume
than the other. This same behavior is exists when the HK930 preamp
drives its own amp.

3. When the other receiver is connected via the preamp out jacks to the
"main amp in" jacks of the HK930 and the tuner is selected on the other
receiver, the main amp of the HK930 is successfully driven by the other
receiver.

Probable cause:

Each channel on this unit has its own separate power supply. Everything
was working fine until I accidentally soldered together the two B+ lines
of the two separate power supplies.

Here is a link to the tech manual for this unit:

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

It is minimal but it has the schematics. The phono stage is referred to
as the preamp. The preamp is referred to as the "tone control board".
The main amp is referred to as the predriver.

I replaced all the electrolytic caps on this unit and the input
differential pairs of transistors in the main amp section. After this, I
attempted to switch the power supplies in an effort to isolate another
minor problem that could have arisen from the power supply, the preamp
or the main amp. That's when I accidentally soldered the two B+ lines
together. A very very thin filament of soldered bridged the very narrow
space between the two soldering posts.

I checked all the transistors and electrolytic caps in the preamp for
shorts and found none. The schematics do not have any voltages on them
except those of the B+ and B- lines and they check out okay.

Does anyone have any hints or suggestions on how I might isolate this
problem? Each channel in the preamp has a pair of 2SC644's at the input
stage and a pair of 2SC644's and a pair of 2SC828's at the output stage
with the 2SC828's being the final pair. The voltage into the preamp is
exactly half of the voltage between B+ and ground. The section of the
power supply that powers the preamp also powers the FM tuner and the FM
tuner appears to be working okay so I am going on the assumption that
the voltage to the preamp is okay.

I don't know exactly what to infer from the fact that I am getting at
least some output from each channel of the preamp output, with one
channel being weaker than the other. My guess is that even though none
of the transistors are shorted across any of the three leads in any
direction, the problem lies with the transistors. I am considering
borrowing the 2SC644 transistors from the phono stage to use in the
preamp. I guess my question would be which end of the preamp would the
faulty transistors most likely be located, at the input of the output?

Thanks,
Jack

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