I must start this thread with a gracious thank you. I've gleaned so much from this forum that it seems fitting that I give back with a small post of success. Most of these solutions have been suggested by others on this forum and so have made this possible. I will simply post what I have done to my HK 730 after lots of reading and testing. I hope this will help someone else keep their amp in service and safe to use.
My HK 730 was exhibiting popping, fuzz, static, and random noise in the left channel. It did this no matter the amount of time turned on, temperature, test equipment connected, line voltage at different levels, etc... It did this without the pre-amp-power-amp loops in, so it was definitely in the power-amp and/or power supply.
All changes were made with before and after observations on an oscilloscope, observing the artifact noise as well as overall rms average changes.
There were only a few reasonable possibilities, which I attacked in this order:
-refresh the bias pot - de-ox it D5
-noisy electrolytics
-cold/cracked solder joints
-noisy transistors
-maybe noisy diodes - nope
The bias pot responded nicely and was fully functional. No change from original problem.
No change was noted after replacing all the electrolytic caps in the power supply and power amp driver boards, except a slightly lower noise floor.
I did find a burn mark on one of the connector pins of the B+/- to the driver board. This got cleaned up and all connector pins were re-flowed. No change in the original problem, but there was a reduction in mocrophonics when I whacked the heat-sink.
I began to replace the transistors in sub-stages. I started with the differential input pair and associated current source transistors. No change. Then the driver transistors for the main outputs. No change, even with the main outputs removed - still noisy. Then the bias transistor. No change. The noise persisted. I then replace the remaining two transistors, just in front of the bias adjustment. The noise became non-existent. There it was, Q408 and Q412.
At this point, I've replace all the electrolytic capacitors and all the transistors except the original output transistors. It should be noted, again (it is on this forum), that there is an error on in the service manual. 2SD549 is supposed to be 2SB549. Here's my substitution list:
Original - Substitute
2SC1400 - KSC1845
2SB549 - MJE253
2SD415 - MJE243
2SC853 - BD139
All caps were replaced with original value or upgraded voltage Nitichicon or Rubicon. I did notice that the bias voltage was not quite able to get to service manual specs, but I noticed no corssover distortion at 25mv. I also noticed the sound was more transparent and airy, not as creamy as the unchanged channel. This was expected with all the new components.
Good luck on your own unit. These are great pieces of gear and I couldn't have repaired this one without the help of real people on this forum. Thank you, all, who have posted about this amp before me and helped on so many other electronics challenges.
My HK 730 was exhibiting popping, fuzz, static, and random noise in the left channel. It did this no matter the amount of time turned on, temperature, test equipment connected, line voltage at different levels, etc... It did this without the pre-amp-power-amp loops in, so it was definitely in the power-amp and/or power supply.
All changes were made with before and after observations on an oscilloscope, observing the artifact noise as well as overall rms average changes.
There were only a few reasonable possibilities, which I attacked in this order:
-refresh the bias pot - de-ox it D5
-noisy electrolytics
-cold/cracked solder joints
-noisy transistors
-maybe noisy diodes - nope
The bias pot responded nicely and was fully functional. No change from original problem.
No change was noted after replacing all the electrolytic caps in the power supply and power amp driver boards, except a slightly lower noise floor.
I did find a burn mark on one of the connector pins of the B+/- to the driver board. This got cleaned up and all connector pins were re-flowed. No change in the original problem, but there was a reduction in mocrophonics when I whacked the heat-sink.
I began to replace the transistors in sub-stages. I started with the differential input pair and associated current source transistors. No change. Then the driver transistors for the main outputs. No change, even with the main outputs removed - still noisy. Then the bias transistor. No change. The noise persisted. I then replace the remaining two transistors, just in front of the bias adjustment. The noise became non-existent. There it was, Q408 and Q412.
At this point, I've replace all the electrolytic capacitors and all the transistors except the original output transistors. It should be noted, again (it is on this forum), that there is an error on in the service manual. 2SD549 is supposed to be 2SB549. Here's my substitution list:
Original - Substitute
2SC1400 - KSC1845
2SB549 - MJE253
2SD415 - MJE243
2SC853 - BD139
All caps were replaced with original value or upgraded voltage Nitichicon or Rubicon. I did notice that the bias voltage was not quite able to get to service manual specs, but I noticed no corssover distortion at 25mv. I also noticed the sound was more transparent and airy, not as creamy as the unchanged channel. This was expected with all the new components.
Good luck on your own unit. These are great pieces of gear and I couldn't have repaired this one without the help of real people on this forum. Thank you, all, who have posted about this amp before me and helped on so many other electronics challenges.
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