Harman Kardon 430 - startup question

[xrayspex77]

Can't find the thread where it's explained but on the HK730, C405/406 are part of the Slow Start Circuit.



From the last Post in this thread https://audiokarma.org/forums/index.php?threads/hk-730-one-channel-slow-to-warm-up-question.168496/

"In this circuit, the final voltage across the cap is set by the voltage drop in the be junction (for Q405) and the voltage drop in the diode (D403). Assuming 0.6V for each, the starting time would be 6.6 seconds:

dt = RC dV/V

R = 1 Meg Ohm
C = 220 uF
dV = 1.2 Volt
V = 40 Volt

The starting time you get is close to this theoretical value, well within the tolerance of the electrolytic caps.

As you said, 5.5 seconds in one channel and 6.0 to 6.5 seconds in the other channel is... close enough."

EDIT

The HK430 appears to have the same circuit

 

[dlucy]

Can't find the thread where it's explained but on the HK730, C405 (is part of it)

Wow! Nice find!

 

[Darren James]

Thank you all for the info..... May the karma gods smile upon you this holiday season!!

 

[tdwpgtp]

I know I'm late to the party, but I have a 430 that does the same (or at least a similar), but also with extremely high DC offset upon start up, and one blown woofer to show it. When I power up my 430, the woofers move out to the limits of their excursion over the course of 1-3 seconds, along with a fairly nasty hum sound. I know powerup thumps are fairly normal, but there is something abnormal going on with mine. DC offset jumps around from the milivolt range up to 14(!!!!!) volts, at least according to what my meter says. I also noticed that the voltages at each pin of most of the amp board transistors jump around, too. These higher voltages vary for around 20 seconds, before the transistors seem to settle out near where the service manual says they should, though the DC offset, particularly in the right channel, remains high. I have replaced all of the electrolytics in the amp board (aside from one 470uF, as I only ordered one), and it has not changed. I also pulled each transistor, aside from the outputs, and checked them with a transistor tester, showing no bad transistors.

Does anyone have a clue what to check next? Or is this better asked in a new thread?

 

[milo63]

I know I'm late to the party, but I have a 430 that does the same (or at least a similar), but also with extremely high DC offset upon start up, and one blown woofer to show it. When I power up my 430, the woofers move out to the limits of their excursion over the course of 1-3 seconds, along with a fairly nasty hum sound. I know powerup thumps are fairly normal, but there is something abnormal going on with mine. DC offset jumps around from the milivolt range up to 14(!!!!!) volts, at least according to what my meter says. I also noticed that the voltages at each pin of most of the amp board transistors jump around, too. These higher voltages vary for around 20 seconds, before the transistors seem to settle out near where the service manual says they should, though the DC offset, particularly in the right channel, remains high. I have replaced all of the electrolytics in the amp board (aside from one 470uF, as I only ordered one), and it has not changed. I also pulled each transistor, aside from the outputs, and checked them with a transistor tester, showing no bad transistors.

Does anyone have a clue what to check next? Or is this better asked in a new thread?

*I'm an idiot with a 430 problem myself, so here I am..
The DC offset can be improved by replacing Q401/403 and Q402/404 with gain matched KSA1845.
All of the 2SC1345s should be replaced as they get noisy.

 

[tdwpgtp]

*I'm an idiot with a 430 problem myself, so here I am..
The DC offset can be improved by replacing Q401/403 and Q402/404 with gain matched KSA1845.
All of the 2SC1345s should be replaced as they get noisy.

Done and done. I've replaced all transistors on the preamp board and all but Q409, 410, and 413-416 on the amplifier board. New diodes on the amp board, and all new electrolytics on every board but the tuner board. The thump/hum seem to have improved slightly, but I still get very high DC offset swings on startup (into the 2v range momentarily, if my meter is to be believed). I get swings up to around 100mv for around 30 seconds before things start to settle into the single digit mv range. Even after 10 or so minutes, I still get ~10mv fluctuations.

I do have oscilloscope here, but my knowledge on using them is basically limited to probing and adjusting the scale and position. I'm a complete beginner with scopes, essentially.

 

[milo63]

Done and done. I've replaced all transistors on the preamp board and all but Q409, 410, and 413-416 on the amplifier board. New diodes on the amp board, and all new electrolytics on every board but the tuner board. The thump/hum seem to have improved slightly, but I still get very high DC offset swings on startup (into the 2v range momentarily, if my meter is to be believed). I get swings up to around 100mv for around 30 seconds before things start to settle into the single digit mv range. Even after 10 or so minutes, I still get ~10mv fluctuations.

I do have oscilloscope here, but my knowledge on using them is basically limited to probing and adjusting the scale and position. I'm a complete beginner with scopes, essentially.

The high initial DC seems to be normal until everything stabilizes, mine does it too but I've got issues too so I may be wrong.
I've got +26V between Q417 and Q419 and .6VBE on 419 so it's a head scratcher.

 

[rickl]

The high initial DC seems to be normal until everything stabilizes,

I recently worked on a 730 and replaced the muting and diff pair transistors and muting cap (Q401,403, 405 and C405 per schematic above). I still keep the speaker switch 'off' for around 30 seconds waiting for voltages to settle down.

my main system are separates so this is SOP.

 

[buzshaws]

I recently worked on a 730 and replaced the muting and diff pair transistors and muting cap (Q401,403, 405 and C405 per schematic above). I still keep the speaker switch 'off' for around 30 seconds waiting for voltages to settle down.

my main system are separates so this is SOP.

I have a 430 with (thankfully) no known problems yet always keep the speaker switch off before turning the unit on for at least a minute to stabilize, and also switch the speakers off prior to powering down. It's just become a natural procedure I don't even think about it. I've done this for the last 3 years after reading about the soft-start circuit since there's no relay to take for granted.

 

[Hooligan1]

I too have a 430 and 730. Early on in my owning the 430 I noticed that the woofer would push and out for 3-5 seconds upon power up. However, I did not notice this on the 730. As the two prior posters suggested, I have engaged and disengaged the speaker switch after power up and before power down on all my units. SOP. That seems to have had a positive influence on the movement of any woofers/speakers connected to the 430. I guess it doesn't solve the DC offset swings but...