Harman Kardon 930 dim bulb question

[eX86]

Hello everybody!

I´m working on a HK930. After replacing all the capacitors on preamp and poweramp, I turned the receiver on with a DBT. DBT goes bright and dims down as it should with a healthy unit. Now, when I hook it up with test speakers, DBT goes bright and stays bright. Output transistors are OK. With speakers not connected DBT acts normal and it sounds nice with headphones.

It shows 70mV and 50mV on speaker terminals with DBT.

I've read that some HK units can act weird with DBT and speakers.

Any experiences?

 

[painttoad]

I've read that some HK units can act weird with DBT and speakers.

i've had the same experience with a 930 a couple years ago.

 

[eX86]

How did it end?

 

[eX86]

I'll put everything back together tomorrow and try to connect speakers AFTER receiver has settled and see how it acts then (should've done this before).

 

[painttoad]

How did it end?

with an extremely happy owner

 

[DatunWalnut]

For starters I'd say the offset wasn't too good at 70mV / 50mV.

 

[kirbjas]

I've seen the same thing on a 930 and also with a 630. They were both fine despite that when starting up without the dbt. I found that if I started it up with the speaker switch off on a dbt the bulb would go bright and then dim, and then I could turn the speaker switch on after that and the bulb would stay dim.

 

[kirbjas]

For starters I'd say the offset wasn't too good at 70mV / 50mV.

I think a commonly used threshold for max acceptable dc offset is 50 mv although some people use 100 mv. I had to change the differential transistors on the main amp board on mine to get it down to an acceptable range (was over 130 mv). It was very much worth doing, after that it sounded really good, has been my main receiver since I did that a few months ago.

 

[malden]

It shows 70mV and 50mV on speaker terminals with DBT.

did you take measurements without the DBT?

 

[rickl]

I was able to dial in DC offset down to 15mV/13mV.

did you read this? See post #3.

https://audiokarma.org/forums/threads/hk-930-dc-offset-voltage-problem.183587/post-13136673

or this?

https://audiokarma.org/forums/index.php?threads/hk-930-dc-offset-voltage-problem.183587/

 

[DatunWalnut]

I think a commonly used threshold for max acceptable dc offset is 50 mv although some people use 100 mv. I had to change the differential transistors on the main amp board on mine to get it down to an acceptable range (was over 130 mv). It was very much worth doing, after that it sounded really good, has been my main receiver since I did that a few months ago.

Yes. 100mV is way out in my book and I like to try for below 30mV if I can. The last high offset (150mV) I had (after changing the diff pair) turned out to be a leaky pos rail coupling cap. I should have noticed but neglected to take my own advice.

 

[Anaxas]

I was just fixing a discarded Harman 630 for the past few weeks and it behaved the exact same way. Dim bulb with speakers selected and attached on power up would cause the bulb to stay bright.

Everything was original, tested the 4 main caps and some on one of the power supply boards and the amp board, but everything was good.

At full power it behaved normally, takes about 5-10 seconds for the bias to settle down (swings wildly) but doesn't cause any issues with the speakers. At full power after a few minutes I set the bias to 10mv across the output resistor and then I measured DC offset at 40mv on one speaker terminal and 30mv on the other

In my case the dial lights would dim considerably when it would 'loose control' on the DBT, but at full voltage they do not. Still took a little bit of bravery to power up full voltage with the speakers connected, but in the end everything was OK.

 

[eX86]

I powered the unit on and allowed it to settle...after a few moments engaged speaker selector.
DMT now acted as I would expect - bright at first and then dims down.
I was now confident enough to power the unit on without DBT.

I forgot to mention earlier that dif. pair transistors were matched and replaced before (KSA992, also coupled with thermal paste and heat shrink), while replacing caps.

After turned on, while still warming up, I managed to measure DC on speaker terminals - now ca 32mV and 42mV.

BUT then...noticed that left side radiator was generating noticeable amount of heat (hot when touching) and turned off the unit. Right side was still cold.

Hope it's the bad bias setting.

Manual says to adjust bias control COUNTERCLOCKWISE until 15mV:



This unit has made me maybe a bit timid. Since there are some other errors in the adjustment procedure and schematic, can someone confirm that if I want to adjust it to 0mV at first, is it fully CLOCKWISE?

 

[painttoad]

This unit has made me maybe a bit timid. Since there are some other errors in the adjustment procedure and schematic, can someone confirm that if I want to adjust it to 0mV at first, is it fully CLOCKWISE?

put it back on the dim bulb. confirm which direction is up/down. set it at zero then go back to full power.

 

[kirbjas]

Checking/adjusting bias seems like a good next step. If you are seeing a quick increase in heat I would make sure not to leave it powered on for long while doing this though. Other things you might want to check are that all new components were installed correctly (ie. right polarity, no solder bridges). I'd especially look at the part of the circuit that is supposed to adjust for temperature (make sure thermal diodes/varistors not open and that their connections to/from amp board are not loose or broken, make sure c608/628 installed correctly).