I have an HK990 that functions properly but the right channel runs 10 degrees F hotter than the left when measured at the heat sinks. Is this enough to be concerned about? (I'm a novice at circuit level electronics, so it's something I don't feel qualified to chase down and repair if it is.)
Also, if anyone knows of a good technician within 75 miles or so of Lansng, MI that would be good at servicing this unit, I would appreciate the referral. (Blackies in Grand Rapids, MI is the closest authorized HK service center, but I wasn't overly impressed with them.)
Thanks for your thoughts.
Also, if anyone knows of a good technician within 75 miles or so of Lansng, MI that would be good at servicing this unit, I would appreciate the referral. (Blackies in Grand Rapids, MI is the closest authorized HK service center, but I wasn't overly impressed with them.)
Thanks for your thoughts.
10F difference sounds pretty small tbh if you say that the heatsink on one side may for example be 120F and the other 130F.
A very small difference in bias current between the channels would account for something like that, as would any asymmetry in the physical layout internally.
Based on just a 10 degree difference I don't think you have anything to worry over. You could check and compare bias currents for interest though.
A very small difference in bias current between the channels would account for something like that, as would any asymmetry in the physical layout internally.
Based on just a 10 degree difference I don't think you have anything to worry over. You could check and compare bias currents for interest though.
Thanks, Mooly. I pulled the cover and noticed a couple caps on the left channel (the cooler side) seem to have a brown substance around their base that's messy in contrast to the same caps on the right channel which have a neatly applied cream color there. So, it would seem that a sloppy adhesive was used on the left channel or they're leaking? 
Looks like I may have it checked out by an authorized HK service center, but the closest one that I might trust is 115 miles away in Toledo.
Anybody know of a good independent shop?
Looks like I may have it checked out by an authorized HK service center, but the closest one that I might trust is 115 miles away in Toledo.
Anybody know of a good independent shop?
Both of those are glue.
In the first picture, the plastic sleeve of the cap suddenly goes very dark at the base. Is that just an effect of the camera ? The second picture has the sleeving as a uniform colour all the way down (as it should be).
The slits cut into the top of the cap are the safety vent. Often when there is a problem with a cap the vent pushes out due to excess pressure internally. These look fine.
In the first picture, the plastic sleeve of the cap suddenly goes very dark at the base. Is that just an effect of the camera ? The second picture has the sleeving as a uniform colour all the way down (as it should be).
The slits cut into the top of the cap are the safety vent. Often when there is a problem with a cap the vent pushes out due to excess pressure internally. These look fine.
Just a bump to this thread. Today, I finally dug into the 990 and was able to check and adjust the bias. The TP's are difficult to reach and the service manual is hard to interpret, but I found the cooler side to be running at half the spec (19mV). I adjusted both to the 38mV spec and both channels are within 1 degree of one another.
Can I hear the difference? Maybe I sense a little more smoothness and depth to the image. Can't say for sure, it's helped my audio OCD.
Thanks for the replies, Mooly.
Can I hear the difference? Maybe I sense a little more smoothness and depth to the image. Can't say for sure, it's helped my audio OCD.
Thanks for the replies, Mooly.
good to hear you were able to sort it out.Did it continue to sync temp? Exactly this has been tried out in another post, 1 were 119mv other 38mv - both adjusted to 38mv and left. Just to fire it up the next day, and the bias was down to 18mv and second still 38mv.
Just wondering how yours were doing.
Could you perhaps do a video of how to adjust it?
The temps continued to track within about 5 degrees of one another, but one of the fans would never go on. A few months later, the volume control quit working intermittently, and the amp would go into protect mode and shut down from time to time.
Because:
1. I have no ability to diagnose and repair
2. I'd already sunk $365 in to a main board replacement
3. There are no reputable/knowledgeable repair centers in my area, and
4. The nearest good HK service center was 2 hours away, I decided to stop throwing good $ after bad....
I dumped the HK and bought a Parasound Halo Integrated. Very happy with my decision.
Sorry I couldn't help. Hope you have better luck.
When you do adjustments the amplifier has to be warmed up. It will take a while for the output standing current to settle during this time. When the currents are settled that is the time to make a series of adjustments to each channel to get them into the same zone progressively - allowing time for the adjustment to take effect on the change in dissipation.
If you had done the adjustment of the 119 mV when the amplifier was cold and your initial reading was 38 mV and the 38 mV channel setting was correct temperature could account for the drift up to 119 mV.
When and where you do your adjustments is another consideration - if you did this in a workshop where it is colder than where the equipment is used this could result in a false setting.
I think you should let the equipment warm up for at least half an hour in the environment where it will be used and sample the readings periodically making fine adjustments to each channel if necessary over the next half hour.
Another new bump for this thread. Hello @jkasch, I am trying to adjust the bias for this amp. Managed to get top and side panel open. Able to see R53 the potentiometer and top rows of measurement point, but could not access the bottom part. In short, please shed some lights how did you adjust in detail? Did you need to remove power amp board with the heat sink to do the adjustment? Thanks a lot in advance.
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