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Harman Kardon 930 Short out - need help!

Doug, again, can’t thank you enough for the great advice. I’ll definitely try what you said when I have time.
The HK930 service manual is horrible. Not enough info and full of mistakes. Very hard to follow.
I’m pretty certain the issue is somewhere in the driver board and / or output part. It runs as a preamp with no issue now. Also, the short occurs when I turn the volume higher. I’ve read it somewhere that the output stage of 930 is pretty prone to failure. So, I’ll probably rebuild the output part some day.
I haven’t thought about selling it yet. A 930 in this cosmetic condition with the original wood box is hard to come by. And at least I can use it as a preamp for now. It also serves as a pretty good headphone amp, driving my HD650 very well.
 
Thanks for chiming in. However, can’t really follow what you are saying. Are you using voice to text input?

Harryconover said:
I read most of watt you’ve done the temper Tracking transistor is a very good choice they get very flaky all Wise look for bad or broken contacts the pot is all so a very good shot the front end difference pair has much improved thermal tracking if thay are heat sronk together with a dab of heatsinkgresse between them this requires care and maybe longer leads but if you’re replacing them your set , I love these 330,730,and the 930 so I think I will become there guru , I allwise wanted to one Any way
Click to expand...
 
Hi Ulysse,

Did you ever find a resolution to this issue?
I picked up an HK 930 and it exhibits the exact same thing. I'm going to replace the output transistors as I've already replaced just about everything in the power supply boards, Driver board, and preamp. The unit works fine if I pull the jumpers and use the 930 as a preamp input to another receiver. My HK 930 seemed to start exhibiting the same conditions as yours after I repopulated the preamp board, but perhaps, it may have started after I repopulated the driver board, but I missed it due to the intermittent condition. Bias is fine even with the DBT glowing brightly. Turning off the speakers, DBT dims down. Reengaging the speakers, It sometimes works perfectly and other times trips the DBT. Other times it runs fine and then the DBT trips after some time. Turning on the unit with the speakers engaged trips the DBT most of the time. All caps replaced in the power section including the main filter caps. Replaced all for common anode cathode devices, no effect. Replaced both of the transistors in the power supply section.....no affect. The originals were fine as a test of them and believe one was D317 NPN.

Checking the power transistors, at the Collectors, with DBT tripped, and when it functions normally, and getting some strange readings. About 33 volts at the outside transistors, and little to no voltage at the inside output transistors....with DBT tripped.

Any thoughts before plunge into changing the output transistors?
 
I couldn't fix it with my limited knowledge and time. Sold it as parts in the end


TonyFinn said:
Hi Ulysse,

Did you ever find a resolution to this issue?
I picked up an HK 930 and it exhibits the exact same thing. I'm going to replace the output transistors as I've already replaced just about everything in the power supply boards, Driver board, and preamp. The unit works fine if I pull the jumpers and use the 930 as a preamp input to another receiver. My HK 930 seemed to start exhibiting the same conditions as yours after I repopulated the preamp board, but perhaps, it may have started after I repopulated the driver board, but I missed it due to the intermittent condition. Bias is fine even with the DBT glowing brightly. Turning off the speakers, DBT dims down. Reengaging the speakers, It sometimes works perfectly and other times trips the DBT. Other times it runs fine and then the DBT trips after some time. Turning on the unit with the speakers engaged trips the DBT most of the time. All caps replaced in the power section including the main filter caps. Replaced all for common anode cathode devices, no effect. Replaced both of the transistors in the power supply section.....no affect. The originals were fine as a test of them and believe one was D317 NPN.

Checking the power transistors, at the Collectors, with DBT tripped, and when it functions normally, and getting some strange readings. About 33 volts at the outside transistors, and little to no voltage at the inside output transistors....with DBT tripped.

Any thoughts before plunge into changing the output transistors?
Click to expand...
 
Thanks for the quick response. Sorry to hear that you had to part with your HK.

Perhaps someone else will chime in with some ideas. Otherwise, I think I'll start checking more voltages to try and diagnose the issue. In my opinion, these cap couples units, although great sounding, are very hard to diagnose and work on, with my level of experience. I've had better luck with most- to-all of the direct coupled units I've restored..... and a few times with the help of some more experienced AK'ers.
 
thanks eiraved,

Good info, but unfortunately it did not help me much further in my diagnosis. I did however seem to narrow the issue, and take some voltage readings.

1. At low to mid volume I can get the unit to play for extended amounts of time, but it will trip the DBT if I have the speakers engaged on power-up. I can power up with the speakers dis-engaged, and then engage them starting from lower volume up to about the 11 o'clock position before the DBT trips again.

2) See pics of my restoration top and bottom side, as well as the Predriver board voltages from the schematic. Perhaps some more experience than I can spot something that I am missing.

Any comments are gladly welcomed.

Thanks.

Tony
 

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BTW....those are the voltages I took from my HK 930. It was on DBT as I was afraid to probe around the predriver board without the DBT. Unit on Aux, no volume, speakers engaged. I did take one reading with the DBT out......43V at -B1 and -B2.
 
You mentioned it will trip the dim bulb tester (DBT). Does your DBT have a breaker or fuse?
 
Perhaps poor usage of the word "trip" on my part. Trip = Bulb shining brightly on my Dim Bulb Tester.
 
You mentioned you can power up the 930 with the speakers disengaged, but engaging the speakers at volume causes the issues. So I have a few questions.
What model speakers are you using?
Have you checked the speaker resistance with a multimeter? What were the readings?
Have you tried other sets of speakers with the 930? Have they had the same results?
Any loose strands on the speaker wire are you using which might make contact?
What wattage bulb are you using on the DBT? Is it an incandescent bulb?
 
Hi, thanks for the quick response.

Realistic Minimus 7's both measure about 7.5 ohms each. NOTE: This HK 930 worked perfectly before I started restoring it as I have done with many other receivers. I also checked the Minimus 7's with my Sansui Eight and they work perfectly.
Additionally, I tried another set of speakers, with the HK 930 and still have the short upon speaker engagement with volume.....same results.

No loose strands of speaker wire, as I always check for that as it is easy to miss.

Incandescent bulb of 60 watts. After power up, with speakers NOT engaged and volume to minimum, I then engage the speakers, and slowly bring the volume up. In doing this, it works fine (music plays nicely) and I can see the bulb slowly beginning to brighten, as you presume, but if I take the volume up more than about the 11 o'clock position, the music stops, the front panel lights go out, and the DBT bulb glows brightly.

Like I may have stated earlier, this started after I rebuilt the preamp board to my recollection. Maybe I loosened or pulled, or broke, a wire that I cannot see. I've look everywhere. It seemed to work fine after rebuilding the driver board. I have since put new power caps in, new output transistors, and new caps to the power supply boards.

Here is what has been done thus far in entirety (see pics):

PreDriver Board:
New caps, new transistors (KSC2690, KSA1220A, and diff. pairs KSA992's, all where applicable and I understand the pinning of transistors pretty well)
2 New 25 turn bourns trimmers. The idle current set perfectly at about 14mV at the test points.

Preamp Board:
New caps and 4 new transistors (KSC1845's)

Power supply boards
New caps and 2 new transistors (KSC2690 and MJE15032G where applicable)

Misc.
4 New Power caps (6,800uF / 63V) (upon checking the old ones, they were fine)
4 New output transistors (MJ21194G) (upon checking the old 2SC897's, they were fine)

thanks,

Tony
 
Hi again eiravid & fellow AK'ers,

I have more information that may help someone more experienced than I diagnose this issue with my HK 930

Idle current: as measured at the driver board test points: about 14mV each channel regardless of whether or not the DBT is bright (in protection mode) or dim.
DC Offset:
Left Channel: fluctuates between 15mV and 30mV when unit is operational (Power on, wait a few seconds, volume at 0, then engage speakers)
Right Channel: stable at 36 or 37mV when unit is operational.
Left Channel and Right Channel at about 4.6V each (not mV) when the DBT is tripped (bright bulb stays bright) either at mid-volume, or any volume if powered on with the speakers already engaged)

One thing I noticed is that when the DBT is tripped, and then I disengage the speaker switch, I hear as slight pop through the speakers (like a cap discharging) and the DBT goes dim. Then I wait a few seconds, and the DBT gives one minor pulse of light, and then I can engage the speakers again, and it works fine at lower volumes. Again, it seems like a cap discharging, and I have all brand new Nichicon caps of the correct value. I've check all the solder joints and re-flowed them on the driver board. Perhaps I damaged a new cap upon installation?

Could it be those Varistors on the HS that are giving me trouble? Not sure how to check a Varistor.

Any thoughts would be much appreciated whether they help or not.

Thanks kindly.

Tony
 
The 60W bulb in DBT is too small for 930, it acts as bottle neck. Go for 100W bulb and check operation again.
You didn't mention which fuses were blowing?
 
Thanks. I will try a 100 watter. I think AK'er, eiravid, was trying to take me along that path, but I just wasn't getting it. I read further up on usage of my DBT and what you stated makes all the sense in the world.
I wasn't blowing any fuses because I always had it on DBT. Ulysse was the originator of this thread and he was blowing fuses.
I used a 60W light bulb successfully on many other receivers, that consumed more power, and it worked fine, but I think that Is a comparison of apples in oranges versus this cap coupled design. I will report back shortly.

Thanks again.

Tony
 
Anybody reading this topic would come to a conclusion you have fuses blowing there as you said on #43 your 930 exhibits the same symptoms as OPs.
Yes, I wouldn't be surprised, if you didn't have a problem at all then. Just a wrong size bulb in the DBT.

930 is not a cap coupled receiver even though you might think. There are plus and minus voltage rails and filter caps for each rail.
 
Hi Mr. Nick,

I found something interesting. While the DBT did trip with both 75W and 100W bulbs, I decided to take a chance with my somewhat junker speakers used for testing.
While measuring the DC offset (at the speaker terminals) with my DMM, I powered on the unit (no DBT), and I got a quick spike of about 1800mV, but it quickly tapered down to about 10mV within a few seconds, and the music started playing. I check each channel, and both channels exhibited this condition.
This split second DC offset spike is more diminished when I power on, and then engage the speakers, but I still get a somewhat large split second spike which is probably why I'm tripping the DBT with a 100W bulb. The bulb is then absorbing that spike and sucking all the power from the unit acting as a bottle neck.

I read some other threads here on AK, and people have mentioned that they hear a small pop of the speakers when they power off. They get around it by disengaging the speakers before powering off, and when powering on, wait a few seconds before re-engaging the speakers. No fuses blew for me, and I checked to see if the woofers in my speakers got hot or not (for fear of destroying voice coils within them), and they did not. So I think I'm o.k.

Perhaps this is the way the HK 930 is designed? or perhaps there is a failing component? Maybe there is a modification out there that I can do to alleviate this condition?

Any thoughts? I really love the sound of this receiver! It is right up there with my restored Sansui 4000, and 2000a, as well as my restored Pioneer SX-737. Comparing the sounds of these units is like apples and oranges. Sometime I like apples, other times oranges.

Tony
 
You are correct and thanks for pointing that out, as I do not want to mislead anyone. My speaker fuses never blew, and I assumed it was because I was on DBT all the time before my latest response. So, whoever is reading this, please take note of this, as it appears there were 2 separate issues. One by Ulysse and one by me. I actually should have started my own thread I believe. Hindsight is always 20/20.

Thanks again.
 
Mr. Nick,

I think our posts crossed in cyberspace as we were posting about at the same time. Thanks again for the info. I'll stop calling this HK 930 cap coupled. + and - rails, makes sense.
 
TonyFinn said:
While measuring the DC offset (at the speaker terminals) with my DMM, I powered on the unit (no DBT), and I got a quick spike of about 1800mV, but it quickly tapered down to about 10mV within a few seconds, and the music started playing. I check each channel, and both channels exhibited this condition.

I read some other threads here on AK, and people have mentioned that they hear a small pop of the speakers when they power off. They get around it by disengaging the speakers before powering off, and when powering on, wait a few seconds before re-engaging the speakers. No fuses blew for me, and I checked to see if the woofers in my speakers got hot or not (for fear of destroying voice coils within them), and they did not. So I think I'm o.k.

Perhaps this is the way the HK 930 is designed? or perhaps there is a failing component? Maybe there is a modification out there that I can do to alleviate this condition?
Click to expand...

So as I understand now everything runs smooth, no problem apart from the pop powering off or on?
I have the same pop on my 930, currently I've gotten around it just like many do: using the speakers off/on switch.

As a first remedy I have replaced the Atlas Oil capacitors in the power supply and next suspect are the bridge rectifiers or the spark killer, a snubber cap across the power switch. Service manual does show "spark killer", but no value for it, so I forgot to check on it as I ordered the new components.
I'm 100% sure it's a failing component, because I had 630 without any pop issues while power on or off.
 
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