On Sunday, October 7, 2018 at 1:06:45 PM UTC-7, Steff wrote:
Den 2018-10-07 kl. 03:24, skrev :
On Friday, October 5, 2018 at 8:54:11 AM UTC-7, Steff wrote:
Well just lost a channel at my Harman Kardon 930 Vintage geek as I am I
want to keep my great Reciver and repair it..

What I done sofaar

Check Fuses ( They are OK)
Moving the speakers between the channels left-Right And A and B speaker
outlets.

With this info I understand that the left Channel is ALMOST dead turning
the balance to Left channel I can hear a faint OK sound. (Like the tiny
transistors trying their best before the end power transistors do their job.

My conclution.

1 Not enough power to the end transistors on the left channel.
2 Broken End Trasistors (two 2SC897 according to the scematics)
3 End of preamp broken on left channel.


Most likely End transistors..

Is my thinking OK or did I miss something ?

I am a beginner in HiFi electronics but have basic skills in electricity
as an electrician for 25 years and do know how to solder..

You got some very good advice already. Just want to point out that failed output (end) transistors nearly ALWAYS short out and blow fuses. I've replaced thousands over the years.

I also want to point out that soldering as an electrician is probably different from soldering in a receiver. I recommend getting an old piece of electronics to try replacing a component and see if you can do it reliably. Better to find out on a piece of junk. Older circuit boards have a higher failure rate due to excessive heat. I had an old Sony board where the pads lifted too easily with a 700° tip but were OK with a 600° tip (Metcal).

I like old HK gear and would prefer to hear you have it working again.

G²

I love HK products and specially the older septon ones. So giving life
to my old 930 have high priority for me as well. Mu dad buy it new and
had some JNL speakers with it.. I got is when I left home in 1979 as a
19 year old and it have been with me ever since, so ofcourse I will try
to keep it alive. Best Amp I ever had. I did have a Sansui that was OK
but still this one is the best.

I also have an oscilloscope and multimeter så I will try to separate the
pre amp from the poweramp and se if the prombel is located in the power
amp for sure. I already order some effekt transistors to prepare to
change them since i Still belive it is in the "end transistors"
I found 3 off them in Italy for a decent price (25 USD for 3)

So thanks so far for all advise I keep updating until it works hopefully
with all you guys help.

Sorry for my English My native language is Swedish so bare with me please..


Hi Steff,

Your English is way better than my non-existent Swedish.

So you have a scope and meter and are way ahead of the game. Most folks
don't realize they also have a test generator called their computer. I make test signals frequently with Adobe Audition 3. Audacity is free and works as well.

I would bet money the output transistors have not failed. I got the service manual from HiFiEngine and noted the only thing between the speaker connector and the amplifier is the speaker switches for local and remote speakers. There is no 'protection' or fault detection or relays

The first thing to do is verify all the power supplies are good. I can't make out if the amplifiers run on +/- 34 or +/- 39 Volts. While the power amps have twin power supplies, the signal system (AM,FM, phono and tone controls) run off the left channel transformer (+B2, -B2) The B3 supply should be around 20 Volts positive and should have nearly no ripple. You'd see ripple with your scope set to AC coupling and turn up the sensitivity to 50 mV per division. I would expect it to be 0-5mV.

If I had it on the bench I would run a 1KHz signal into AUX 1 or 2 and verify normal signal at the 'preamp out' jumpers on the back of the unit. If you want to test tone controls you can generate a 'chirp' (frequency sweep) in Audacity and observe the output on the scope. I predict no problems at preamp output. With no speakers connected (speaker switch off is
the same thing) there should be sound on the headphones even if the speaker fuses are blown.

Good luck and let us know.

G²