Requesting Troubleshooting Help - HK 730

[NickDeO]

Hello again, AK!

A few months ago, I picked up a Harman Kardon HK 730 Twin Receiver for around $160. I forget if the issue was "no sound at all" or "no sound in one channel" (I misplaced my original repair notes... whoops), but I had hoped that a recap and some DeOxit would get everything working again.

While working on the main amp board, I decided to test the main output/driver transistors. A few of them failed the diode test, so I ended up replacing all of them with new matching pairs.

Upon testing the HK 730 following the cleaning/recapping/transistor replacement, I was still unable to get a sound out of the left channel.

Unfortunately, I had been pretty busy between school/work/life as well as fixing up a friend's receiver, that my Harman Kardon had been moved to the back burner for the past few months following the initial work mentioned.

Now that my own gear is finally at the top of the to-do list, I was able to look further into the dead left channel. When I was looking for the service manual online, I actually noticed an additional "service bulletin" released by Harman Kardon that was specifically concerned with the loss of one channel. Nice!

I'll attach the PDF of this bulletin, but the main idea here is that the feed thru plug in connections used in this family of receivers was prone to issues that would kill one of the channels. For such a highly-regarded amp, I was surprised to see these type of connections being used, although I'm sure the corporate bean counters at HK would have appreciated the lower production costs.

Anyway, I looked over all of these connections, making sure to disconnect them, clean with DeOxit, and replace correctly. I even went over all of the solder joints and resoldered each individual pin on the mentioned boards. Everything then received an ADDITIONAL dose of DeOxit, including the Tape Monitor buttons on the front panel that I assume could also be a potential culprit in this case.

After everything dried, I tested the FM channel using headphones. Still, nothing coming out of the left channel!

On pg. 5 of the service manual, Power Amp Idling Adjustment is mentioned, but I don't see anything about Bias or center voltage adjustments for this particular model. To carry out idling adjustment, I need two 8ohm 50W resistors, which I should have by Friday. Could this kill the entire channel?

In the meantime, I think I might try switching sides on the pre/main in/out jacks on the back of the receiver. I assume that if i can get the right channel preamp to come out of the left channel main amp output, I can narrow the issue in the left channel down to somewhere before the signal reaches the power amp (i.e. the selector knob, tape mon. button, or maybe a semiconductor or poor connection in the pre-amp section). I'll keep everyone posted!

With all of that said, does anyone have suggestions as to where to look next? Thanks in advance!

 

[NickDeO]

Results from the aforementioned test:

• (R) Pre-Out to (R) Main-In: Works fine, same as stock connector
• (L) Pre-Out to (L) Main-In: No sound at all, completely dead. No static or noise even when unplugging cable. Same as stock connector.
  
• (R) Pre-Out to (L) Main-In: No sound at all, completely dead. Leads me to believe that the problem lies somewhere in the left channel path after leaving the preamp PCB.
• (L) Pre-Out to (R) Main-In: Works fine, proving that the left channel is working fine up to and including the preamp section.

With that said, I wonder if connections could still be the culprit. After cleaning the switches so many times, I doubt they could still be a problem. This belief is really based on the fact that there is absolutely NO sound at all coming out of the left channel.
Either that, or there may be issues with components on the power amp section.

Any thoughts?

 

[jheu02]

You've ruled out everything prior to the amp board with the above test. So, the issue lies somewhere between the Main In RCA jack for the L channel and the Speaker Connector.

If you're meter has continuity tone check:

1. Place one meter probe on the center pin of the Main In jack and the other on the (+) leg of C402 (L channel Input Cap on the Main Amp Board. If no tone, either a loose wire, broken solder joint, etc. You mentioned reflowing several boards, but I didn't see you mention the main amp board and connector pins. Check solder joint for the connector plug in (should be labeled CO-B1 If you have a good tone, check the following...

2. One probe on the leg of coil L402 that is not connected (trace) to the 0.47ohm resistors R432,434 (or the leg of ceramic cap C420 that's tied to the coil). The other probe In the L channel (+) speaker connector. If you get a tone, you have an issue in the L Main amp channel.

If NO Tone, move the coil probe to connector pin CO-B6 on the main rectifier board. If you then get a tone, it SHOULD be the connector Pin or it's joint. If still no tone, most likely culprit is the speaker protection fuse.

If it's a ground issue, you can do many of the above checks just moving the probes to ground side and seeing where the continuity stops. Use the Service Manual's wire diagram and scat to see where to place your pins.

 

[NickDeO]

You've ruled out everything prior to the amp board with the above test. So, the issue lies somewhere between the Main In RCA jack for the L channel and the Speaker Connector.

2. One probe on the leg of coil L402 that is not connected (trace) to the 0.47ohm resistors R432,434 (or the leg of ceramic cap C420 that's tied to the coil). The other probe In the L channel (+) speaker connector. If you get a tone, you have an issue in the L Main amp channel.

Thanks for the help!
I carried out the tests and it looks like the issue is somewhere on the main amp board. After removing the output transistors, I tested them to find that one of them in the Left channel was blown. This is interesting, since it was already replaced. I'll have to test the rest of the transistors on the left channel of the main amp board and then replace everything that has blown again. After that, I hope that setting the idling adjustments properly will keep anything else from breaking down! I'll continue to post on this thread as progress is made. Thanks for the help!

 

[jpreza]

Thanks for the help!
I carried out the tests and it looks like the issue is somewhere on the main amp board. After removing the output transistors, I tested them to find that one of them in the Left channel was blown. This is interesting, since it was already replaced. I'll have to test the rest of the transistors on the left channel of the main amp board and then replace everything that has blown again. After that, I hope that setting the idling adjustments properly will keep anything else from breaking down! I'll continue to post on this thread as progress is made. Thanks for the help!

Did you make any progress? I had an HK 430 receiver with similar symptom - i.e. no L channel output. Replaced all noisy transistors on pre-amp, amp, and EQ boards. Also replaced output transistors with same results.

 

[dlucy]

When output transistors blow, there is often a resistor (sometimes a cap) immediately nearby that shorted and let too much current through... and burned out the output. Always check the components immediately "around" the output transistor, as far as the schematic goes, after getting a burned out output and before reapplying power.

Sometimes a failure earlier in the power amp circuit, say the driver stage, causes too much signal to go to the outputs. This is often discovered by comparing the expected voltages, transistor by transistor, from the service manual to the actual voltages in your circuit. Good service manuals typically have such a chart.