Fixing non-working HK 930- need advice

[kotofei]

Got a Harman Kardon 930 that was sold as "does not producing sound". Found SM with schematics; however, it appears to be very different from what is seen in the actual unit so I’m seeking advice of those who have some experience with this receiver.

First test, with 1 kHz tone fed in the AUX input showed no signal at the speaker outputs. Fuses are good. Testing at amp/preamp jacks revealed that input signal is not seen at “pre out”; however, if “power amp in” fed with the signal it’s amplified without distortion. Further tests showed that the signal is present at the input of the preamp/tone control board but not at its output and no power is supplied to that board.

The preamp board is powered by a small power supply or regulator board located at the bottom of the amp. It has four in/out connections numbered 1-4 and DC voltages at them are as follows:

#1: +41.87V, this is +B2 supply voltage, terminal connected to +of C908 filter cap;

#2: +20.32V, connected to + of C909 1000uF/35 V cap;

#3: +0.45V, connected to power terminal of the preamp. Most likely an output “B3” on schematics;

#4: 0 mV, connected to ground.

If terminal 3 is disconnected from the preamp, voltage at it jumps to +3.81V.

The problem is that the schematics of the preamp PS shown in the service manual is very different from what I actually saw on that little PCB! The transistors are different too: TR906 which should be 2SC829 is actually C1384 and TR905 which should be 2SC931 is 2N5492. It doesn’t look that someone worked on this plate before.

I measured voltages at transistor outputs: TR906, C1384: E=+19.8V; C=+41.94V, B=+20.32V; TR905, 2N5492: E=+1.15V; C=+41.94V; B=+19.8V. The TR905 emitter is connected with Terminal 2 (or B3) not directly as in schematics but through some black thingy the size of a resistor but wider, with the blue dot at one end and marking “s-05” shown on the first picture.

I’ll try to draw a schematic of this preamp PCB based on what is really in it.

What could be wrong here? Any help and advice will be greatly appreciated.

 

[Tom B]

The unit has been worked on. Those replaced transistors are not original. HK didn't use RCA transistors and that 2SC1384 is a replacement as well.
I suspect that the RCA transistor is bad (open base to emitter), and the other transistor 2SC1384 could be OK as it is rated a lot higher than the original, and its voltage reading looks fine. I think that diode D901 (21v zener) is OK because you are getting the 20.3v on C909 which is in parallel with that diode. Dont worry about the S-05 component, as its just a diode put in series with the pin 3 output to the preamp board. Lastly ohm check across D902 (12V zener) and make sure it isn't shorted.
But I suspect that there may be a short on another board causing this regulator to fail because that 2n5492 was rated way higher than the original, that must have failed previous. Before you power it up, leave that pin 3 disconnected on the phono preamp board, and ohm the connection on the board to chassis ground. The reading should be very high ohms. there is another connection going to the tone amp board coming from P3. Ohm it out reading to ground after disconnecting thate wire coming from P3. should be high ohm reading. Lastly this P3 supply connection goes to the FM multiplex board as well on pin 2. Disconnect the wire and ohm out pin 2 to chassis ground. Should be high ohm reading.
After replacing the RCA transistor with something like a TIP41C or other reasonable replacement, and leaving the wires disconnected at the phono preamp, tone control and FM MPX board, power up the unit using a dim bulb tester. Measure the voltage on P3. It should be somewhere between 20vdc and 30vdc. If it is still very low, check the other components on the regulator supply board for problems, as well as examining those 3 boards and the power supply boards for signs of previous repairs and look for shorted connections etc.
Good luck

 

[sregor]

^^^ +1


Original C931 transistors were total junk-all failed. The 2n5492 is open, everything else is probably OK. My 2 cents.

 

[kotofei]

Tom,

You are right on the spot!

The 2N5492 is open in all directions and the other transistor, 2SC1384, is good. The D902 is not shorted.

I disconnected pins on the preamp board, FM multiplex board and the Phono board from B3/B5 feeding wires and then measured resistance from each of pins to chassis ground. All are in the range of 12-14 MOhm or even OL ( FM MPX).

I will order TIP41C and see what will be going on.

Many thanks for your detailed input and let the Karma be with you!

I

The unit has been worked on. Those replaced transistors are not original. HK didn't use RCA transistors and that 2SC1384 is a replacement as well.
I suspect that the RCA transistor is bad (open base to emitter), and the other transistor 2SC1384 could be OK as it is rated a lot higher than the original, and its voltage reading looks fine. I think that diode D901 (21v zener) is OK because you are getting the 20.3v on C909 which is in parallel with that diode. Dont worry about the S-05 component, as its just a diode put in series with the pin 3 output to the preamp board. Lastly ohm check across D902 (12V zener) and make sure it isn't shorted.
But I suspect that there may be a short on another board causing this regulator to fail because that 2n5492 was rated way higher than the original, that must have failed previous. Before you power it up, leave that pin 3 disconnected on the phono preamp board, and ohm the connection on the board to chassis ground. The reading should be very high ohms. there is another connection going to the tone amp board coming from P3. Ohm it out reading to ground after disconnecting thate wire coming from P3. should be high ohm reading. Lastly this P3 supply connection goes to the FM multiplex board as well on pin 2. Disconnect the wire and ohm out pin 2 to chassis ground. Should be high ohm reading.
After replacing the RCA transistor with something like a TIP41C or other reasonable replacement, and leaving the wires disconnected at the phono preamp, tone control and FM MPX board, power up the unit using a dim bulb tester. Measure the voltage on P3. It should be somewhere between 20vdc and 30vdc. If it is still very low, check the other components on the regulator supply board for problems, as well as examining those 3 boards and the power supply boards for signs of previous repairs and look for shorted connections etc.
Good luck

 

[Tom B]

Excellent, Hopefully everything will be fine.
One thing I didn't mention was to check connection B3 to ground. It should read ~100 ohms to ground in one direction, and high resistance in the other direction.
If you get less, check the 12 Volt zener D902, and the 2 caps D911 and D912 for short or leaky condition.

 

[kotofei]

Found a KSC2073TU transistor. Could it be used instead of 2N5492?

2N5492: NPN, Pd=50W, Vcbo=75V, Vceo=55V, Veb=5V, Ic=7A

KSC2073TU: NPN, Pd=25W, Vcbo=150V, Vceo=150V, Veb=5V, Ic=5A

The original 2SC931 was Pd=10W, Vcbo=50V, Vceo=50V, Veb=5V, Ic=3A

The KSC2073 is an overkill in terms of voltages and has bigger Pd than the original 2SC931.

 

[Tom B]

Yes it should work. I am not familiar with them though. Just make sure they are pinned the same as in ECB or BCE

 

[kotofei]

It's working!

Soldered in the KSC2073TU, assembled everything (DBT passed) and checked voltages before connecting preamp PS to sources. The B3 voltage was+20.8 V DC. Resistance from disconnected wires leading to preamp phono and FM multiples was around 400 Ohm.

Solder back the preamp/FM/phono wires and powered her up. DBT OK although if used tuner as input the bulb glowed half-power after initial drop to very dim. Anyways, proceeded and.. the HK930 IS producing sound! With tuner as a source everything sounds OK after 20 or so minutes of powering up. The tuner is very sensitive.

The HK needs further work: some deoxit to tone defeat switch, new idling current preamp pot in place of severely abused one, recap, and possibly better phono preamp. However, it's working now.

Thank you Tom B for help.

 

[schoenradt]

Got a Harman Kardon 930 that was sold as "does not producing sound". Found SM with schematics; however, it appears to be very different from what is seen in the actual unit so I’m seeking advice of those who have some experience with this receiver.

First test, with 1 kHz tone fed in the AUX input showed no signal at the speaker outputs. Fuses are good. Testing at amp/preamp jacks revealed that input signal is not seen at “pre out”; however, if “power amp in” fed with the signal it’s amplified without distortion. Further tests showed that the signal is present at the input of the preamp/tone control board but not at its output and no power is supplied to that board.

The preamp board is powered by a small power supply or regulator board located at the bottom of the amp. It has four in/out connections numbered 1-4 and DC voltages at them are as follows:

#1: +41.87V, this is +B2 supply voltage, terminal connected to +of C908 filter cap;

#2: +20.32V, connected to + of C909 1000uF/35 V cap;

#3: +0.45V, connected to power terminal of the preamp. Most likely an output “B3” on schematics;

#4: 0 mV, connected to ground.

If terminal 3 is disconnected from the preamp, voltage at it jumps to +3.81V.

The problem is that the schematics of the preamp PS shown in the service manual is very different from what I actually saw on that little PCB! The transistors are different too: TR906 which should be 2SC829 is actually C1384 and TR905 which should be 2SC931 is 2N5492. It doesn’t look that someone worked on this plate before.

I measured voltages at transistor outputs: TR906, C1384: E=+19.8V; C=+41.94V, B=+20.32V; TR905, 2N5492: E=+1.15V; C=+41.94V; B=+19.8V. The TR905 emitter is connected with Terminal 2 (or B3) not directly as in schematics but through some black thingy the size of a resistor but wider, with the blue dot at one end and marking “s-05” shown on the first picture.

I’ll try to draw a schematic of this preamp PCB based on what is really in it.

What could be wrong here? Any help and advice will be greatly appreciated.

I had the same problem I isolated to that nasty Sanyo 2SC931 the zener holds precisely at 22.0 Volts and the B3 supply voltage increases to about 19VDC. It will run for hours on end then suddenly the dial lights dim down and it hums then comes back up.