I'm restoring a Harman/Kardon HK 750 for a friend, it was in a very sorry state. Almost 80% of the capacitors were leaking and a lot of transistors had cracked solder joints. I did a full recap and did all the solder joints. When I powered it on I had about +/- 4v on all rails of the secondary power supply instead of the +/-22v. I suspected on of the other boards was pulling down the voltage so I disconnected the phono equalizer board and the voltage went to 17v, not exactly what it should be but I have the dim bulb in.
I narrowed it down to The part fed by R643, when I lift this from the board the rest has the correct voltages. The resistor itself is good and so are the two transistors.
Also the little relay doesn't close.
I narrowed it down to The part fed by R643, when I lift this from the board the rest has the correct voltages. The resistor itself is good and so are the two transistors.
Also the little relay doesn't close.
so, you lift one side of R643 and the rails come back up?
and you are sure that Q605 and Q606 are good?
did you check the zener D601?
R643 biases on the current sinks Q605 and Q606 that essentially activate each of the gain stages.
if that zener fails open, ugly things will happen to gain stages ....
good luck ,
mlloyd1
and you are sure that Q605 and Q606 are good?
did you check the zener D601?
R643 biases on the current sinks Q605 and Q606 that essentially activate each of the gain stages.
if that zener fails open, ugly things will happen to gain stages ....
good luck ,
mlloyd1
Yes that's it, Q605 and 606 both test 0.64 from B to C and B to E and open from E to B and C to B. I just checked the zener on Ohmmeter I got read open in both directions but with the diode test I got 0.62 in the forward direction. If I'm not mistaking that's a 5v 500mW zener right ?
Thank you mlloyd1
Thank you mlloyd1
i would guess 5.1v @ 500mW would be OK, but for the record, I'm not very familiar with Japanese diode part numbers.
how did you verify the zener is not properly "zenering" 🙂 at about 5.1v?
In the forward direction, it should indeed read like a conventional silicon diode; that won't necessarily confirm that it correctly behaves as a zener.
and how did you verify that Q601 through Q606 are all good?
regards,
mlloyd1
how did you verify the zener is not properly "zenering" 🙂 at about 5.1v?
In the forward direction, it should indeed read like a conventional silicon diode; that won't necessarily confirm that it correctly behaves as a zener.
and how did you verify that Q601 through Q606 are all good?
regards,
mlloyd1
I checked the datasheet and it is indeed a 500mW zener (between 4.85 and 5.15v) is 5.1 a standard or just 5v ?
I didn't verify if it is properly "zenering" (nice one) but most tests suggest to just verify the resistance and ( around 400kr in forward direction and very high or open in the other) and it reads open in both so unless my meter is faulty that doesn't seem right.
I had all the transitors pulled from the circuit and test in diode mode, they all seem fine, of course they could very well fail in circuit, it wouldn't be the first time.
I didn't verify if it is properly "zenering" (nice one) but most tests suggest to just verify the resistance and ( around 400kr in forward direction and very high or open in the other) and it reads open in both so unless my meter is faulty that doesn't seem right.
I had all the transitors pulled from the circuit and test in diode mode, they all seem fine, of course they could very well fail in circuit, it wouldn't be the first time.
Little update, the zener actually tests fine (I had a bad connection in my multimeter leads) so I'm going to leave that alone for now. concerning R643 I should have 22v on one side and -17.4v on the other. When it's in place I have 4.3v and when it's out I have 16v and -15.6v.
Q605/606, R615/616, D601, C617, R643 all form the Constant Current Sources for the input diff pairs. Looks like the +22 and -22 volt rails are shorted together until you break the circuit by removing R643. You checked the Zener, how about C617? New and installed with correct polarity?
Craig
Craig
Hi Craig, yes C617 has been changed by a 50v10µF nichicon FG cap, I followed the polarity written on the board, compared it to the cap that was in it before and to the schematics and they all line up the same way.
Try desoldering the bases of Q605/606 to see if the voltages come up, I kind of doubt it. Try pulling C617 and see what happens.
Craig
Craig
If you can't find anything else on the phono board let's go back to the power supply. Check the collector of Q1 for +40VDC and Q2 for -40VDC. Q1 base should be +24VDC and Q2 base should be -24VDC. Then check R4,7,8, and 9. All new caps installed with correct polarity?
Craig
Craig
OK so I got Q 605 out and not much changed but when I removed Q606 I got back to 14.5v.
I will check the rest tomorrow, it's quite late here. The power supply has new caps (I always check several times for polarity but will do again tomorrow). From what I measured before the voltage on the whole secondary power supply drops whenever the phono stage is giving trouble.
I will check the rest tomorrow, it's quite late here. The power supply has new caps (I always check several times for polarity but will do again tomorrow). From what I measured before the voltage on the whole secondary power supply drops whenever the phono stage is giving trouble.
Well R4 and R7 (47R) are both dead open. I don't have them around so I will get some new tomorrow and get back to you.
You were chasing the symptoms of the real problem, dead regulators. Now the million dollar/euro question, why were they open? Might as well check R10/13 also. There are big ! flags around all four of those resistors!
Craig
Craig
I guess yes, I've had this case several times with pioneer (a short in the preamp pulling down the voltage of the secondary power supply) that's why I immediately went in that direction. R10 and 13 Are good but I think I will replace them anyway. The transistors mounted on the little heat sink get quite hot and almost all the solder joints were broken, those could be the reason for the resistors opening, they're slightly discoloured so maybe they got hot as well. I will get some higher wattage replacements.
Replaced R4, R7, R10 and R13 and it goes strong again +/-22.5v. The thread name isn't quite wrong though because phono input 1 works fine but as soon as I press phono input 2 the little relay on the back opens. I checked, when input 1 is selected I have 0.2v across the relay but when I press phono 2 I got 20v. It does seem logical because the cathode of the LED is at 0v and when it turns on it get 2v across which means the anode is at 2v. The anode of the LED for input 1 is connected to power supply through R91 but the anode of the LED for phono 2 is connected to on side of the relay and the other side is directly connected to the 22v rail. When The LED for phono 2 turns on one side of the relay drops to 2v and the other stays at 22v. My explanation might sound a little confused but it's clear in my head 😀
OK, Phono 1 works as does the LED, correct? Phono 2 does not work but the LED does?
When in Phono 1 mode the relay should NOT be energized, high voltage on both sides of the relay coil. Looks like the relay coil provides the current limiting function for the Phono 2 LED as R91 is not in the that LED circuit. When Phono 2 is engaged the relay should energize switching the RCA inputs and lighting the Phono 2 LED. The Phono 2 button should provide a ground for the relay and LED.
Craig
When in Phono 1 mode the relay should NOT be energized, high voltage on both sides of the relay coil. Looks like the relay coil provides the current limiting function for the Phono 2 LED as R91 is not in the that LED circuit. When Phono 2 is engaged the relay should energize switching the RCA inputs and lighting the Phono 2 LED. The Phono 2 button should provide a ground for the relay and LED.
Craig
Yes that's it exactly.
Actually that could be what's happening. I thought that when I pushed the phono 2 button it opened the relay but it might instead close it, I can check with an ohmmeter. The thing is, both inputs go to the relay. Could it be that the relay closes one circuit when it's open and the other when it's closed and that one of the two (the closed position) doesn't make good contact ?
Actually that could be what's happening. I thought that when I pushed the phono 2 button it opened the relay but it might instead close it, I can check with an ohmmeter. The thing is, both inputs go to the relay. Could it be that the relay closes one circuit when it's open and the other when it's closed and that one of the two (the closed position) doesn't make good contact ?
Look at the relay as a DPDT switch (double pole, double throw) with the output taken from the common center terminals. In one position Phono 1 contacts are shorted to the center terminals and the other terminals are open. In Phono 2 the terminals that were open are now shorted to the center terminals. You just might have a bad/dirty relay especially if Phono 2 was never used. Is it a sealed relay?
Craig
Craig
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