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| Re: K150-6 pumps out DC voltage when powered on. [message #27722 is a reply to message #23707] |
Mon, 09 March 2020 12:54   |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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You may have figured the problem out already. The voltages around Q4 are off, especially at the collector. The collector should have about -3 volts there and not the -33 volts that you have.
That would lead me to believe that the transistor is breaking down when hit with the full voltage of the circuit or that C7 is shorted. As you don't have the correct replacement for Q4, remove C7 and see what happens.
As for the CR4 question, that diode is part of the limiter circuit. That -25 volts is being blocked by the diode and with a negative voltage being applied to Q7 base and emitter, I would expect the transistor to be turned fully off, preventing any voltage from showing up at the collector.
The power amp does not need the limiter circuit there to operate, so you could remove both CR4 and CR5 from the circuit for testing.
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| Re: K150-6 pumps out DC voltage when powered on. [message #27732 is a reply to message #23707] |
Sat, 14 March 2020 21:58 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Yes, there is a way to reuse the heatsinks, but it is not a simple job.
For testing, you don't need to have the heatsinks on the replacement transistors. The amp will need the heatsinks if you drive the amp hard and for an extended period of time.
Did you try testing Q4 or removing C7?
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| Re: K150-6 pumps out DC voltage when powered on. [message #27738 is a reply to message #23707] |
Mon, 16 March 2020 16:10 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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So the cap was shorted. Good to know that you found the problem.
So let's see, have you reinstalled all of the original transistors again? Is the amp keeping fairly stable and not going into failure mode? How does the amp sound?
If the amp is sounding good and staying stable, I'd just replace the bad cap and call it good. The cap is a ceramic cap 47 pf (picofarad) and rated at 100 volts or higher.
The fluctuating 1/4 volt on the output is fairly normal and may go away or lessen once C7 is replaced.
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| Re: K150-6 pumps out DC voltage when powered on. [message #27745 is a reply to message #27744] |
Sat, 21 March 2020 15:10 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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stevem wrote on Sat, 21 March 2020 11:26It's a 1N3754.
Electronic parts outlet in the Netherlands has some for E7.50 each.
Do not operate the amp without atleast a 1n1004 or 7 in its place .
E7.50, Ouch!
A long time ago I started to test other diodes to use as a replacement for these diodes. Generally little glass diodes like 1N914 or 1N4148s seemed to work the best. The problem comes in mounting them in a way to get them to react to the heat of the output transistors.
If you don't want to buy the expensive NOS one, get a 1N914 type and solder two thin wires to the ends. Heat shrink the connections so that there is no chance of shorting out to the heat sink. Then using as thin a bead of silicon glue as you can, glue the diode to the center of the heat sink near the clip that held the original diode. Then run the wires to the pc board making sure that you keep the correct polarity.
This diode string ties the lower half of the power amp to the upper half. If the diode string breaks circuit, the upper half of the circuit turns fully on and will cause the power amp transistors to fail. So don't run the amp without the diode in place.
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