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Nakamichi PA 5E class A Stasis amplifier hum problem
Nakamichi PA 5E class A Stasis amplifier hum problem
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Old 4th February 2018, 02:37 PM   #1
Sixtron is offline Sixtron
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Default Nakamichi PA 5E class A Stasis amplifier hum problem - SOLVED

Hello,

maybe you can help me with a humming about 30 years old Nakamich PA 5 please ?

If i don't connect input cable then there is no hum.
If i connect just one input cable for one side then there is no hum.
If i connect two for stereo input this amplifier makes a hum that you can hear from about 5m distance.
I tryed different cables.
I can connect my smartphone or my TV direct to the PA 5e, all the same hum.
I had only connected the PA 5E to the Power net for testing, nothing else was plugged in,
input with my smartphone and have the hum.
I can also connect the CA 5E II pre amplifier without power plugged in and have the hum.

I tried to use a AC powerfilter with no success.
I bought new 47.000 uf capacitor, no success.
i installed new trimming potentiometer for exactly adjust the bias.

The problem appears when i set the bias to 40mah like the instructions say.
If i set it to zero then there is no hum.
If i use an input filter the hum is gone also with the disadvantage
of worse sound.

I have attached a screenshot from a part of the circuit plan.
Do you have an idea wich component is maybe broken ?

Best regards from Germany

Edit: Problem solved. Solution on page four.
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File Type: png Screenshot_2018-02-04-14-30-34-990_com.adobe.reader.png (609.9 KB, 291 views)

Last edited by Sixtron; 14th February 2018 at 04:35 PM.
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Old 5th February 2018, 12:06 PM   #2
AndrewT is offline AndrewT  Scotland
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you need to do three things.
a.) read D.Joffe's paper
b.) examine the PCB and see how Signal Return is connected to Speaker Return, on both channels. A continuity tester may help trace the routes/connections.
c.) determine if D.Joffe's HBRR & HBRL can be fitted in the voltage reference link between Signal Return and Speaker Return.
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regards Andrew T.
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Old 5th February 2018, 05:31 PM   #3
Sixtron is offline Sixtron
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Thank you. Unfortunately i don't understand to much :-)
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Old 5th February 2018, 07:01 PM   #4
Sixtron is offline Sixtron
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Is it usual that i can measure with a normal voltmeter 274 Volts AC over the plus and minus from the two 47.0000uf filter capacitors measured in one direction only. The other direction 0V AC.
Or is this causing my hum ?
DC Volts is 124.
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Old 5th February 2018, 10:03 PM   #5
classAchap is offline classAchap  United Kingdom
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Quote:
Originally Posted by Sixtron View Post
Is it usual that i can measure with a normal voltmeter 274 Volts AC over the plus and minus from the two 47.0000uf filter capacitors measured in one direction only. The other direction 0V AC.
Or is this causing my hum ?
DC Volts is 124.

The information you give is confusing but those capacitors should not have 274V AC anywhere. The D.C. is o.k. I would first check the bridge rectifier (D401 is it?) Iíd want to know thatís ok first - itís the easiest and most obvious thing to check - if there is indeed AC on the caps.
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Old 5th February 2018, 10:14 PM   #6
Sixtron is offline Sixtron
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Is there a way to test it with a multimeter or do i have to order a new one to test ?
Looks like there is AC on the caps, but i can not say if it's only few ripples or much. Is it not normal that i can measure this ripples with a standard multimeter as AC voltage ?
Sorry for confusing.
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Old 5th February 2018, 10:51 PM   #7
phase is online now phase  United States
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Maybe the smaller capacitors are dried out perhaps?
You should be able and detect ac on the + and - rails with a multimeter.
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Old 5th February 2018, 11:11 PM   #8
Sixtron is offline Sixtron
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Maybe, but they all look fine. When i measure the smaller capacitors by ohms they go up to infinity. So i think they charge ? I could change them later if there is nothing more suspicious like the rectifier.
I had many years ago a crash. I plugged the input cable in while the amplifier was on and two power transistors where burned and a big resistor exploded. I replaced them but maybe the rectifier suffered from this overload also ?
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Old 8th February 2018, 03:56 PM   #9
Sixtron is offline Sixtron
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I have changed the rectifier with a 400v 35A but it didn't help to get rid of the hum. I can still measure 271 Volts AC over the capacitors from minus to plus.

As next i try maybe to change the small capacitors and try to use the 10 Ohm Hbr's.

Any more ideas ?
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File Type: jpg IMG_20180208_174507.jpg (1,018.5 KB, 161 views)
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Old 8th February 2018, 04:02 PM   #10
Mooly is offline Mooly  United Kingdom
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Nakamichi PA 5E class A Stasis amplifier hum problem
Quote:
Originally Posted by Sixtron View Post
Is it usual that i can measure with a normal voltmeter 274 Volts AC over the plus and minus from the two 47.0000uf filter capacitors measured in one direction only. The other direction 0V AC.
Or is this causing my hum ?
DC Volts is 124.
Something is amiss with the measurement.

One possibility is simply that you are not making a good connection with the meter probes and one is 'floating'.
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