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Chinese DIY EAR 834P Phono Preamp Build - What has gone wrong?


Spider27

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After Pass ACA Amp Kit Build, I thought that I can handle this but I was mistaken. This is a different league of its own. There is so certain manual came with it and had to look closely finished build photo and do some google research.

 

After spending many hours, finally finish building it but there is no sound.?     

 

Here are things that I checked so far.

 

+ 3 x 12AX7 Tubes. Checked with 2 different sets (each set 3 tubes) and result is same so no tube issues.
+ Signal In and Out cables (RCA sockets to PCB Joints) are checked with Multimeter for conductivity and they are fine.
+ All solder joints were triple checked and made sure no cold solder
+ Original 2K 3W resistors were out of value so got replacement one from RS Online and replaced.

+ DC voltage from Power Supply Board measuring 6.49v and Main Board DC IN is 6.34v with tubes installed so it seem fine.

 

My guess is that tube stage cathode voltage is lacking because tube is just barely glowing and hard to see glow.

 

Can anyone help me how to measure the cathode voltage out of 3 tube stages? And if you have built one before and had same issue, please do share.

 

Hope other forum members who are thinking of building one learn from mistakes made here. 

 

* I understand that tube amp involves high voltage so will be cautious.

 

Thank you for sharing suggestion and ideas.

 

Schematics Diagram

Hb16c03a8313c43f1949ca8de58850632Y.jpg

Completed PCB Board

 

Y5F621q.png

 

Cathode Resistors Highlighted

 

8wiAc3J.jpg

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A quick google will bring answers:

 

To measure voltage drop across the cathode resistor set your multimeter for DC Volt measurement, clip the black probe to chassis ground (so you can use just one hand to do the measurement) and put the red probe on the tube socket cathode pin . You can also put one probe on each leg of the cathode resistor.

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2 minutes ago, Gryffles said:

A quick google will bring answers:

 

To measure voltage drop across the cathode resistor set your multimeter for DC Volt measurement, clip the black probe to chassis ground (so you can use just one hand to do the measurement) and put the red probe on the tube socket cathode pin . You can also put one probe on each leg of the cathode resistor.

 

My My... Why I could not find this when I do research..  

 

Dumb question.. Does it have to be tubes installed? I am a bit worry about tubes getting damaged during measurement. 

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Its all good mate 

 

Yes with tubes installed. It wont damage tubes but if you are worried connect the probes with power off and then turn on. Without tubes you wont get accurate readings as no load

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First thing, always ,measure the power supply voltage.   What is the voltage on the output of the regulator (on top of first 22uF filter cap, junction with emitter of the transistor).

 

1 minute ago, Spider27 said:

Dumb question.. Does it have to be tubes installed?

 

Yes

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8 minutes ago, aussievintage said:

First thing, always ,measure the power supply voltage.   What is the voltage on the output of the regulator (on top of first 22uF filter cap, junction with emitter of the transistor).

 

 

Yes

 

Thank you for the suggestion measuring power supply voltage.       Ah... Another super dumb question..

Are you referring MJE13009 transistor? if so, how do I measure supply voltage?  

 

IMG_8426.jpg

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Just measured cathode Vol resistor reading and here are the findings.

 

Stage 1

Cathode resistor R4 (2.2K) : Left Channel 9.8mv, Right Channel 10.6mv

 

Stage 2

Cathode resistor R5 (2.2K) : Left Channel 0.8mv, Right Channel 0.7mv

 

Stage 3

Cathode resistor R6 (68K) : Left Channel 32mv, Right Channel 31.6mv

 

Not sure how far they are from target value....

 

 

Cathod Vol Reading.jpeg

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3 minutes ago, Ihearmusic said:

does not sound right.

Have you checked your supply voltage yet?

Thank you for confirming and I doubted that the figure would not be right since there is no sound and tubes are not glowing bright enough.

 

Re; supply voltage, it is embarrassing to admit and I do not know how to measure it..  ?

 

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2 hours ago, Spider27 said:

how do I measure supply voltage

 

Black lead connected to ground/common,  red lead on the junction of the 22uF cap, the 2k resistor, and the emitter of the regulator transistor.

 

oh,  and be careful

 

 

16 minutes ago, Spider27 said:

Stage 1

Cathode resistor R4 (2.2K) : Left Channel 9.8mv, Right Channel 10.6mv

 

Stage 2

Cathode resistor R5 (2.2K) : Left Channel 0.8mv, Right Channel 0.7mv

 

Stage 3

Cathode resistor R6 (68K) : Left Channel 32mv, Right Channel 31.6mv

 

Not sure how far they are from target value....

 

 

Way too low.   

 

Edited by aussievintage
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1 minute ago, Spider27 said:

Thank you for confirming and I doubted that the figure would not be right since there is no sound and tubes are not glowing bright enough.

 

Re; supply voltage, it is embarrassing to admit and I do not know how to measure it..  ?

 

Next to the R6 resistor in the 3rd stage is a fat blue 100 ohm resistor. Measure on either site the voltage reverenced to ground. That will give us some indication

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19 minutes ago, Ihearmusic said:

Next to the R6 resistor in the 3rd stage is a fat blue 100 ohm resistor. Measure on either site the voltage reverenced to ground. That will give us some indication

 

Voltage through 100 ohm resistor on left channel is 0mv and right channel is 2.5mv which is almost nothing..

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15 minutes ago, aussievintage said:

There's a white triangle on that board that reads 285V.    Obviously a test point.  Measure it (ref. to ground)

 

edit:  two of them actually

Just measured those two points after running about 10 mins and both test point voltage measured that Left channel 0mv and right channel 2.5mv.

 

If I understood correctly, the test point should provide 285v so supply voltage definitely went wrong...

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Just now, Spider27 said:

 

Voltage through 100 ohm resistor on left channel is 0mv and right channel is 2.5mv which is almost nothing..

 

I don't think you have any high voltage supply.  Maybe that regulator isn't working - check the 285V test point.  If nothing there, go back to before the regulator to the raw DC on the 100uF caps

 

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1 minute ago, Spider27 said:

Just measured those two points after running about 10 mins and both test point voltage measured that Left channel 0mv and right channel 2.5mv.

 

 

Doesn't need to run for 10 mins.  The supply is solid state, voltage should be there as soon as you turn it on.

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3 minutes ago, aussievintage said:

 

I don't think you have any high voltage supply.  Maybe that regulator isn't working - check the 285V test point.  If nothing there, go back to before the regulator to the raw DC on the 100uF caps

 

 

Thank you. Yes, I fully agree that there is something wrong with Primary Voltage. My toroidal transformer primary AC Vol out is 240v and Secondary 6.3v.  Secondary Out AC 6.3v seem working through Power Supply Board to Main Board because the DC Vol measurement shows 6.29v.

 

However, primary AC voltage 240v does not seem to transferring correctly.

 

First of all, I may need to check AC 240v out from Toroidal transformer is working. 

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Toroidal Transformer seem working fine. Primary AC out 253v and Secondary AC Out 6.5v so it looks like voltage regulator might be the issue. 

 

May I ask how to check if regulator is working? If regulator is the issue, do I just change the MJE13009 NPN Power transistor?

 

Thank you very much for the guidance. 

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1 minute ago, muon* said:

Don't you ,mean secondary out in both instances, the primary side is the input to the transformer.

 

Sorry but was bugging me ;)

You are right. Thank you for correcting me. :) 

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10 hours ago, Spider27 said:

Toroidal Transformer seem working fine. Primary AC out 253v and Secondary AC Out 6.5v so it looks like voltage regulator might be the issue. 

 

May I ask how to check if regulator is working? If regulator is the issue, do I just change the MJE13009 NPN Power transistor?

 

Thank you very much for the guidance. 

 

OK, you have ac out of the transformer.  Next, check that you have raw DC after the main rectifier.  There should be a high voltage (330 volts or so) on the positive terminal of the first 100 uF capacitor.  The 1K3 resistor may be a convenient point to measure it.

 

If there is raw DC, check the voltage reference at the base of the transistor (top of the string of zener diodes).  If there is voltage there, check the output of the regulator right at the emitter of the transistor (the 2K resistor should be a convenient point to do this)

 

Also, generally check for bad solder joints.  Follow the high voltage, measuring it at various points between where it is present, and where it was not (the 285V test points).  Try to establish exactly where it stops.

 

 

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2 hours ago, aussievintage said:

 

OK, you have ac out of the transformer.  Next, check that you have raw DC after the main rectifier.  There should be a high voltage (330 volts or so) on the positive terminal of the first 100 uF capacitor.  The 1K3 resistor may be a convenient point to measure it.

 

If there is raw DC, check the voltage reference at the base of the transistor (top of the string of zener diodes).  If there is voltage there, check the output of the regulator right at the emitter of the transistor (the 2K resistor should be a convenient point to do this)

 

Also, generally check for bad solder joints.  Follow the high voltage, measuring it at various points between where it is present, and where it was not (the 285V test points).  Try to establish exactly where it stops.

 

 

 

Thank you very much. I will check them out soon.

 

Just one quick question. Is 2K resistor correct one? Because I can see 1K3 resistor but could not see 2K resistor. I see 3K resistors just beside 1K3 resistor.

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