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Old 29th July 2012, 02:57 PM   #1
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Default Yaqin MS-22b Phono Amp

Hi Guys
I have a Yaqin MS-22b phono stage with a hum. I have shorted out the input phonos and the hum is still there. I disconnect the input leads from the circuit boards easy to do because of the connectors and shorted both pins of the connectors on the board and the hum stopped. I examined the cable from the inputs to the board and it is 2 pair unscreened. Do you think that this should be screened cable and the screen earthed. Any other ideas would be welcome. I have tried all the normal things moving it around, changing cables etc. This last test was done using a different amplifier. You only notice the hum when there is no music playing.
Many thanks
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Old 29th July 2012, 10:42 PM   #2
kevinkr is offline kevinkr  United States
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Are the RCA jacks isolated from the chassis? If they are not isolated this creates a ground loop that would be responsible for the hum you hear. Twisting should normally be sufficient if inside a metal chassis and not near the power transformer and choke, otherwise low capacitance shielded cable can be used.
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Old 30th July 2012, 09:04 PM   #3
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Hi

I checked the RCAs both input and output and they are insulated from the metal case. There is also an earth terminal and this is also insulated from the metal case. I disconnected the earth from the board and desoldered an earth that looks like it came from the transformer - none of these worked the hum was still there. I also wrapped the input cables in metal foil to simulate a screen and earthed that but the hum was still there. Does anybody have any other ideas?
Many thanks
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Old 9th August 2012, 12:03 PM   #4
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I'll be interested to know the answer to this too as I have the same issue...

I have attached the schematic of the MS 22B

I used a multimeter to control the 12AX7s heater voltage and here is what I got:
11.7VAC & 25VDC, are these ok ?
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Old 9th August 2012, 12:40 PM   #5
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Default Yaqin MS-22b Phono Amp

Hi
Following on the the above.
1) Used shorted phono plugs on the input - hum still there.
2) Disconnected the inputs from the PCB using the on board connectors.
3) Made up some flying leads with a socket on one end and connected it to the PCB on board connectors - hum all but gone - only the noise you would expect from a cartridge turned up full.
4) After a lot of head scratching traced the leads back to the phonos and found that both the earth/ground L & R were connected together. Split these to make two seperate circuits - Hum gone - same niose as above.
5) Connected my input phono leads shorted them out and hum gone. Connected up my DIY SUT and hum returned - so now trying to sort that out. Pretty sure it will be a ground loop - but it does take time.


Hope this helps

Please let me know.

Regards
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Old 9th August 2012, 03:01 PM   #6
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Hey,
Thanks for this explanation.
Would have pix of what you've done?

I m not sure I get the step 3 and 4
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Old 9th August 2012, 03:02 PM   #7
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btw, I made an error in typing earlier, I meant:
"I used a multimeter to control the 12AX7s heater voltage and here is what I got:
11.7VDC & 25VAC, are these ok ?"
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Old 9th August 2012, 03:34 PM   #8
DF96 is offline DF96  England
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Phono cartridges generally use a 'floating' connection so the signal return wire should only be grounded once at the preamp input - typically at the point the signal connects to the circuitry. The separate 'ground' connection is for grounding the arm/deck, not the cartridge.
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Old 9th August 2012, 05:06 PM   #9
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I noticed that the hum was also here even when the pre amp was switched off...
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Old 9th August 2012, 05:53 PM   #10
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Default Yaqin MS-22b Phono Amp

Hi

Reference my previous post. If you disconnect the leads from the input phonos to the circuit board by using the PCB board connectors. You will see two pins on each connector. With the volume control on the amp turned up with a small screw driver short each out. Just like putting two shorted phono plugs in the input sockets. The hum should disapear. This means whatever is casing the hum is further down the line. After reconnecting the PCB connectors I found the problem.

As I said I tracked it down to the common earth returns on the input sockets as soon as I split these making effectively two seperate circuits the hum went away.

Hope this helps
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