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Old 12th August 2012, 08:50 PM   #21
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Hey Guys

The hum is this time TOTALLY gone! :-)

We reviewed the Yaqin this morning with a friend, split the input RCA ground and he also has had a mini "69" circuit (diodes, cap and R) to clean the ground signal going from the pcb to the chassis :-)

Now with new Russian oil paper caps and chassis raised on spikes (due to the caps size) this little Yaqin sounds incredible! :-)

Next step: Sylvania NOS tubes on the way

Last edited by SDIESEL77; 12th August 2012 at 08:58 PM.
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Old 13th August 2012, 09:52 AM   #22
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Default Yaqin MS-22b Phono Amp

HI SDIESEL77

Glad you have managed to sort the problem and it sounds as if you have managed to improve on it as well.

Could you post details (circiut diagram) of the 'mini 69 circuit' so that I can try it on mine.

What caps did you actually change? (Russian paper in oil) and I might give that a go as well.

Many thanks
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Old 13th August 2012, 11:02 AM   #23
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Hey Will,

The caps changed were the C4 C7 C11 C12 (1uf 275v) in the schematic by Russian Oil paper ones (1uf 600v but if you find lower voltage it's fine too and will take less space).

For the grounding circuit (Big Thanks to my friend Fran for his genius idea ): You'll need 2 x 1n4007 diodes, a 10-30R >5W resistor and a 0.001-0.02uF 400V cap. Quality or type of any of these is not important.

Diodes: QL79L, QL78K QL77J or QL79L will do fine
Resistor: L6R8, L8R2, L10R, L15R or L22R
Cap: any cheap ceramic cap low value 0.001-0.02uF ~400V or so

Click the image to open in full size.
Click the image to open in full size.

Last edited by SDIESEL77; 13th August 2012 at 11:04 AM.
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Old 13th August 2012, 01:38 PM   #24
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Hi SDIESEL

Just to check. With the '69' circuit you disconnect the earths on the board. Solder the '69' to the board and then solder the earths to the other end of the '69'. Is that correct?

I have tried it this way - did'nt cure the hum completely but quietened it down quite a bit. The problem I guess for me is the SUT still looking to cure this.

Many thanks for sharing.

Regards
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Old 13th August 2012, 01:43 PM   #25
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Hey,

Actually, you don't disconnect both earth (chassis and TT input), you only disconnect the cable going from the PCB to the chassis.
Then as you said, you solder the "69" to the board and the other end to the cable that is screwed to the chassis.

This really cured completely the hum for me, then if it's still there for you, I'll indeed look at your SUT.

What do you use the SUT for ?
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Old 13th August 2012, 04:18 PM   #26
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Default Yaqin MS-22b Phono Amp

Hi SDIESEL

I use the SUT for a MC cartridge feeding into the Yaqin. I reconnected the circuit as you suggeted with only the chasis earth connected to the '69', still got a bit of a hum but not that much.

In all the messing about the blue LEDs have stopped working. Not a problem, I'll sort it ought if I ever get round to replacing the caps. However it may be just fancy but I think it sounds better without them working. As you know there is a socket on the other side of the board which should make connecting them and diconnecting them really easy. I remember reading somewhere that LEDs introduce noise. So for now I just use it without.

The SUT thing and ground loops seems to be a bit of a mystery, trial and error it would appear. But at present it is running really well. Thanks for your help.
Regards
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Old 13th August 2012, 05:01 PM   #27
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Happy to see that it helped
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Old 20th August 2012, 09:57 PM   #28
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Default Yaqin MS-22b Phono Amp

Hi SDIESEL

A couple of posts back you said that you had changed the 1uf caps to Russian POI what type were they? I've done a bit of research and there seems to be quite a few types to choose from, so not sure which ones would be best.

Regards
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Old 21st August 2012, 07:37 AM   #29
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Hey Will,

You can look on eBay for any NOS oil paper ones 1uf and minimum 275v

I found the following ones, however they're 250v (instead of 275v), it'll be good to check what's the actual voltage going through the original ones in place in the Yaqin.
ex:
K75-10 V Paper In Oil PIO Capacitor 1uF 250V 10%, Lot of 20 | eBay

Last edited by SDIESEL77; 21st August 2012 at 07:40 AM.
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Old 22nd August 2012, 09:50 PM   #30
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I've created a video people who wanna try these mods:
http://youtu.be/Xm5oqrll2ZY
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