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Old 2nd August 2010, 02:04 PM   #4291
Salas is online now Salas  Greece
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Quote:
Originally Posted by RCruz View Post

What is CFP arrangement ?

I decided to invest in an oscope.... Is this suitable ? VELLEMAN SA - HPS10 - OSCILLOSCOPE, HANDHELD, 10MHZ VELLEMAN SA|HPS10|OSCILLOSCOPE, HANDHELD, 10MHZ | Farnell Portugal

I chose it for the lowest cost because I am now quite short in tim€
Its the compound pair of BJT buffer transistor and output Mosfet. Those are good to be very intimate in layout.

A very practical handheld instrument with many calculating and holding functions, yet only 2MHz and 100VDC in. If you evolve your troubleshooting skills and want to play with tubes maybe you will outgrow it. Of course it will show a gross disturbance on DC but you could browse European Ebay sites for 230VAC import hassle free used desktop CRT full scopes for under EUR100 too before you buy.
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Old 3rd August 2010, 02:22 AM   #4292
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While waiting for parts for the RIAA section, I thought it would be a good time to build the chassis. Attached are photos of chassis taped together for review. Still some fitment to complete, but the layout can be seen. The chassis is made of aluminum with a wooden faceplate for cosmetics. The RIAA stage will be installed in a secondary aluminum box seen within the main chasis. Power will be fed from the shunt regs through a couple of small holes. The Force and sense wire will be Belden Blue Hose (shielded two conductor) 22awg cunductors. It is good wire with low capacitance 17pF/ft.

Inside the RIAA shield, the output caps will solder direct to the RCA connectors. The input wire will be cardas two conductor shielded from the RCA post to the board. All runs will be less than 100mm. I put the caps and board on top of the inner box for reference.

Since the main chasis is completely metal enclosed, I did not include the shunt reg in the inner box. My only concern now is the magnetic fields from the PT and some EMI (AC mains) from the input side of the PS board seen in the center of the chasis. I have tested, using the oscilloscope, this arrangment and I still cannot detect any ripple/noise from the shunt regulator. With the RIAA further shielded I feel that 60 Hz pick-up from the AC mains would be minimal if at all.

Any thoughts or improvements or a critical flaw which I can correct before I start final assembly? I would prefer not to go bigger on the case, and for right now I want to keep the unit as a single chassis.

Thanks for your opinions, all are welcome.
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Old 3rd August 2010, 07:31 AM   #4293
massimo is offline massimo  Italy
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Originally Posted by SGregory View Post
The RIAA stage will be installed in a secondary aluminum box seen within the main chasis.
Nice idea, but aluminum as shield has a very limited effect (unless 1/2" thick).
It's most probably better to use just a thin sheet of mild steel vertically, between the two heatsinks.
My 2c
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Old 3rd August 2010, 09:46 AM   #4294
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How about if I make the box from steel? The AC mains will come from the back corner next to the RIAA boards. The idea of a piece of steel between the heat sinks is appealing as it is very simple but I am afraid it may not shield it from the power line noise.
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Old 3rd August 2010, 10:42 AM   #4295
AndrewT is online now AndrewT  Scotland
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Originally Posted by RCruz View Post
In the riaa board, blue is GND and brown (red in real life.. camera to blame) is (+).

Maybe I should try to interlace those lines to minimize the loop area....
the wide loop formed by the power lines will act as a receiving aerial for electromagnetic radiation.
The wires show the problem. The spacing of the tapping points show the problem.
The circuit formed by the supply lines, flow and return, must be small loop area. This philosophy must also exist in the traces on the PCB.
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Old 3rd August 2010, 10:52 AM   #4296
RCruz is offline RCruz  Switzerland
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Thank you Andrew

I will revise the layout and come back with the results.
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Old 3rd August 2010, 10:55 AM   #4297
RCruz is offline RCruz  Switzerland
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Originally Posted by SGregory View Post
How about if I make the box from steel? The AC mains will come from the back corner next to the RIAA boards. The idea of a piece of steel between the heat sinks is appealing as it is very simple but I am afraid it may not shield it from the power line noise.
Steel is quite effective even in small squares that must be connected to GND.
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Old 3rd August 2010, 11:10 AM   #4298
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@SGregory
Could give details of your PSU, inductor, caps used & schematic?
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Old 3rd August 2010, 11:32 AM   #4299
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The tranny is a 2x18V 4amp connected in series from an old recycled project. It feeds a RCLC network. R=.75R (mostly for voltage control) C1=13,400uF L1 = 0.25mH C2= 24,000 uF. Caps are long life low ESR. Ripple per PSUDII is approx 100uV. It is undectable using my scope at 2mV/div. The rectifiers are MUR 100V 3A schotky's. I don't have a schematic on hand to post but it should be easy enough to draw one from the above info.
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Old 3rd August 2010, 11:35 AM   #4300
AndrewT is online now AndrewT  Scotland
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Hi,
does your R=0.75ohms include the resistance of the secondary windings and the resistance of the wires from secondary through rectifiers to the first stage smoothing capacitors?
What is the resistance of the 0.25mH inductor?
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