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Converting SRD-7 to pro voltage?


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Could someone post how to do this, or a link?

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Here is the SRD-7 Mk2 schematic with the bias supplies on the bottom. It's pretty straight forward with a 150k resistor on the input and then the z1100 bidirectional zener but you can use two 81v units instead facing each other. The rest is easy and it doesn't take long to build on a perfboard.

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  • 7 years later...

Hi Birgir:

Resurrecting an old thread.

I have a SRD-7 I am thinking about converting to Pro bias to fill in while I build my KGSSHV. The link you posted in this thread is broken. Any chance you can link me to a schematic?

Thanks!

Edited by mwl168
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I am a bit embarrassed. I did google it and came across this image but did not know it's what I was looking for :-)

 

Thanks Birgir!

 

Edit: For reference for others that may be interested in this topic, here is the normal-bias SRD-7 schematic.

post-4147-0-61740300-1425151222_thumb.jp

Edited by mwl168
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I did this using the schematic above, but kept one socket at normal bias, taking the voltage for it from halfway up the zener ladder.  One thing I did was to glue a nylon 4-40 screw into middle hole in the socket I converted to pro bias.  This prevents you from accidentally putting a normal bias phone into a pro bias circuit

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There are two things very wrong with that schematic though, under no circumstance should that capacitor me after the ballast resistor and the ballast should be 4m7 and not 2m2. 

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There are two things very wrong with that schematic though, under no circumstance should that capacitor me after the ballast resistor and the ballast should be 4m7 and not 2m2. 

 

To be clear, are you saying that the SRD-7 MK II schematic was wrong and that R104 should be 4.7M and C108 should be at a different position in the circuit.?

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Thanks Birgir. I'll have to fabricate it on a perf board then. 

 

To confirm the right way to convert the SRD-7 to run on Pro bias (580V):

 

1. I disconnect the factory bias portion of the original circuit after the 150K resistor and neon bulb.

2. Construct the bias circuit of the SRD-7 Mk II in post #6 but skip C108 and change R104 to 4.7M.

 

Thanks again for your help!

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Excellent points by Birgir.  When I built my version I did as he suggests and had the 4.7M resistor after the last cap - been awhile and I forgot that- early senility sucks sometimes.  ::)

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S5277G and Z1100 diodes are both obsolete. Can someone recommend current production replacement? Preferably Mouser stocked parts - I am about to place a big order with Mouser for a KGSSHV build.

 

Thanks! 

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  • 1 month later...

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