altec 436 rectifier replacement

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QUEEF BAG

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Jun 12, 2009
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563
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haven't really found an answer searching the forum...
just the suggestion to do so.

our 436C is sick and getting worked on.

while it's on the slab, err. bench, i'm thinking it's a good idea
to replace the selenium rectifier.
one,  because its selenium,
two,  because the voltages are a little low through out the circuit.

the question:
if i put in a couple of 1N4007 diodes in it's place, will i need a resistor to drop the voltage
to a reasonable level since selenium has less of a voltage drop than silicon diodes?

 
The 2 can caps (80, and 80-40-10) are leaking horribly on a 436c I am working on.  I suspect the rectifier is bad which caused the caps to go and I know these Selenium  rectifiers tend to go.  If you replaced this with 2 diodes can you tell me the results you got and how you did it?

:)

Adam
 
as per the client's wishes, the selenium rectifier was left in.


one clue may be this...

 

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ripped this off the net<

"Snubbers are frequently used in electrical systems with an inductive load where the sudden interruption of current flow leads to a sharp rise in voltage across the current switching device, in accordance with Faraday's law. This transient can be a source of electromagnetic interference (EMI) in other circuits. Additionally, if the voltage generated across the device is beyond what the device is intended to tolerate, it may damage or destroy it. The snubber provides a short-term alternative current path around the current switching device so that the inductive element may be discharged more safely and quietly. Inductive elements are often unintentional, but arise from the current loops implied by physical circuitry. While current switching is everywhere, snubbers will generally only be required where a major current path is switched, such as in power supplies. Snubbers are also often used to prevent arcing across the contacts of relays and switches and the electrical interference and welding/sticking of the contacts that can occur.
RC snubbers


A simple snubber uses a small resistor (R) in series with a small capacitor (C). This combination can be used to suppress the rapid rise in voltage across a thyristor, preventing the erroneous turn-on of the thyristor; it does this by limiting the rate of rise in voltage (dV/dt) across the thyristor to a value which will not trigger it. An appropriately-designed RC snubber can be used with either DC or AC loads. This sort of snubber is commonly used with inductive loads such as electric motors. The voltage across a capacitor cannot change instantaneously, so a decreasing transient current will flow through it for a small fraction of a second, allowing the voltage across the switch to increase more slowly when the switch is opened. Determination of voltage rating can be difficult owing to the nature of transient waveforms, and may be defined simply by the power rating the snubber components and the application. RC snubbers can be made discretely and are also built as a single component."


inverters have problems with EMI so a lot of "snubbing going on there too.
only it is the transistors making all the racket,

 
You can leave the selenium rectifier there if the client wants it for an authentic appearance but using it in circuit is asking for trouble.
Disconnect the wires but leave it mounted on the chassis.
These things literally can explode and release toxic gas.
We began replacing them in pro audio and broadcast equipment back in the 60s, about the same time we started replacing tube rectifiers with plug in solid state replacements.
You must make adjustments for the proper output voltage, though as diode bridges usually put out more voltage than the seleniums or tubes.
 
I appreciate the feedback.  Does anybody have a pic of the board in a 436 that has had a Selenium rectifier replacement?

I see the JPG that was posted above appears to have a full diode bridge.  I may be mistaken Isn't the Sarkes 58D a half wave rectifier?  I would love to see a board that was built to replace this rectifier.

If no pics are available can someone depict exactly what needs tpo occur for a proper implementation.  I could use a little hand holding on this one. 

Thanks

Adam
 
ab0mber said:
Does anybody have a pic of the board in a 436 that has had a Selenium rectifier replacement?

It is just two diodes like in the schematic below.! The Altec tube compressors don't have a circuit board - they are wired on turrets.

I'm possibly doing a similar conversion next week, once I find some suitable cap rings (grrr...) . I'll post pictures, but can't quite say when.
 

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Cool. since the selenium rectifier has more resistance, i assume no resister will be needed to compensate for the slightly higher voltage?
 
Sorry - I had a pile of other jobs to do, so my stuff gets  put to the back. I haven't even ordered the right size of rings for my capacitors yet. Will probably be another couple of weeks. I just checked and this has been on my bench since Feb 2012!

But you have everything you need in the info above. Just swap them out as in the schematic and tweak the resistors to get the right voltages.

I build a clone 438 some years ago using a couple of silicon diodes - it worked just fine once the resistors were tweaked.

Cheers

Stewart
 
I'm not sure why I keep replying....since no one is listening.... have never changed anything but the caps and the rectifier on these, and they work exactly right.  This is a case of extreme over-thinking.  Just do it. 
 
What is the best way to mount these diodes in there.  Should I remove the selenium diode and mount a solder strip in there?

I was doing a rough test to see if the voltages would be correct so i took the secondaries and wired up two diodes and a 80mfd cap.  It seems like the voltages i was getting out were low (192vdc compared to the 270ish vdc i should be seeing.  Do you think this is because i did not have the other can cap hooked up.  (80-40-10)?  I am still learning a bit so excuse me if these questions sound dumb but I am just trying to vison how this should look without the selenium diode in there.

Yes I know the caps need replacement.  :)
 

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Multiply the percentage of line variation from your AC wall outlet by target B+, and see what normal voltage swing is.  This is not a regulated supply, those aren't precision resistors, and these are tubes, which draw wildy varying amounts of current, which in turn changes B+.  This is why there is a meter adjust control. 

Your voltage is low because this is a voltage doubler circuit, and you don't have it all there. 
 
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