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Mywing 27th January 2009 05:53 PM

Threshold 400A Power supply help
2 Attachment(s)
Hi guys,
I need two recommendations regarding my 400A Psu.
I noticed that the blue cap (330 mF) at the foot of the large electrolytics next to the back panel (it seems to be part of the power supply circuitry), is swelling to an end and is likely to arrive at the end of its life. I think it's a good thing to replace it. I would replace also a resistor connected at the cap: also it is damaged, cracked. But I don't know its value.
The 400A schematic in my possession, relate only to the boards of the two-channel transistors. I do not have the power supply schematic.
Does some of you have it could kindly post me? Or knows where I could get it?

Another question.
I bought two-Evox Rifa 22000/100V to replace the old big cans but I did not care for their height: in fact they are higher by nearly three centimeters: the wires that connect them to the trafo and the boards are too short. I do not think I be able to open the transformer to replace the wires ...
The only solution that comes to my mind is to buy two lower big cans, but I can not find good quality and values necessary. Unless instead of 22000/100V suggested by Jon Soderberg and other friends of the forum, I install two 15000/65V.

Who can suggest the best solution?
Thanks in advance

GRollins 27th January 2009 06:06 PM

Resistors do not crack open of their own accord. Something else is wrong. I suggest you begin trouble shooting to find out what's going on or you're likely to destroy any replacement parts you put in.
As far as the caps go, you've only got two options--replace the wiring harness or buy shorter caps. Which you do is up to you. The only wire that would go directly from the power transformer to the power supply caps is the ground wire. The AC leads go to the rectifier first, then the caps. Putting in longer wires from the rectifier to the caps shouldn't be that big a problem. To increase the length of the center tap (ground) wire from the transformer, you'd have to splice in another piece of wire.


Mywing 27th January 2009 07:44 PM

Thaks for your help Grey,
1) About the big cans I'll follow your advice, because the wires from the transformer to the caps are just a ground wires, I'll increase their lenght and replace the other.
2) The resistor is not exactly broken, but only slightly damaged, probably because its old age ... If broken, the amp I think would not work, while he goes. I would like replace it for safety.

In any case, I need the power supply schematic.
Someone else can help me or tell me what value is that resistor?


GRollins 27th January 2009 10:02 PM

I repeat...resistors do not crack with age.
I suspect that the part has been subjected to excess heat. If there's been excess heat, that means there's been excess current. If there's been excess current, something's been drawing that current...and that something (or somethings) is likely to cause problems with any new parts you put in, even if it takes a while to become obvious.


Nelson Pass 28th January 2009 08:01 PM

IIRC that is the cap for the front panel display.


Mywing 28th January 2009 08:12 PM

Thanks boss. But the resistor value?....

m2003br 19th February 2009 10:35 PM

In my own 400A, the resistor in series with rectifier is 1 ohm.
I think it is intended to operate like a fuse.
This supply is only for VU meter, as Mr. Pass stated.
The output is about 30Vdc.
Cap is 330uF 35V


m2003br 20th February 2009 12:15 AM

2 Attachment(s)
Power Supply schematic:

VintageAmp 4th May 2009 02:46 AM

Sometimes the carbon resistors bulge with age and stress. They hold value, but have been damaged.

The cap, resistor and diode at the base in the rear is the display supply circuit. The resistor is a 10 ohm. It only has to handle 15 V for the display logic op amps. It could be omitted and the amp would still work. Of course the display would not.

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