Inrush current tripping circuit breaker

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Hi guys, I have a problem with an amp that trips the circuit breaker most of the times I turn it on. When it doesn't, everything works fine.

I have measured the four 15000 µF caps in the PSU and they are fine. There's a circuit on the primary side that I suspect could be faulty, or be modified to work better. Anybody got any ideas?

The current limiter circuit looks like this:

ksfFpKm.jpg
 
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Hi Welcome,
What a flaky soft start circuit! From what they say about the way it works, then yes. You can slow the main soft start resistor cut out down by increasing that capacitor. However, I'm willing to bet that the resistor might open if it is too slow.

-Chris
 
Thanks, man!

I would've checked that earlier, but these parts are quite hard to get to. After much fiddling, I managed to get the 4R7 resistor out, and it measures over 500k! I will replace it and report back.

Thanks again. :)


EDIT: I don't have a 5W 4R7 resistor, only a 2W part... not gonna risk that. Unless you guys tell me to. I guess the original burned due to being undersized. I will have to order one, and maybe make it a 10W part instead.
 
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Hi Welcome,
It might burn out the first time you try it, or maybe a short time down the road. If you have safe, extra space, a 10 watt resistor might be worthwhile to put in. Also consider the tubular 20 watt resistors that mount on a pair of metal supports. Then you can use stranded wire to connect with them, and the resistor can not move into contact with anything else.

-Chris
 
Hi Welcome,
It might burn out the first time you try it, or maybe a short time down the road.

What makes you think that it will burn out right away? Do you suspect further problems to be the cause of this? This amp is from the 70's and has worked presumably for quite a while with this design.

I have a 25W 3R9 resistor. Maybe I should try that. There is space for it. :)
 
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Hi Welcome,
Simple. The 5 watt resistor burned out, and the 2 watt version has much less thermal capacity. It's only in circuit for a brief time, so the power it handles would probably exceed a steady state 5 watts. The 2 watt resistor will heat more quickly than the 5 watt one did. Also, the original was wire wound. Your 2 watt might be metal oxide. Wire wound resistors are great for momentary overloads. Better than other types.

I was merely trying to explain why the resistor burned out if it did in your test. Extending the time it is in circuit only makes the situation worse.
This amp is from the 70's and has worked presumably for quite a while with this design.
Isn't it amazing how long parts can sometimes last when being abused? It may have, but don't count on this as normal.

Try it, and good luck. I would say that the 2 watt part is a stop gap measure until the right one comes walking through the door.

-Chris
 
No no no, I misunderstood you, or you misunderstood me. I thought you meant if I put in another 5W part, that would burn out right away. As if the original circuit shouldn't have worked in the first place. I have no intention of trying the 2W part. :)

I'll try the 25W 3R9 one tomorrow. Will have to get creative mounting it, though.
 
^^^^^^^^ that.

Chimed in to say the same, you are ignoring an obvious clue: those relay contacts are welded close, it's *always* on.

No time delay or resistor replacement will work if you don't solve it first.

EDIT:
or, at least some time, the relay did not short the current limiting resistor and it burnt because it stayed always in the main current path, again not a time delay problem.
Unless you count "never" working as the time delay , that is ;)
 
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Hi Welcome,
Ahhh, misunderstanding is right. You figured it out though.

Be extra careful with that 25 watt resistor. You are switching line voltage, so any contact with a metallic surface is extremely bad news. That's why I mentioned the 20 watt tubular resistor.

Normally the contacts on these relays do not burn out. They can become badly oxidized, then burn the surfaces. With the open resistor, the chances are there that the contacts are damaged on that relay. You can inspect the contacts in that relay directly by taking it apart. If in doubt, you should replace it. Same goes for the other relays.

With most power amplifiers, the speaker protection relay will be bad by now. It still works, but replacing it might make a tremendous increase in sound quality.

-Chris
 
A direct off the mains circuit is a dangerous way to construct anything that does not have adequate "design for safety" built in.

I would throw away the direct off mains "delay" circuit and use a small 3VA to 10VA mains transformer to generate the required DC voltage that the relay needs.


BTW, this Forum usually refers to this circuit as a "soft start".
There are many Threads that discuss properly designed soft start to allow transformers to start up without blowing the close rated mains fuse.
 
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Hi Andrew,
This circuit application is normally run off the mains. It has no parts that are in touch-vulnerable areas. They are normally located on the chassis floor, behind the face plate or near the rear apron. These circuits are also UL and CSA approved as long as they are not modified in such a way that contact is possible. That would suggest that the original 5 watt type resistor must be used to maintain it's safety acceptance.

Without having that amp on my bench, there is no way I can offer advice on repairs / modifications (usually not something you want to do).

-Chris
 
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