Power Amp Repair

amp

are you using a Slow blow fuse?

T = Time Delayed. you need a slow blow.

but

amp

That points more to a possible design fault like ultrasonic oscillation.

The Behringers are reliable and pretty much unbeatable value. They are based on the QSC RMX models btw and are very similar.

Graham

the

I suspect I know what the problem is. I'll bet it does use a toroidal transformer and the cheapskates haven't bothered fitting a surge-guard device.

This all points to poor/skimpy design I'm afraid.

Graham

Let me suggest that perhaps the idling current is set too high. Then the output transistors start to warm up and after a while there is thermal runaway and the fuse pops.

Check the idling current and adjust it if needed. Another possibility is that the temperature sensor, usually a transistor or diode or thermistor, is not properly attached to the heat sink and thus doesn't react to warming output transistors.

Bob

Would that be the cause of the fuse blowing at Power On though? That seems to be the problem as of now. As soon as the unit recievies that first initial surge of power the fuse pops. I'm keeping track of all the suggestions if i would have had this info at the start i would have by-passed the repair shop and did this on my own. I'm a newbie at this but i want to learn as much as i can. I find it really facinating. And if i can learn to fix stuff like this on my own even better

On Sun, 04 Feb 2007 11:43:48 -0800, Haaky Has Frothed:

Did you say if you were using a slo-blo fuse or not?

No i believe it a "T" timed fused i think that's what there called?

In one of the earlier posts, you stated "On the back of the unit it says the fuse type is 250v T 7A. The fuse that was in it was a 250v don't know if it was a "T" or not 6.3A."

If the original fuse was rated for 7A, and it's being replaced by a 6.3A fuse, that could be your problem. I suggest that you try the correct fuse before replacing any more components. If no more fuse blowing, then you might be good to go. If the correct fuse blows, I would suggest that you replace all of the output transistors at the same time. They are all available from MCM Electronics.

If you can scan the schematic and post it on abse, we can probably come up with some pretty good suggestions for you to go with. Does the schematic that you have give the instructions for setting bias for the output stages?

OK. That's definitely a slow blo fuse that it needs, and if it is on the back, then it is almost certainly the line power fuse. If there are no signs of distress inside the fuse, just a broken wire with no signs of melting, then this would normally indicate that it is breaking just because of metal fatigue, which is exactly the symptom you get when a fast blo is wrongly fitted in this position. A big tranny is almost a short at power up for a few cycles, until the core gets magnetically 'charged'. If you have a straight fuse in there, you can watch the wire sag until it almost touches the glass at power up. A " T " rated fuse is designed either not to do that ( spring-loaded types ) or to withstand the effect ( spiral types ). So for a start, I would make sure that the next fuse you fit, is definitely a " T ".

It is common practice in a repair shop to scribble " C " and " A " on the heatsink over the transistors, just to make sure that when you have them all out, the new ones go back in in the correct places. The transistors are standard Japanese types and are actually 2SCxxx and 2SAxxx. They always leave the " 2 " off the on-device print. The " A " is a PNP type, and the " C " is NPN, so it's very important that they go back in right.

If the company were happy to send you service info just as a result of an e-mail enquiry, then in my experience, they would probably talk to you on the phone.

Arfa

was

good

the

I was affraid to put in a 7A fuse even though that what is on the back of the unit. Ever since i've owned the amp it has had a 6.3 fuse it. I might try a 7 if it keeps popping. Thanks for the link to the websites. I might order me some just to have as spares

I'll post the schematics of the amp

No it sounds like magnetising current actually. Toroids are notorious for it because of the low reluctance of the magnetic circuit.

Graham

Actually he needs a T type to IEC specs.

Slow-Blow is unique to the USA and has different characteristics. In fact all US fuses are different to IEC ones.

Graham

That is the correct type to use.

Graham

Pretty unlikely actually. 7A is not a preferred value so it'll be hard to come by. You could always try 8A though to see if it makes a difference.

Graham

They may have missed a failing bias diode or another bias related part may be failing. Doesn't sound like they have much experience with these types of intermittent recurring failures.

Mark Z.

My confidence in this repair shop went down the tubes. I thought they would be good. They have a good rep, are always busy, very helpfull Supposedly the repair guy has 30 years experience. But i wonder if it's in electronic repair?:):):)

by. You

Wouldn't i be running a bigger risk putting in an 8A fuse instead of a

I'de rather the fuse blow instead of a more expensive part.

I have built things that have had seemingly intermitant problems. A transformer will pull different currents depending on time of the AC waveform. You can probably figure what percentage the waveform will be max, and most of the time it will not blow. Then you hit it and it blows. Also tends to give off loud noises. This might be part of the problem. For the price of that unit, a good shop could not afford to play with throw away equipment, or, your not going to like the bill a good shop will charge.

greg