Nady XA-300 Amplifier

[bluto]

Got this one thrown my way because it was dead. The fuse was good but there were absolutely no signs of life. I cracked it open not knowing what to expect. At first glance there was nothing grossly wrong looking. I partially removed the power supply and started probing around on the primary side. It didn't take long until I located a disc shaped component that looked a little toasty. It turned out to be directly in series with all the input filters. However there was no continuity across it. Handily, the manufacturer printed the part number on the silkscreen.

It turns out to be a 7A current inrush limiter/thermistor. Digikey to the rescue! Three days later the part is installed and the amp came back to life.

The pics show the old and new parts as well as the internals. Although not visible, the part in question is near the back of the chassis between the right filter capacitor and the yellow capacitor seen just above it.

Digikey part number: http://search.digikey.com/scripts/Dk...173-nd&x=0&y=0

[Maxxarcade]

Re: Nady XA-300 Amplifier

I had to fix one of those a while back. The two big caps on the power supply secondary were bulging, and the B+ was a few volts too high. Also replaced a couple small caps on the output modules. It's been running fine since.

If I recall, this amp is 100W per channel into 8 ohms.

[bluto]

Re: Nady XA-300 Amplifier

Good to know, thanks! On this one the caps all looked fine. This was a charity job for a local KC hall type place. It came to me dead and was returned not dead so they got what they paid for. If it breaks again I'm telling them to get a new one.

[Caryw]

Re: Nady XA-300 Amplifier

I know I am bumping an old thread, but I have this exact amp and it worked fine but now goes into protect mode as soon as the power is turned on. The fans spin for about 2 seconds then powers down.

All Nady will say is that it is a bad relay and want 70 to fix it.

any idea from looking at the pictures above, where the protection circuit relay is? Nothing looks bad or out of place on mine at all.

I have randomly unplugged everything trying to nail down where the problem is but it just goes straight to protect.

Thanks

[bluto]

Re: Nady XA-300 Amplifier

I'll let you know if I get my hands on this one again. The one that I fixed has a twin brother in the rack, so I am sure that eventually I will have to work on one of them again.

[budm]

Re: Nady XA-300 Amplifier

I am surprised that they did not use the relay to bypass that in-rush limiter a few AC cycle after the caps are charged up, that way the Inrush limiter will be used only during turn on, this will also reduce current limit compression, otherwise it will get pretty hot, I never design inrush protection circuits without the relay, may be they try to safe money, I also wonder what the resistance value is when the Thermistor cold and warm.

[bluto]

Re: Nady XA-300 Amplifier

This particular one was 50 ohms cold and according to the datasheet it approaches zero when warmed up. I agree it seems like a poor design, but then again this was not exactly a high end amp. Your sentiments about cost cutting probably play into the design.

[budm]

Re: Nady XA-300 Amplifier

You mean 5 Ohms, so worst case max current at start-up will be about 120V/5 Ohms = 24Amps.

[bluto]

Re: Nady XA-300 Amplifier

Yes that's right, 5.0 not 50. It can't do that much. I thought about just bridging it for a few minutes...

[Jukie812]

Re: Nady XA-300 Amplifier

Mine is doing the same thing. Nothing is burnt looking and the caps check out fine. As soon as I power on it goes to protect. I was running 4 ohm mono. What do I do next

[lbfire]

Re: Nady XA-300 Amplifier



I had the same amp go into protect mode moments after I turned power on.

The lowest hanging fruit is the power supply.
1. I did a visual inspection of the electrolytic capacitors that might be bulging, fans that do not spin, then burnt marks, odd looking component shapes that don't look normal, and/or blown components. Found nothing.
2. Next I took a look at the outputs and a visual on the output board. I took an ohm meter and check for a short or a near zero reading on the output terminals. On the XA-300, you can disconnect the output boards (they have connectors) and check them independently with an ohm meter. I quickly found it was channel 1 that had a shorted output. I ohmed each output FET and found my shorted FET very quickly. I only found 1 bad FET. The output FETs are IR (International Rectifier). Unfortunately, IR was bought by Infineon. I couldn't find an original IR FET, so I stuck a Vishay FET in there. The gain pot sensitivity seems to have changed a bit. I probably should have replace all of the FETs with Vishay for uniformity on the gain. I might do this in the future.

[petehall347]

Re: Nady XA-300 Amplifier

yes change them all and check/reset bias current after