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Old 3rd March 2011, 04:16 PM   #41
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Hi,
I think the only one bad is the 2N5320. It is shorted. Replace it.
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Old 3rd March 2011, 05:16 PM   #42
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I can see you have gone to a lot of trouble with the readings...

The 2N5320 is short as are both 40636's in the duff channel.
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All (of this sort anyway) silicon transistors read similar. It doesn't matter whether it's a power device or a tiny small signal one, the readings are always in the same ballpark give or take.

Taking the BF258 which we know is good should read from B to E and B to C with the red lead on the base. Are you sure your identifying the connections OK and also old devices can have oxidised leads that hinder good contact with a probe.
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Old 3rd March 2011, 06:36 PM   #43
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Well, I'm struggling to understand the readings my multimeter is giving me... but that will be down to me not understanding the electronics behind whats actually going on, I can read up on that in time. Right now I'm going to right ahead and change the components I've highlighted as damaged. You were right about the BF258 Mooly, I hadn't copied the results right from paper to computer, they are both reading out as you expected.
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Old 3rd March 2011, 06:41 PM   #44
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Just take it slow... and don't power up without the bulb tester in place.
As you have removed components be very careful to replace all back correctly.
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Old 30th March 2011, 04:33 PM   #45
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Well, after weeks of waiting for some free time...

I refitted all of the components, checked and double checked them to make sure they were in the right places and soldered up correctly. I tested the power transistors to make sure they weren't shorting to the chassis and from what I can gather they don't appear to be any different from the working right channel. Then I inserted the 100w tester bulb in series with the amplifier to try firing it up.

I first switched the amp on with only the right channel fuse in place. The bulb lit up initially then went dark. I tested with a signal and the right channel is still working fine with the replacement components that I changed. Anything I had to replace on the left channel I changed on the right channel as well.

I then switched the amp on with both fuses in, the bulb glowed dimly for a couple of seconds then begun to increase in brightness, simultaneously a wire wound resistor started to smoke. I turned the amp off immediately.

The resistor appears to be this one:

Click the image to open in full size.

Both the fuses on the amplifier are still intact, I guess the bulb saved them.My guess is that the channel is shorting somewhere, any ideas welcome?!?
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Old 30th March 2011, 05:03 PM   #46
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Remove the fuses and check both channel resistance of both channel and compare them. Do the test to the load side of the fuse. This will let you know if you still have a short.
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Old 30th March 2011, 05:45 PM   #47
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Both channels responded the same. They both started out at about 8 kOhms, then kept climbing, charging caps I guess. So I guess I don't have a short, but what else can be going wrong?
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Old 30th March 2011, 06:49 PM   #48
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Firstly just measure the resistor that burnt and make sure it reads OK or thereabouts.

The fact the bulb started to increase in brightness points to possible bias problem... maybe just requiring adjusting but this will prove it.

On the bad channel apply a shorting link as shown between the bases of the two driver transistors.

Now switch on again... the bulb should remain dim.

The outcome of that determines what we do next.
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File Type: jpg Armstrong 621.jpg (262.2 KB, 78 views)
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Last edited by Mooly; 30th March 2011 at 06:53 PM.
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Old 30th March 2011, 07:21 PM   #49
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Hi,
Does the relay close after few seconds. The resistor it is use only in the power up. Once the relay timed out it will short the resistor. Why the resistor it is getting hot? Means the relay is not closing the contact.
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Old 31st March 2011, 05:54 PM   #50
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I can't find a mechanical relay anywhere inside the amp, for all I don't really know what it would look like anyway.

I tested the resistor and it reads out at 46.7 so all safe there. I did the modification that you suggested Mooly, Upon power up the bulb didn't even light initially so I checked the voltage rails and I was getting a healthy 79v.

So it would appear that whatever you suggested has made the amplifier more comfortable. I tested the pot adjusting the quiescent current before and it appeared to read out ok, or is that nothing to do with it?

Thankyou!
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