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Old 21st August 2008, 01:17 PM   #21
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by jaycee
If push comes to shove, leave it out - it will just make some interference when you switch it on/off. A better place for the capacitor wouldve been across the switch contacts IMO.

Quote:
Originally posted by AndrewT
no Jaycee.
You are referring to two different capacitors with different purposes.

Quote:
Originally posted by jaycee
Yes, in the second case I was referring to supressing sparks across the switch contacts, which wear away the contacts and create switching noise.
and that is not a better place to retain or insert mains interference suppression.
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Old 21st August 2008, 05:31 PM   #22
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Thanks guys for all of your help so far. I hope that I'm doing the right thing by what I've done below.

I can now report back on what I've found.

With the secondaries disconnected, as I've mentioned before, the fuse blows when connected to the mains supply without the light bulb tester connected. I'm wondering if I've mainly seen it blowing on startup and not during longer use with the 500mA fuses.

With the secondaries disconnected and the light bulb tester connected, on switching the unit on, not every time, but I see a small flash from the bulb.

With the secondaries connected up to the circuit board, the power LED illumates, an I get a greater flash on starting up the unit from the bulb.

In either the above cases, the bulb is not illumated when the amp is just sitting there, on, doing nothing.

(Originally when the 7.5 amp fuse was fitted, the fuse failed with the amplifier under load 1 hour (say) into use.)

Regarding the fuse rating, in the service manual it indicates 800mA antisurge, but in the user manual it says 500mA, and on the schematic it also says 500mA, so this is a bit misleading.

I've replaced the C210/C211 6800 microfarad capacitors now as suggested with 63v rated ones, bearing in mind what jaycee mentioned about the replacement of the transformer (if needed), and I put the 100nf X2 250v capacitor back on the power supply as the one I fitted was not labelled "X2". I can remove it temporarily if need be to discount it, but the fuse was blowing with it and when it was removed.

I've checked for cr@p on the circuit board, and I found a few cut off component legs sticking to the board which shouldn't have been there, but I don't think they were causing the problem.

I've looked in the service manual how to convert to 120V AC mains, and this does appear to be set up for 240V AC - the track marked with an "x" is not cut.

You can see a picture of the current state at ...
http://www.waltersfamily.org/docs/arcamalpha.jpg

I hope that I've explained everything where I'm up to at the moment.

Best wishes,

twosheds.
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Old 21st August 2008, 05:49 PM   #23
Mooly is offline Mooly  United Kingdom
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This will turn into a marathon like the other thread. Are you saying it sits there now all powered up, nothing getting hot ( output transistors )
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Old 21st August 2008, 05:56 PM   #24
Mooly is offline Mooly  United Kingdom
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The flash from the bulb is normal. 500 ma is small for a mains fuse. Let me just say I am NOT saying increase on safety grounds but I have seen fuses fail like this on commercial equipment ( it was a Sanyo beta VCR ) and the cause was the mains filter cap and the low value of the fuse combined. We are going back years but the official cure ( Sanyo ) was to either remove the cap or up the fuse from T315ma to 1 Amp -- sorry can't remember which of those two was the official answer now. We had dozens failing at the time. No matter.
Keep a check on the temperature of the outputs and you may use the amp with the bulb in place, actually if you can do so. Make it sweat a bit -- let's see whats going on.
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Old 21st August 2008, 06:50 PM   #25
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I've cranked the volume up, and the bulb flashes quite brightly once the volume is really loud.

The heatsinks on the back are moderately hot - I'd say the output transistors are all at the same temperature.

The two hottest components are the 15v voltage regulators, but they're without heat sinks - would expect it?

It sounds very nice with the bulb in place!
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Old 21st August 2008, 09:36 PM   #26
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I've got hold of an 800mA fuse which I've substituted for the 500mA one, and I've removed the light bulb tester.

I've powered the amplifier on and off a few times, and played music for about half an hour, and the fuse does seem to be holding.

The temperature is as before - normal I suspect.

Do you think that I'll be okay letting it run like this, or should I run some tests first?

There was obviously something wrong with it to start with. Is it possible that replacing the capacitors has sorted out the problem?

It failed after some use before, so I guess I might have to let it run for a bit first to ensure that it's okay before being confident about it.
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Old 22nd August 2008, 07:07 AM   #27
Mooly is offline Mooly  United Kingdom
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Hi,
Get's a bit confusing this. The mains filter cap could have been playing up---it does happen.
You will only really know by using it. I would suspect that if the output stage's were playing up, for eg going into thermal runaway, it wouldn't just pop the fuse. It would start humming loudly through the speakers first as the current drawn keeps on increasing. That hasn't happened has it ? The same if one of the diodes in the bridge was playing up, or a PSU fault.
This sounds silly but have you checked the mains plug -- no loose fuse holder etc. Same with the fuseholder in the amp.
Hot for the output transistors means above 60 degrees C or so.
The mains fuses you replaced, could you see in them, were they just "parted" or were they blackened.
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Old 22nd August 2008, 07:58 AM   #28
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When it failed originally with the 7.5A fuse in, it just died - no audible signs of humming or anything. (According to my son I was driving the amp much harder last night with the 800mA fuse in it, and it was working fine then.)

Since that time, I have been powering it up without the speakers attached and with a 500mA fuse, and (when not using the light bulb tester) the fuse has continuously blown whether secondaries were connected or not.

The original 7.5A fuse was parted after it blew - a gap in the wire. The 500mA fuses were white in colour, not glass, so the only indication of the blow was the fuse giving a quick glow when it burned out.

The mains plug on the amplifier looks ok internally, although I do need to replace it as the screw on it is dodgey.

Actually, I am running the amp at the moment without the mains filter cap (100nf X2 250V), although I earlier said that I had put it back again. Can't remember why I removed it again, but may be I should replace it with a new one?

Last night the output transistors were lower than 60 degrees I reckon - they really didn't feel that bad.

Just one thing which is subjective - when we were first using the amp when I bought it, I thought it was getting slightly hot and thought at the time that it was probably normal. I think it was running cooler last night even when driven harder.
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Old 22nd August 2008, 09:02 AM   #29
AndrewT is offline AndrewT  Scotland
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If I'm reading these posts correctly, it appears the amp has only three changes from it's faulty condition.
1.) removed X2 interference suppression cap.
2.) changed to alternative/new smoothing caps.
3.) fuse reduced from 7.5A to T800mA.

Replace the X2 cap and check again.
I think you have found and solved the problem.

Remember use the light bulb start for EVERY modification.
Check the output voltage before attaching speakers.
Check for temperatures.
Finally, power up normally.
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Old 22nd August 2008, 10:16 AM   #30
Mooly is offline Mooly  United Kingdom
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I too suspect the filter cap. To pop a 7.5 amp fuse takes some doing, a short on the secondaries I don't think would do that.
You have just got to try it.
Are you sure it was a 7.5 amp fuse, it's a funny value is that. They usually go T1 T1.5 T2 T2.5 T3.15 T4 and T6.3amp.
We are not that far off 2 killowatts at 7.5 amp.
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