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.
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.
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.
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.
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!
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!
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Glad to see you have it sorted This is a really early Alpha amp as it has an EI transformer instead of a toroid. If you find the fuse still pops, I'd go up to T1A, but no higher. You couldn't really fit a toroid in there as a replacement. Replacing the main supply capacitors *will* have been worth it though.
The heatsinks getting quite hot is normal - as you can see they're not exaxtly substantial! In my opinion this, along with no proper VBE multiplier, was a weak point in this amp.
This is a really early Alpha amp as it has an EI transformer instead of a toroid. If you find the fuse still pops, I'd go up to T1A, but no higher. You couldn't really fit a toroid in there as a replacement. Replacing the main supply capacitors *will* have been worth it though.The heatsinks getting quite hot is normal - as you can see they're not exaxtly substantial! In my opinion this, along with no proper VBE multiplier, was a weak point in this amp.
The fuse which originally blew was definately rated at 7500mA, and had a glass case with coloured bands on it, although I seem to have misplaced it now!
I have yet to replace the 100nF X2 250V capacitor - I'll get around to that in the next week.
It does get rather hot doesn't it? It is probably running about as hot as it was when I first got it after prolonged use. Still, in the handbook it warns of this too, so I'll ignore that unless it really cooks.
Thanks for all your advice guys. I'll let it run on for a few more days before I'm totally convinced though
It's for my son, connected to his PC, who'll play mp3s and youtube videos using it (sacralidge for such a good amp I guess!)
Of course it is an orignal alpha, not an alpha plus re: my previous post - I got a digit wrong when I looked at the s/n.
Best wishes,
twosheds
I have yet to replace the 100nF X2 250V capacitor - I'll get around to that in the next week.
It does get rather hot doesn't it? It is probably running about as hot as it was when I first got it after prolonged use. Still, in the handbook it warns of this too, so I'll ignore that unless it really cooks.
Thanks for all your advice guys. I'll let it run on for a few more days before I'm totally convinced though
It's for my son, connected to his PC, who'll play mp3s and youtube videos using it (sacralidge for such a good amp I guess!)
Of course it is an orignal alpha, not an alpha plus re: my previous post - I got a digit wrong when I looked at the s/n.
Best wishes,
twosheds
Have you checked the quiescent current. With the amp hot turn the volume right down to zero, then disconnect the speakers and check as below. If it makes it easier to measure, solder a couple of insulated wires to the appropriate test points and leave the meter connected. It's a critical adjustment in the sense that if it's too high the amp will tend to run hot and may approach a thermal runaway situation.
In the first instance don't alter anything, just measure for eg from cold, and when it's red hot, but you must do it with no signal and no load attached.
Post the readings here if you want
Edit.-- A 7.5 amp fuse makes it more likely to be the cap playing up I think.
In the first instance don't alter anything, just measure for eg from cold, and when it's red hot, but you must do it with no signal and no load attached.
Post the readings here if you want
Edit.-- A 7.5 amp fuse makes it more likely to be the cap playing up I think.
Attachments
Thanks for the tip.
I havn't checked this. I need a voltmeter which goes down to mV, which I don't have, so I'll put it on the bench at work and test it next Tuesday.
Will it be safe for my son to use the amp until then do you think?
Like has happened before, there is a discrepancy. The warning which you've kindly posted tells me to set it to 8mV, whereas the schematic mentions 7mV. Will this matter?
I can see 4 x 0.22 ohm resistors, R41, R42, R141 and R142.
It says in the warning to only check one, so I guess the reading should be the same across all 4.
I havn't checked this. I need a voltmeter which goes down to mV, which I don't have, so I'll put it on the bench at work and test it next Tuesday.
Will it be safe for my son to use the amp until then do you think?
Like has happened before, there is a discrepancy. The warning which you've kindly posted tells me to set it to 8mV, whereas the schematic mentions 7mV. Will this matter?
I can see 4 x 0.22 ohm resistors, R41, R42, R141 and R142.
It says in the warning to only check one, so I guess the reading should be the same across all 4.
Hi,
You do it for each channel. Just ohms law. So it's 8 mv/0.22 = approx 36 ma. So 7 mv is about 32 ma. To be honest on this amp lower is better. Go too low and you will hear crossover distortion from the output transistors having insufficient bias. Too high and the power dissipation increases. 36 ma with a transistor that has say 35 volts across it is disipating 0.036*35=1.26watts (and there are four of them) doesn't sound much does it. Increase the current to 50 ma and it's 1.75 watt per transistor.
Thermal management is a real weak point on this amp, it's marginal at best , so you have to err on the safe side. Make sure you check it when the outputs are really hot and adjust back down to 7 or 8 mv. Don't then readjust it as the amp cools. The current will fall but thats fine as the outputs cool.
I got the figure from the manual in the other thread mentioned.
You do it for each channel. Just ohms law. So it's 8 mv/0.22 = approx 36 ma. So 7 mv is about 32 ma. To be honest on this amp lower is better. Go too low and you will hear crossover distortion from the output transistors having insufficient bias. Too high and the power dissipation increases. 36 ma with a transistor that has say 35 volts across it is disipating 0.036*35=1.26watts (and there are four of them) doesn't sound much does it. Increase the current to 50 ma and it's 1.75 watt per transistor.
Thermal management is a real weak point on this amp, it's marginal at best , so you have to err on the safe side. Make sure you check it when the outputs are really hot and adjust back down to 7 or 8 mv. Don't then readjust it as the amp cools. The current will fall but thats fine as the outputs cool.
I got the figure from the manual in the other thread mentioned.
Last post of the day Look at the circuit. The 0.22 ohms are in series effectively. You may find it easier to measure across the two together, that is to say from the emmiters of both output transistors. In that case you are looking for 16 mv (8 mv across each) but that should be do-able on an AVO and easier to see.
Adjust the pot slowly, and remember that as the outputs get hot the reading will change. It's not an exact thing, just make sure that with the amp as hot as it gets it's no higher than 8mv across each resistor.
Double checked the AVO manual. 100 mv FSD use the 50 microamp range. Steady hands required, have fun
Look at the circuit. The 0.22 ohms are in series effectively. You may find it easier to measure across the two together, that is to say from the emmiters of both output transistors. In that case you are looking for 16 mv (8 mv across each) but that should be do-able on an AVO and easier to see.Adjust the pot slowly, and remember that as the outputs get hot the reading will change. It's not an exact thing, just make sure that with the amp as hot as it gets it's no higher than 8mv across each resistor.
Double checked the AVO manual. 100 mv FSD use the 50 microamp range. Steady hands required, have fun
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