The parts are on order. I am replacing all the ones suggested, mosfets, Q3, Q4, Q5 and Q14. It is not clear in the service manual (at least to me 
which type of fuse is specified, slo or fast blo, so I ordered the fast to be safe.
The fuses are ordered from an Ebay ad from Hong Kong, 10 for a dollar. The local "The Source" in the nearest mall charges $5.00 each and they didn't have the correct amperage anyway. Now there's a healthy markup for you (or rather for them.
which type of fuse is specified, slo or fast blo, so I ordered the fast to be safe. The fuses are ordered from an Ebay ad from Hong Kong, 10 for a dollar. The local "The Source" in the nearest mall charges $5.00 each and they didn't have the correct amperage anyway. Now there's a healthy markup for you (or rather for them
.
If it is the one to the mains trx it will blow pretty much every time I would say. Mains transformers draw huge currents for a fraction of a second untill the magnetising field within the core is established.
The parts finally arrived. I installed them all. I hooked up a 60watt bulb across the fuse terminals. I turned the trimpots all the way anti-clockwise. I hooked up my meter across R101 as the service manual says. Plugged in the power cord. Press the power button. And the moment of truth...
The bulb glows brightly for a second, then goes out. Was it supposed to glow at all? I don't know. At least it doesn't seem to have blown the outputs again. The power LED on the amp glows orange for a few seconds, then green, exactly as it should, indicating normal operation. The power relay makes a soft clunk. Just as it always does when powered on. Everything looks like it is working as it should. Now I am supposed to adjust the trimpot to read between 2.3 and 2.8 mv across R101 so I very slowly turn it. No response on the meter. Adjust meter setting downward. My meter only goes down to 1 volt! How can I read 2 millivolts with this meter? It only has DC volt settings of 300up, 100, 30, 10, 3 and 1.
I could turn the trimpot back to where it was (I had made a little mark) and take a chance but that's probably foolish.
So here is the situation; The amplifier seems to be working but I can't set the trimpot to the proper voltage. Maybe there is a way to put something else in the circuit between the test probes that will change the scale of the meter? Probably not accurate enough. I don't know. Kinda stumped here...
The bulb glows brightly for a second, then goes out. Was it supposed to glow at all? I don't know. At least it doesn't seem to have blown the outputs again. The power LED on the amp glows orange for a few seconds, then green, exactly as it should, indicating normal operation. The power relay makes a soft clunk. Just as it always does when powered on. Everything looks like it is working as it should. Now I am supposed to adjust the trimpot to read between 2.3 and 2.8 mv across R101 so I very slowly turn it. No response on the meter. Adjust meter setting downward. My meter only goes down to 1 volt! How can I read 2 millivolts with this meter? It only has DC volt settings of 300up, 100, 30, 10, 3 and 1.
I could turn the trimpot back to where it was (I had made a little mark) and take a chance but that's probably foolish.
So here is the situation; The amplifier seems to be working but I can't set the trimpot to the proper voltage. Maybe there is a way to put something else in the circuit between the test probes that will change the scale of the meter? Probably not accurate enough. I don't know. Kinda stumped here...
I dont know about Canada, but here in the UK, and im sure elsewhere in the world you can pick up a cheap digital multimeter for around £5 that does DC volts on 200mV 2V 20V 200V up to 600V, AC volts much the same, resistance, current etc. This will do the job. Something like this:
http://www.rapidonline.com/netalogue/zoomed/Large/M031042W01.jpg
http://www.rapidonline.com/netalogue/zoomed/Large/M031042W01.jpg
You need a suitable meter.
With the bulb still in place and the amp on just confirm the DC offset at the speaker terminals is essentially zero volts DC.
If you very slowly turn the bias pot up, the bulb should begin to glow a little. If it's OK then turn it back down again. That would confirm the bias circuit is OK.
But you still need a meter.
With the bulb still in place and the amp on just confirm the DC offset at the speaker terminals is essentially zero volts DC.
If you very slowly turn the bias pot up, the bulb should begin to glow a little. If it's OK then turn it back down again. That would confirm the bias circuit is OK.
But you still need a meter.
After looking more exactly to the schematic and adjustment instructions of this amp I am confused concerning ARCAM's recommend idle current of only 30mA through the HEX-FET's of quasi complementary output buffer.
This I have never see before by a commercial amplifier.
Are there very special front end features in the topology, that allows such low idle current values for MOSFET's (naturally only used by BjT power devices) ?
I cannot find such features.
BTW - without oscilloscope and rule out any oscillation you cannot say nothing about the working of this amplifier.
Check out also the clipping behaviour and gate current flow in both MOSFET's without and with clipping.
This I have never see before by a commercial amplifier.
Are there very special front end features in the topology, that allows such low idle current values for MOSFET's (naturally only used by BjT power devices) ?
I cannot find such features.
BTW - without oscilloscope and rule out any oscillation you cannot say nothing about the working of this amplifier.
Check out also the clipping behaviour and gate current flow in both MOSFET's without and with clipping.
OK I've ordered a meter. Another 2-3 weeks wait. Unfortunately the lowest range on it will be 0-200 mv so I don't know how accurate it will be down at the bottom end of the range but maybe it's not that critical? I was unable to find a reasonably priced one with a range of 0-20mv or even better 0-5mv. Hopefully high precision is not needed. Thanks for the pic Jaycee, I used it as a guide for ordering.
200mV range will be plenty for setting bias. It doesnt have to be EXACTLY 30mA, just close enough.
tiefbassuebertr, i dont think there is anything really special, but these amps do have pretty small heatsinks so there is not a lot of dissipation for high bias currents. Having said that, they sound pretty good to me with only this "low" bias.
tiefbassuebertr, i dont think there is anything really special, but these amps do have pretty small heatsinks so there is not a lot of dissipation for high bias currents. Having said that, they sound pretty good to me with only this "low" bias.
Hi again Mooly and jaycee. My meter finally arrived and setting the bias was a piece of cake. I removed the bulb, hooked up speakers and with minor trepidation turned it on. It works perfectly! Thanks very very much to you both for all your help. I really would like to express my appreciation. It was a great learning experience as well. The cost of having an electronics shop do the repair would no doubt have been prohibitive so thanks to you for saving this excellent amplifier.
Resurrecting an old thread but this was what drew me to the forum in the first place.
I am completely new to electronics but of sound (ish) mind and judgement and facing the same issue as the IP. I am going to follow all the advice above but am struggling on the initial parts ordering.
When i go to RS components for example and type in IRF540 I get a few option (10 to be exact). They differ as far as I can tell in ampage, is this just a tollerance? Can I use any of these? I am open to any and all suggestions.
Thanks advance for your help. Reading this post I'm amazed by the time some have put in on this forum to help others.
I am completely new to electronics but of sound (ish) mind and judgement and facing the same issue as the IP. I am going to follow all the advice above but am struggling on the initial parts ordering.
When i go to RS components for example and type in IRF540 I get a few option (10 to be exact). They differ as far as I can tell in ampage, is this just a tollerance? Can I use any of these? I am open to any and all suggestions.
Thanks advance for your help. Reading this post I'm amazed by the time some have put in on this forum to help others.
IRF540
Half of the RS listed types I saw as UK stock were SMD types so you can forget them but the rest (TO220) were from
different manufacturers and only had small detail variations. Pick the best price one (watch out for min. quantities) that
matches brands to your existing ones and pair them up if you are buying as a complement to IRF 9540, for example.
Also try CPC for a better retail deal.
Half of the RS listed types I saw as UK stock were SMD types so you can forget them but the rest (TO220) were from
different manufacturers and only had small detail variations. Pick the best price one (watch out for min. quantities) that
matches brands to your existing ones and pair them up if you are buying as a complement to IRF 9540, for example.
Also try CPC for a better retail deal.
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