Roger! Thanks for the tips.
R416 and R417 aprox 0.3
Q403 - 2sc2229 is new
Q404 - 2sc1740 rd is faulty - bought 2sc2240 gr 3c
Q406 - 2sa965 0 seams ok - bought 2sa1013 y
Q405 - 2sc2235 - i have a new one
Q407 & q408 seams ok.
Can u confirm me if replacements that seller recommended me are suitable ? Ty. And the point for the bulb to connect pls.to be safe.
Thanks !
R416 and R417 aprox 0.3
Q403 - 2sc2229 is new
Q404 - 2sc1740 rd is faulty - bought 2sc2240 gr 3c
Q406 - 2sa965 0 seams ok - bought 2sa1013 y
Q405 - 2sc2235 - i have a new one
Q407 & q408 seams ok.
Can u confirm me if replacements that seller recommended me are suitable ? Ty. And the point for the bulb to connect pls.to be safe.
Thanks !
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2SC2240 should be fine for Q404.
2SA1013 should be OK for the driver.
The bulb goes in series with the live mains supply to the amp. One easy connection point (but its up to you to make sure you are safe with the connections) is to remove the mains fuse from the amp and to solder a bulb across the fuse holder. So the bulb takes the place of the fuse.
Powering Your Radio Safely with a Dim-bulb Tester
Also turn the 330 ohm bias preset to MAXIMUM resistance before switching on. This will set the bias current to minimum. So the preset needs to be turned to make it appear as a 330 ohm resistor.
2SA1013 should be OK for the driver.
The bulb goes in series with the live mains supply to the amp. One easy connection point (but its up to you to make sure you are safe with the connections) is to remove the mains fuse from the amp and to solder a bulb across the fuse holder. So the bulb takes the place of the fuse.
Powering Your Radio Safely with a Dim-bulb Tester
Also turn the 330 ohm bias preset to MAXIMUM resistance before switching on. This will set the bias current to minimum. So the preset needs to be turned to make it appear as a 330 ohm resistor.
Thanks ! Back with result, which is very good i think.
1. Checked all resistor to be sure everything is ok (values are usually a little bit lower)
2. Checked q401/q402
3. Replaced q403,404,405,406 with same or equivalents from above
4. Replaced 1A fuse with 75w bulb
5. Powered up
Result :
1. Bulb lighted shortly when started up then went off - removed bulb and started to measure
2. Voltage on output (aprox 0.01-0.04v on both channels)
3. Made a test with signal and a apeaker on both channels (low volume cause radiator for q407 and q408 is not mounted) - both channels were up and running.
4. I made a few measurements because it seems to be a difference in voltages between the two channels:
ECB
outputs : q407a -0.04/41.2/0.43 q407b +0.25/41.1/0.4
q408a -0.04/-41.3/-0.04 q408b -0.04/41.3/-0.04
q405a 0.04/41.3/1.0 q405b 0.04/41.3/0.9
q406a -0.4/...../-1.0 q406b -0.4/...../-1.0
where "a" was the problem channel
q407a has the collector pin broken and soldered (don't know if it's a problem but i'll change the outputs with originals or some more suitable when i find some).
Another possible problems:
1. main voltage - 41V ? isn't too high ?
2. when powering off , voltage (aprox 10v and slowly dropping) appears on both channels.
1. Checked all resistor to be sure everything is ok (values are usually a little bit lower)
2. Checked q401/q402
3. Replaced q403,404,405,406 with same or equivalents from above
4. Replaced 1A fuse with 75w bulb
5. Powered up
Result :
1. Bulb lighted shortly when started up then went off - removed bulb and started to measure
2. Voltage on output (aprox 0.01-0.04v on both channels)
3. Made a test with signal and a apeaker on both channels (low volume cause radiator for q407 and q408 is not mounted) - both channels were up and running.
4. I made a few measurements because it seems to be a difference in voltages between the two channels:
ECB
outputs : q407a -0.04/41.2/0.43 q407b +0.25/41.1/0.4
q408a -0.04/-41.3/-0.04 q408b -0.04/41.3/-0.04
q405a 0.04/41.3/1.0 q405b 0.04/41.3/0.9
q406a -0.4/...../-1.0 q406b -0.4/...../-1.0
where "a" was the problem channel
q407a has the collector pin broken and soldered (don't know if it's a problem but i'll change the outputs with originals or some more suitable when i find some).
Another possible problems:
1. main voltage - 41V ? isn't too high ?
2. when powering off , voltage (aprox 10v and slowly dropping) appears on both channels.
That all sounds basically good
The two important checks are:
1/ That there is less than 100 millivolts across the speaker terminals. It will probably be a lot lower than this but 100mv would be an upper limit.
2/ That the bias current of the amp adjusts OK. This is set with NO speakers or signal. I would aim to get this at around 6 to 8 millivolts as measured across either of the 0.33 ohm resistors. That will give a current of around 25 milliamps at most.
Seeing a falling voltage at the output is normal and is caused by the supply rails falling as the reservoir capacitors discharge. That causes the amp to develop a high DC offset as it 'loses control' due to the low supply voltage. Once a speaker is connected that voltage will fall very quickly.
Normal mains variations won't cause a problem, 41v is fine and would fall anyway when the amp was under load.
The two important checks are:
1/ That there is less than 100 millivolts across the speaker terminals. It will probably be a lot lower than this but 100mv would be an upper limit.
2/ That the bias current of the amp adjusts OK. This is set with NO speakers or signal. I would aim to get this at around 6 to 8 millivolts as measured across either of the 0.33 ohm resistors. That will give a current of around 25 milliamps at most.
Seeing a falling voltage at the output is normal and is caused by the supply rails falling as the reservoir capacitors discharge. That causes the amp to develop a high DC offset as it 'loses control' due to the low supply voltage. Once a speaker is connected that voltage will fall very quickly.
Normal mains variations won't cause a problem, 41v is fine and would fall anyway when the amp was under load.
Ok. I've switched to sb688.
I got -50mv and 60. On the good channel i got down to 0.01 from vr 401.
On the problem channel i cannot go lower than -48mv from vr401.
I don't understand where is that"jumper lead" they say into service manual to make the "idle adjustment" as they say.
Apperently everything is ok - tested 1/2 h on a low volume - my child is sleeping.
The only thing that worries me is that q403,404 and 406 are getting hot (on the problem channel). U cannot keep finger on more than 7-8 secs. Q405 has same values as the brother on the other channel. But it's hot. How can it be ?
I got -50mv and 60. On the good channel i got down to 0.01 from vr 401.
On the problem channel i cannot go lower than -48mv from vr401.
I don't understand where is that"jumper lead" they say into service manual to make the "idle adjustment" as they say.
Apperently everything is ok - tested 1/2 h on a low volume - my child is sleeping.
The only thing that worries me is that q403,404 and 406 are getting hot (on the problem channel). U cannot keep finger on more than 7-8 secs. Q405 has same values as the brother on the other channel. But it's hot. How can it be ?
A small DC offset won't make anything run hot and DC offset is what VR401 controls. Forget that for the moment.
Have you checked and adjusted the bias current using VR402? You set this by measuring the voltage across either of the 0.33 ohm.
What voltage do you see across these resistors? (no speakers connected for this)
I haven't got the manual in front of me, only the circuit, but the bias should be set to give around 7 millivolts (a safe working value). If it won't turn down that low then try increasing R410 from 680 ohm to either 820 ohm or 1k.
Have you checked to make sure the amp isn't oscillating?
Q403 will run pretty hot and should be around the same temperature as the one in the good channel.
Have you checked and adjusted the bias current using VR402? You set this by measuring the voltage across either of the 0.33 ohm.
What voltage do you see across these resistors? (no speakers connected for this)
I haven't got the manual in front of me, only the circuit, but the bias should be set to give around 7 millivolts (a safe working value). If it won't turn down that low then try increasing R410 from 680 ohm to either 820 ohm or 1k.
Have you checked to make sure the amp isn't oscillating?
Q403 will run pretty hot and should be around the same temperature as the one in the good channel.
Sorry for my late answer. I was away for a few days.
I don't know how to check if it's oscillating cause i don't have the tools needed for that (oscilloscope). On the other channel transistors are not heating. I will follow your recommandations this evening and i'll come back with results. Thanks for your patience. Beside the fact that we are so close to the finish, i've learned a lot of stuff with this occasion, and i really like that !
I don't know how to check if it's oscillating cause i don't have the tools needed for that (oscilloscope). On the other channel transistors are not heating. I will follow your recommandations this evening and i'll come back with results. Thanks for your patience. Beside the fact that we are so close to the finish, i've learned a lot of stuff with this occasion, and i really like that !
Hello Mooly!
I'm back again on my L3. I've set bias voltage to aprox 6-6.5v as you said on both channels. Voltage on outputs are 1mv and -38mv(on problem channel). Everything seems ok and sounds ok except the fact that even if I run it at max 10% power, on the problem channel q405,q406 and q403 are running hot. I cannot keep my finger on them more than 2 sec. On the other channel only q403 is a little hot but not like that. Are there any measurements that I can make or should I leave it this way? If bias voltage is ok I presume that we can forget about oscillation, am I right? Thank you again for your time.
I'm back again on my L3. I've set bias voltage to aprox 6-6.5v as you said on both channels. Voltage on outputs are 1mv and -38mv(on problem channel). Everything seems ok and sounds ok except the fact that even if I run it at max 10% power, on the problem channel q405,q406 and q403 are running hot. I cannot keep my finger on them more than 2 sec. On the other channel only q403 is a little hot but not like that. Are there any measurements that I can make or should I leave it this way? If bias voltage is ok I presume that we can forget about oscillation, am I right? Thank you again for your time.
A long time has past 
so I've just quickly skimmed back on the thread.
Given that its basically working OK but also running hot does make me suspect oscillation... and yes, you do need a scope to check for that.
Bias should be adjusted as per post #70 where you are looking for around 7 millivolts (so very small value) across the 0.33 ohm resistors. Have no speakers connected while adjusting.
That leaves the possible stability issues. I would definitely keep using the bulb tester while working on this.
Two things we can try.
1/ Try adding a small cap across R409. Something like 10pF to 22pF (pico farads) but no higher than that.
2/ If that doesn't do anything then we could look at altering C406 to a slightly higher value.
You really do need a scope though for this kind of work because we are just guessing.
so I've just quickly skimmed back on the thread.Given that its basically working OK but also running hot does make me suspect oscillation... and yes, you do need a scope to check for that.
Bias should be adjusted as per post #70 where you are looking for around 7 millivolts (so very small value) across the 0.33 ohm resistors. Have no speakers connected while adjusting.
That leaves the possible stability issues. I would definitely keep using the bulb tester while working on this.
Two things we can try.
1/ Try adding a small cap across R409. Something like 10pF to 22pF (pico farads) but no higher than that.
2/ If that doesn't do anything then we could look at altering C406 to a slightly higher value.
You really do need a scope though for this kind of work because we are just guessing.
That level of bias is perfect, however you still have to find out why those driver transistors are running so hot.
If they were passing the same level of DC as in the other channel then they would be at the same temperature. The fact they are so hot suggests something else is going on such as instability. That is where the caps come in, but without a scope you are working blind.
If they were passing the same level of DC as in the other channel then they would be at the same temperature. The fact they are so hot suggests something else is going on such as instability. That is where the caps come in, but without a scope you are working blind.
That would be great
And remember, the oscilloscope is really just a voltmeter that traces out the voltage plotted against time. So checking for oscillation is fairly straightforward with a scope. An analogue scope or a good digital scope is probably best, a cheap USB type wouldn't respond to the frequencies involved, which can be many Mhz.
And remember, the oscilloscope is really just a voltmeter that traces out the voltage plotted against time. So checking for oscillation is fairly straightforward with a scope. An analogue scope or a good digital scope is probably best, a cheap USB type wouldn't respond to the frequencies involved, which can be many Mhz.
Haven't got luck until now with oscillo.
I've tried to find out how important is hfe - dc gain on transistors.
Cannot find the answer on what is happening if u replace a transistor with abother one with identical name but with different specification. In my case all 4 transistors q403,404,405,406 are replaced with others with same name but with bigger hfe.
I've tried to find out how important is hfe - dc gain on transistors.
Cannot find the answer on what is happening if u replace a transistor with abother one with identical name but with different specification. In my case all 4 transistors q403,404,405,406 are replaced with others with same name but with bigger hfe.
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