ok, i'm confused. on the first page of schematic, it shows Qe7 as a round-ish transistor with a small tab stuck out of the casing. on the board, Qe7 looks identical to the two driver transistors the 5401 5551. post production change? solder looks pristine
aidan
aidan
Probably just a change in production.
I actually think that your amp is fine and working OK at this point and that all we need to do is to tweak one resistor value to get the bias in range.
I'm just asking you to measure these things to confirm and prove its all OK.
I actually think that your amp is fine and working OK at this point and that all we need to do is to tweak one resistor value to get the bias in range.
I'm just asking you to measure these things to confirm and prove its all OK.
well, that's good news! in the service manual, Qe7 is supposed to be 2SA620WB, but the transistor on the board is marked A721 T69 . sound legit?
aidan
aidan
Its sure to be fine. Lots of manufacturers changed device types in production and for different markets.
Emitter is the top one with the arrow. Collector is the bottom one.
Emitter is the top one with the arrow. Collector is the bottom one.
1.9 volts as a maximum ? and turning the pot reduces that ?
That sounds OK but not quite high enough... I'm just writing something down for you 🙂
That sounds OK but not quite high enough... I'm just writing something down for you 🙂
I'll have to leave it for a while shortly.
I think you'll find that voltage (across Qe7) varies smoothly with the bias preset, the problem is that its just not going quite high enough to turn the outputs on. That is because of the slightly different characteristics of the new drivers.
Assuming that the voltage does vary smoothly this is what we do. And we use the bulb tester while sorting this bias issue.
1/ Firstly make sure that the good channel is turned back down to zero bias.
2/ Turn the repaired channels bias preset back so that it is as a 1 k resistor. Remember that maximum resistance equals minimum bias current.
3/ Look at the circuit. Do you see Re27, a 1.8k that is in series with the bias preset. We need to reduce the value of that resistor by a small amount. Now going down to a 1.5k may be to much of a jump so I would suggest adding a 22k across it (making 1.66k) and again seeing if the bias now adjusts. That 1.9 volts you measured should now go a little higher.
Its trial and error. The bulb tester and variac make it safe for the amp so keep reducing the value of that 1.8k either by adding resistors across it or if it gets to say 1.5k then by replacing it.
I think you will need to see around 2.4 to 2.5 volts across Qe7 before the output transistors start to conduct.
I know you want to try the amp. Providing it is just a low bias issue we have at the moment then the amp should work correctly. You could try it with headphones and if that was OK a speaker BUT KEEP THE LEVEL VERY LOW because the bulb will limit current and cause the rails to collapse which could cause the output to suddenly latch to one or other rail. Not good 😀
And connect the speaker AFTER the amp is powered up because again the bulb will limit current and not allow the amp to stabilise.
I think you'll find that voltage (across Qe7) varies smoothly with the bias preset, the problem is that its just not going quite high enough to turn the outputs on. That is because of the slightly different characteristics of the new drivers.
Assuming that the voltage does vary smoothly this is what we do. And we use the bulb tester while sorting this bias issue.
1/ Firstly make sure that the good channel is turned back down to zero bias.
2/ Turn the repaired channels bias preset back so that it is as a 1 k resistor. Remember that maximum resistance equals minimum bias current.
3/ Look at the circuit. Do you see Re27, a 1.8k that is in series with the bias preset. We need to reduce the value of that resistor by a small amount. Now going down to a 1.5k may be to much of a jump so I would suggest adding a 22k across it (making 1.66k) and again seeing if the bias now adjusts. That 1.9 volts you measured should now go a little higher.
Its trial and error. The bulb tester and variac make it safe for the amp so keep reducing the value of that 1.8k either by adding resistors across it or if it gets to say 1.5k then by replacing it.
I think you will need to see around 2.4 to 2.5 volts across Qe7 before the output transistors start to conduct.
I know you want to try the amp. Providing it is just a low bias issue we have at the moment then the amp should work correctly. You could try it with headphones and if that was OK a speaker BUT KEEP THE LEVEL VERY LOW because the bulb will limit current and cause the rails to collapse which could cause the output to suddenly latch to one or other rail. Not good 😀
And connect the speaker AFTER the amp is powered up because again the bulb will limit current and not allow the amp to stabilise.
That's surprising.
Lets jump in at the deep end and test the amp with heaphones/speaker as I outlined above.
Lets see where its all at.
ok mooly, thanks very much. that should keep me busy. i have a 2.5k pot here i'll wire in and see what happens.
have a good day sir. will report back tomorrow morning with the results.
best,
aidan
have a good day sir. will report back tomorrow morning with the results.
best,
aidan
well, good news. it works, AND, funny but now the bias adjusts the C-E of Qe7 from 1.7 to 2.3 Vdc, but, no reading across bias, i.e. emitter of Qe23 to emitter of Qe21. also, left channel distorted. 🙁
i meant as per the manual, adjusting the 1K bias pot does not affect the bias reading across Re47 to Re45
you have to go, mooly. i'll talk to you later sir.
we made a great deal of progress. THANK YOU!
aidan
we made a great deal of progress. THANK YOU!
aidan
Excellent 🙂
Now unless something is drastically wrong its distorted simply because there is no bias, you are hearing crossover distortion.
2.3 volts I think is right on the edge of where it will set up and adjust so turn it all off and turn the bias back down again (always start with it on mimimum) and try fractionally reducing that 1K8 resistor.
Go back to having NO speakers attached again and then see if it now adjusts.
Now unless something is drastically wrong its distorted simply because there is no bias, you are hearing crossover distortion.
2.3 volts I think is right on the edge of where it will set up and adjust so turn it all off and turn the bias back down again (always start with it on mimimum) and try fractionally reducing that 1K8 resistor.
Go back to having NO speakers attached again and then see if it now adjusts.
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