What's the voltage drop across R635 now? Seems like your new Q615 doesn't do a thing. You did test it, right?
And are you sure you didn't lift a trace on the board or something? Verify Q615 does in fact connect to all the points it should. You can try adjusting DC offset back up to where it was and check again, maybe that's the cause.
Also, how can the bases of Q631 and Q633 have differing voltages if they are (or at least should be!) directly connected? Bad solder joint?
BTW, part numbers on the other channel should be offset by either +1 or -1. Shouldn't be too hard to find out.
As a rule of thumb, if the amplifier seems to work (sound) fine save for an offset, chances are that loop gain still is high enough and thus the offset can pretty much only come from the input. You have confirmed this by your measurements. I'm not sure why you're messing around with the VAS / driver stages.
And are you sure you didn't lift a trace on the board or something? Verify Q615 does in fact connect to all the points it should. You can try adjusting DC offset back up to where it was and check again, maybe that's the cause.
Also, how can the bases of Q631 and Q633 have differing voltages if they are (or at least should be!) directly connected? Bad solder joint?
BTW, part numbers on the other channel should be offset by either +1 or -1. Shouldn't be too hard to find out.
As a rule of thumb, if the amplifier seems to work (sound) fine save for an offset, chances are that loop gain still is high enough and thus the offset can pretty much only come from the input. You have confirmed this by your measurements. I'm not sure why you're messing around with the VAS / driver stages.
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I think we should sort out this quoting stuff. If you want to split a quote into multiple sections, you insert a "/QUOTE" (in square brackets obviously). For the next section, copy/paste the first "QUOTE=author;postnum" (again with square brackets).
BTW, ideally Q613 and Q615 should be the same, as the operation of a current mirror hinges on identical transistor geometries. Voltage drops across R629 and R635 should be compared. From what I recall you wrote that voltages across R673 and R675 were the same, so I didn't suspect anything wrong with the current mirror.
What did hFE measurement say for your dead Q615? The settings for pnps hopefully differed from those for npns?
Either this, or each end to whatever constant potential you like, in which case the difference will be the same.
BTW, ideally Q613 and Q615 should be the same, as the operation of a current mirror hinges on identical transistor geometries. Voltage drops across R629 and R635 should be compared. From what I recall you wrote that voltages across R673 and R675 were the same, so I didn't suspect anything wrong with the current mirror.
What did hFE measurement say for your dead Q615? The settings for pnps hopefully differed from those for npns?
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I have never been good in quoting, so I thought I do it with different colors...
R635 voltage drop is 3,6 volt.
Q613 and Q615 has been now both exchanged.
hFE measurement for dead Q615 has been not possible as legs were to short to read in the multimeter. But I have been able to measure hfe from Q621.
This is more then 200. I exchanged this now also, as I am not sure if my multimeter shows hfe that this speaks for a working transistor...
I measured the whole bag 2SB631 which I received from Reichelt. They are all around 120. Is this too less? I don't know...
When I put music in the amp it plays, but there is a lot of humm now. DC offset reads more then 2 volt now after the changes I made! Dc offset is possible to adjust between 2-3 Volts
Thats because voltages drift even after the amp is running for more then 10 minutes. Solder joints have been checked.
With or without a load connected? At least you can't say your new 2SB631 is doing nothing. Current does, however, seem to be almost twice as high as it should be (and actually was) before you swapped Q615.
What about R629? It almost seems like the differential amp is extremely unbalanced.
How did you find it to be dead then? :?
As I understood it, you determined that in hFE measurement?!
Seems like a normal value for a medium-power transistor.
Doesn't mean it can't be substantially different from the original 2SB631s. If, for example, the new ones are substantially slower in terms of fT, stability may suffer. The originals were quite speedy beasts at 110 MHz fT, without too much beta droop. Old original Philips BD140s may work there (fT 160 MHz), the generic ones you get these days may not (apparently about 50 MHz only, a specification is carefully avoided in datasheets). The specs for your ISC 2SB631s are as good as for the originals, but that doesn't necessarily mean the parts actually are.
Congratulations, you have successfully misimproved the poor thing. Let's hope it's not oscillating now.When I put music in the amp it plays, but there is a lot of humm now. DC offset reads more then 2 volt now after the changes I made! Dc offset is possible to adjust between 2-3 Volts
That's exactly the kind of thing that I feared might happen.
Now will you try a new pair of 2SA970s for Q603/605 please? If that doesn't get things back in order, swap in the old 2SB631s again.
That doesn't make a difference
It measured 100R
I simply replaced the transistor, and the I was able to adjust the currents again
Thanks for this info.
After I put in a new pair the amp is playing music again
I also measured voltages on the other channel then. Found out that one transistor was also broken. After replacing both of the pair also this channel is playing music now
It was possible to adjust the Vr601 and VR602 that my multimeter reads 2,5mv
But adjusting DC offset is still not possibel: about 180mv on both channels
Here are the voltages from the power transistors:
Q631 Base 178mv Collector -33,4 Emitter -362mv
Q633 Base 181mv Collector -33,5 Emitter -361mv
Q621 Base -0,9 Collector 1,3 Emitter -331mv
Q629 Base 185mv Collector 33,2 Emitter 0,7
Q627 Base 185mv Collector 33,2 Emitter 0,73
Q628 Base 229mv Collector 33,2 Emitter 0,8
Q630 Base 228M Collector 33 Emitter 0,8
Q622 Base -0,87 Collector 1,37 Emitter -242mv
Q634 Base 218mv Collector -33,2 Emitter -300mv
Q632 Base 219mv Collector 33,4 Emitter -299mv
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more transistor voltages:
Q601
Emitter 32,4v
Collector 0,7V
Base 31,7v
Q607
Emitter 32,5v
Collector -9,4V
Base 31,9v
Q613
Emitter 32,3v
Collector 31,8V
Base 31,8V
Q615
Emitter 32,5v
Collector 1,44V
Base 31,7V
Q623
Emitter 0,83v
Collector 34,1V
Base 1,43V
Q603
Emitter 0,57v
Collector -10,8V
Base 23,6mv
Q605
Emitter 0,63v
Collector -10,9V
Base 97mv
Q609
Emitter -10,5v
Collector -31,8V
Base -11V
Q611
Emitter -10,6v
Collector -31,8V
Base -11V
Q617
Emitter -32,4v
Collector 22,4V
Base -31,9V
Q619
Emitter -32,2v
Collector -0,89V
Base -31,7V
Q625
Emitter -316mv
Collector -34V
Base -0,89V
To me they all look ok. Also when I compare them with the voltages in the schematic. But B1 and -B1 Voltages are 7 Volt less then they should be (as stated in the beginning somewhere)
Is it possible that the transformer is faulty? But still the amp should work ok with less voltage as also stated before...
Hmm....
Q601
Emitter 32,4v
Collector 0,7V
Base 31,7v
Q607
Emitter 32,5v
Collector -9,4V
Base 31,9v
Q613
Emitter 32,3v
Collector 31,8V
Base 31,8V
Q615
Emitter 32,5v
Collector 1,44V
Base 31,7V
Q623
Emitter 0,83v
Collector 34,1V
Base 1,43V
Q603
Emitter 0,57v
Collector -10,8V
Base 23,6mv
Q605
Emitter 0,63v
Collector -10,9V
Base 97mv
Q609
Emitter -10,5v
Collector -31,8V
Base -11V
Q611
Emitter -10,6v
Collector -31,8V
Base -11V
Q617
Emitter -32,4v
Collector 22,4V
Base -31,9V
Q619
Emitter -32,2v
Collector -0,89V
Base -31,7V
Q625
Emitter -316mv
Collector -34V
Base -0,89V
To me they all look ok. Also when I compare them with the voltages in the schematic. But B1 and -B1 Voltages are 7 Volt less then they should be (as stated in the beginning somewhere)
Is it possible that the transformer is faulty? But still the amp should work ok with less voltage as also stated before...
Hmm....
Ok, I fiddled around now long enough, and also learned something.
I will exchange all parts in the input stage. Especially these horrible trimpots!
If then the offset disappear fine, if not I put in into ebay.
By looking again through the voltages from the transistors:
Q623
Emitter 0,83v
Collector 34,1V
Base 1,43V - this seems a bit high. In the schematic its only 0,54 volt.
I did change Q623 to new one, no difference
Q625
Emitter -316mv
Collector -34V
Base -0,89V - seems also a bit high but closer to 0,54 volt from the schematic.
Also I did measure all resistors around and capacitors, they look fine....
Q623
Emitter 0,83v
Collector 34,1V
Base 1,43V - this seems a bit high. In the schematic its only 0,54 volt.
I did change Q623 to new one, no difference
Q625
Emitter -316mv
Collector -34V
Base -0,89V - seems also a bit high but closer to 0,54 volt from the schematic.
Also I did measure all resistors around and capacitors, they look fine....
In a DC-coupled amplifier everything is connected. Distinguishing between cause and effect can be very difficult.
About the only thing that's reliable is a base-emitter voltage drop between 0.5 and 0.7 V. In order to find out what you should get at the drivers, you must work backwards from the output. (Besides, the +/- 0.53 V indications on the schematic are in the wrong spot.) Plus, as long as the amplifier works halfway properly, it'll have a lot of open-loop gain and be input-dominated. (Zum Thema Open-Loop-Verstärkung...)
First of all, your supply voltages seem rather low now. Lower than they were in the beginning, actually. (IIRC they were about +/-37 V.) There may be some oscillation going on - do you think you can get a scope somewhere? Does the amplifier get unusually warm? How much AC voltage do you measure between the two fuses? Which DC levels do you measure on R901/902?
Voltage offset at Q603 base looks better now. It's not as low as I'd like to see, but a lot lower than it was. (I think the rest is from output DC offset, which should contribute about 19 mV on its own via the feedback. It's an inverting amp...) About 60 mV of difference between Q603/605 also lines up with simulation.
And you say you still can't adjust DC offset properly? What kind of range do you get? What is the minimum resistance you can obtain between the wiper and each end of VR603? Maybe it's just a dirty pot that's playing tricks on you.
I did a bit of simulation again. It could be that the kind of voltages you saw on Q605's base (90-97 mV) are indeed too high to be compensated via the DC offset adjustment. (BTW, an extra 25 mV already results from having 5 megohms(!) in parallel to Q605's B-E junction. The circuit board better be clean as a whistle, with no flux residue in sight.) You may have to decrease R671 in value, to about 180 kOhms.
Seems like a bit of a hack, but it's the best idea I can offer for now.
Quite possibly the value of R671 was chosen empirically at the time, with all-new transistors and little reserve left for aging.
About the only thing that's reliable is a base-emitter voltage drop between 0.5 and 0.7 V. In order to find out what you should get at the drivers, you must work backwards from the output. (Besides, the +/- 0.53 V indications on the schematic are in the wrong spot.) Plus, as long as the amplifier works halfway properly, it'll have a lot of open-loop gain and be input-dominated. (Zum Thema Open-Loop-Verstärkung...)
First of all, your supply voltages seem rather low now. Lower than they were in the beginning, actually. (IIRC they were about +/-37 V.) There may be some oscillation going on - do you think you can get a scope somewhere? Does the amplifier get unusually warm? How much AC voltage do you measure between the two fuses? Which DC levels do you measure on R901/902?
Voltage offset at Q603 base looks better now. It's not as low as I'd like to see, but a lot lower than it was. (I think the rest is from output DC offset, which should contribute about 19 mV on its own via the feedback. It's an inverting amp...) About 60 mV of difference between Q603/605 also lines up with simulation.
And you say you still can't adjust DC offset properly? What kind of range do you get? What is the minimum resistance you can obtain between the wiper and each end of VR603? Maybe it's just a dirty pot that's playing tricks on you.
I did a bit of simulation again. It could be that the kind of voltages you saw on Q605's base (90-97 mV) are indeed too high to be compensated via the DC offset adjustment. (BTW, an extra 25 mV already results from having 5 megohms(!) in parallel to Q605's B-E junction. The circuit board better be clean as a whistle, with no flux residue in sight.) You may have to decrease R671 in value, to about 180 kOhms.
Seems like a bit of a hack, but it's the best idea I can offer for now.
Quite possibly the value of R671 was chosen empirically at the time, with all-new transistors and little reserve left for aging.
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