Well, the 100K pot had to go into it's amp, so I can no longer use it 
I do remember that both bases measured 'about' 350mV, but I couldn't tell you if they were off by a mere 8mV or not.
My gut instince tells me that the servo should have no problem balancing this stage if all that is needed is to pump 8mV into one of the FET's. There is a flaky ribbon cable that joins this assembly to the main driver section and output. I've checked it before for continuity, but I'm a gonna check it again. If the servo was not getting it's signal from the output, then there would be no way for this to balance, so rather than a component problem I'm beginning to believe I have intermittent interconnection problems.
Seems like it ought to work, eh?? If the servo did it's job this should make music...............
??
I do remember that both bases measured 'about' 350mV, but I couldn't tell you if they were off by a mere 8mV or not.
My gut instince tells me that the servo should have no problem balancing this stage if all that is needed is to pump 8mV into one of the FET's. There is a flaky ribbon cable that joins this assembly to the main driver section and output. I've checked it before for continuity, but I'm a gonna check it again. If the servo was not getting it's signal from the output, then there would be no way for this to balance, so rather than a component problem I'm beginning to believe I have intermittent interconnection problems.
Seems like it ought to work, eh?? If the servo did it's job this should make music...............
??
EchoWars said:
How you want that the servo do his job, if without the VAS transistors ,the input can not comunicate with the output???
If you don't try the sugestions that people give you ...why to ask??
For the the third time (and last time)...put the missing transistors
or in the year 2010 will be still triyng to balance the amp...
I had suspected that you might be right Tube_Dude, but I had to make sure that the stages had the capability to balance. Now that I see that all it needs is 8mV on the FET to balance, I think that the transistors will be going back in to see what happens.
Thing is, you were the only one suggesting that the root of the problem I saw was the lack of feedback to the servo...if one or two others had suggested the same I might not have doubted you.
Anyway, I'm going to add parts to it this evening and see what blows up...thanks all..
Thing is, you were the only one suggesting that the root of the problem I saw was the lack of feedback to the servo...if one or two others had suggested the same I might not have doubted you.
Anyway, I'm going to add parts to it this evening and see what blows up...thanks all..
For some reason, the differential-mode gain of the first stage is higher than it should theoretically be (it should be about 577 for small signals with ideal bipolar transistors). Since you found several discrepancies between the schematic and the real circuit, the missing FET's for example, it could be that someone has been modifying it.
If I were you, before blowing up anything, I'd check if R709 and R711 are really 10kohm, if R729 is really 180 ohm and if there is indeed 0.9V across it and, most important of all, if the voltage at the collectors of Q703 and Q705 in the manually balanced state is indeed about 3V to 4V below the 66V supply.
If someone made a change that significantly increases the difference between the 66V-supply and the common-mode output voltage of the first stage, that will also cause an increased bias current through the resistors that were smoking.
When you are going to test with all transistors in place, you could connect big power resistors between the supply filter capacitors and the amplifier, chosen in such a way that the total amplifier bias current will cause a voltage drop of about 10%...20% of the unregulated supply voltage. These resistors then act as a rough current limit, possibly reducing any damage. Connect decoupling caps from the amplifier supply lines to ground when the big resistors are in place, to prevent possible instability due to the increased supply impedance.
If I were you, before blowing up anything, I'd check if R709 and R711 are really 10kohm, if R729 is really 180 ohm and if there is indeed 0.9V across it and, most important of all, if the voltage at the collectors of Q703 and Q705 in the manually balanced state is indeed about 3V to 4V below the 66V supply.
If someone made a change that significantly increases the difference between the 66V-supply and the common-mode output voltage of the first stage, that will also cause an increased bias current through the resistors that were smoking.
When you are going to test with all transistors in place, you could connect big power resistors between the supply filter capacitors and the amplifier, chosen in such a way that the total amplifier bias current will cause a voltage drop of about 10%...20% of the unregulated supply voltage. These resistors then act as a rough current limit, possibly reducing any damage. Connect decoupling caps from the amplifier supply lines to ground when the big resistors are in place, to prevent possible instability due to the increased supply impedance.
I just realised that the higher than theoretically expected DC voltage gain can also be due to positive thermal feedback in the double NPN Q703. The half which draws most current automatically gets hotter than the other half, which again increases its collector current. The thermal resistance between the two halves of Q703 has to be of the order of 100K/W to explain the difference between the measurements and theory.
I think 100K/W between the NPN's is extremely unlikely if they are integrated on the same piece of silicon, but it sounds plausible if these are two separate packaged devices, selected for equal VBE and held together by a piece of metal or something.
I think 100K/W between the NPN's is extremely unlikely if they are integrated on the same piece of silicon, but it sounds plausible if these are two separate packaged devices, selected for equal VBE and held together by a piece of metal or something.
The resistors all measure OK. I have checked the surrounding circuitry in my quest to locate anything else that may have been modified. As of now, all parts are stock value except the input FET...since the original device is no longer available...and the servo, where the mods simply allow for a bit more correction range.
To me, it doesn't seem odd that only 8mV is required to balance the stage. Should be an easy task for the servo as long as the loop is unbroken (which I'm assuming was one of the problems and hopefully will have an opportunity to dig into this evening).
To me, it doesn't seem odd that only 8mV is required to balance the stage. Should be an easy task for the servo as long as the loop is unbroken (which I'm assuming was one of the problems and hopefully will have an opportunity to dig into this evening).
Well...it damn near works!!!! Found that a cheezy ribbon cable that I have checked and rechecked had a break in the middle of it. It read fine when checking it (had to flip the board up), but as soon as you tucked it back in, it lost connection. Found almost the exact same ribbon here off of an old TV, just had too many wires. Removed the wires I didn't need with an exacto knife, and installed it along with all the transistors.
The servo balances the input just fine. However, one channel has a nasty humm, and the output relay won't stay engaged even though the offset stays below 5mV.
So I apparently got a lot more work to do. I do very much appreciate youre suggestions guys...I would have never gotten this far without the help... Thank you...
EW
The servo balances the input just fine. However, one channel has a nasty humm, and the output relay won't stay engaged even though the offset stays below 5mV.
So I apparently got a lot more work to do. I do very much appreciate youre suggestions guys...I would have never gotten this far without the help... Thank you...
EW
I want to thank you and Tube_Dude for demonstrating how much influence thermal runaway can have on the low-frequency gain of a simple bipolar differential pair biased at a few milliamperes. This has serious consequences for a simple balanced microphone preamplifier I'm designing, and I'm glad you helped me discover that before building it!
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