I have another old Fluke 5101B on the bench and it has a problem with runaway AC voltage output and a circuit I am unable to figure out. At one time it appeared to be working, then I looked over and saw that it was cooking my DMM with 1345+ volts. DC outputs are stable and work properly. The problem has been narrowed down to the AC converter circuit on the analog control board. I actually have two analog control boards for this and they both have problems with the AC converter circuit, although the problems are not exactly the same. On one board, we'll call it board M, the output is occasionally OK but usually jumps up to around double what it should be. The other, board F, occasionally works almost OK with some minor instability, but usually 'runs away', the output creeps up and then jumps to the maximum that the power amplifier can put out in whatever range it is in. I stay out of the high voltage ranges now just to avoid any unpleasant consequences.
Here's the schematic:
The description given in the theory section of the manual is:
The way that it operates is that it receives a feedback input of 0.2 to 2.0V
rms from the ranging module (scaled from the output) and converts it to a 0.2 to 2.0V
DC output that goes on to the control circuitry which adjusts the output power amplifier accordingly. The input is BUFFERED AC and the output is the upper left corner. he ranging and control circuits are working properly--they work fine on DC and they produce the expected results from the wrong output of this AC converter. So the problem is in this circuit. My main issue at this point is that I do not understand how this circuit works. There is a troubleshooting section and I've taken some of the suggested measurements, I can test components, etc, but since I work on these and have two broken units at hand, I'd like to actually understand the details of this circuit and as of now I'm a bit baffled--both as to how it works and why they designed it this way. It needs to convert a known sine wave to DC as accurately as possible over the range of 0.2 to 2.0 volts, it doesn't need to do TRMS and it needs to be linear. At first I glance I thought U42 was a comparator and this was a half-wave rectifier type AC converter, but it isn't that. C39 and the description indicate that U42 is an integrator. In normal operation, the signal levels at the gate of U36 are very low, so I don't see how it can be called a 'high-impedance' amplifier, as the feedback apparently cancels out the input at the gate. I think that would be more like a transimpedance amplifier with the input resistance of 10k +/-20 ohms converting the input voltage to a scaled current.
I also am having some difficulty finding datasheets on some of the parts. I'll list the part numbers I have and then I'll move on to a second post in a little while with my measurements and tests I've done so far.
Q36 is a N-JFET, Siliconix J2908, no datasheet.
U42 is an Analog Devices
A3902 AD3092, no datasheet and I'd really like to know if it is anything special or weird...
U37 is an LM318H.
Q44 is a 2N3906 and yes they forgot the emitter arrow (should be on top).
CR37 is not really a diode and the 3mA and arrow are not just a helpful note. It is a Siliconix J2901 3mA N-JFET constant current diode assembly. It can be tested by looking for ~1.5V across R37 and that has always been good.
Q45 and Q46 are just some small-signal transistors MPS6520 and MPS6522.
The various inductors are just ferrite beads or ferrites with a few turns of wire. Q45 and Q46 have ferrites on their base leads to prevent oscilliation--and the circuit will oscillate at about 5MHz if you probe it the wrong way.
CR36/37 are FD7223 small signal diodes, but they state 'selected'. That sounds ominous.
Any ideas on the theory behind this thing? I'm especially puzzled that they connect the input to the output through C30 and R30, I have no idea what that accomplishes.