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| Anritsu / Wiltron S331A - Service Manual / Schematic |
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| rg58:
So that PNP transistor to the right of the peg seems to be OK, although in-circuit measurements were a bit inconclusive on C-E. At least not shorting, assuming that part is good. However, the ERA-3 is officially dead. Although from the DC current draw of 32mA it looks OK (35mA according to datasheet), it measures a DC resistance of 73 ohms on it's output which can't be right, but adding up in my calculation now: 7.9V/(196+73ohms)=29.4mA + current drawn on the input. Because the datasheet of the ERA-3+ tells us the (simplified) internal circuitry. There are 2 transistors in darlington configuration inside, plus 4 biasing resistors. The 2nd transistor seems to be shorted, so we are left with 1 resistor to GND probably in the region of... 73 ohms. This makes sense now and explains the signal loss of -30dBm - -16dBm = -14dB instead of a +23dB gain. The datasheet also tells us the supply voltage to be 3.5V at 35mA, but it breaks down to 1.62V at 32mA. Checking transistor NE021 in circuit again. Diode test: Vbe=0.78V, Vbc=0.78V, Vce=nothing, reverse polarity all 3 nothing. Looks a bit HIGH for silicium, otherwise that part seems to be OK. But why the loss in signal? At 50MHz: Base: -30dBm, Collector: -55dBm, another -25dB loss! DC voltages in operation: Vb = 0.94V, Ve=0.51V => Vbe=0.43V (unusual LOW this time!) and Vc=0.62V. I can't find any RF short on it's output, it only continues to a SMD capacitor of 10pF (as my LCR meter says). Behind that... oh, I see it now. These are 2 switching diodes, behind the upper diode some low impedance circuitry. Makes sense now, this could be about right. The 2nd transistor acts as a power amplifier it seems, so the current is not enough. Hence, my 50 MHz signal nearly disappears in the noise while a weak 49.8 MHz signal appears here now. Will replace ERA-3 when I received it and let you know the outcome. Hope this solves the problem. |
| metrologist:
I don't believe that transistor is biased in baseband. The bias line is from the feedthrough to the left on that small blue resistor. Consider why that mar is fried and the low impedance circuitry on the other side. There is little to protect the samplers. Also, follow the trace from TP29 to the J310S fet, which is next in line. Then 2N5087S and a few 2N7002S around there. Similar circuit on the reflection side (other side of bridge). Assuming a mild overpower input, that should cover it. HV destruction then toss it but I've seen these with a melted bridge and black charred PCB so it might be salvageable. |
| rg58:
Yesterday I received the ERA-3 and just soldered it in. Here is the result: ERA-3 input: -19dBm ERA-3 output: -5dBm = Gain: +14dB (good!) Vcc: 3.05V (good!) Level at N-socket: -9.8dBm (good!) All looking good now! Well, except the device still doesn't show valid measurements :( The SWR is still stuck down at 1. The via (feed-through) behind the blue resistor at the base of NE021 ends up unconnected in the middle of a ground plane. Is this a 4 layer board? I can't tell from where the DC is coming. The cap at the other end of the trace is connected to ground, then there is a coil (choke) in a black package connected to it. All this makes sense, but where is the DC coming from? When I approach such repairs I always think what might have been fried first, so my main interest was in those samplers right from the start. In the meantime I did some diode testing (in circuit) and the 3 diodes in both samplers seemed to be OK, although I am not 100% confident with the results. One could just be damaged, not completely fried. Would need to investigate them more thoroughly, signal-wise. Next is the transistor connected to TP29 which you identified as J310S. I can't find a datasheet for this. Marking code "6TU" is not in my lists either. I did find a datasheet from Fairchild for MMBFJ310 (6T) and MMBFJ309 (6U), so telling from the marking code its a combination of the two ;D Assuming the pinout is correct, it has a D-S short. You were spot on once again! Thanks! :-+ As for the remaining transistors around there, none of them is shorted as it seems. Will try to source the J310 with an "S" at the end or look for the MMBFJ310 which *should* be the same. Once received I will let you know again. Btw, my time for electronics is very limited these days. |
| rg58:
Received the MMBFJ310 today and put it in. To my surprise it seemed to be blown again. That's when I realized it's a J-FET!! Silly me :palm: Of course I knew it, but somehow overlooked the fact. The J310 is quite common and the name already suggests it. Oh well... Back to square two... |
| rg58:
While the device was switched on and sitting quietly on my table, the display suddenly disappeared for no reason. Nothing is shown on it anymore. The backlight is still working. When I turn the device off and on again, about 3-4 horizontal lines appear for a fraction of a second, same as it did normally when it started up. It still reacts to keypress with beeps and I can turn on and off the backlight. I have already checked the voltages from the test points in my earlier post and on the voltage regulators themselves, but everything looks fine as far as I can tell. I have also taken out the display again, removed the ribbon cable, wiggled it while it was on, but still no display. |O Has something suddenly died ? What happened ??? |
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