My four-channel 35670a dynamic signal analyzer has two bad channels, channels 1 and 2. Even with the inputs grounded, The half-level and overload LEDs for channel 1 are always on and the half-level LED for channel 2 flickers. These channels are on separate A2 analog input input cards, so there's damage to both A2 cards.
I've gone through the service manual block diagram and traced the points where the signal goes bad: the output of buffer U5 (TP7). I pulled U5 suspecting it was bad and found that the voltage at TP7 is still the same: 9VDC. This should be an input node, so there's something broken pulling this node to 9 volts. The service data shows this node driving the half-range det, differential overload det, analog switch and 25 khz AAF. Unfortunately the service data gives no circuit details for these blocks, so I've hit a dead end.
Does anyone have an actual schematic for this section of the A2 card? Or is anyone aware of a common part failure for the A1 or A2 card? Any suggestions on debugging/fixing this problem would be greatly appreciated.
Mark R
I've gone through the service manual block diagram and traced the points where the signal goes bad: the output of buffer U5 (TP7). I pulled U5 suspecting it was bad and found that the voltage at TP7 is still the same: 9VDC. This should be an input node, so there's something broken pulling this node to 9 volts. The service data shows this node driving the half-range det, differential overload det, analog switch and 25 khz AAF. Unfortunately the service data gives no circuit details for these blocks, so I've hit a dead end.
Does anyone have an actual schematic for this section of the A2 card? Or is anyone aware of a common part failure for the A1 or A2 card? Any suggestions on debugging/fixing this problem would be greatly appreciated.
Mark R
I've made some progress on this. U460 (analog devices ADG201HSKN analog switch) was blown on both boards. This switch corresponds to the two-pole switch driven by TP7 on the block diagram on p 8-15 of the service manual.
This repair completely fixed one of my two bad A2 cards. It improved the second A2 card, but there's still something else broken. After self-test, I get:
INPUT AAF/BYPASS FAILS
50 KHZ PATH FAILS
25 KHZ PATH FAILS
Also, the half range detector for the bad channel stays on, even with no input. I'm seeing evidence that the half-range detector fault damages replacement ADG201HSKN chips.
Once again, thanks in advance for any advice.