HP8591A Counterlock (A25) Troubleshooting

[sreaves]

Does anyone have any experience troubleshooting the Counterlock PCB ASM in the 8591/8592A/8593 series spectrum analyzers? Or if you have access to one of these analyzers please read on....

I have a unit that occasionally loses lock. The C107 adjustment yields only +/- 150mV or so of signal at TP1 and TP2 for the A25C107 adjustment. I also note that the polarity of TP1 and TP2 are reversed in the HP procedure but I checked this with the schematic and the polarity is reversed from the sampler match adjustment procedure. The schematic is correct polarity wise. I would be interested to know what the levels are for TP1 and TP2 on a functioning analyzer. Checking them is simple: Power up the analyzer with the cover off. Select Preset. Enter 194MHz as the Center frequency. This sets the sample oscillator on the A25 assembly to the center of its range.

Read the DC voltage at TP1 and TP2

When the displayed signal unlocks it gets noisy and will sometimes return to lock. The unit has run several hours locked onto the calibrator or external signal before unlocking. I have also noted that disconnecting the LO feed to the counterlock does not significantly change the voltages read at TP1 and TP2. If I disconnect this LO feed (J4 on the A4 assembly) and put it close to the pin I can leak enough signal to make the displayed signal noisy and if I contact the center pin of the cable to A4J4 the analyzer will lock, assuming that it is not in its failure mode. I was tempted to move the cable from A4J4 to A4J5 which is the tracking generator feed (an option that this analyzer does not have installed) but the J4 feed has a divider and a coupling cap instead of a direct feed off of the L.O. and since I do not have a detailed schematic of the sampler on A25 I did not want to risk damaging the sampler with overdrive or a non AC coupled source.

Thanks,

Sam



The A25 assembly has die cast shields on both sides that are screwed together. I do not know if the board will operate properly without the shields for troubleshooting.

I would appreciate any help you can give me.

[jry]

Steve,

You may want to try the HP AGiLENT group on Yahoo ...someone there may have some input.

My 8591A has always worked flawlessly

[sreaves]

Hi John,

Any chance you could measure those two testpoints for me and tell me what levels you see with regards to the test procedure. The sampler match test procedure is on page 2-16 of the service manual. Set the analyzer to 194MHz after power up and check the voltages at TP1 and TP2 and please tell me what you find. I think you may find that the polarity of the actual signals is opposite of what the manual says. It says that TP1 is positive and TP2 is negative. If you look at the A25 counter lock schematic TP1 is biased negative and TP2 is biased positive unless the signal reverses this due to the current from the sampler source.
Thanks,
Sam