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HELP - HP 8567A SA Troubleshooting Frustration

Question asked by K1TVV on Feb 27, 2011
Latest reply on Jun 27, 2011 by JohnMiles
First Post - so hoping to get LUCKY!.........................

I have an HP 8567A SA I've been troubleshooting for weeks now and am running out of ideas......The problem first evidenced itself when running a CAL the point RBW = 1 KHz; VBW = 1 KHz; SWP = 5 KHz; and SWP = 300 ms, the sequence halts with the message "ADJUST AMPTD CAL" ....but there is too much variance to be adjusted with the front panel pot.  So switching out of the CAL routine and simply measuring the level of the internal calibrating signal, at RBW=1KHZ it is exactly +20 dB too high, i.e., +10 dBM, while for all other RBWs > 1 KHz the level is the expected -10 dBm.

As this indicates attenuation problems, I first troublshot the IF-Display unit's A4A8 ATTN BW Filter (10-20 dB ATTN) via the std Step Display method.... Voila! the 10-20 dB steps are missing, all other step gain control lines appear to be correctly operating. Indeed, checking the A4A8 10dB and 20dB ATTN control lines they do NOT have the necessary (Low state) signals to activate the attenuators.  Stepping back to the A4A9  IF Control module, I found that the IOB8/9 control lines are always LOW, indicating possible shorted/grounded bus lines....HOWEVER, those control lines properly show both high and low states when (multiplex) used through other latches with other enabling addresses.  So going back further to the RF Unit's A15 Processor data bus circuitry that generates those signals, I can find no indications they ever provide the correct control signals for the 10-20 dB Attens, but via my logic analyzer, do see valid signals generated for the eother control iines to those IF-Display modules.

So that leaves me to conclude there could be a problem with either the decoding logic PALs or the ROM program that generates the 10-20dB ATTN data bus control signals. However, the A15 board passes the reset POP and "long" POP checks and it seems unlikely that the 10-20 dB ATTN control pgm ROM errors would be such that the ROM check would still pass!

OBTW - I hope I'm not missing something basic as I'm using an 8566B manual for the A15 board --- i.e., the board I"m shooting is an A15 85670-60002, but the schematic I'm using is the 85660-60245 A15 module which appears (for all I can see) to be identical?
More INFO:  8567A RF UNIT SN: 2841A00542;  F/W: rev 7.4.87 and ROMs 85670-80019 to 22  870407 A15U37 to 40

I also have been unable to find the Repair Manual for the 8567A , i.e., the 08567-90003 (08567-90017 supplement) so have ben using the 85662B manual for the most part.

Would appreciate any hints/ideas on what to tackle next

Thanks in advance>>>>>>>>>>>>>>>>>>>...