Definitely look at the column of electrolytic capacitors in the middle of the power supply board. It seems most 70900s have succumbed to failures of those capacitors now. There have been YIG failures, but in my experience nearly all of those capacitors will be bad.
They do need to be low-ESR types, but they're not difficult to find. I'll bet that, once replaced, your 70900 will spring to life.
-Dave
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On 11/20/21 9:02 PM, Gene Silvernail wrote:
Thanks Vladan,
That¡¯s a good postulate because I had to replace one of the fuses on the PS board to get this far in trouble shooting. BTW those fuses are rare and damned expensive. They are from the early NASA days and listed as very fast acting fuses. Got a few on order but not wanting p-off $10 dollar fuses in trouble shooting I improvised by breaking open standard fast 2A fuses reclaiming the fuse wire. I pinned the wire in the 2 pins of the socket using the blown fuse. I extended the wire free of the socket by 3mm to free it from any thermal sink. ?If I was going to blow fuses they would be cheap fuses. Before refusing I could not get all the keys on the 702005A to respond, i.e. row column selection on A-Map. Also none of the front status LEDs on the L.O. were illuminated. Best guess was with that fuse out, inboard fuse, some mutual bus was being inhibited. Refusing cleared 70205A functionality and LEDs were lit, albeit maybe dimmer than the RF section. Of note, now the IF LEDs were out where prior to fuse replacement at least one, I think both, were lit.
I think next step is to pull the PS board and¡
1. Visual on full board top and bottom
2. Run C and ESR readings on the electrolytics
3. Verify what the bad fuse fused then verify all actives in that path
I still don¡¯t know if the 70905A/L.O. should be seen in the Address Map
*From:* [email protected] <[email protected]> *On Behalf Of *pianovt via groups.io
*Sent:* Saturday, November 20, 2021 5:36 PM
*To:* [email protected]
*Subject:* Re: [HP-Agilent-Keysight-equipment] I'm old 70+, the 70K system is old; good or bad omen?
Gene,
I don't know the answers to your configuration question, but one possibility is that a power supply in the LO module is down. After years of service, the electrolytic capacitors in that module tend to fail. You may be able to see what's going on by removing the mainframe cover and the LO module cover.
Vladan
--
Dave McGuire, AK4HZ
New Kensington, PA
Dave McGuire