Thanks for your commments. On startup the HP-IB address is displayed on the screen (18). I have tried to address it directly with no result. Same with polling.
I have already ordered the two different bus drivers U30 and U31, they are cheap. So changing them will be easy. If this does not work I have to dig deeper into the circuit.

There is another problem: BATTERY is displayed on startup. The battery is good and the battery voltage is present on the RAM ICs. So maybe one or more of the SRAMS are bad.
Howerver Self Test does not show any problem.

Ernst

On Feb 6, 2019, at 4:21 PM, Steve Hendrix <SteveHx@...> wrote:
...

Be aware that the 75 ohm Type N are different from the 50 ohm Type N and you could damage them by attaching the wrong one. Someone else may have a better explanation but 75 ohms seems to explain the different dimensions.

Yes, someone else already pointed that out to me, and an article on N connectors on Wikipedia that explains the differences quite well…. I think I'd better make some time to apply my micrometer to my collection of various N connectors and adapters, to be sure what's what.

Steve,

Perhaps you would report any connector numbers you find and which is which.

I have a dial calibers here that would let me figure our what connectors I have here.

Thanks.

Roy

Roy Morgan
K1LKY since 1958
k1lky68@...

On 2/7/19 9:46 AM, gregdunn@... wrote:

I checked DCV on the 1V range, and it reads 0.03 whether open or short.? So, 30mV?? That seems quite a bit too high.

That's suspicious, and if A and B volts of Q15 are different, the feedback is not working. (have not looked at rest of schematic though). Does the manual say what I ref should be, and is it? (can calculate from volts across those measured
and good R52 or R53 resistors.)

They're all pushbutton switches, and I have (at least twice) cleaned the contacts with Deoxit.? It helped with some flaky behavior on the voltage ranges but did nothing for Ohms.? The upper 4 ohms ranges read garbage before I replaced Q11; now they at least are somewhat consistent, though way too high.? U3 actually measures good voltages at all the marked locations; but despite that, the gates of Q15 read different values.? The manual suggests shorting C1 and then checking gate voltages - it didn't make any difference.? I haven't changed the pot settings since replacing Q11, but I will go back in and measure the test points again before proceeding. If necessary I'll pull the pots and measure them.

The divider resistors R52-56 all measure proper values; you can force the switches to "all open" which essentially takes them all out of circuit and makes measurements possible without removing them.

I checked DCV on the 1V range, and it reads 0.03 whether open or short.? So, 30mV?? That seems quite a bit too high.

I'll take another look at some of the test points today and report back if anything changed markedly after swapping Q11.

On 2/7/19 12:06 AM, gregdunn@... wrote:

Replaced it with a known good part (though not an exact replacement), and it only made a slight improvement.? Now the readings are 2-3x what they should be up to about 100KΩ (steadily getting worse at higher resistances) but it still reads OL on a 1MΩ resistor.

Have you tried checking the volts at output of U3 and the pots R59 R64 and the rotary switch contact next to R59?
Was it dirty at all? If so, is there conductive dirt on rotary switch?

Are R53 R52 good value?

Yes 75 ohm N connectors are dimensioned differently than 50 ohm connectors. The difference is the center pin diameter. NEVER try and mate a 50 ohm to a 75 ohm as the 50 ohm pin is larger than the 75 ohm connector

/RANT
NEVER SHOULD HAVE MADE 2 VERSIONS WITH THE SAME OUTER THREAD
/RANTOFF

Content by Scott
Typos by Siri

Hello Greg, it's nice to see you back with a new problem ;-D

Just a quick check, just to make sure the problem is not being caused by some leakage in the mainframe (the 34740): When switched to DC volts, on the 1V range and the input open, does it read 0.000 ? Or how much ? We're checking for leakage in the 10 nanoamps range here.
Cheers,

Joel

?

As promised, the followup to my HP3490A repair.? I found this 34702A/34740A at a hamfest for $12 and thought it deserved a home.? Upon hooking it up, the DCV and ACV seemed to measure pretty accurately but the Ohms measurements were way off.? Not too bad on the low ranges, but it eventually went OL at or above the 100K range.? This one has been frustrating.

I initially suspected a leaky Q15 (the dual FET) but replacing it and Q16 its current source made no difference.? Maybe U2, the op amp that controls the current?? Nope.? The manual suggests that out of spec on the highest range could be caused by Q11 (JFET) and so I pulled it and it tested bad.? Aha!? Replaced it with a known good part (though not an exact replacement), and it only made a slight improvement.? Now the readings are 2-3x what they should be up to about 100KΩ (steadily getting worse at higher resistances) but it still reads OL on a 1MΩ resistor.

I pulled Q13 and Q14, and they tested OK and nearly identical so I didn't replace them.? The voltages look reasonable; Q14 was questionable in-circuit but it may just be because of the defect disturbing its bias.? It seems to me that the current source is not pushing enough current through the unknown, and that's why it's seeing higher resistances than it should.? I'm trying hard not to just go on replacing parts until it starts working, but I'm about halfway there already.? ;-)

Does anyone have a couple of 75/50 ohm N pads gathering dust?

I picked up a semi-antique 8754a with a 8502b test set and I'm
unearthing various bits of RF gear (SA, 8640b etc) I've accumulated for
a project. As a consequence, I need to stock up on various adapters
without killing the budget.

Essentially, right now I'm looking for the HP 11852A and B or their
equivalents.

thanks!
--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows

At 2019-02-06 01:34 PM, dk1vi wrote:

the unit does not give any reaction when addressed with correct IB? address. Have checked HB-IB clock which is OK. I do not have the special extender board, so further testing inside circuit is almost impossoble. So maybe best would be to change IC U6 (TMS9914AJTL), U30 (75161) and U31 (DS75160). Or any other suggestions?
Still wondering, what the options mean. Firmware ROMs are standard matching the serial prefix.

I'm jumping into the middle of this, not having seen any preceding discussion, but here are some ideas:

1) The unit might have IEEE-488 disabled in firmware, either as a factory option, or as a user option. Can you see what address it thinks it's using on the local display?

2) Any chance you've flipped the sense of the address switches? It's not uncommon on IEEE-488 to see generic DIP switches labeled On and Off, and the On position pulls down to 0 whereas the Off position allows that bit to float to 1, so the numeric value is opposite to what you might expect.

3) If possible, have your bus controller poll all addresses one at a time. That might find the unit if it's not at the address you expect, for whatever reason. My KISS-488 includes that function, precisely for such a scenario. When your instrument recognizes itself as being addressed, it should pull \DAC low.

4) It's a bit tedious for extensive troubleshooting, but I've done such things as checking Vcc, checking chip clocks, etc., inside an instrument where I can't reach with everything plugged in, by temporarily tack-soldering some #30 wire wrap wire to pins, leaving it long enough to connect a scope or meter outside the instrument. You probably don't want a very long wire hanging off a high frequency line, but you can get away with this trick usually up to several MHz, which is usually plenty for these older instruments.

Steve Hendrix

开云体育

The thing that scares me about this now is that trimpot.? I have doubts that it has survived.? Might be good to replace that DIP socket on general principal too.? The other components are likely OK.

Daun

Yes, Orin, you are correct. After taking a second (and third)? look, I see that I misread the 'B' for an '8'. Maybe I need some better glases...

开云体育

Thank you all but the book has been sold.

Gedas, W8BYA

Gallery at 
Light travels faster than sound....
This is why some people appear bright until you hear them speak.

OK, thank you. You all are right, I obviously exaggerated with the vinegar. I washed a second time as Ed recommends and now it is improved a little bit. What is remained is the tarnish on the weldings and the surface of the traces that is not shining like earlier but I do not see any corrosion or interruptions of the traces. So... apart of the aesthetics, what can be the problem?
And a second question.... I don't want to use the oven but I prefer to cool air dried... how many days do you suggest? After washing I used a jet of compressed air.

At 2019-02-06 09:19 AM, peter bunge wrote:

Hi Steve, an HP8714B just sold for $565 US.
I suspect you paid much less for the 75 ohm version and as much for shipping..?

Somewhat less, but not all that much. And shipping wasn't bad, as it came from the same side of the continent as me.

My suggestion is to keep it and use it with 75/50 pads to learn about it while watching for a cheap 50 ohm version, not working if you have repair abilities, and use your 75 ohm version to troubleshoot and repair.

That's my intent exactly, and it'll likely be a while before I'll have need of that much additional capability.

Do you have the pads? I have a couple of new Greenpar 502-4718-701, 90-48 that you can have for the price of shipping. Contact me offline.

Rats! I already order some and they're enroute. Thanks for the offer, though.

Be aware that the 75 ohm Type N are different from the 50 ohm Type N and you could damage them by attaching the wrong one. Someone else may have a better explanation but 75 ohms seems to explain the different dimensions.

Yes, someone else already pointed that out to me, and an article on N connectors on Wikipedia that explains the differences quite well. It appears HP used some good-quality connectors that are built for 75 ohms, but have enough compliance in the fingers that grab the inner pin, that they won't be damaged. Now that I know the difference, I think I'd better make some time to apply my micrometer to my collection of various N connectors and adapters, to be sure what's what.

Thanks for all the help!

Steve Hendrix

开云体育

FS is a near mint condition book by Michael Hiebel. Considered one of the engineering standards in the field with 420 pages of theoretical & practical information with lots of quality color plots, pictures & tables. It is a hard cover book and has two dents, one on the top-front & the other on the bottom-back.....no torn or bent pages & no writing or hand notes. If interested pse contact me off list. Thank you.

Gedas, W8BYA

Gallery at 
Light travels faster than sound....
This is why some people appear bright until you hear them speak.

Hello,
the unit does not give any reaction when addressed with correct IB? address. Have checked HB-IB clock which is OK. I do not have the special extender board, so further testing inside circuit is almost impossoble. So maybe best would be to change IC U6 (TMS9914AJTL), U30 (75161) and U31 (DS75160). Or any other suggestions?
Still wondering, what the options mean. Firmware ROMs are standard matching the serial prefix.

Regards Ernst.