Agilent 34401A internal voltages

[go0fy42]

Hello everyone...

I got hold of an Agilent 34401A which had one of the internal tantalum caps blown out (C311 for those playing along at home ;-) ). I think I have narrowed the aftermath down to C311 and CR306 (3.3V Zener) which have blown due to unknown reasons. The actual regulator voltages (5V, +/-18V) are OK.

Unfortunately when I run the full self test suite (the unit actually powers up just fine, but measurements simply don't work) I get another error which I somehow cannot correlate to the problem (all other errors fit the problem with the blown cap/zener, which cuts power to several components in the the measurement circuit): I get a "608 serial readback failed". When I check the power rail for components U309/U311 (74HC4094 serial shift regs) I only get 1.8V directly across the Vdd/Vss pins of those devices, which seems odd, but isn't completely implausible either. According to the schematics power to these chips comes from the internal Vcc rail, which is at 5V (verified). But then in the schematics there is R310 (which has a different value than what is noted in the service guide: service guide is 46.4ohms, actual resistor on the PC is 220ohms). It sits between Vcc and the power pins of quite some devices and there is a drop of ~3.2V across this resistor leaving only ~1.8V at the shift regs. The schematics call this voltage "5A", which seems to imply 5V (15A is ~15V, -15A is ~-15V etc.) but I didn't find anything in the service manual to confirm the "5V assumption".

Now my question: can someone confirm those voltages? I am especially interested if someone could just crosscheck the power supply voltages of U309/311. Do you also get 1.8V or do I have another fault in the meter which I would have to track down? Also if someone could enlighten me to the purpose of that R310 resistor in the 5A voltage (page 9-11 service manual, box on the bottom right corner of the page)?

Any help is greatly appreciated!

regards and thanks in advance,
Patrick

[Rerouter]

it likely implies there is a greater load than intended on the serial portion (separating voltage rails with a low value resistance creates a low pass filter for the rail)

check any remaining tants or zeners you can find, they are a common fault in a large number of test gear around that vintage

equally if there are any darling-ton pair transistors powered off that rail check them, as if one part of it shorted that may account for the voltage

[Dr. Frank]

Well,
those labels 15A / 15B / 5A really imply the value of the voltages they are intended for. (Didn't measure that in situ).

The AC (!) circuitry needs filtered power supply, i.e. blocking ripple from e.g. digital circuits outside.
The filtering / noise blocking is done by those tantalums C311, C312, C322, C323 and the RC lowpass filter R310 / C310.

I assume, that simply tantalum C311 failed, as they sometimes do by themselves, in first instance shorting CR306 to GND, causing a high current through that zener and damaging it.

Did C311 really burn?

In this case, C311 would in 2nd instance be high ohmic afterwards, releasing 15B from GND.

CR306 might fail in two different ways by the over-current.
The pn junction could have been open afterwards. In this case, no 15V or 18V would have been present @ 15B afterwards, and no further damage would have happened, probably.
Or the junction could have alloyed to a short, causing  overvoltage of 18V at 15B afterwards. You might check that, by measuring CR306s resistance, if you still have it.

This might have blown one or several ICs on 15B, as U306,305,312,307,308,310,...

Check their datasheets, what the absolute max. supply would be, either 30V or 36V?
Candidates for damage are those 30V devices. The OpAmps normally have 36V.

Then, one of those could now draw too high a current from 5A.
Best candidate is U306, as it also is connected to 5A.

5A definitely must be around 5V, as AD77524 (U302) and 74HC4094 (U309,311) will not work at all from 1.8V!

U309,311 obviously cause your 608 error, as the steering data for the AC circuit and others are clocked in here.
This serial data stream is read back to the processor during test run, via U150 => U101A => SERRBK => µP, obviously. So, if the data gets stuck @U309, there's no feed back, and the test fails.
That also explains, why absolutely nothing happens, not only in AC mode. There will also be no relays and multiplexer actions on page 9-9.

To my "gut feeling", R310 must have 46.3 ohms only, as in the 20yrs. old schematics.
If the resistor is really labelled 220Ohm, then perhaps there were later revisions of the PCB, with newer logic ICs accepting smaller supply voltages, i.e. 5V +/-10%  instead of originally +/-5%. and with less current consumption (old: <5.8mA, new: <2.3mA).

But in the actual online service manual, from 2012 (!), R10 is still 46.3 Ohm, and no further change was done..

So I assume, that you measure 220 Ohm, and that is nOK!!
Perhaps, its value is even higher, and the 220Ohm are measured inside the whole circuitry only.

In this case, replace R310 back to 46 Ohm. Check current over 5A, max. ~ 5mA, something should have caused that increase / damage of R310.

Hope, my assumptions are right and lead you to the failed resistor R310 or failed IC.

Frank

[go0fy42]

Hello...

OK, so that confirms my assumption that I need to see 5V on all of those devices on power supply "5A".

Did C311 really burn?

Yes, C311 is completey charred. No doubt that this cap has failed. I measure 2.5KOhms across that burnt cap, but this is in circuit, so the only thing I take from this, is that it is not dead short (are there caps failing to short? Never came across that... yet...)

CR306 might fail in two different ways by the over-current.
You might check that, by measuring CR306s resistance, if you still have it.

As the 34401a still powers up I measured the voltage across the zener and I get the full 18V across it indicating that it (at least in the end) failed to open. I think under magnification in can even see a small burn mark on the zener, but it is so small that I would not put a bet on this (and it could be debris from C310, as the cap really is burnt to a crisp...). Further this would indicate that at first it may have failed to short and then burnt open?

This might have blown one or several ICs on 15B, as U306,305,312,307,308,310,...

OK, let's just hope it is one of those smaller components, as I will have to go back to "SMD soldering 102" for bigger ICs ;-)

U309,311 obviously cause your 608 error, as the steering data for the AC circuit and others are clocked in here.
There will also be no relays and multiplexer actions on page 9-9.

I can hear absolutely no relays clicking during initialization. But this also confirms my gut feeling that this is "just" a follow-up error from something else failing and thus pulling down 5A.

To my "gut feeling", R310 must have 46.3 ohms only, as in the 20yrs. old schematics.

I probably have the same service manual that you are referring to (recently d/led from agilent.com), and yes mine also says 46.4ohms. But R310 clearly has "221" on it, which is 220ohms and that's also what I measure in circuit. As far as I have analysed the schematic that resistor should be OK to measure in circuit (just some bypass caps on that path, no parallel loads).

Thank you very much for the detailed reply! From this and my measurements I think I have a way to go forward.

My working hypothesis now is exactly as you said: C310 blows, CR306 shorts, kills something on 15B, which in turn pulls 5A down to 1.8V. Shortly afterwards CR306 blows to open and there we are. Or something like that :-) I will probably at first restore 15B to working condition and then see what happens before I start sucking chips off of the PC. I have had those analog switches go funny with me in another project when the supply voltages were out of whack... Maybe (very optmistic, but that's probably just me...) this brings current consumption on 5A back in line already. If not I will go through the ICs there one by one... Fortunately there are no really weird or expensive parts in there (the DAC is probably the worst and that goes for like ~7€ in single quantities at digikey...)

BTW: the 15B pins on the ICs are hovering around -0.7V could be a coincidence, but that's awfully close to one pn junction drop...

thanks,
Patrick

[go0fy42]

So...

I had some more time to ponder on the problem and that 0,7V pn Junction drop between turned out to be the right clue: the missing 15V supply caused the analog CMOS switches to basically "hang" in the reverse input protection ("source to body diodes"), which in turn then draws power from the 5V supply 5A which then drops 3.2V across that R310 resistor.

I bodged the burnt tantalum cap and the 3.3V Zener Diode with "matching" through-hole components I had lying around and revived supply rail 15B and lo and behold: the 5A rail returned to 5.018V.

The device now passes the extended self-test with flying colors! 

Next: Order the SMD parts from somewhere and make it a nice repair job instead of a dodgy bodge.

Thanks to everyone, especially to "Dr. Frank" for the insightful analysis! 

best wishes,
Patrick

[Arngill]

Thanks to this thread I have fixed my 34401A which was driving me crazy. Mine was failing multiple self tests and not measuring anything. I found the 5a (5V) supply was down to about 2.22V, the other side of the series resistor R310 (schematic said 46R but actual marked value was 215R, same as on another (good) unit, so I disregarded this) correct at 5.014V. So I assume something pulling it down, only 5 IC's supplied by this rail so I started lifting legs one at a time, results inconclusive. Every chip I disconnected from 5a, it came up a little but not much. C310 disconnected, no change either. So after a further poking around I thought maybe an internal board fault? but resistance measurements compared to a good unit showed nothing. Then I searched for "34401A 5a" on this blog and bingo. Went and checked my 15b supply, it's zero, 3V3 Zener CR306 O/C. Why didn't I check these other supplies first I wonder?
Thanks to all for assistance 

[coromonadalix]

my r310  is 46.4 ohms ??

Just recapped another 34401a of mine, all tants and chemicals changed

here's an image taken from page 148 of the service manual

I circled in red all caps, tants  and crXXX diodes zenners, rectifiers,   it helped me search parts on the pcb

[dorkshoei]

Just recapped another 34401a of mine, all tants and chemicals changed

@coromonadalix

Do you happen to have a parts list?  DigiKey or Mouser?

Just trying to save myself time.

[coromonadalix]

Sorry  no   

All the tantalums came from my job, i simply ordered the "lytics" caps at digikey, and the fuse holder cap.

[dorkshoei]

Sorry  no   

All the tantalums came from my job, i simply ordered the "lytics" caps at digikey, and the fuse holder cap.

Do you know the sizing.  I'm not that familiar with SMD caps.    I know some are D size but I don't think all are.