U110 is a 40 pin UV erasable EPROM marked:
03458-88887, Check Sub = B26A, DS 8061
U140 is a 20 pin IC marked:
03458-88838, SLRW FN, Check Sum = 2F48 8063
U121, U122 are Dallas Semi DS1230Y-150 Nonvolatile SRAMs (date code 9746H)
U132 is a Dallas Semi DS1220Y-150 Nonvolatile SRAM (9751D)
The three Dallas Nonvolatile SRAMs are all a couple of years past their 10 year lift. The internal batteries may be dead!
I've heard the batteries should be good for another few years. If they were bad there might be an error message about the SRAM checksum.
Inguard Power Supply 03458-66504B
The diodes are 1N5365B. What has shorted out to cause CR9, CR11 & CR12 to die? These are 5W 40 V Zeners.
Why is CR10 still OK?
There is a connection between the Zener diodes and the 5 pin header nearby. So the problem may be on one of the boards that's receiving the 40 Volts. Maybe a bad cap?
A new A4 board is on order with a 33 day lead time. 29 Jly 09 + 6 wks = early Sep 09.
Working on schematic.
This may be what happens when 220 VAC is applied when the rear panel straps are set for 120 VAC?
Layout to get schematic
to the transformer secondary. P1-1 and P1-3
supply the +18 V and -18 V half wave rectifiers with
the center tap to P1-2. Zener CR12 has
it's anode connected to P1-3 and it's cathode to CR11
anode. CR11 cathode to Gnd screw 1.
In a like manner CR09 cathode connects to P1-1, CR09 anode to CR10 cathode, CR10 anode to Gnd screw 1. So the Zeners are input over voltage protection.
T1 Transformer in upper left.
A6 Below transformer is Outguard power supply.
A4 Central rectangular box is where the Inguard power supply sits. The red connector goes to the transformer.
A2 AC Circuitry 66502E in the upper right.
A3 A/D Converter & Inguard logic 66503 in the lower right.
There does not seem to be any troubleshooting information ( VAC or Ohms readings for the transformer nor is there any removal or installation info.
I'd prefer to not completely disassemble the DVM to change the power supply.
31 July 2009 after: replacing T1
|With new transformer
|Inguard p.s. voltages
with new transformer:
+ 5: +5.0
+18: +18.8 (spec: 16.2 to 19.8)
-18: -23 (spec: -16.2 to -19.9) <****OUT***
The Outguard assemblies are on the right and include the power transformer and below it the outguard power supply and on the other side the A5 controller.
The outguard assemblies connect to the inguard assemblies (on the left) mainly via fiber optic cable pairs.
The central hole is for the inguard power supply.
A1 DC circuitry
A9 DC Reference
A10 Front Rear Switch
|1 Aug 2009 - Installed
new In guard power supply. The voltages are
now: 5.00, +18.5 and -18.5.
But the same error message.
There are some memory chips that have an internal backup battery and some instruments are getting old enough that the battery is going dead causing the loss of calibration data.
Thanks to the work of Poul-Henning Kamp there's a way to read out the data from these chips.
All you have to do is enter the commands TRIG HOLD QFORMAT NUM MREAD 393216 MREAD 393218 MREAD 393220 ... MREAD 397308 MREAD 397310 and save the results. But that's 2048 MREAD commands, so having a program to do it would be a lot easier...
and here is some sample data: HP3458Sample.txt
|I also picked one up, a
mixed one, ie old analogue PCB from the 90's and a
younger processor board from 2001 (perhaps replaced
due to SRAM battery discharge). Cost me about 3k
instead of of 8k new, and its working perfectly - but
stable environmental temperature conditions are
required, that's the weak point of the design.
Maybe its an old design, but currently, no other DMM or calibrator can beat it essentially in certain parameters.
It has got the best (differential) linearity, over Keithley 2002, Fluke 8508A, Datron 1281, Fluke 57xx and even the Primary Ratio standard 720A! Only the JJ array can test its linearity!
OK, the 3458A's internal references are not that stable, but it is not intended as a secondary volt or ohm standard.
For that you need additionally something like a 732B and a SR 104, or the quantum standards, respectively.
But all others of the above mentioned, newer DMMs are not much better.
For DCV, the 3458A obviously has been designed for a very broad temperature range of 0..55°C (military use??), which gives an internal temp of at least up to 80°C in a rack mount. Placing its internal volt and ohm standards in a lower and more stable environment would have been better, but then, it could not have been a DMM-in-one-box.
So the internal LTZ1000A reference has to be running on 90°C.
If powered constantly, this gives at least 20 times higher drift rates over time compared to a Fluke 7000A, which is running on 45°C.
Other DMM are specified for meteorological temp. range and have certainly slightly better drift rates (two times).
I have set (pimped) the LTZ to about 55°C for lower drift.
The HV divider cannot be corrected for power dissipation effects, so the 1000V range is quite mediocre.
I have built my own 100:1 Hammond type divider (~ 752A) to get around 1ppm for 1000V.
The ohm ranges obviously is its weakest mode.
It relies on an elder hermetically sealed Vishay resistor, with high time and temp. drift, and additionally the resistor is exposed to the strong internal temperature variations.
Today, by using a selected VHP202Z resistor, one might improve time drift to <1ppm/year and stability (with respect to temperature) to < 0.2ppm over the complete "meteorological" ambient temp range.
Additionally, its ohms range resistors are very sensitive to temperature changes.
But it is possible by using it in absolutely stable amb. temp. conditions (+/- 0.2K), to make 10k Ohm measurements / transfers on sub ppm stability level.
Currently, I'm working on external 10k standard resistors; but still in discussion with Vishay.
Report will follow.
ACV is also unparalleled by other DMMs, due to its (patented?) digitizing algorithms, and this can be further improved by Swerleins Algorithm. So I think, only Flukes AC standard 792A is "better", if using standard electronics.
So my advice, get one used 3458A, it's simply a nice and ultra precise box, and build yourself some standards which you would need also if you got a newer DMM.
Btw.: A good, absolute calibration is nearly impossible to acquire for us amateurs. Even Fluke and Agilent obviously offer 2nd grade calibrations only.
The Linear Technology LTZ1000 is the heart of many voltage references see LT app note 86 appendix I Fig 12.
LTZ1000 chips: eBay seller polida 2088
HP Journal - April
1989 the 3458A is the Cover Feature