Help with 34970a that I managed to fix, then break again

[julian1]

I have a rather dead 34970a.

When mains-power is applied and the front-panel power button pushed there are no beeps, no fan, and no-life on the front display panel

Current situation,

- have a 3v bench supply connected in place of the battery
- have all rail power including +5V ER and 5V flt (not Backplane power, which also controls fan and is turned on by u305).
- have front-panel power +18, 13 volts
- have 12Mhz clock on controller u205
- reset pin on controller u205 is correctly pulled low - in response to the front-panel on button being pressed - eg. no connection issues
- i have tested pulling u102 (ER reset) low and nothing happens
- vbatt on u104 is correct at 3.0v
- vout on u104  is 5v.


The unit had been working with no errors for a few-hours. The series of events that led to the current state are,

- replaced old battery showing 3mV charge with new one
- unit then ran well for a few hours, when I noticed a lot of heat and so disconnected the main-power
- after re-powering on the unit - errors were shown in the display-panel
- after several power cycles - 34970a stopped working entirely
- noticed that the new battery had discharged (to 1.9v) over a couple of days indicating a short
- found cr110 - a bav99 dual-diode - was physically damaged and testing showed that it had failed (may be the cause of the battery discharge)
- replaced the diode with a new one

However the unit still won't power on.

At this moment, I believe that it's my power-cycling or the heat that has lead to other damage.

Any ideas on what to check next?

[HighVoltage]

Is it possible that you mixed up the polarity when you installed the new battery?
I have done this procedure before and it did not cause any problems but I remember that I had to be careful with the polarity

[julian1]

Ok, some success.

I had replaced tantalum c108 which was near the broken diode cr110 which I thought was also bad due to a measuring mistake.

The replacement cap matched the spec of 1uf and 35volts. But I just now re-soldered the original back-in and the unit now powers on! The possible explanations are ESR sensitity - the new replacement has esr of 4.6ohm and the original was around 2 ohm. Otherwise perhaps the new one wasn't really rated for 35volts, or else I must have completely messed up the soldering.

The unit now emits a single beep. followed by two sets of three beeps. Nothing is displayed on the front panel display, but it is responsive to the front-panel buttons and will beep when they are pressed.

[julian1]

Front display filament power comes from an AC transformer tap.

But it's not being turned on, via the FILPWR pin of the ucontroller. u107 is a lm393 op-amp which switches mosfets to control AC power to the display -  but it only has 0.2V against the 2.5v ref that it needs.

So I suspect that the ucontroller is choosing not to switch on the filament in software due to some other system error(s).

   

[julian1]

Soldered a bodge wire from +5v flt to pin 3 of u107 op-amp, which control the mosfets that power the front display filiment.

power on, and the display now lights up and unit is half-working!.

There's some heavy flicker of the lhs 'scan' menu item of the vfd that I don't remember seeing before.

Beeps on boot - sometimes one, sometimes two.

I can now go through the errors - Have 740, 741 747, 748 which I've read indicate failure of fram. I'm somewhat surprised since there's no dmm card.

Edit, I think those errors are for the fram on the main board, - as there's no DMM installed.

740 Cal data lost: secure state
741 Cal data lost: string data
747 Config data lost: HP-IB address
748 Config data lost: RS-232

[HighVoltage]

Be aware, that the FRAM liked to die on the 34970A.
You probably need to exchange it.
But you will loose all calibration data and the instrument needs a full new calibration.

[julian1]

Is it possible that you mixed up the polarity when you installed the new battery?
I have done this procedure before and it did not cause any problems but I remember that I had to be careful with the polarity

I think it's unlikely - I can remember checking and re-checking the polarity, since it took a couple of goes to figure out how to solder it. The unit wouldn't start at all without the battery, and I suspect that would be the same if it was reversed.

After the second failed battery was removed there was a puddle of corrosive liquid under it - so it probably shorted (somehow).

[julian1]

Be aware, that the FRAM liked to die on the 34970A.
You probably need to exchange it.
But you will loose all calibration data and the instrument needs a full new calibration.

Yep. It looks like this is on the cards.

I'm still puzzled why I need the bodge wire to turn on the display.

[julian1]

Soldered in a non-initialized fm24c043-g fram

ran the intialization suggested in these forums,
:DIAG:POKE 15,0,0
:DIAG:POKE 0,0,0


It remembers the rs232 configuration between power cycles, which it wasn't doing before, which I think indicates the fram replacement was successful.

It still gave 4 errors (740, 741, 747,748) at power-on for about 5-mintes, even after erasing them.

Now it has settled down to a single beep on soft or full power-on, indicating no errors.

However it still flashes the error sign in the vfd - and reports a single 741 error which is weird.

Edit.  Actually the last 741 is probably due to the blank initialization.