Now the question is whether installing the display will result in the chip getting damaged again.
Keep in mind, none of the display pins were shorted to (chassis) ground, however, I'm also uncertain exactly how the display works to have an idea which pins could be susceptible to shorting or cause the IC to blow.
Any suggestions on what to look for before I solder the display? The time to solder it isn't necessarily an issue, it's more than I don't want to keep applying heat to all the pads. The IC pads have already taken a beating with hot air twice and soldering twice; not including having to reflow pins that didn't flow correctly to the pads.
Then turning on the power for a few seconds to confirm that the segments light.
I measured all the pins on the VFD. Filament pins were about 9.3ohms,
2- to 36 were shorted (but the traces have them tied together), all other pins were high impedance.
Shorted? I don't understand. Oh, you mean only the pin2-to-pin36 pair was shorted. That may be acceptable since pin 2 and pin 17 aren't normally connected; ie. may serve an internal structural need.
+5V ---47R filament resistor--- pin1, then pin 18 to ground.
+30V ---10K grid resistor--- Gn pin (where n=1 thru 12)
+30V ---10K segment resistor--- SEG_m pin (where m=A thru Q, DP1, DP2, COMM)
Left it powered for about 90s, turned off power, left it unpowered for about 3-5s, turned on power, no display, and the 38V was about 12V. Several power recycles and still no display.
I did notice the +38V takes time to discharge when I turned off the unit after the first power up and wonder if this is an issue.
On a side note, I may have only one IC left. So one more failure, and I'll be ordering more and/or repairing pads that eventually lift.
Bummer !
Could be turn off, or turn on, or turn on into a partially charged load, that kills things.
It survives initial ON, so a cold start appears to be OK ?
QuoteShorted? I don't understand. Oh, you mean only the pin2-to-pin36 pair was shorted. That may be acceptable since pin 2 and pin 17 aren't normally connected; ie. may serve an internal structural need.
I used the word short (which they were), but it could have been a bit misleading. The follow up somewhat clarified they are suppose to be "shorted" since I followed the trace showing they are connected electrically.
I'm only thinking out loud and NOT rejecting testing the VFD and also elaborating a bit. Upon first powering after installing the new chip and the display worked, the DC/DC was not getting warm at all. It was actually cold. If the VFD had a short, rhetorically asking, why would the 518 continue working and the DC/DC staying cold? Also, this display worked fine (ignoring the bad segments) until I began installing the replacement 518's somewhat implying the VFD isn't shorted and something funky is going on with these new chips.
Upon repowering, it's not even the display shows something for a brief moment, it just doesn't come on at all, the DC/DC begins getting warm, and the +38V never goes beyond approx. 12V. I did notice the +38V takes time to discharge when I turned off the unit after the first power up and wonder if this is an issue.
The failure mode in my opinion indicates the IC is latching, but, if it's latching, why does it power fine the first time (unless the +38V taking long to discharge after the first power up/down is a hint).
Quote+5V ---47R filament resistor--- pin1, then pin 18 to ground.
+30V ---10K grid resistor--- Gn pin (where n=1 thru 12)
+30V ---10K segment resistor--- SEG_m pin (where m=A thru Q, DP1, DP2, COMM)
I'm scared of blowing up the VFD, so I want to confirm.
Connect +5V to to pin 1 with a 47ohm resistor in series. Pin 18 gets connected directly to ground?
+30V to Gn pin with a 10k resistor in series - obviously to pin 1 - 12 one at a time. The ground on the +30V channel also gets connected to pin 18 (or the ground banana jack on channel 1 that's being used for +5V)?
Now I take the same 10k that's connected to (let's just say G1) and also touch it to segment A (then B, C, etc...) at the same time? I know this is the answer because segment A can't light until a G line activates the proper digit, but I want to make sure. One wrong move with the VFD and I'm really in trouble.
On a side note, I may have only one IC left. So one more failure, and I'll be ordering more and/or repairing pads that eventually lift.
A different 10K (from +30V) to the segment of choice. See attached diagram.
What V/div? I see Ch. 2 at 1V/div for a noisy 5V, and Ch. 3 at 2V/div for 11V.
The HV518PJ appears different (BIDFET) than the SN75518 (Bi-CMOS) output pin uses a BJT vs MOSFET to source current.
My money is on the tube having a short inside. Or a 5V rail cap is low value, it looks noisy.
On the power sequencing, where is the trace of the 38V rail?
I was interested in the rail sequencing with a good IC, not with an overloaded DC-DC converter. So we don't know what the 38V rail normally does.
If Q12 is shorted then SW+5V could stay up too long keeping the 38V rail up after the 5V rail is down, on power down.
Where does the 5V for the HV518 come from? There is a DPAK linear regulator but what provides the input?
If Q12 is shorted then SW+5V could stay up too long keeping the 38V rail up after the 5V rail is down, on power down.