Vision Microsystems VM1000 & EPI800 Displays -- How the Backlights Work & Fail
This is the first in a series of planned posts regarding the Vision Microsystems VM1000 and EPI-800 engine management systems. I'll plan to hit various topics with tidbits I've learned over the years regarding these great systems--I'm certainly open to requests if you have a specific area of interest. Up front, I don't consider myself a VMS guru and I'm not a certified avionics technician, just wanting to use acquired knowledge to help others out just as I was helped when I first started out. I was introduced to these units in 2002 when I was in the market for an experimental, test flew a RV-4 with an EPI-800 system and loved it ... I eventually bought a Glasair I-RG and it too had an EPI-800, I’ve been flying behind it ever since. When VMS ceased production a number of years ago, I began to learn all I could in hopes of keeping my 1987 vintage EPI-800 system viable for years to come. As a member of EAA, I worked with a number of EAA folks addressing VMS installation and troubleshooting issues, learning a wealth of information in the process and gathering quite a bit of tech data too. Last year, some folks suggested that there's a lot more VMS owners out there that could benefit so I started advertising services on the internet and running a few eBay auctions for parts and services. As a member of the forum for just a little while now, I thought I would let the community know about my services and maybe teach some others about VMS systems, so here's the first post, more to follow periodically.
I've recently expanded my VMS repair services into the arena of VM1000 and EPI-800 display back lights. After completing a number of back light replacements on both systems, I want to pass along several observations to hopefully help your back lights to last a little longer, or at least provide some thoughts on how they work and why they may fail.
I think one of the coolest features of the VMS systems is the back light system, little power usage and very readable even at quite low luminosity levels ... but it's very frustrating when you preflight for that beautiful clear night flight and find your display is black!
A little background first (for those who already know about the back lights and how they work, skip to the next paragraph). The VM1000 display and EPI-800 indicators use an electroluminescent panel (ELP) with a phosphorescent coating encased in a thin plastic laminate. The ELP receives an 80 volt AC excitation voltage at a frequency of ~400-800Hz from the small DC to AC inverter inside the VMS data processing unit (DPU). This AC voltage travels via the two outermost leads in the display ribbon cable to the VM1000 or EPI-800 units. In the presence of AC, the ELP glows with brightness modulated according to the level of the AC excitation which is in turn controlled by your aircraft dimmer input to the DPU.
An inop back light system is generally caused by either a failed DPU inverter or a failed ELP in the display unit. There are other possibilities such as chafing of the ribbon cable, incorrect or inop aircraft dimmer input, connector corrosion, etc. but I'm going to concentrate on the ELPs only in this post.
Because the ELP has a phosphor coating, it is susceptible to moisture damage from direct contact or high humidity. Hence, most ELPs today are hermetically sealed in plastic laminate. As I have found in repairing these units, this was not the case in the days when VMS manufactured their systems--almost 30 years ago (EPI-800s) or 20 years for VM1000s. Easily 80% of the ELP failures I've found appear to be due to humidity/moisture invasion of the phosphor coating via unsealed edges of the ELP, either not properly edge sealed or were cut and sealed with regular cellophane office tape which degraded over time. Once moisture contaminates the phosphor coating, it no longer glows. Based on the sample set I've seen, improperly sealed ELPs seem to last on average about 12 years or less. See picture 1 below.
ELPs also naturally degrade over time, this is usually characterized by the panel slowly getting dimmer with age at a given input level or perhaps developing "dark spots" at various points. In our aircraft usage profiles, ELPs seem to last about 15-20+ years if moisture has not been an issue before dying of old age. Some of the ELPs in my ’87 vintage EPI-800 displays are still working, although they are noticeably dimmer than the newer replacements in some of the other displays.
Finally, ELPs can fail due to excessive vibration or shock/blunt trauma (e.g. dropping the display). Now all aircraft vibrate, what I mean is really high frequency/high intensity vibration experienced at the display location. Such can be the case if the instrument panel is not adequately stiffened and/or not shock mounted and it resonates in flight at a particular engine RPM. The small PC board connecting tabs can fail in this mode (See picture 2 below). Similarly, damage can occur if there are longer/larger instruments tucked tightly against a display unit that amplify or transmit vibration to the display in a resonant mode. Thus far, I've only replaced one ELP that appeared to be damaged from vibration but there were telltale signs of fretting on the frame mount face and a large rub area on the side of the frame where another instrument was contacting it--worth doing a double check of your install to make sure you don't have these issues.
So the question becomes how to address these failure modes in a new ELP when it is time to fix the back light. First, the new panel must be air tight sealed with reinforced edges that will not readily degrade over time--cellophane tape does not cut it here! Next, the ELPs should be robust enough to handle normal aircraft vibration with good service life. Finally, they need some form of shock mounting inside the display to guard against the effects of resonation. See picture 3 below.
In my installations, each issue is addressed. First, by using a high quality edge sealing tape, double wrapped on all edges, moisture should be kept at bay. This is a super high quality poly tape, temp range of -100 to +120C, adheres well to plastic materials and has an adhesive that will not readily degrade, costs me about $45 a roll; good news is a roll can cover quite a number of panels. Second, a stiffer backing with larger edge electrodes should translate to longer service life and reliability than the earlier generation ELPs; my supplier provides an excellent ELP to address this issue. Lastly, I provide a barrier against vibration and resonation by using quality/non-abrasive foam mount pads between the ELP and PC board and strain relieve the PC board mount tabs; this should also translate to longer service life. Time will tell, but I'm very optimistic that these techniques and better quality ELPs mean no more back light headaches. So if I can help you out with a backlight repair/replacement, please feel free to email direct at firstname.lastname@example.org
Last edited by Glas467; 02-10-2012 at 11:01 PM.
Reason: Misspelled word & paragraph formatting.
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