Electroluminescent panels

[Swampy]

Hey everyone,

I have an UE LA-610 mic pre and opto compressor on the bench that had a short in the power supply. I've managed to get it operational but the compressor section is playing silly buggers on me now.

There is signal at the electroluminesent panel, and when I take off the lightproof can and flash light on the photoelectric cells I can see the VU dip and hear the signal drop in volume, but there seems to be no response to audio.

How sensitive to light are the panels? Would hours under a fluro prevent them from operating as per usual?

Matt

[J M Fahey]

The LDRs should have no problems with exposure to fluorescent light.
The electroluminescent panel neither, afaik.
*Maybe* they are not getting all the signal they did before, because of a problem in the driver circuit.
A fainter signal may be just enough to make them glow anyway ... but faintly.
I'd measure audio across them, I think they use over 100V AC, although I may be mistaken.
Never actually repaired one of them, but the electroluminiscent panel on Ampegs , which I think is about the same technology .... and they needed a pretty hefty AC voltage.

[Steve Conner]

There's a missing link in your troubleshooting: Do the EL panels give off light when they get signal? Maybe they've degraded with age and don't work any more.

[jrfrond]

I repair a lot of Urei LA-series opto compressors, and they all experience EL panel failure at some point in their life cycle. You've already verified that the LDR is working, so if you don't see the panels glowing when you remove the opto cover, there is a 99.9% chance that the panels are dead.

New optos are pricey, but Anthony DeMaria Labs rebuilds them for a fraction of the cost.

[Swampy]

I measured about 8 volts AC at max preamp gain and max peak reduction settings with a test tone and about the same with a mic connected. Are you certain there should be around 100V p-p out of the driver circuit?

I can't see the panel glowing when I take the cover off, nor can I see any light emmisions when I run audio through the unit. It seems the panel is the panel is kaput.

[J M Fahey]

Hi swampy. I have experience only with the clasic LA2, and yes, they drive the EL panel *hard*.
They use a power tube (6AQ5) with a 10K resistor in its plate, +B around 250V. Don't know your particularr unit.
If it's tubed, now you know; if it's SS they probably use a transformer to get such a high voltage. Just guessing.
No matter what the driver, 8VAC seems too low to make them glow.
Post the schematic if you have it.

[Swampy]

My mistake - I replaced the plate resistor of the power tube with a lower value then the circuit was designed for (put in 220 R, should have been 22K).

We shall see what happens when the correct value is in...

[J M Fahey]

OK, post results.

[Swampy]

Good as. Must double check values before installing in future.

Thanks for your help everyone,
Matt

[kscience]

EL panels require a minimum of 80vac typically, at say 1khz; at 60 hz, 115 will do. They are capacitive so the higher the frequency the more they conduct, hence lower voltages will trigger them.