Magnatone MP-3 Electroluminescent Tape

[twilightofthedogs]

Hey all, trying to fix up an old Magnatone MP-3 amp, the kind with the electroluminescent tape behind the panel board. I have the amp up and running pretty well, but have been trying to figure out the EL tape. I got this stuff:

https://www.adafruit.com/product/415

It says it runs at 60-250vac. I have attached the schematic for the amp below, the tape circuit is at the bottom right coming off the rectifier diodes. When I take the voltage measurement where it meets the tape, I get a sweep of about 304-315vac when i turn the dimmer pot. The tape will not run at this high of a voltage. When I bring up the amp on a variac with the tape connected, it glows until about 70% on the variac and shuts off. All the individual parts in the dimmer circuit seem to be to spec. Should i try to lower the ac voltage by increasing the 56k resistor? Any other thoughts? Thanks!

[The Dude]

Increasing the 56K resistor value should work just fine. It's basically a voltage divider circuit with resistor(s) and the tape.

[twilightofthedogs]

Thanks Dude! I'll play around with it to see what works. I'll up the wattage on the resistor in the 56k spot just for fun too

[twilightofthedogs]

And for those of you keeping track at home, 270K got me right in the 100-120vac range. Thanks!

[J M Fahey]

Cool

The Magnatone circuit is fine ... but the tape is NOT what they had in mind, so .....

Instead of increasing the 56k to 270k, I would put , say, 100-150k in parallel with tape to lower voltage it sees, matching spec given by Adafruit. (between 60VAC and 250VAC)

They do NOT give a current spec.

In any case, if it works .... so be it.

Absolute worst case, you can replace the high voltage Electroluminiscent strip by a low voltage LED strip which typically expects 12VDC, fed from rectified 6.3VAC from filaments, which will give you some 8VDC.

Probably still too bright for its intended use, you might add a series resistor to dim it somewhat.

Original dimmer pot will not work any more, of course.

If at all possible, please show a couple gut pictures showing how is strip mounted in that chassis, it will be helpful to future users with a similar problem.