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Author Topic: Alpha 86 Dial Lamp Voltage?  (Read 1954 times)
AB3CX
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« on: April 13, 2008, 10:44:38 AM »

My Alpha 86 dial lamps went out. I had some spare bulbs sent by Alpha, but they turn out not to fit the sockets properly. The low voltages in the amp are 28V, 5V DC. Does anyone know the correct bulb voltage?
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K0BG
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« Reply #1 on: April 13, 2008, 12:27:26 PM »

I don't mean to sound condescending, but that would be easy to measure with a voltmeter. I know they were 12 vdc in the one Alpha I owned.

Alan, KØBG
www.k0bg.com
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N3JBH
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« Reply #2 on: April 13, 2008, 12:39:07 PM »

Well Alan you beat me to it. but i looked over the manual and yes sir 12 volts Type 85 bulb.
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N3JBH
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« Reply #3 on: April 13, 2008, 12:47:58 PM »

Funnythough a type 85 bulb is indeed a 28V / 0.04A Bulb

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AB3CX
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« Reply #4 on: April 13, 2008, 03:28:40 PM »

Yes, 28 volts. In case any of you need an inexpensive source, Allied Electronics sells them at 10 bulbs for $2.60, whereas Alpha sells them for $2.50 apiece. It would not be a good idea to try to measure bulb volatge with the amp cover off while it's turned on...
the interlocks tend to shut it down for safety...
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K6AER
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« Reply #5 on: April 13, 2008, 04:54:34 PM »

The interlocks do shut down the amplifier. A jumper hold down strap has to be fabricated to defeat the AC interlock. In addition an isolation tube needs to be slid over the HV interlock tab on the top center wall. At this point you are working around lethal voltages. The usual precautions apply to working around 2600 VDC.

The lamps are lit by 28 VDC. The power supply is the same source that runs the relays and several control voltage points on the control PCB. The lamps tended to be fragile after they have been in use for many months. Vibrations and sudden banging will kill the filaments in the back lit bulbs.

I replaced them with white LED’s. I used a 1500 ohm, ½ watt resistor for each LED to limit the current. Since the modification the lamp problem is no longer a concern.
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AB3CX
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« Reply #6 on: April 14, 2008, 01:27:54 PM »

While Googling around last night, I also found a source for an LED built as an exact plug in replacement for the #85 bulb. They run $10.00 apiece, but avoid any issue of modifying the unit directly...
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