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Old 25th September 2008, 01:40 PM   #1
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Default Dim Bulb Testing: What to Expect

I finished rebuilding a Marantz 2285B amp. It kept blowing fuses, traced the problem to shorted drivers and outputs. Did not check "everything" in the amp, just the usual suspects. All drivers and outputs as well as electrolytics on the amp section replaced.

I now have it plugged into a dim bulb tester. I started with 20, 40 watt bulbs, not enough to energize the relay. The 60 watt bulb did the trick, relays clicked and the bulb got a tad brighter, but it is still quite dim and has held that way for 10 minutes or so. My questions:

1. Am I out of the woods at this point because my bulb did not go bright or could I still have problems as I move up to 75 and 100 watt bulbs?

2. If my repair would not have been successful and there was still a short somewhere, I'm guessing that the bulb would have gotten brighter and brighter and perhaps eventually burned out, is this correct? Does this happen slow enough that I can catch it and flick off the power before causing damage to the unit?
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Old 25th September 2008, 02:13 PM   #2
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Can you specify : Exactly the parts you restore in there ?
And did you check the voltage selector on the back of the amp ?

also you can look at :www.irebuildmarantz.com/ remove-solder.jpg

Sorry bout the link size, but i hope it will help ya
http://ipetkow.olx.bg/ - Електронни компоненти .
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Old 25th September 2008, 02:26 PM   #3
wg_ski is offline wg_ski  United States
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Once it comes up stable, move up to 100W bulb, and play music at low volume. Then disconnect the speakers and play at full clip. If it passes these tests without drawing high current and lighting the bulb you're probably home free. Even with a 100W bulb, you probably can't hurt anything even if something is wrong. A brightly glowing bulb means little voltage at the amp. Set the bias to where the outputs are barely cracked open (a milliamp or two) because it will go up at full voltage with no bulb. Then hook it up w/o the bulb and set the bias to target (do this slowly, monitoring over the course of an hour or so).
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Old 25th September 2008, 02:27 PM   #4
h_a is offline h_a  Europe
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the bulb would have gotten brighter and brighter and perhaps eventually burned out
In case of a shorted amp the bulb would work just as if the amp wasn't connected at all, means it would just glow normally - and give you light to search for the short No burn out.

Have fun, Hannes
High Octane phono preamp Wiki
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Old 25th September 2008, 05:54 PM   #5
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The extent of my restoration of this unit is described here:

Blow Output Transistors Identified...What Next?

I also replaced the few caps on the amp board with Panasonic FCs. As has been the case with almost every '70s vintage amp I have swapped out caps for one reason or another, they test good with a cap and esr tester. A few grey elnas, mostly chemicons.

The bias voltage is presently set to the absolute minimum. The pot is at maximum, 300 ohm. Interestingly, though both pots look identical and have identical model numbers, one is reading right on at 250 ohm, the other 300 ohm, which is as per the schematic. The 250 ohm pot was on the originally good channel, so I don't think it is a problem.

I'll hook up some speaks and see what happens, go with a larger bulb, and eventually plug it in. This is kind of scary....
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