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Old 28th April 2006, 03:58 AM   #1
infinia is offline infinia  United States
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Default Finished Adcom 555 mods

Hi All
An earlier thread inpired me to dust off the old 555 for a minor tune up. 2 mods were completed with one remaining.
GFA-555 parts upgrade/substitutions?

Mod 1 ) Add inrush current limit. This mod is mainly to prevent documented premature failure of the on/off power switch and generally a good thing for larger power amps.
Installed a 3 ohm 15W in series with the Xfrmer primary. Use a 13A NO relay contact to short the resistor when the secondaries come up. The 125 Vdc relay coil senses across both rails of one Amp. Added a 1N4005 free wheel diode on the coil. A 5K resistor drops the +/- 82 V rails to a nominal 120 relay volts.

Mod 2 ) Add film bypass caps across each of main caps (15000 uF 100V 4 places).
I added 2.7 uF 250V mylar film. Some people say to add 100-22 uf electro.


Mod 3 ) Change driver transistors. I still have this to do. I am having trouble finding sources for mje15030/31 TO-220 parts.
Checked digi and mouser so far.
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Old 28th April 2006, 04:01 AM   #2
infinia is offline infinia  United States
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Relay using socket with standoffs
resistor mounted on standoffs.
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Old 28th April 2006, 04:02 AM   #3
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closeup of relay
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Old 28th April 2006, 04:05 AM   #4
infinia is offline infinia  United States
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bottom view
Location of four added fasteners, only had to drill one hole.
Watch out don't drill your toroid.
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Old 28th April 2006, 04:07 AM   #5
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Bypass caps and two 2.4 K resistors to the relay coil.
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Old 28th April 2006, 07:06 AM   #6
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Digi-Key normally stocks the MJE15032/33 but is out of stock right now.

You could get the MJE15034/35, they have stock on these(and cost about $1 more each).
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Old 28th April 2006, 04:57 PM   #7
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Dear Mr DJK,
Thank you for helping me find the driver transistors!
The alternate parts you suggest have about 1/2 the current and x2 voltage. My main concern is the hfe specs are quite different (much lower from Ic=1A on). They are both 50W devices.
Do you think this is OK ?
Also, are SOA specs a good thing to compare for alt. drivers?

FYI I have started to remove the old parts today. Darn Adcom they super glued the thermistor to the TO-220 driver case. I hope a razor between can seperate them. I really don't want to hunt for new thermistors too.
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Old 28th April 2006, 10:58 PM   #8
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The stock part Hfe drops like a rock after 100mA or so, that's why it needs to be replaced.

1A driver current is way more than enough to drive the output stage into full power, even at low impedance.
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