From: steve@zero.com (Steve Urich)
Newsgroups: comp.sys.3b1
Subject: Expansion Box PS fix story
Summary: Fixed Dead PS without prints.
Keywords: Expansion-Box PS Story
Date: 13 Oct 92 19:24:27 GMT
Organization: Beyonet - Zero Network


	Hello Expansion Box fans. Last weekend I woke up to a dead 3b1 system.
	I was very unhappy to find out that my expansion box had stopped 
	working. Its my source of 6 ttys and the use of the DOS-73 board
	which I am still learning how to setup properly. I still have one
	slot free for another board for the future.

	Anyway it was dead, the +5V and -+ 12V leds were not lit and the 
	fuses were good.

	I took apart the box and removed the switching power supply to 
	inspect its components. After the inspection I noticed a diode that
	was near the Fan end of the PS PCB next to the Hi voltage switching
	transistor that is mounted on the upright single heat sink. It was
	showing a bad solderjoint at the heatsink end of the trace of the
	diode. The diode checked ok and I resoldered that bad joint.

	Well, I put 2 + 2 together and took out the HV switching transistor
	heatsink and all. I checked it and it was totally shorted. 
	Since I don't have any prints on this PS I had to rely on the 
	"check all of the semiconductors that drive the HV transistor 
	and look for shorts and opens in caps and resistors" process :-(.

	[Tech note]
	The bad thing about switching power supplys are that they tend to
	take a kamakazi routine when 1 part fails usually. They are designed
	to stop working instantly if the feedback process of the design is
	halted. It usually isn't that simple :-)

	I replaced the HV transistor (2SC3461 Japanese) with a ECG-2300
	(Sylvania) sub. I foolishly tried the power supply after the replace-
	ment thinking that was the total problem, it wasn't. Kids don't
	repeat this mistake it might burn up your new transistor that you
	have installed. Fortunately it didn't burn up but gave me a good
	Idea what to look for next because the voltage readings were much
	different leading me to the startup pulse circuit and oscillator
	driver that controls the switching process of the HV transistor.
	You see the 2SC3461 fed approx 167 volts of dc and it switches
	that to the primary input of the transformer at an AC rate. 
	
	[Another Tech note]
	This is a cheap and dirty layman description of what the PS does.
	The 120vac gets isolated from the Electric company by AC coupling
	caps after the fuse on the PS PCB. It gets converted to DC by
	a fullwave Bridge rectifier and doubled by a bunch of electrolytic
	capacitors. This gets fed to the HV switching transistor.
	The HV switch get jump started by a resistor, cap combo and is
	controlled via other components after that. The transformer takes
	the switched dc in the primary and converts it to AC again (Stupid
	huh :-). then it gets converted to DC again (here we go again)
	and filtered and regulated by a gauntlet of electrolytics and 
	regulators. 

	Well there is a vertical circuit board soldered onto the PS PCB
	with pins. You have to unsolder that board to get to the controlling
	components. 

	I found a electrolytic (The only electrolytic on that board) to be
	shorted. I was the 330uf @ 16V. The Q2 that was off that cap was
	shorted out also. Its an 2SD468 which regulated the switching of
	the 2SC3461. 

	All of the other semiconductors were ok. 

	Last but not least it still didn't work after I put the components
	in but at least they didn't burn up. I had to find something open
	in the startup pulse circuit that energizes the switching HV transistor
	just like in a TV set. Everytime you turn on a switching PS device
	you will get the whistle of a HV sound. Its a very short whistle.
	You get them when you turn on a computer monitor, TV etc etc.

	I found the resistor which looked perfectly good on the outside but
	was open electrically. It was a .33ohm flameresistant (Thats why
	it look good :-). It was located on the Fan side of the PS PCB
	next to the electrolytic caps. I believe it was R4 if I remember
	correctly. 

	I replaced that resistor and voila! The expansion box was back in
	business. So that was what I was doing all day yesterday, trouble-
	shooting the PS and running around getting the parts. I had to
	wait till monday to get the parts anyway right :-).

	I hope this will help those with expansion boxes get an idea  of
	what to look for even if they don't have any prints available to
	them.

	Share and enjoy!!!
	Steve



	

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Newsgroups: comp.sys.3b1