Re: Is there a fuse on the main board of the 9114A? Message #16 Posted by Tony Duell (UK) on 3 Dec 2002, 5:59 p.m., in response to message #15 by Daniel
OK, Vc is fine. The +12V testpoint will be at battery voltage (+6V) if the +12V chopper isn't running. That's probably right too if the +5V line is missing. So let's start by sorting out the +5V PSU.
HEre's how it should work : U105 (MC1403 I think) is a +2.5V voltage reference. The output of that goes (amongst other places) to the top end of the potential divider formed by R105 and R104. This maintains the +ve input of U101a (LM358) at +0.5V.
The -ve input of U101a comes from the tap on the potential divider formed by R107 and R106. The top end of that goes to the +5V line. R017 and R106 are chosen so that the -ve input of U101a is at 0.5V when the +5V line is correct. Thus U101a acts as an error amplifier, comparing what the 5V line _is_ (tap of R107 and R016) with what it should be (tap fo R105 and R104).
The outptu of U101a goes to the base of Q102 via R102. Q012 drives the base of Q101, which is the pass transsitor between the battery voltage and the +5V line. If U101a detects that the +5V line is too low, then it drives Q102 a little harder, which in turn drives Q101 a little harder, thus bringing the +5V line up a bit.
There is one more circuit to consider.Q103's collector is connected to the base of Q102, and its emitter is grounded. Therefore id Q103 is turned on, Q102 and Q101 will be turned off, thus turning off the +5V line to save the battery.
OK, how to debug it. Start by measuring the output of U105 (e.g at the -ve input of U104b (MC3302). If it's not 2.5V, check U105, CR102, and look for shorts on the reference line.
Then check the votlage at the +Ve input of U101a. If it's not 0.5V, check U101, R105, R104. Then check the voltage on the -ve input of U101a. If this _is_ 0.5V (even though the +5V rail is low), then check U101, R106, R107.
Now check the Q/ ouput (pin 8) of U6b (74LS74). This should be a TTL low. If it's not, then the PSU is being turned off under software control. Find out why. Check U6. Check U15 (very unlikely). Check the output of U104b (MC3302) with a 'scope. This is a reset line for the HPIL chip and power-on logic, and should go low briefly at power-on). Check CR101 too.
OK, if it's not being turned off by software control, then check the 3 transistors Q101, Q102, Q103.
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