Re: TECH: System 80 Power Supply and CPU - 5V voltage drops
- From: Matt <sirmistermatt@xxxxxxxxx>
- Date: Fri, 18 Apr 2008 06:48:52 -0700 (PDT)
On Apr 17, 6:57 pm, Matt <sirmisterm...@xxxxxxxxx> wrote:
On Apr 17, 4:40 pm, Ken <jetsurg...@xxxxxxxxxxxxx> wrote:
On Apr 17, 9:53 am, biscuitbe...@xxxxxxxxxxxx wrote:
On Apr 17, 5:33 am, Matt <sirmisterm...@xxxxxxxxx> wrote:
Guys,
Interesting observation onGottliebSystem 80 power supply, when I
have it hooked up to the CPU board. (This is a little bit "mental
mastubation," but I can't work on my board now and I am super-
curious...if anyone would care to indulge me.)
I have done the repairs on most all parts that appear to have been
corrosion damaged (taken it down to bare copper, vinegar scrub, wash
off, etc.)
My power supply tests perfect without the CPU attached, both led's
strong, voltages correct.
There are more that might be damaged obviously, but here is my LED
behavior with the CPU board connected:
- Fire up the game.
- Both LED's at roughly full brightness.
- 5V led starts to "blink" like a street light
- 5V led dims from there on out (until I restart again, of course).
By this time I can get over there and test the voltage which has
dropped to 1.8V, post "blink period."
This is the same voltage I was ending up with when I tested the CPU
"pre-mods."
I put the Dallas/Maxim transistor in to replace the reset section in
the interim. I think at this point the board only has a few tiny
capacitors and a diode that I have not yet changed out in the reset
section area.
Also, pin 40 of U1 registers 0V, same as before.
Any idea of how typical this is and what typically causes this? The
pinrepair.com guide mentions the existence of voltage drop but doesn't
specifically say what typically causes it when it is there.
As for the clock section, I still have to replace Z2 and a few other
parts that have some mild corrosion damage...but from the guides and
so on it doesn't seem like those come before the reset section in
terms of ability to test pin 40 of U1.
Any ideas?
Best,
Matt
Hello Matt,
in addition to Ken's fine treatise on System 80 power supplies, you
should simply replace Q3 with a new piece; this is a workhorse in the
circuit, Also, the filter cap for the 12 VDC in the cabinet bottom
needs to be replaced with a new one. Perhaps you have already done
this, I do not remember.
Regards,
Dan- Hide quoted text -
- Show quoted text -
The main point I wanted to make, is that my Q3 was brand new. It was
the thru-holes that were to blame, not the transistor... Yes,
definetly replace this, although, IIRC, it comes in the GPE kit.
-Ken
Preliminaries:
- I tested for continuity between the said transistor's case and the
ground plate. There was none.
- I replaced the orange filter cap with a brand new computer-grade
one, just like in guide.
- I replaced all of the "required" replacement parts on the power
supply per the pinrepair.com guide (not the big silver transistor
though).
- The PS tests *absolutely* perfect with no cpu attached.
- All the pins on my A1-J2 cable have silvery color and great
continuity.
Which is surprising...so you are saying that the big transistor might
spit out perfect voltages with nothing attached, but then when it has
heavy lifting, it craps out?
I finished the work on the board tonight per the guide. The voltage is
now at 1.5 at the 5V test point, so it dropped somewhat. Also, I am
getting a 0.3V signal from pin 40 of U1.
Does this change any of the above advice?
Please advise.
Best,
Matt J.
I
MAN GOOGLE GROUPS IS MESSED *UP*!
No posts are showing up beyond maybe 8PM last night...wild.
Anyhow, Ken, Ed and everyone, I saw your responses on another
newsreader (pasted above):
To answer Ken's questions:
- The PS tests *absolutely* perfect with no cpu attached.
<This is where I want clarification -- By perfectly, you mean all test
points on the PS show correct? Or are you checking the outputs on the
connectors as well? Very importnat because if you have a broken
solder joint or crispy connectors, you basically have a resistor in
there dropping the voltage>
No did not check connectors, but did verify that the cable itself has
very little resistance on it end to end. Maybe 0.2 ohm, but then again
my meter is a $20 Home Depot special...full sized with diode test
setting, but that's about it.
I will check connectors and pins while PS is fired up. Moving on...
I finished the work on the board tonight per the guide. The voltage is
now at 1.5 at the 5V test point, so it dropped somewhat. Also, I am
getting a 0.3V signal from pin 40 of U1.
<something is definetly wrong here (thank you Mr. Obvious . Again,
do you mean the PS test point or are you measureing at the common
capacitor test point on the MPU. If I read this correctly as at the
PS test point, then your voltage is droppping as soon as you connect
the MPU? This is the exact same problem I had and it was Q3. I also
has cracked solder joints, but didn't worry about them as I had
planned to (and did) change all of the J connector pins as well as the
connector contacts.>
For the first test of the 5V test point, I put black lead on Common
test point and red lead on 5V test point.
For Pin 40 of U1 test, I put the black lead of my DMM on the ground
plate of the PS, and the red lead on pin 40 of U1 on the CPU. That is
when I got the 0.3 VDC reading.
As for the pins on the PS, I did resolder the existing pins' board
connections (they all looked fairly silvery, just old), but have not
actually tested their output directly from the board or through the
cables themselves. Will do so tonight.
<Hope the above responses help. You oviously have some voltage drop
occuring. Will need to know if you get 5V when disconnected and then
it bogs when you add the MPU or if you are intermitant on the 5 V as I
was. You may want to tap around the connectors and Q3 transistor to
see if you get your 5 volts back (however quickly they come and go)
and if you are getting some kind of pulse that correlates with your
tapping, then suspect a bad connecton involving the Q3 and / or solder
joints / connector pins.>
Yes, I do get 5V when the PS is "standalone." Nice, solid 5V.
Yes, it bogs when I attached the MPU and then repower. It sputters (as
described above) with blinks, then dims and I read that 1.5V at the 5V
test point on the PS.
I will do the tap test as well and report back.
Dan, I will check the bridge rectifier. If the connection test dosen't
solve the problem, I will replace the Q3 transistor.
Also, will fix the remaining corroded parts.
Ed,
When you say measure the current draw, what does that mean in terms of
what test points to use?
Also, I replaced all parts on this board with the recommended
replacements from pinrepair.com. I also replaced the trim pot, which
works fine (I can move voltages up and down just fine with it, in the
correct range).
I will perform the load test as described.
Thanks!
Thanks for the help so far fellas.
Best,
Matt J.
.
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