Minimum 7904 supply currents from H?kan's earlier message:
-50 V: 0.08 A
-15 V: 0.8 A
+5 V: 0.4 A
+15 V: 1 A
+50 V: 0.2 A
+ 130 V: 0.036 A
+5 Lights: 0.3 A
Don't forget to check the caps after the rectifiers but before the regulators. You can go with a DVM across each cap and check for a short, some tantalums on the supply look like rectangular dipped mica capacitors but they are tantalum caps. One of them was bad on my 7904.
Supply resistances measure higher than the troubleshooting manual, likely a difference in DVM used as you also guessed.
Ozan
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On Wed, Jan 8, 2025 at 01:44 AM, William wrote:
Thanks in advance for the excellent help I have received already just by
reading.
In an nutshell: 7904 from fleabay advertised as good work but is not - no
trace no beam find no sawtooth. Seller very kindly offered me partial refund
as seller doesnt want the BA back.
Teardown -> -15V supply at -0.6 V (yes, I know that number ;)) all other
supplies that I had the tools to check are within 1%. Pulled power to each
board 1 by one -> still -0.6V. Of note, pulling Z axis board power beam
unblanked but no sweep or nuthin.
Pulled PS and it is in tik tik mode when disconnected as it should be. It
also goes into tik tik mode if no plugins, which I do not think its supposed
to do.
All supply resistances much much greater than listed in manual - I assume
autoranging DMM is driving the semiconductors different than Tek forsaw a half
century ago
-15V transistor fell apart while desoldering from board - not sure if stigmata
of overheating or I was just cavemanning it. Replaced and replaced its
driver. Still no love.
- Pulled main interface board thinking C60 might be culprit - nope! no short.
Ugh now gotta reassemble.
- Measured resistances of the tantalums on the rectum fryer board - seem all
OK or at the very least not dead shorts.
- have to get it together to test further. I might be able to borrow a
variac. I don't really feel like putting together a fancy dummy load setup
just for this one thing (lets all remember I thought I was buying a working
scope to use on my OTHER projects) but would be willing to spend a little on
some resistors to finish the troubleshooting.
but -17 seems ok and during the burst the - 15 is at about -17. The
waveform, though, isnt pretty as shown in the manual - it looks more like the
-17 or so is actually the envelope of a much higher freq signal.
The ribbon cable ends are turning to dust in my hands.
Any assistance you might offer in next step troubleshooting, or some minimal
load current values to keep it out of burst mode while troubleshooting would
be great help.
Thanks in advance!
Bill
