DEC H7104-D debugging...

Mattis Lind mattislind at
Thu Mar 24 05:25:47 CDT 2016

> So I had a few minutes tonight to investigate a few things:
> - The startup PSU is running, and the relay in the 5V supply is firing and
> stays energized until the supply is powered off.
> - The +5V, and +/-12V bias voltages are being properly generated. This
> makes sense, since the 2.5V supply is working fine.  From my understanding
> of the supply, this means the 300VDC is also present (also validated by the
> relay firing.)
> - The 20Khz clock generated on the bias board is present (measured from
> pin 8 of J1 on on the bias board), at the proper rate and duty cycle.
> - 5V AC and DC LO signals (measured at J1) are .3 and .9V, respectively.

Then a lot things seems to be working. Is the main chopper transistor

First check that you have a signal on the primary side of the pulse
transformer  T2 on the mother board? Driven through B22 / B24 from the
control board. Q6 collector.

If not then check backwards towards the NE556, Q6 and Q15. E9 pin 9 and pin
5. E8 pin 3. These outputs are OR:ed together via diodes and drive Q15 base.

If yes then you need to investigate the secondary side of the pulse
transformer T2 on the mother board. (Now it is getting dangerous if you are
not careful). Is there base drive on the power transistors?

> So far, so good.  I also measured the 5V Overcurrent and 5V Crowbar on J1
> (pins 2 and 19) and they're at 12V and 8V respectively.  I'm not sure I
> understand what this indicates; the Overcurrent lamp is not illuminated on
> the power controller, for example, and per the tech manual (section 4.4.2)
> if an overvoltage occurs and the crowbar kicks in, the supply should shut
> down for ~1 second, which I'm not seeing any indications of (I'm not seeing
> the voltage reach anything over .2V with a small load attached).  The
> overcurrent protection appears to work similarly.  I don't see an
> indication of what the levels for the signals on J1 are supposed to be,
> however.

The NE555 will trigger on a low going TRIG signal, so if either the CROWBAR
SENSE (which is generated by a SCR that sinks this singal to ground when
fired) OR the +5V OVERCURRENT goes low it will trigger. These signals are
active low. I.e you don't have a over current or over voltage condition.

> I'll hopefully have more time to debug later this week, thought I'd report
> my findings and see if anyone has any bright ideas.

Good luck!


> Thanks as always,
> - Josh

More information about the cctech mailing list