DEC H7104-D debugging...

Josh Dersch derschjo at
Fri Mar 25 22:21:43 CDT 2016

On 3/24/16 3:25 AM, Mattis Lind wrote:
>> 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.
It does seem that way.  Thanks for the suggestions, I have a few (likely 
head-scratchingly obvious) questions below.  This is not my area of 
expertise by a long shot so some of these things are less than obvious 
to me.  Thanks for being patient :).

> Is the main chopper transistor driven?
How do I identify which transistor is the main chopper?  I've looked 
through the schematic and the tech reference and I've come up empty.

> 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.
These are the pins on J4 (on the motherboard) labeled "-TRIGGER" and 
"+TRIGGER" ?  Is this safe to probe with a standard 10x oscilloscope 
probe (assuming I bring some leads out to where it's safe...)?

> 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.

Makes sense.

> 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?
What precautions do you recommend taking here?

Thanks again.  I may have some time this weekend to play with this, but 
I have family coming to visit, so probably not :).  I'll let you know 
how I get on if I do, though...

- Josh

>> 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!
> /Mattis
>> Thanks as always,
>> - Josh

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