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Hi all,

Scored a very nice condition 7104 and plugins at auction for a great price. Long story short it was working during the item viewing hours the day before the auction. Got it home this morning and it's clicking...anyway...

I believe the issue lies in either the +5V rail or the +/-15 V rails. However, specifically, the manual says the typical resistance for the +5V rail when everything is connected up is 0.005 Kohm. I'm measuring 3 ohms. Is there any chance that is a typo and it should be 0.050 Kohm and not 0.005 Kohm? It's the former for 7904 mainframes.

Here's my readings:

With the mainframe circuits connected:

+50 is at 1.82 kohm
+15 is at 73 ohm
+5 is at 3 ohm
-15 is at 84 ohm
-50 is at 4.494 kohm

With the mainframe circuits disconnected:

+50 is 9.28 kohm
+15 is 154 ohm
+5 is 7.53 kohm
-15 is 8.55 kohm
-50 is 30.2 kohm

Thanks,

Sean

On 8/9/19 4:42 PM, [email protected] wrote:

I believe the issue lies in either the +5V rail

Seems likely. Can you rig a little current to flow through a resistance to be mild like
20 milliamps total, then do you have a 6 or 7 digit voltmeter? Probing will show dropping volts along power
rails until you get next to the offending tantalum cap, then not change much if probing farther along than that...

On Fri, Aug 9, 2019 at 03:39 PM, John Griessen wrote:

On 8/9/19 4:42 PM, [email protected] wrote:

I believe the issue lies in either the +5V rail

Seems likely. Can you rig a little current to flow through a resistance to be
mild like
20 milliamps total, then do you have a 6 or 7 digit voltmeter? Probing will
show dropping volts along power
rails until you get next to the offending tantalum cap, then not change much
if probing farther along than that...

Input it where?

I've been poring over the schematics trying to map out in my head where everything is.

On 8/9/19 5:47 PM, [email protected] wrote:

On Fri, Aug 9, 2019 at 03:39 PM, John Griessen wrote:

Can you rig a little current to flow through a resistance to be

mild like
20 milliamps total, then do you have a 6 or 7 digit voltmeter? Probing will
show dropping volts along power
rails until you get next to the offending tantalum cap, then not change much
if probing farther along than that...

Input it where?

Right where the 5V is made, at its SMPS source. or rectifier and caps, (I have not looked at a 7104 schematic today), and relative to a ground point that is the 5V return.

So, that way, as you probe along in the 5V loads, and/or hold up caps, one will be more of a short than the whole
and you will notice that lowness.

If you no longer suspect the 5V and switch, *DO* switch the return GND point also to be related to the new suspect.

Most, if not all, of the boards have "harmonica connectors" for the supply input. When I am faced with this, I connect the ohmmeter at the PSU perhaps, and, noting the resistance, remove the connectors one-by-one, checking to see if the resistance reading jumps up a bit. If it does, you have likely found the board with the shorted cap. From there it is usually a simple matter to check the resistance across the caps on the board.
It usually is about this simple, especially on the 7000 series. It is also possible that the short is in the PSU proper, again, disconnecting it from the main scope should reveal the location.

So I had a considerable breakthrough. While I still don't know WHAT is shorting out, I have determined that there is no electrical problem as far as I can tell. It's mechanical. I "fixed" it with my tinkering...then it started clicking again when the SMPS module was reinstalled in the mainframe. Thinking something is pushing on something in there when the module is installed.

It's the fan. If I pull on the power leads that go to the fa, the SMPS will start. Maybe something's rubbed up in there. Have to take a look.

Happy ending: the problem was the fan wiring all along. I rearranged it before I inserted the SMPS for the last time, and we have life.

Cangratulations!? I like when stuff is easy.
Bob

Please explain the solution. How does a fan interfere with the switching
power supply?

--
Gary Robert Bosworth
grbosworth@...
Tel: 310-317-2247

On Sat, Aug 10, 2019 at 09:19 AM, Gary Robert Bosworth wrote:

Please explain the solution. How does a fan interfere with the switching
power supply?
Hide quoted text ( #quoted-71154881 )

On Fri, Aug 9, 2019 at 9:49 PM <[email protected]> wrote:

Happy ending: the problem was the fan wiring all along. I rearranged it
before I inserted the SMPS for the last time, and we have life.

The best way to explain it is probably a picture: /

Note that little tab? It was intermittently touching the chassis. This morning I made a neat sandwich of electrical tape to put underneath it and now the problem is solved.

Sean

I would think that a better way would be to properly align
the resistor so that it is away from the chassis. It got
bumped.

-Chuck Harris

[email protected] wrote:

On Sat, Aug 10, 2019 at 11:41 AM, Chuck Harris wrote:

I would think that a better way would be to properly align
the resistor so that it is away from the chassis. It got
bumped.

-Chuck Harris

I did. The electrical tape is just to stop it from happening again if it loosens.

Sean

Hi Sean,

My question should sound obvious, but... Did you try to power on the 7104
without the plugins?

I have that oscilloscope (now under repair, due to the "usual" issue at the
mains filter...) - as well as a 7904A and a 7854. In the 7904A I
experimented a PSU issue due to a bad 7A26 plugin: once removed, everything
turned to normality.

Tomorrow I could check the values of resistance on my 7104 and share them
with you.

Alberto, IZ2EWV

The tape certainly won't cause any harm, but there were
tens of thousands, and still are hundreds of these scopes that
never suffered this failure on their own.

My guess is someone got aggressive with the wire, and tugged
it hard, which caused the resistor to rotate into the chassis.

Good job in finding the problem.

-Chuck Harris

[email protected] wrote:

On Sat, Aug 10, 2019 at 02:43 PM, Chuck Harris wrote:

The tape certainly won't cause any harm, but there were
tens of thousands, and still are hundreds of these scopes that
never suffered this failure on their own.

My guess is someone got aggressive with the wire, and tugged
it hard, which caused the resistor to rotate into the chassis.

Good job in finding the problem.

-Chuck Harris

Or someone at the auction preview decided to monkey with it, knowing it would cause a short and make it seem like the scope is dead. Either way, it's working now.

Sean

On Sat, Aug 10, 2019 at 02:08 PM, Alberto, IZ2EWV wrote:

Hi Sean,

My question should sound obvious, but... Did you try to power on the 7104
without the plugins?

I have that oscilloscope (now under repair, due to the "usual" issue at the
mains filter...) - as well as a 7904A and a 7854. In the 7904A I
experimented a PSU issue due to a bad 7A26 plugin: once removed, everything
turned to normality.

Tomorrow I could check the values of resistance on my 7104 and share them
with you.

Alberto, IZ2EWV

Hi Alberto,

I actually got it working (was a dumb short to the chassis caused by that resistor under the fan), but I'd be interested in seeing your numbers just as a data point for future reference.

Thanks,

Sean