Re: 7904 Mainframe damaged

Chuck Harris

The problem is, the current from the probe's ground
lead is directly connected to the chassis at the BNC
connector on the plugin's front panel, and at the
finger stock around the front panel, and at the side
rails along the bottom and top of the plugin slot...

And then the chassis of the scope is directly connected
to the ground pin on the power cord... Which was
connected to the ground pin on the isolation tranny,
which was connected to the building's ground, and at
the service panel to the mains ground lead (neutral)...

So, how much current were we talking here?

Enough to vaporize the ground wire? Not from what we've

Did it even burn the plating off of the ground clip?

Enough to blow a fuse somewhere? Not from what we've
heard.. AFAICR.

And, exactly what was shorted by the ground wire?

Mains? A filter cap on a switching supply? An 800V
power supply on a monster tube amp?

I'm having more than a little trouble believing the
presumed cause fits the symptoms. Something else is
going on.

A very popular failure in the 7904's 50V power supplies
is a 50V tantalum capacitor on the Horizontal amplifier
board. It doesn't seem to show up on the schematic in
the obvious place, and I don't remember if it is on the
+50, or the -50V supply, but it is there. Usually, when
it shorts, it incinerated a little 10 ohm resistor placed
there as a fuse, and eliminates the horizontal amplifier
from the circuit. If the resistor is not killed, the
resistor takes down the current overload, and shuts down
the supply.

I'd look there.

-Chuck Harris

Robert Hay wrote:

Just to re-cap, your initial message indicated your probe ground lead contacted a
high voltage point on a device under test with a common ground. This would raise the
scope vertical input ground and cause high current to find its way thru your scope.
This apparently caused a failure in the scope.

Your resistance tests don't point to a short on any of the LV supplies. The author
of the old article used a VOM so I would tend to compare VOM readings to those given
in the article. If you tested using both Fluke and VOM did you find much differences
in readings?

The minus 51 V load resistance is much higher than the article measurement. The plus
5 is lower resistance. So, those might be the places to look. Did you check the
diodes and caps for the -51 volt supply? If they are good, you may be looking for
something failed open on the mainframe side since your measurement of 31.6k with the
LVPS disconnected is much higher than the 2k given in the article.

Usually, the tic-tic mode is the power supply letting you know there is a short on
one of the supplies and saving itself from further damage. As far as testing it
without load to see if you still get the tic-tic or not it would be good if one of
the more experienced members with testing the 7904 supply jumps in here.


On 4/10/2018 4:37 PM, JJ wrote:
In reading that doc, I'm concerned that the problem is in that inverter
control chip. That's probably impossible to find. I found the schematic of
what's inside it:

Z-axis Board voltages PSU in Mainframe
+130 8.3k
+50 3.7k
+15 90.1 ohms
-15 205 ohms
-50 4.2k
+5 47 ohms

+5V lamp 2 ohms

Z-axis Board voltages PSU out of Mainframe
+130 9,9k
+50 4.6k
+15 92.3 ohms
-15 216 ohms
-50 31.6k
+5 48 ohms

+5V lamp 23k

On Tue, Apr 10, 2018 at 6:55 PM, Robert Hay <> wrote:

Ok, you said the resistances were very high, how much higher than those
suggested by the article?

+130 volts 6k
+50 2k
+15 90 Ohms
-15 100 Ohms
-50 250 Ohms
+5V lamp 800 Ohms


On 4/10/2018 1:28 PM, JJ wrote:

Bob, I found that the diodes seem OK when I disconnected one lead and
measured both resistance and diode forward and reverse voltage. I found
that the short across those two diodes was due to a shorted capacitor on
the rectifier board - I was actually measuring the winding tap resistance
through a weird path. I reconnected the PSU back into the mainframe after
changing the cap. There was no glory - a very low tick coming from the

I'll need to continue debug by following the procedure in the that
document. I measured the resistances in z-axis board as the procedure
indicates while the PSU was out of the mainframe - they are pretty much in
line - a couple of resistances were much higher than the table - none were
lower . I'll measure them now while the power supply is in the mainframe,
Also, I get different results using a VOM and a DVM. Maybe there's
something wrong with my VOM - it's pretty old. Is a VOM required to get
proper measurements as indicated in the table?


On Tue, Apr 10, 2018 at 2:41 PM, Robert Hay <> wrote:

They say in the article that a low resistance indicates a problem in the
mainframe which is true if you are looking at something shorting to
ground. But, in your case I would not discount that high current caused
open circuit while trying to get back to the source. I suppose you
looked for any signs of burning or smoked components, leads, traces, etc.


On 4/10/2018 7:44 AM, JJ wrote:

Yes the resistances were very high. According to that doc, the issue
be on the PSU. I'm going to put the PSU back into the mainframe, connect
the cables, and make those voltage measurements on the LV regulator
I didn't check for voltages before taking the PSU out because I didn't
at that time that the PSU needs to be under minimum load.

I found a shorted cap C1360 on the rectifier board (+54v filter cap on
output of Pi filter). I'm going to lift one lead to ensure it's the
problem. I'm also going to validate that the two power diodes that I
are shorted - by disconnecting the wires going to those diodes and
measuring. If the diodes are bad, I'll replace the bad 10A diodes with
5A diodes in parallel temporarily - hoping that's OK, I'll then hook up
mainframe's cables to the PS and check the voltages. I think the 3
are long enough for the PSU to hang out the back. If not, I plan on
extender cables.


On Tue, Apr 10, 2018 at 10:16 AM, Robert Hay <> wrote:

Have you checked resistances as suggested in the Service Scope article?

And then voltages with the supply outside the scope but still


On 4/10/2018 6:38 AM, JJ wrote:

Hi Tony, So, based on your findings, you have confirmed that the the

needs to be under load in order to be functional. I will check all the
on those 3 boards as well. Thank you for the info - it's very, very
I hope the experts on this forum can help you out with the remainder
your 7904's issues.


On Tue, Apr 10, 2018 at 9:29 AM, Yiu On Tony C via Groups.Io <> wrote:

Hi JJ :

I am from Hong Kong ! I just complete the repair of PSU of TEK7904 , I
lucky , I find a SHORT E-Cap. once I replace it and turn on PSU
alone ,
still sound shut-down ! once I connect to main unit , it come normal
I found few E-Cap short on A9 H amp board, A2 Main interface board
A12 Rectifier board
For the power diode , I did check each by de-solder the to wire only
My 7904 still have other issue , H ok but the read-out still error
units , Y are not function -- position knob no response .

RegardTony CheungAPR 10 2018

From: JJ <>
Sent: Tuesday, April 10, 2018 7:16 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Dave, I went through hundreds of files in the files section of the
and wasn't able to identify the schematic describing the minimum
requirements for power supplies. Tried searching, browsing, ad
glory. Are you sure that Jerry uploaded it?


On Mon, Apr 9, 2018 at 4:54 PM, Dave Daniel <>

On some of the 7xxx supplies (SMPS supplies, I believe), one needs to

a minimum load on the power supply output in order for the supply to
I don't remember if that is true for the 7904 power supply.

Jerry Massengale built one of these loads. I think he uploaded a
the schematic to the files section back in maybe2015.


On 4/9/2018 12:22 PM, JJ wrote:

I removed the power supply from the mainframe - it's on my bench.

I'll remove the wire from the diode terminal and check to see if the
is actually shorted as others have suggested. I was thinking of
the 10A diodes with two 5A diodes in parallel that I have available
parts bin temporarily to see if I get all the other voltages back -
now there are no voltages at the test points of the rectifier board

low voltage regulator. I wouldn't think there would be that much
the supply removed? That way I can continue to debug. Thoughts?

On Mon, Apr 9, 2018 at 2:06 PM, Jeff Urban <>

Incidentally, that stuff about the mini EMP is not very likely.

does it actually have to happen at a strength to do something, it

needs to be oriented the right way to do something. Also I erred
vaporized foil on the board, it was about ½ cm., not inch.

Anyway, if you are sure about the diodes I guess you know what to

possible. It might be rough to gt everything hooked back up for a

without actually assembling it. Changing the diodes right away ?
do that. And of course watch for mounting screws that also

ground, that has tripped me up a couple of times.
Since you have one + and one - diode bad, assuming you don't have a
to ground, I would check see if there is a short between the + and

of that supply. At this time I have no idea what those sources feed

could be a series arranged push pull output to something and while
the output devices could be shorted, the load isn't low enough

or maybe even electrostatic, to read a short to ground. It happens.

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