Re: 7904 Mainframe damaged

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 a open circuit while trying to get back to the source.  I suppose you already 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 should
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 board.
I didn't check for voltages before taking the PSU out because I didn't know
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 found
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 two
5A diodes in parallel temporarily - hoping that's OK, I'll then hook up the
mainframe's cables to the PS and check the voltages. I think the 3 cables
are long enough for the PSU to hang out the back. If not, I plan on making
extender cables.


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

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

And then voltages with the supply outside the scope but still connected?


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

Hi Tony, So, based on your findings, you have confirmed that the the PSU
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 of
your 7904's issues.


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

Hi JJ :
I am from Hong Kong ! I just complete the repair of PSU of TEK7904 , I am
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 and
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 in
units , Y are not function -- position knob no response .

RegardTony CheungAPR 10 2018

From: JJ <jajustin@...>
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 loading
requirements for power supplies. Tried searching, browsing, ad nauseum!
glory. Are you sure that Jerry uploaded it?


On Mon, Apr 9, 2018 at 4:54 PM, Dave Daniel <kc0wjn@...> wrote:

On some of the 7xxx supplies (SMPS supplies, I believe), one needs to put
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 copy of
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. First,
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 in
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 and
low voltage regulator. I wouldn't think there would be that much load
the supply removed? That way I can continue to debug. Thoughts?

On Mon, Apr 9, 2018 at 2:06 PM, Jeff Urban <JURB6006@...> wrote:

Incidentally, that stuff about the mini EMP is not very likely. Nott
does it actually have to happen at a strength to do something, it also
needs to be oriented the right way to do something. Also I erred about
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 do,
possible. It might be rough to gt everything hooked back up for a live
without actually assembling it. Changing the diodes right away ? You
do that. And of course watch for mounting screws that also function as
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 but
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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