Re: Tek 7904 help!

Harvey White

Comments interspaced.  While I have a 7904, I don't remember these particular problems, but these ideas might help.

On 7/8/2020 7:30 PM, Bill Riches via wrote:
My 7904 just stopped working.  I have had it for about 5 years - no problems - used several times a week.
1. No blown fuses
No blown fuses says that the bulk supply, rectifiers, and filter capictors are likely ok, but does not say anything about the bulk capacitors not being open.  Check them by looking at ripple on the bulk supply if you have another scope.
.2.  Powered with metered Sencore variac -  it draws about 10 watts after initial kick and meter pulses slightly.  I assure it is in pulse mode.
It would say that, and it makes sense.  Been there, think I've done that.

3.  High voltage warning light is on and when power is removed it goes out after a few min.
Not sure that this is significant, depending on where the HV supply is derived.  Things at this point suggest that you start disconnecting power supply and see if it works.  Note that supply may need a minimum load on one supply to regulate.  Not sure about this.
3. Removed all plugins and it still would not power up.
Plugins therefore are not the only problem.  If using a plugin in a working scope brings the supply down, then it's a plugin.

    4. Removed ps and checked all caps and diodes for shorts.

Then the PS, unless it needs a minimum load, should be ok for shorts.  If it will work without a load, look at the supply that has an adjustment.  It will be a reference supply.  Tektronix liked to use a negative supply.  That must be very close for the other supplies to be within tolerance.

5. Checked resistance readings of supply voltages with ps connected and unconnected - they are close.

I think here you need to define "close".

The supply generally ticks (in startup mode) when there's too much current drawn.  Something's got to be causing that.

I have not checked out ps with 20 volts input and using test scope yet.
Go for the test scope.  See if any supply actually gets to the desired voltage before the scope PS shuts down.  Storage scope would be good, but slow sweep and dim light can give you an idea.

I would like to rule out ps problems and wonder if any of our group would have a working ps that I could purchase.
Nope, not here but it's diagnosable... and fixable, generally.

Question is:  what supply is shutting the scope down.  IIRC, there's a resistor from each supply to a summing junction, and then that junction has to be right for the main supply (not bulk) to be running.

So your problems can be twofold, one is that there is a supply demanding too much current, another is that there's a supply overvoltage.

Since you have a variac, you might want to run the voltage up from zero, looking at the outputs of a supply and the inputs to that regulator.  Until you get to about 80 or 90 volts, you can expect the supplies to be a little under.  If any supply overshoots sufficiently, then you're messing with the overvoltage protection.  SImilarly, look at the overcurrent protection on any supply, it's generally a transistor BE junction across a sense resistor, and that sense resistor in the output of the supply just after the regulator.

Watching that voltage (either one) as the supply ticks should show you a lot.


Any help would be appreciated.
Bill, WA2DVU - (good on QRZ)Cape May, NJ609 425 8651

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