Re: Tek 7904 help!

Chuck Harris

There are two tantalum capacitors that every 7904 I have had
has blown. One is on the +50V supply, and doesn't show up in
my schematics, and the other is on the -15V supply, if I recall
correctly. They both are rated at too low of a voltage.
They are on the Horizontal board, which is on the left side of
chassis, as I recall. One has a choke feeding it, and the other
a 10 ohm 1/4w resistor.

You should be suspicious of any tantalum capacitor that is rated
less than 2x the applied voltage. 6.3V caps on 5V, and 16V caps
on 15V are especially bad. Anything on a 50V supply is a problem
because the maximum voltage a dry slug tantalum can be is 60V.

Be warned, there is a blue harmonica connector on that board
that turns to dust when you touch it.

The power supply itself is usually pretty solid. Look
at the rest of the scope for a blown tantalum.

-Chuck Harris

Bill Riches via wrote:

Thank you Raymond and Harvey.  Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell!  Will check them in the nether regions!
On Wednesday, July 8, 2020, 08:53:08 PM EDT, Harvey White <madyn@...> wrote:

When I used a variac, I looked at the current draw.  If it turned out to
be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran
it up to the 80 to 90 volt range if the current draw was excessive.  I
used the variac to check if the main bulk supply was shorted.  When it
wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies
draw excessive current because the switch doesn't come on (and off)
until a minimum voltage, that's the reason for being sneaky in this
case.  You try to avoid that overcurrent draw due to the switching
circuit not working.

More of a turn it up and see what it draws, turn it right back down,
then figure out why.

think that'll work?


On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul  9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.


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