Reparing a dead FG504


The following lists my findings and maybe some hints for others fighting with the tricky (but ingenious) construction of the Tek 504 while trying to repair it.

The unit was dead - no lights, no output. Obviusly someone had it plugded in/out under power...

Measuring the supply voltages showed some volts on the - 25V line, the others were zero.
Fuses 1616 (+25V) and 1715 (-15V) were blown.
Since the 25V lines depend on the 15 V - and both share an overload crowbar switch - circuit isolation had to be the next step.
Internal connections to the +-25V auxiliary power supply board were removed.
Fuses were replaced.
The Main board only (with the +-15V pwr suplly) was powered via an extension cable from a TM500 mainframe.
This showed both 15V supplies being operative.
Externally supplying +-25V (via the testpoints on the main board) showed a high current flowing at +25V.
Measuring the +-25 V supplies (on the testpoints on the main board) against ground with an ohm meter showed a short circuit on the +25V.
After checking several suspects (all power transistors and diodes connected to +25V) without success, finally the typical culprit was found:
a tantal capacitor (C978) between +25V and ground - it did not show any outer signs of damage.
Removing the tantal brought the complete circuit back to life.
After replacing it with an identical new type - it blew again next time at power on!
Obviously the initial voltage after power up (and to much current) do this damage.
Replacing the tantal with a 50V rated type fixed the problem.
When checking the functions everything worked except the square wave function on the main output.
Again - the tricky Tek circuit is generating the square wave out of the triangle via kind of a feedback switch around U375.
Following the signals showed it to arrive at the base of Q450 which was obviosly killed by the preceeding power supply problems.
As this SPS6927 is "unobtanium" I was lucky to find the information in another post that it it is equivalent to the MPSH81 - which is stil available at below 1€/pce at ebay Germany.
Now all works well.

Just one last comment on modern vs. good old analog Scope technology:
I did all checks for the repair with my small digital tek TBS1152 - working fine.
But for the calibration - step 15 requires you to do an amplitude flatness check of the swept output and monitor the envelope curve.
That proved to be impossible with the (damned) digital thing - but no problem at all with my nice 547/1A4 combination with stable external trigger.

Bests, Gordian