After another trip to Germany, another HP 8753C to fix. This unit had option 020, 006, a 6 GHz unit, but there is no 6 GHz test set.
First, we need to get a suitable YTO, found a good ASF-8751M, from Israel. Cleaned it up and gave it a proper test.
It is a 4-8 GHz unit, but I easily got good power down to necessary 3.6 GHz. It is a well-behaved unit, with reasonable power consumption running of +15 and -5 Volts. The heater may be better run with 24 Volts, but there is only 15 Volts in the 8753C, and it is good enough it seems.
Some modification of the PLL board, as described before, to approximately double the tuning current, installed a 20 Ohms sense resistor, and installed a BD249C transistor on a good heatsink.
A quick drawing of the heatsink, should you need it. Use 1 mm aluminum sheet. Don’t cut yourself, when cutting the metal!
The YTO, installed in the veritable source assembly. Pretty confident that this will last for a while.
This time, all worked well and the pretune correction functioned immediately, no further adjustments needed. Phase lock seems very stable at all frequencies, scan rates, and band transitions.
Out of curiosity, did a phase noise test of the 8753C in CW mode (fixed frequency mode), getting well below 100 dBc. Pretty good. Maybe better than the original YTO.
For the current unit, I also wanted to update the firmware, and install the 010 option (time domain analysis). The option installation (and EEPROM backup), done like described in an earlier post, but desoldering the EEPROM, and changing three bytes…
The unit is still running pretty old firmware.
Should be easy enough to program some 27010 EPROMs, but the devil is in the detail. After a number of incorrectly programmed EPROM, finally figured out the once of the CD4015 CMOS of the EPROMMER had failed! Fortunately, I had some in stock to fix it.
After these efforts, the 8753C is starting up with the latest (albeit, dated) firmware, and all options.
A few tests with filters and such, a very useful and well working unit. The CRT also very good, no need to install a LCD.
