So after almost 10 I decide finally to have this instrument fixed. I bought this back 2014 from ebay and found some burned components, some zener have burned the board.
Than the problems really started:
- Had to source a front panel boards as I broke the VFD during the initial repair apparently
- Than I was able to turn it on for the first time, I quickly realized that something was wrong in the output amplifier, as I got some clipping on the waveform.
- Than one day the output was gone I traced it back to the main DAC been bad.
Than phase 2 of the repair started:
I started bought components from China to replace the DAC and some the transistor I though were bad, replace them and burn again the DAC.
When I had to change it again I destroy some of the pads on the mainboard
so the first stop moment started.
After a lot of years I decide to give another go at this, so I bough a new DAC and hack it on the remain pads with a lot of bodge wires, still it work for some time than the DAC was bad again.
So basically I give up.
Years passed and now I wanna get this working again finally!
So I decide to try to repair the broken pads with some PCB epoxy and the SMD pads kits you now find on ebay,
than I will resolder yet another DAC and give another go. This will start next month.
Now that you know the full story, I can begin the real post I wanna make about something that always bothered me:
Apparently the AGND you found in the schematics is not connected to the GND, the supply voltage of chips in the analog output section are referenced to AGND.
Those are supplied by a +15V volts that is generated from the +18V via a zener diode. The 18V is generated by U1002 that is referenced to GND.
The strange part is that the bypass caps on the 15VA is referenced to AGND!
In the schematics you can see that apparently GND/AGND/AGN2D/PGND are connected together to JM1006 but I cannot find any such component on the PCB (I suspect it is the big middle hole where the plastic support is inserted by the way).
When I measure the voltage between AGND and GND, I see around 6.5V!, If I connect the two ground together using a multimeter in current mode is see around 0.152A of current flowing between the grounds!
Logically I think those ground should be the same but I cannot really confirm it in any way. If the ground should be the same I see why I am destroys DACs as probably the output are biased above the maximum allow value due to this voltage difference!
One of the burned caps that I had to replaced (C707) did burn the board and I had to dig in it to remove the burned FR4, so I could have destroy an important trace, that with my luck was the one that connected the grounds together?
Anyone have any idea about this ground situation nation
?
Or have a 33120A that can open to test if there is any continuity between AGND and GND? AGND can be found at pin 2 of U603 as indicate din the subscreen, GND is the ground of J401 SMB connector
.
Despite all I still expect to have a failed output amplifier part as I was never able to really debug it due to the main DAC always broke before I can do any serious measure, so yeah this is really a never ending story
PS
In this 10 years I got a lot of interesting instruments but all results in a easy repair as changing a cap or reconnect a cable (or not possible repairs due to hybrids gone bad
), so really this could be said that is the first "real" repair that I am doing ahah
Also I need to investigate this before installing yet another DAC because I get bored of broken DACs
)