GW Instek GDM 8255A problem

[Analogdevice]

Hello guys,

I'm back here and I have a problem with my GDM8255A. I buy it for 50€ for repair. In the input part there were some chip resistor dead, two zero "resistor" was under the shunt resistor, someone measure HV or something... I don't know, but this is now irrelevant.

Take a look on the VFD part. After power up the instrument I hear relay clicking and one beep, but no image on the VFD. On the display board there are two pieces of PT6311 VFD drivers, whitch have digital input marked as DIN. On this pin, there is nothing, still Log 0. It shall be serial communication.
This serial line goes to CPLD Lattice iM4A5 256/128 to the I/O pin 38. I measured resistance between this pin and GND and it's 0,82 Ohms when instrument is switched off and 0,06 Ohms when instrument is powered on. No communication, no data. I don't know how it's programmed, is it a problem or it may be normal? I'm guessing that it's ok and this serial line to VFD is dependent of some other input to CPLD. Also the local keyboard is not working; this part also comes thru PT6311.

I tried to connect the instrument via RS232 and it communicates. When I send command "*rst" thru RS232, I hear relays and one beep just like at startup, also I can read fw version and all that stuff and I tried to use GDM software to set the measurements and read the display and it works (but thanks to bad input part I have read 10 in every measurement, i.e. 10.000A, 10.000V etc.). When I ask GDM for errors using system commant (syst:err?) it answers me "No error". So the communication is working.

Do you have any hints? Or schematic, service manual? The PCB have min. 4 layers including shielding, I can't see the routing.
Thanks for any advices!

[coromonadalix]

Never saw any schematics for the gdm-8251, 8255, 8261  series of meters

It would take some reverse job of your side, or maybe find another identical meter to do some comparaisons ??


Maybe ask Instek for a service manual  under an non disclosure agreement ??

[Analogdevice]

No way
I texted some messages to goodwill service and they only ask me, why I need it, then no reply.

So I have two options:
1. F..k that sh**
2. Ask someone on EEVBLOG for help using his meter.

So first I try second option. Is there someone, who have GDM 825XA meter? I need some voltages and waveforms. THX.