Repair Rohde & Shwarz FSH3 Portable Spectrum Analyzer

[Jay_Diddy_B]

Hi group,

I was inspired by the excellent Video posted by Shahriar on the Signal Path YouTube Channel:



And discussed here in the EEVBlog Forum:

https://www.eevblog.com/forum/testgear/teardown-repair-analysis-of-a-rohde-schwarz-fsh3-3-0ghz-spectrum-analyzer/msg723839/#msg723839

I bought a FSH3 at the Rochester Hamfest today. I was hoping to get the same problem ;-).

I have been given a different challenge.

My unit turns on, it beeps, and displays a blue screen.

I have used a scope and probed around the LCD connector. I can see some signals that look like video signals. For example I can see a narrow pulse with a Frequency of 63Hz. This looks like the Vertical Sync.

I don't have any schematics.

I hear that some of the circuitry is similar to the Fluke 190 series scopemeters.

The display is khs057qv1cr-g01.

Does anybody have an information on the display module or a schematic for either the FSH3 or a color Scopemeter LCD section?

Thank you for your help with this.

Regards,

Jay_Diddy_B

[Jay_Diddy_B]

Hi,

I have done a little more troubleshooting. I have found that the Fluke 192B/196B-C/199B-C Service manual contains some useful information.

I found that this section matches the same section on the R&S FSH3 SA:



If I measure the outputs on the 33174 Quad Op-amp, I got proportional voltages, but all of them were low. I measured the voltage on pin 4 of the 33174 it was also low around 5V instead of 30V.

The power supply on my FSH3 does NOT match the power supply that is documented in the Scopemeter manual:



(The schematic seems to match the power supply in Shahriar's FSH3).




I examined the board carefully, and I found a burnt resistor. I also measured 6 Ohms across two decoupling capacitors. I removed the capacitors, but the 6 Ohms remained. I removed a chip with a topmark 'LBHM'. A quick google search revealed that the chip is an LT3471.



Link: http://www.linear.com/product/LT3471

This chip is a Boost / Inverter in a 3 x 3mm DFN.

This could be used to generate the 30V needed by the LCD supply.

I have ordered the part from Digikey.


Regards,

Jay_Diddy_B

[thm_w]

Is the 84P diode on the left cracked?

[Jay_Diddy_B]

thm_w,

I am away from the unit at the moment, I only have the photographs to look at. I will check when I get home.

I suspect the diode is a B4p. B4 is used for 40V 0.5A Schottky diodes.

Thank you for the tip.

Regards,

Jay_Diddy_B

[scopeman]

Were you able to fix yours? I just bought a FSH6 at a hamfest and it appears that someone bypassed the attenuator chip. What a mess.

Sam
W3OHM