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Repair Rohde & Shwarz FSH3 Portable Spectrum Analyzer
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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
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