Author Topic: Agilent E4421B RF generator restoration project  (Read 16616 times)

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Offline nctnico

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Agilent E4421B RF generator restoration project
« on: December 27, 2014, 03:31:33 pm »
About a year ago I bought a 'for parts' E4421B from Ebay. The most common fault I found when researching these machines is the output amplifier going bad. I more or less assumed this machine was most likely to have the same problem. -Spoiler alert- I was wrong  :scared:


The most obvious problem was the display. Did someone stick a post-it behind it? After some reading I discovered these DSTN screens can also create a yellow color when overdriven.


After opening it up I got a bad feeling. Everyone who has worked at a repair shop knows the look of a test unit. The unit with the boards which can't be fixed but are good enough for experiments or to try something quickly. Every module has a blue cross on the outside... To sum up the problems with my E4421B:
  • Frequency slightly off
  • Output level too low and distorted
  • Display not working
  • No AM modulation
  • Dead backup battery
The first thing I attacked was the frequency problem. According to the service manual (which only has block diagrams and no schematics) the reference board should be the place to look for that problem.
« Last Edit: December 28, 2014, 12:58:02 am by nctnico »
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Offline nctnico

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Re: E4421B restoration project
« Reply #1 on: December 27, 2014, 03:43:41 pm »
The reference board
The reference board generates the reference frequency and a 1GHz signal which is used on the output board to generate frequencies in the range from 100kHz to 250MHz. The AM and FM modulator are also on this board.

After opening the reference module it quickly becomes clear a chip labelled U13 is missing  :'( That is about the worst that can happen when trying to repair something without a diagram. Oh crap... :palm:

Some probing around learns me that the missing chip is connected to the reference input of a DAC (circled in red) which seems to have been replaced given the later production date than the other chips and lots of flux residu. So U13 must be some kind of reference.
Fortunately Mike (Mikeselectricsstuff) did a teardown and he took a closeup of the chip:

Some Googling for a Linear Technology device labelled 2110 revealed it is an LT1021 10V precision reference. Let's order one but first see why it was removed. I hooked up some wiring to supply 10V from a bench PSU and measured the current:

It shows more than 11mA is drawn from the reference chip. Since the maximum current is 10mA it is no wonder why the reference chip was removed. With the bench power supply set to 10V the output frequency is spot on though. I tried to find other loads connected to the LT1021 but the circuit gets fuzzy quickly. Now how to proceed?? Since the reference can source and sink current I decided to live with the 11mA and added a 470 Ohm bypass resistor from the input to the output (this is a suggestion from the LT1021 datasheet). This should limit the current through the LT1021 to around 5mA.

I suspect the AM modulation not working could have something to do with the high current draw on the reference but I think I just leave that.

I declare the reference board 'fixed' for now.
« Last Edit: December 27, 2014, 04:20:39 pm by nctnico »
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Offline G0HZU

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Re: E4421B restoration project
« Reply #2 on: December 27, 2014, 04:39:31 pm »
Interesting thread :)

It's a shame that HP/Agilent/KS won't release schematics of these old signal generators. They do seem to be suffering from various reliability issues and I'm not sure I envy your task in terms of repairing your list of faults...

Quote
I suspect the AM modulation not working could have something to do with the high current draw on the reference but I think I just leave that.

In case you don't already know this, the AM often comes for free on a sig gen that uses ALC to keep tight control on the RF output level. This is because the AM modulation is often injected into the ALC system as a modulated reference for the ALC to aim for and track. So as long as the ALC response time is faster than the AM modulation then the ALC will track and AM modulate the RF output faithfully.

So if the levelling is faulty then I'd expect the AM to be faulty too. But I don't have any experience of working inside these E44xx series generators even though I own a couple of them. So maybe this doesn't apply.

What I will say is that these aren't really lab grade generators in terms of AM modulation quality. So don't expect too much in terms of low distortion if you dial up AM modulation from the menus after you repair it. Both of my E44xx generators are quite poor in this respect.






 

Offline KJDS

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Re: E4421B restoration project
« Reply #3 on: December 27, 2014, 05:57:46 pm »
I've recently found a couple of dead units in the back of my storage lot. One has a label on it saying "display broke". No information on the other so I've just tested it and the display works but it looks like it has a lot of other faults on it.

Offline nctnico

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Re: E4421B restoration project
« Reply #4 on: December 27, 2014, 07:40:21 pm »
The output board
The picture below shows the output board and the signal paths (0.1 to 250MHz in blue, 250MHz to 3GHz in purple). There is a lot going on here. Besides the end amplifier this board also contains the filters to filter unwanted harmonics from the output signal and the auto-level circuitry.



The big giveaway was the blue cross on the end amplifier (1GMI-420). Ofcourse this chip is unobtanium so I had to somehow replace it. The first thing I tried was to replace is with a wire between the input and output. This gave some signal but at very low (and distorted) levels. I needed something stronger so I made a small board with an ADL5602 MMIC as an amplifier replacement.




This gave me an idea on the amount of amplification needed. This little board had a major problem though: it sang like a bird due to the poor layout. From this board it became clear there was also something wrong with the auto leveling. After taking parts of the log-amp circuitry apart it became clear the detector diode (2 diodes in one package; blue circle) was broken as well. Fortunately there was a diode with the same markings on the board and even more surprisingly only one diode was used (red circle) and that diode wasn't the one that was broken in the detector diode. After swapping these over the automatic levelling started to show some signs of life.

I did some measurements on the input and output levels of the amplifier and it appeared that a lot of harmonics at the output where produced because the input signal was already overdriven. This was especially true for the <250MHz range because the input signal is around 3dB lower. I guestimated the original amplifier had an amplification of at least 30dB. I was unable to find a low noise MMIC with that much amplification. I also needed a low noise (low noise figure) and low distortion (high IP3) part. I decided to cascade 2 MMICs with a gain of around 15dB each. I choose the Triquint TQP3M9028 devices because of their low noise and low distortion figures. A problem with MMICs is that they don't perform well at low frequencies which isn't surprise because their outputs are shorted to the power supply. I did a test with a resistor in series with the inductor and this greatly improved the low frequency performance. I had some PCBs made by Seeedstudio which have the same size as the original amplifier. The replacement PCB has two layers; the bottom layer is a solid ground plane.



The attached PDF shows the schematic of the amplifier.

I also found the root cause of the amplifier failing:

The aluminium block it is mounted on has a very rough surface. This is extremely bad for heat transfer. I don't understand how HP/Agilent/Keysight managed to overlook this! If the aluminium block and the module housing where milled so the block and the housing had smooth surfaces the amplifier most probably wouldn't have died.

Anyway, with this fix the output signal works over the entire range albeit with some distortion at the lower frequencies (100kHz to 5MHz). The maximum output level is about +3dBm (more between 250MHz and 2GHz) but that is good enough for me.
« Last Edit: December 27, 2014, 08:18:14 pm by nctnico »
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Offline G0HZU

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Re: E4421B restoration project
« Reply #5 on: December 27, 2014, 08:19:21 pm »
I'm a bit confused because your output board looks like it has the IQ modulator on it. Is it the original board for this generator? It looks like an ESG-D board but I thought the E4421B was an ESG-A signal generator.

I nearly spat my drink out when I saw your ADL5602 board. Definitely an oscillator with that layout :)
 

Offline nctnico

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Re: E4421B restoration project
« Reply #6 on: December 27, 2014, 09:20:56 pm »
@G0HZU: Let's just say the board came with the generator. It's part number is E4400-60003. According to the service manual this can be used in both the ESG-A and ESG-D series.

Time for short intermezzo:
The attenuator
These generator can come with either a relay based or an electronic attenuator. The attenuator can attenuate in steps of 5dB. So the output board has to regulate the signal between -5dBm and upwards. The problem with the relay based attenuator is that it can wear out. Fortunately my unit has the electronic attenuator. While searching for the cause of the weird output level behaviour I took the attenuator apart. Maybe I should not have done that but it resulted in some interesting pictures:
The top side:


The bottom side:


There are a bunch of analog switches on the board and filtering, and filtering and more filtering. Also notice the 5 leds in the bottom left corner...
When the attenuator is turned on they shine a red light to the inner of what is obviously the attenuator unit which needs some optical effect to work. Google told me nothing. If someone else knows what kind of technology is used here I'd be gratefull if he/she can share some information.


« Last Edit: December 27, 2014, 09:27:38 pm by nctnico »
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Online PA0PBZ

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Re: E4421B restoration project
« Reply #7 on: December 27, 2014, 09:39:53 pm »
When the attenuator is turned on they shine a red light to the inner of what is obviously the attenuator unit which needs some optical effect to work. Google told me nothing. If someone else knows what kind of technology is used here I'd be gratefull if he/she can share some information.

I took apart the attenuator of my E4433B because it was making strange steps, and I didn't notice any leds in there. And yes, it was the electronic one also. Turned out that there was some milling residue shorting out one of the signals, go figure...
Keyboard error: Press F1 to continue.
 

Offline Fraser

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Re: E4421B restoration project
« Reply #8 on: December 27, 2014, 10:00:49 pm »
A very interesting thread. Thank you. I shall continue to follow it with interest.

The Output amplifier MMIC is available as a tested used part from China. I had a Google search some time ago.

Aurora

 

Offline nctnico

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Re: E4421B restoration project
« Reply #9 on: December 27, 2014, 11:13:05 pm »
The display
I took the display apart to see if it could be fixed but that wasn't the case. One of the driver chips obviously was broken. I quickly found a Chinese Ebay seller which claimed to have the display for $140 + $40 shipping. Including customs duties and taxes the display would add up to 180 euros. Because of the age of the equipment the display could be from a salvaged unit with lots of hours and a dim CFL backlight.

I choose to apply my TFT conversion skills. I tracked down an 8" 800x480 screen (TX20D16VM2BAA) from Ebay which has almost the same size as the orginal screen. That left me with a bit of a problem; the original display has a resolution of 640x320. I need some scaling. Horizontal: 800/640=1.25 so I need 5 output pixels for 4 input pixels in the horizontal direction. Also the active area of the TFT screen is heigher than the original screen so I may need some stretching there as well.

The new display mounted: Some 3M double sided tape and stick-on feet keep it firmly in place. To prevent shorts between the backplate and the controller board I stuck some polyimide tape on the backplate.



The primary problem is to convert the data for the DSTN screen into information a TFT can work with. A DSTN screen is black/white and usually has 4 to 8 parallel data lines, a clock and vsynch+hsync signals. To complicate things more the pixels of a DSTN screen are typically 'modulated' to get grey scales. The more consecutive frames where a pixel is 'on' the whiter the pixel and vice versa. This means that a picture is spread in time over several frames. In the case of the E4421B the display has 8 bits and one picture is made up from 4 consecutive frames.

What I need for the TFT screen is to count the number of ones for each pixel over a period of 4 input frames. That is not going to happen on the fly. The refreshrates are also too different for that. Time for a CPLD and SRAM. The TFT screen uses a pixel clock of 32MHz but I'm not going to store RGB data in the SRAM. All I need is the count of the number of ones; 4 bits is more than enough for that. If I use a 16 bit SRAM I can supply 4 pixels of TFT data with one read cycle. The TFT data can be converted to RGB by using a fixed pallette. All in all that leaves three 32MHz clock cycles to do something else or use slower memory. I choose to use slower (45ns) memory so I can use 2 clock cycles for reading TFT data and 2 clock cycles for dealing with the DSTN data. But... remember I need to stretch the TFT data so I'm using 5 clock cycles in total (which means one TFT pixel gets repeated). In total 2 clocks to read TFT data, 2 for DSTN data and 1 for nothing. This means that I have 32M/5= 6.4 million memory time slots for dealing with the DSTN data.

The DSTN data is clocked at a rate of 2.1MHz -ish. I'm using the 32MHz clock to sample the edges of the dot clock, vsync and hsync. When a vsync arrives I reset the line counter and increment a frame counter. The hsync resets the dot counter and increments the line counter. The dot clock increments the dot counter. These counters can be combined into a memory address nicely. There is more than enough memory so no need to make things more complicated the necessary. The most complicated part is counting the number of ones over 4 frames. For each dot clock cycle it means reading the old value from RAM (or force the value to zero when dealing with the first frame), increment the value and store the value to RAM. So processing every DSTN pixel takes one read and one write cycle. That adds up to 4.2 million memory accesses per second. Since there are 6.4 million available memory bandwidth is not going to be a problem. Unfortunately there is a problem... The DSTN data is 8 bits wide. The first 4 bits are for the upper 160 pixels of the display, the second 4 bits are for the lower part. I can't process these in one go. I solved that by using 8 frames to produce one TFT screen display image. From the first 4 frames (1 to 4) the data from the upper 160 lines is used, the next 4 frames (5 to 8 ) are used to create the lower 160 lines. The last step is to use double-buffering to prevent the TFT screen showing how the pixels are counted. After 8 frames have been processed the display areas are swapped so the TFT screen shows steady data.



I designed a small board and had it made by Seeedstudio. The CPLD is a Xilinx XC95144XL; the RAM is a 512kx16 45ns SRAM from Cypress. The CPLD is in a TQFP144 package. I ordered the wrong package for a different project so I opted to design the PCB so I could use that one. By optimising the connections between the RAM and the CPLD the PCB has most traces on the top layer which kept it very compact. The XC9500XL series is 5V tolerant and has internal flash. It is a much nicer solution than an FPGA for a project like this. The only external components are a 3.3V regulator and an oscilllator module.



I also managed to source a connector which is the same as the connector on the original display. This means I can keep the existing display cable and don't need extra wires to the motherboard of the E4421B.

After several hours of VHDL coding and getting the timing right things start to look good but I forgot the active area of the new display is higher than the original display so stretching 320 lines into 480 lines isn't going to cut it:


After some fiddling the height is just right:


The fixed pallette conversion allows any color scheme. Who's afraid of red, yellow and blue?  >:D

« Last Edit: December 27, 2014, 11:33:48 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline nctnico

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Re: E4421B restoration project
« Reply #10 on: December 27, 2014, 11:37:58 pm »
The Output amplifier MMIC is available as a tested used part from China. I had a Google search some time ago.
Any pointers to a reputable seller? The output amplifier is definitely a key part of getting a clean output signal. I have seen them as well but mostly as part of a board which was cut from an output board.
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Offline tautech

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Re: E4421B restoration project
« Reply #11 on: December 27, 2014, 11:42:18 pm »
Wonderful work.  :-+
Quote
The fixed pallette conversion allows any color scheme. Who's afraid of red, yellow and blue?  >:D
Na keep it white on blue.  :-+
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Offline nctnico

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Re: E4421B restoration project
« Reply #12 on: December 27, 2014, 11:49:22 pm »
Conclusion
All in all the generator is working useful again except for AM and probably FM modulation as well. Both these modulations are generated on the reference board so there still are some problems lurking in there. My primary use for this generator is tinkering / learning HF stuff and that usually involves sending sine waves through amplifiers and/or filters.

Yes, I also changed the battery but that was an easy job.
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Offline nctnico

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Re: Agilent E4421B RF generator restoration project
« Reply #13 on: December 28, 2014, 03:50:16 pm »
Fixing modulation
I couldn't resist taking another look at why the modulation wasn't working. First I tried to apply an external modulation signal to make sure the internal modulation signal generator on the motherboard wasn't the problem (according to the service manual the internal modulation signal is generated on the motherboard). The modulation also didn't work with an external signal so the cause of the problem must be somewhere on the reference board. So I opened the reference module again and I noticed a few opamps where running hot. Both these opamps where connected to different DACs. DACs are prone to failure in general but two broken DACs is still unlikely. I tried to find out what the opamps had in common and I ended up with the power supply (+15V and -15V). I could trace the +15V back to the power supply connector but I could not find -15V. I remembered that the board had a blob of solder when I got it (inside the red circle):



I removed it hoping it would not be harmful but appearantly it shorted -15V to ground which burned a filter inductor. I replaced it with a ferrite bead and presto: FM and AM modulation working again!



The hot opamps are circled in green, the removed solder blob in the red circle and the replacement ferrite bead in the blue circle. And yes, I removed the resistor across the 10V reference. I didn't measure the current but I doubt it will be needed.
« Last Edit: December 28, 2014, 03:56:59 pm by nctnico »
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Offline G0HZU

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Re: Agilent E4421B RF generator restoration project
« Reply #14 on: December 29, 2014, 05:21:16 pm »
Quote
@G0HZU: Let's just say the board came with the generator. It's part number is E4400-60003. According to the service manual this can be used in both the ESG-A and ESG-D series.

Interesting... On Mike's youtube teardown his HP4421B A had the areas for the IQ modulator blanked off in the artwork and replaced with a stripline section to bypass this unused area. So he has a different output board. So maybe HP/Agilent decided at some point to use the E4400-60003 output board (with IQ modulator) on both A and D models?

In my opinion the IQ modulator is one of the best features of this generator series. I wonder how much extra work it would be to make use of your IQ modulator?

That was impressive work on the screen replacement by the way :)
 

Offline Mosaic

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Re: Agilent E4421B RF generator restoration project
« Reply #15 on: September 05, 2016, 04:42:09 am »
I got an E4432B  ESG-D, OPT 100,1E5,UN8,UN9,UNA,UND for abt $590 on Ebay auction with an unleveled error.
After much hunting I realized that the burst mod drive on the A9 output board (E4400-60003) was not responding and causing the BURST MOD to be at 0% duty = no signal.

W/o a schematic I cannot resolve this...although the problem is located. Too many buried vias etc. in a multilayer board. I am unable to rev. eng. a schematic.

So, in order to get the unit alive, albeit without burst modulation, I did a hack.

I fed the ALC MOD 8V PIN diode drive  forward over the U180 mmic via a 20K resistor to create  a BURST MOD 100% duty drive.  Smaller resistor values caused the U180 to have +ve feedback causing unleveled errors due to too much signal strength. A 1K? pushed it up over 10dBm!

Here is a pic of the solution which works across the full 3Ghz span. AM & FM modulation etc. are fine.

 

Offline Mosaic

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Re: Agilent E4421B RF generator restoration project
« Reply #16 on: September 06, 2016, 09:15:46 pm »
Something interesting. The burst mod appears to be working with the hack in place. Feeding a waveform into the EXT1 input and activating the burst envelope results in modulation. Not linear though. A 2Vpp sine wave modulation signal results in a +4dBm to -7dBm  modulation of the carrier.Sine wave appears a bit top flattened. Internal AM modulation shows a good linear sinewave modulation. :)

Pulse modulation is also good.  :-+

The zero span view of the sine wave burst mod RF out on my DSA implies a LOG sine wave modulation.
Since I have a UN8/9 and UND option...it appears that the burst envelope IS indeed handled as a LOG signal, with the AM envelope being linear. :)
Thus, the  single resistor, pull up,  hack may have done the trick altogether! :-+

« Last Edit: September 06, 2016, 09:34:28 pm by Mosaic »
 

Offline nctnico

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Re: Agilent E4421B RF generator restoration project
« Reply #17 on: September 06, 2016, 11:44:36 pm »
Perhaps it could be caused by a problem on the board driving the burst modulation. In my E4421B there was a power supply absent in one of the other modules because there was a blob of solder shorting the supply rails.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Mosaic

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Re: Agilent E4421B RF generator restoration project
« Reply #18 on: September 07, 2016, 12:12:03 am »
Yes, that's so.
But w/o a decent schematic or even a block schematic of the UN8 gen, I'd not spend time hunting a bad solder joint or such like as the solution in place seems good.
All voltages on the system show good.

Interestingly there are two units almost identical to mine with identical failure modes on Ebay now. Could be a good deal if the problem is the same.
#262509365736
#252448189801

If I were in the Con US I'd be tempted to  repair and resell them given their option load out.

 

Offline Samogon

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Re: Agilent E4421B RF generator restoration project
« Reply #19 on: September 08, 2016, 04:35:54 am »
Yes, that's so.
But w/o a decent schematic or even a block schematic of the UN8 gen, I'd not spend time hunting a bad solder joint or such like as the solution in place seems good.
All voltages on the system show good.

Interestingly there are two units almost identical to mine with identical failure modes on Ebay now. Could be a good deal if the problem is the same.
#262509365736
#252448189801

If I were in the Con US I'd be tempted to  repair and resell them given their option load out.
Nothing said about failures on this items. No error listings. May be i miss something?
 

Offline Mosaic

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Re: Agilent E4421B RF generator restoration project
« Reply #20 on: September 08, 2016, 05:31:43 am »
This one: 252448189801 specifies no RF and unlevel error...which precisely matches what I had.
I'd suggest that if you're interested that you ask for verification that the coherent output is alive, which then means the A9 board is the fault as mine was.

The other says dual arb error which is the UND option, which can be removed. But may be a voltage issue which makes it an easier fix.
 

Offline Samogon

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Re: Agilent E4421B RF generator restoration project
« Reply #21 on: September 08, 2016, 05:45:28 am »
Oh i must be blind, reading description carefully explains errors. Sorry
 

Offline SoundTech-LG

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Re: Agilent E4421B RF generator restoration project
« Reply #22 on: November 01, 2017, 02:33:09 pm »

Just a heads up...

I followed the fix shown above adding the 20k resistor, also touched up several questionable soldered areas in that circuit. Voila! works great. Then I said to myself, remove the resistor to prove that fixed it, not the touch-up soldering. Removed the 20K resistor. Voila! Still working fine! Chances seem good there are issues with solder, not circuit values (at least with this E4432B).


 

Offline SoundTech-LG

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Re: Agilent E4421B RF generator restoration project
« Reply #23 on: November 03, 2017, 02:19:42 pm »
Had this E4432B running about 36 hrs. w/no problems. One of our engineers took it away to his lab, and used it under load for about 4-5 hours, then brought it back complaining "it's broke". Noticed the supply for the output amp IC was shorted to ground, then noticed all the pins were shorted to it's integral heatsink/mounting. Noticed one of the screws for mounting was loose. Looks like this is another fried 1GM14201 because of poor heatsinking, and mounting. Thanks Agilent! :--

 

Offline TK

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Re: Agilent E4421B RF generator restoration project
« Reply #24 on: September 25, 2018, 12:27:56 pm »
This thread is old but I think it is worth keeping any repair hints together.

Recently I acquired this signal generator and had missing columns on the bottom half of the LCD.  After replacing the LCD, the problem persisted.  While trying to map the LCD pins to the correct part on the motherboard, I used a continuity meter to map the pins.  It shorted one or both HM628512BLFP-7 SRAM chips (I think they are called BootROM RAM) and the motherboard did not want to boot, resetting every 3 seconds due to the power supply being shorted.  I then realized that the motherboard has a 3V lithium battery to keep some configuration alive and I passed the test probes over pins while they were powered by the Battery.  When I removed the SRAM chips, the board was back alive, but stopped with LED diagnostic code 2, and the manual says it is checking for BootRAM RAM, which of course were removed.  After soldering new SRAM chips, the motherboard booted correctly passing all the self diagnostics.  The LCD problem is still there, and I traced the problem to the Video RAM (HM514260CJ6) next to the Chips F65510 VGA controller (The controller seems to be OK because I can measure signals changing on all 8 bits that goes to the LCD).  I am waiting for the Video RAM chip to arrive and I will post the results
 


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