Electronics > Repair

Agilent E4421B RF generator restoration project

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pquadrat:

--- Quote from: TK 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

--- End quote ---

I bought the ESG mainboard from TK on ebay, to repair it. There were some more problems, most of them were easy to fix. One switching voltage regulator was shortet, and two transistors (display and green power LED) were shot.
But the lines in the display were still there.
After analyzing the protocol of the data transfer to the display, I was short before taking my logic analyzer. I even replaced the RAM chip a second time. But then I did some testing with removing signals to the display, and pulling the 8 lines to H one at a time. No result on bit 8.

Problem was a bent contact in the connector of the board to the LCD display! Looking back, it was obvious, because there came no signal for these lines out of the board. With black screen, alll signals were constant L. Pulling one of the 8 bits H there were white lines. But not on bit 8. It just had no contact to the display. But I was also sure that the problem came from the display controller or a bad display RAM chip. Key to the solution was to understand the display transfer protocol, which explained whyonly the lower half of the display was faulty.

TK:

--- Quote from: pquadrat on February 08, 2019, 07:15:09 pm ---
--- Quote from: TK 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

--- End quote ---

I bought the ESG mainboard from TK on ebay, to repair it. There were some more problems, most of them were easy to fix. One switching voltage regulator was shortet, and two transistors (display and green power LED) were shot.
But the lines in the display were still there.
After analyzing the protocol of the data transfer to the display, I was short before taking my logic analyzer. I even replaced the RAM chip a second time. But then I did some testing with removing signals to the display, and pulling the 8 lines to H one at a time. No result on bit 8.

Problem was a bent contact in the connector of the board to the LCD display! Looking back, it was obvious, because there came no signal for these lines out of the board. With black screen, alll signals were constant L. Pulling one of the 8 bits H there were white lines. But not on bit 8. It just had no contact to the display. But I was also sure that the problem came from the display controller or a bad display RAM chip. Key to the solution was to understand the display transfer protocol, which explained whyonly the lower half of the display was faulty.

--- End quote ---
I am glad you found the problem with the board and the display is now working.  Thanks for the follow up.

minsik:
New (second hand) E4421B arrived and was told it "works good" by reputable dealer. Not so, the reference is faulty, being too high in frequency by about 200-300 hz as well as the external Ref says its unlocked! Damm.
Went whole hog in testing in stages and finally tracked down to the 10Mhz TCXO diagnosed as faulty. Dismantled and added a 1.5pf across xtal to tune. Reassembled and tested A1 and external ref now works 100.
Free run testing and the 10Mhz does vary about  3-5hz over cold to hot conditions. At least external Ref is fully operational at all times.

Problem solvered!
Unit destined for a fishing sinker is now pride of place in workshop.

nctnico:
This is the link to Mike's ( Mikeselectricsstuff ) teardown video (as requested per PM):


Nice find! I guess the output amplifier is still working? This was the biggest headache for the unit I used to own.

minsik:
Thanks nctnico. 
I am very pleased with the outcome, a working E4421B! In my dreams I did not think to get this magnificent generator for so little money (relatively speaking)! Yes all levels and all frequencies appear excellent as measured on a Siglent spec-an + FA-3 counter. This thing is beastly good. it reeks quality. So far its life was 34,400 approx hours, 200 power ups,  with lcd being on for 100% of the time. It must have been in a factory or long running in an assembly line or similar. I have turned on screen "light and text saver" and see a difference slowly coming. Even then LCD and backlight is good, maybe a slight yellowing. 


I was able to win an eBay replacement reference module (same as mine, same xtal unit) that is on the way. So I can see if that is better or worse from my doctored one when it arrives. Also then it might allow some experimenting with changing the xtal to a OCXO. CCT seems to be able to cope with many differing mountings and types, sine-square etc. Only thing is if xtal is oven controlled I am not sure how it will react without enabling the "oven cold cct" (5 minute timer) in software, as I don't have any confidence/knowledge (treading carefully) in any software changing, yet! 


Very chuffed I got a working good generator.
On any repair, "Never give up, never  surrender! (Galaxy Quest)".
Minsik.

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