Author Topic: HP 8648C Sig Gen Ebay purchase repair story  (Read 369 times)

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Offline 4thDoctorWhoFan

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HP 8648C Sig Gen Ebay purchase repair story
« on: October 13, 2019, 06:41:26 pm »
I purchased a 8648C off Ebay with the description saying it had no display so unable to test.  Considering the prices these units go for, I think I got a great deal at $250.  The picture showed the unit powered on with no display.  It is the LCD model and the backlight was on.  Well, once I received the unit, it was exactly as described.  No display with backlight.

First repair:  Must get a display.
I was hoping for an easy fix by adjusting the display contrast pot on the rear of the unit.  No luck because there is NO pot.  WHAT?  You could also tell that one was never installed from the factory.  I am assuming at some point, the front panel was switched as the main board was for the VFD display and not a LCD display.  After some research, the main board seems to be the same except for the contrast adjust pot.  So I installed a 5K pot and presto, I now have a display.

Second repair:  Error code.
The display is showing a 608 synth error.  I just happened to watch Defpom's YouTube channel on the repair of his B model with the same error.  Since I had the same symptoms, I decided to first check the inductor that opened on his unit.  Yes, the same inductor was open on the Synth board.  Thanks Defpom for saving me a lot of troubleshooting time.  So I replaced the inductor and now no errors.

Third repair:  Keypad issues.
Now I have a display and the unit powers up normally with no errors.  The RF output also is working fine.  Now about a third of the front panel buttons do not work.  They are just unresponsive.  I remove the front panel and disassemble the keypad.  Well, the problem is the flex circuit membrane has corrosion.  Something is eating away at the traces.  To get the front panel working now, I used a circuit pen to jumper the corroded open spots on the traces.  And to my amazement it worked.  The buttons work now.  I will probably order a new one from China even though they want like $60.  I'm not sure how long the repair will hold since the membrane will continue to corrode.

Anyway, just thought I would post this repair story.
« Last Edit: October 13, 2019, 09:24:45 pm by 4thDoctorWhoFan »
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Offline 4thDoctorWhoFan

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Re: HP 8648C Sig Gen Ebay purchase repair story
« Reply #1 on: October 20, 2019, 07:27:57 pm »

Well, a 4th problem has been found.

Previously, I just did a quick RF test of the frequency & amplitude capabilities.  After a more extensive test, I found that below -75.9dbm the signal would just drop out completely.  This unit is capable of going down to -130dbm.

I disassembled the attenuator and discovered the tiny o-rings on the plungers were in terrible condition.  1 of the o-rings was missing and numerous others were cracked & hard.  Now, I'm glad I looked inside because more problems would have occurred as the o-rings got worse.

Anyway, I replaced all the o-rings and now it works properly all the way down to my measuring capability of -125dbm.

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