HP8656B problem - Page 1
Forum section: electronics / repair

author: Daxxin
author_rank: Regular Contributor
timestamp: January 15, 2018, 06:52:30 pm
content: About the 4 bands100k-123.5M (heterodyned)123.5M-247M (divide by 4)247M-494M (divide by 2)494M-990M (divide by 1)divide 50 Mhz reference , changing frequency from front panel increasing or decreasing 1 Mhz stepsthe output change only every 50Mhz in the first band , 12.5Mhz in the second , 25 in the 3rd and 50 in the last high bandlook like the divisors is working but something else is wrong , tracing with scope found weird signal here at MC10138L chip divisorscheme section:pin 15:pin 14:pin 4 and 12:

author: Daxxin
author_rank: Regular Contributor
timestamp: January 28, 2018, 05:04:24 pm
content: pin 14 look like incorrect

author: Tony_G
author_rank: Super Contributor
timestamp: January 28, 2018, 06:28:07 pm
content: What did you measure at TP 32 & 33?

author: Daxxin
author_rank: Regular Contributor
timestamp: January 28, 2018, 07:02:50 pm
content: TP 32:TP 33:

author: Tony_G
author_rank: Super Contributor
timestamp: January 29, 2018, 04:45:23 am
content: ignoring the levels - You're seeing 50MHz prior to the divide-by-10 circuit and then 5MHz after it. That circuit you showed seems to be working correctly from a frequency division perspective.Service Sheet 16 covers that part of the instrument - Did you tests find anything wrong as you followed the troubleshooting steps there?Also when you set the unit to 100MHz 0dBm what do you get on the output? Is the signal phase locked or does it wander around? What does the display show on the unit show?Finally, as an aside, remember that these devices areMECLnot TTL so the 1 & 0 voltages will be different when you look at the scope.TonyG

author: Daxxin
author_rank: Regular Contributor
timestamp: January 30, 2018, 07:46:51 pm
content: Yes i think too correct about divisions , at 100Mhz output leve is correct but the frequency is 102.520Mhz and never change until i down to 90Mhz then freq output go to 52Mhz.testing ''SS16 reference phase lock'' looks wrong to me the output of flip-flop at pin 5 looks fine the 10Mhz but look at the 2 outputs pin 2 and 3 both unlocked (red led of unlocked still always on)one of the 2 need to be disabled selecting external or internal reference?test table:scheme sect:pin 5:pin 2:pin 3:

author: Tony_G
author_rank: Super Contributor
timestamp: January 31, 2018, 03:14:23 am
content: What is the output level of 50 MHz reference - It looks like it should be between 15-19 dBm in the poor scan that I have. Also what are you seeing on pin 5, 6 & 7 of U45B.I'm wondering if the output of the reference is either too low or being turned off. Do you have another sig gen that you could just use for a 10 MHz input?

author: Tony_G
author_rank: Super Contributor
timestamp: January 31, 2018, 04:59:57 am
content: Thinking a bit more about this the problem may be in the low frequency loop and specifically either the VCO or Divider/Pre-scaler. You should do the check on the 50MHz reference output anyway because it is used to create the other reference frequencies in the unit (10MHz, 100KHz, etc).My reasoning for this is that the VCO is moved pretty much 1:1 in that first band you mentioned previously (well actually from a front panel setting of 40 to 90 MHz).Look at SS9. Do those checks and then take a look at the voltages being used to tune the VCO - They're coming out of the FM Calibration DAC in SS14 and going into the 60 -110MHz VCO in SS9 - Table 1 in SS9 shows the voltage for freequency.In fact if you're careful you could even inject a +/- 15Vdc voltage into the VCO - Make sure that you limit the current to around 5mA. You should be able to drive the VCO across the entire range.

author: Daxxin
author_rank: Regular Contributor
timestamp: January 31, 2018, 08:11:15 pm
content: VCO voltage never change at frequency output selection always -3.11V and also frequency fixed at 85mhz , tracing back at pin7 of U33B have no dc at all for every change of freq. and theU34 very cold if you compare temperature to the other ICs after 30 minutes or mor warms , maybe dead U34? i have ordered alredy it very cheaper DAC ad7533 .

author: Tony_G
author_rank: Super Contributor
timestamp: January 31, 2018, 09:36:55 pm
content: Quote from: Daxxin on January 31, 2018, 08:11:15 pmThat seems to be a problem - Did you go through SS14 troubleshooting step #2? That should show whether the D to A is working.As an aside, have you checked that all the jumper links are present? (A3Wxx parts) - It just occurred to me that maybe someone has left the jumpers in a test mode.

author: Tony_G
author_rank: Super Contributor
timestamp: February 08, 2018, 11:20:19 pm
content: Just wanted to check in and see if you had any more information/progress.Hopefully it's working now.TonyG

author: Daxxin
author_rank: Regular Contributor
timestamp: February 09, 2018, 07:01:35 pm
content: Thanks Tony i didn't any big step due lacking time by my job , tried to replace the dac ad7533 but no changes then i did onlyone check with ss13 and the results is different by the table , seems the opamp not work correctly at input pin1 0.7v pin2 0vby the way eventually can the xr5533(hard to find) replaced by NE5533 ? pin 5 and 11 of u13 toughthave same voltage they are almost identical scheme except for the resistance r622 and r623 value but whythe table expect different results?

author: xwarp
author_rank: Frequent Contributor
timestamp: February 09, 2018, 10:47:59 pm
content: Have you checked those components off U53 pin 14? It looks to me like that signal isn't being pulled down enough.

author: Daxxin
author_rank: Regular Contributor
timestamp: February 10, 2018, 04:56:29 am
content: hi xwarp , how sayd Tony this is ECL signal then if you look at picture TP32 test i post before look great thats amplified by 2 transistors Q54 Q53 look ok to me but maybe i'am wrong.what i get wrong is the minus voltage at pin 11 it came from W4 if i remove it voltage is 0v or small positive but the test need to be done with W4 installed.

author: Tony_G
author_rank: Super Contributor
timestamp: February 11, 2018, 01:10:42 am
content: Have you verified U27A is working?