HP8656B problem

[Daxxin]

About the 4 bands
100k-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 steps
the output change only every 50Mhz in the first band , 12.5Mhz in the second , 25 in the 3rd and 50 in the last high band
look like the divisors is working but something else is wrong , tracing with scope found weird signal here at MC10138L chip divisor



scheme section:


pin 15:


pin 14:


pin 4 and 12:

[Daxxin]

pin 14 look like incorrect

[Tony_G]

What did you measure at TP 32 & 33?

[Daxxin]

TP 32:



TP 33:

[Tony_G]

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 are MECL not TTL so the 1 & 0 voltages will be different when you look at the scope.

TonyG

[Daxxin]

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:

[Tony_G]

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?

[Tony_G]

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.

[Daxxin]

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 the
U34 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 .

[Tony_G]

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 the
U34 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 .

That 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.

[Tony_G]

Just wanted to check in and see if you had any more information/progress.

Hopefully it's working now.

TonyG

[Daxxin]

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 only
one check with ss13 and the results is different by the table , seems the opamp not work correctly at input pin1 0.7v pin2 0v
by the way eventually can the xr5533(hard to find) replaced by NE5533 ? pin 5 and 11 of u13 tought
have same voltage they are almost identical scheme except for the resistance r622 and r623 value but why
the table expect different results?

[xwarp]

Have you checked those components off U53 pin 14? It looks to me like that signal isn't being pulled down enough.

[Daxxin]

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.

[Tony_G]

Have you verified U27A is working?