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8116A Code 42

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MarkL:
An additional thought on the hot chips A2U33 and A2U37... They are ECL and powered by a -5.2V rail.  You probably have gotten to adjust it already, but if the -5.2V rail is too negative due to "pot recentering", it could be causing extra heat dissipation.  The -5.2V rail is adjusted with A1R12, as shown in the table on page 9-3, along with the other rails.

ECL is fairly robust, but if presented with too extreme a supply voltage, one or more chips could have sustained damage.  Just a theory.

Please post back your progress on the adjustment procedures and the option 001 verification when you can.

Mick B:
Greetings, well I got to the end Pg. 9-8. I changed 5 pots for 10 turn pots, yes, the averaging out (for lack a better term) a little here a little there and when there both right move on to the next. It's very time consuming. When I got to Overshoot & Transition Time adjustments, after setting it up correctly the overshoot was always less than<4% but fluctuating. At Steps 5 & 6 is where I noticed a voltage problem at 200mv it was off the screen. I unhooked everything and checked the voltages, and they were way off 2v was 3.19v and so on. Can you explain step 20 Pg. 9.7 I don't understand where to, or how to check the transition time. I'm just not sure what transition they are referring to or how to measure it. 
I kept on, trying to get things in the ballpark. But ended up starting over and decided that to get the first adjustment step5 Pg.9.4 the voltage at 16v is... well look at the square wave photo " The first Procedure." The purple trace is the tracking output. also, the crossover distortion Photo " crossover" seems only to happen at high voltage, I can pin it down. I am wondering if it's because I only changed Q501 Its Hfe was way higher than the one I took out perhaps it's a mismatch, I have a new Q502 in a TO-92 pkg, but I also have the heatsinks for it. also, I just changed Q506 I have to do some checking to find out if I changed Q505 earlier I think I did. will see. I will deal with the IC later unless I'm thinking one problem at a time. but I could be wrong. (it happened once before)   
     

MarkL:
Unless the pots had issues, I would have left them alone.  Depending on what you put in, and where, some 10-turn pots are wirewound and the internal coil of wire adds inductance to the circuit.  In an amplifier that runs up to 50MHz, it could be significant.  I really have to emphasize "if aint't broke, don't fix it".  Especially in the middle of a repair where the thing you're replacing is known working.  It adds more unknowns to what might be wrong.

Transition time (aka rise time or fall time), is how long a signal takes to get from one plateau to another.  A square wave is used for the calculation here.  A change from one voltage level to another doesn't happen instantly, and can be seen as a fast ramp.  The measurement for transition time is the points on the ramp between 10% and 90% of the starting and ending voltage levels, where the difference between starting and ending voltage levels is defined as 100%.  I don't have a Rigol scope, but I'm sure it has a button to measure it automatically for you, probably under the "Measure" menu.  You need to have your horizontal sweep set fast enough so that you can see the ramp of the signal.  If you're curious what it's doing, search for "measure rise time on an oscilloscope" for the old way of manually doing it.

The square wave is not looking great, and it may be related to the crossover issue.  What does it look like at a lower frequency, say 10kHz?  Looks like you have it set to 100kHz in the screen shot.

For the unusually high output voltage, I would start with verifying the amplifier input at TP4 as described on page 10.4-1 1.  If the input isn't right, certainly the output won't be.

In the one scope capture you say the purple is "tracking output"?  Do you mean the trigger output?

The crossover distortion could be caused by very mismatched hFE pairs, or it could be something wrong with the bias in one of the stages.  The transition from the positive side of the amplifier driving to the negative side, and visa-versa, is discontinuous.  You could try changing the frequency of the waveform and observe what it does to the distortion.  If it's less of a problem at lower frequencies, and the distortion gets closer to 0V, it's a problem with the speed of the changeover, perhaps capacitive in nature.


Also, note that the adjustment procedure has you constantly putting in attenuators and taking them out and changing the multiplier on the scope.  This is because HP has chosen an insanely over-powered 20GHz scope for the adjustment procedure that can only do 1mV/div to 80mV/div.  So, all these attenuators are compensating for the scope's deficiencies in this application.  Instead, you can simply put a 50 ohm terminator on your scope (or use the internal 50 ohm termination if it has one) and set the scope according to the needed V/div for any particular step.

MarkL:
Also, it may be worthwhile to re-visit the static operating points on your unit now that it's partially working.  I don't think you need to re-post all fresh readings from your unit since that's a lot of work to transcribe them, but you should compare again to a working unit.  You might find an area with incorrect biasing that is contributing to the distortion.

I've included my readings again below, updated with the two nodes from a few posts ago.

Mick B:
Hello again, just an update. The voltage for the -5.2 rail is set at -5.4 I could not get it to adjust, and then I found this in "UPDATEING/CHANGES/ERRATA Pg.11 this mod had been done. I set the voltages the day I got it, it had no -24v rail. I also did the option 001 verification procedure test it produces a square wave, but it is always on there is no timing function it starts as soon as the output is on and does not stop the counter will continue forever.
And yes, I meant "trigger output" my-bad. and I know what rise and fall times are (you presented a great explanation) DOH! I just couldn't put the two terms together.
I dissected one of my 10 turn pots they are carbon, the ones that were in there should have been 10 turn to start with to call them sensitive is an understatement.
I wondered why all the attenuation, I'm so glad you explained what that was all about. that will shave off a lot of time.
Without the attenuation nonsense I have not been able to reproduce the crossover distortion, YET. I'm going to re-due the calibration today. and take measurements after.
Thanks, Mick
 

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